LS1B PIE 2 Study Guide

In contrast, *slipper limpets* use ______________ determine sex. These small marine creatures form piles on the ocean floor - the first slipper limpet larvae that settles in a particular location will develop into a ____________, and secrete _____________________________________. Other slipper limpets will then ___________________________________________. The males and original female become reproductive partners. Over time, though, ____________________________________________. This causes additional larvae to join in ______________________________________________________.

*location* female chemical signals that attract other larvae to her location join the pile, and develop into males due to their location on "top" of the pile the males switch sex, becoming female and start to secrete attractive chemical signals themselves and develop into males that become reproductive partners with the females below. And, on and on

When analyzing data from experimental crosses and pedigrees, geneticists are often faced with the question of whether or not the data that they have collected are consistent with expected ratios based on the principles of Mendelian inheritance. Deviations from expected ratios are often the first clue that something interesting is going on, but researchers need to be able to distinguish those deviations that are due to chance from those that are significant. For example, when crossing a heterozygous round pea plant (Ww) with a recessive wrinkled pea plant (ww), we expect 50% of the progeny to be round (Ww) and 50% to be wrinkled (ww), based on the Mendelian principles of segregation and dominance. However, if you were to analyze 60 pea plants, you would rarely observe exactly 30 round and 30 wrinkled peas, in the same way that you would rarely achieve exactly 10 heads and 10 tails from a total of 20 coin tosses. In both instances, the results will likely deviate somewhat from the expected proportions (e.g., you may observe something a little different from the expected value, like 35 round peas and 25 wrinkled peas simply by chance). In cases like this, we need to determine ______________________________________________________________________.

*whether the deviation from the expected value (50/50 split) is greater than what we might see due to chance alone*

In humans, there are ________________chromosomes, numbered __________________________________. X and Y are known as the sex chromosomes, and the non-sex chromosomes (Chr1-22) are known as _______________. In eukaryotic cells chromosomes are found in ___________________________. Although each chromosome contain distinct genes, _____________________.

23 unique 1-22 and X / Y, with chromosome 1 being the longest and chromosome 22 being the shortest autosomes the nucleus, bounded by the nuclear envelope each chromosome has the same general structure

Take a look at the cross below: AaBbCCDdEeff x aaBBCcDdEeFf Let's say we wanted to know the likelihood of getting an offspring that has the dominant phenotype for all traits? In this case, we are asking the likelihood of having the dominant phenotype for A, and for B, and for C, and so on. __________________________________________________________________________________________________________ P(B phenotype) = probability of BB or Bb from this cross = 1 P(C phenotype) = probability of CC or Cc from this cross = 1 P(D phenotype) = probability of DD or Dd or dD from this cross = 3/4 P(E phenotype) = probability of EE or Ee or eE from this cross =3/4 P(F phenotype) = probability of fF from this cross = 1/2

Assuming each trait assorts independently, we can determine the likelihood of each individual event (e.g., likelihood of dominant phenotype for A), then multiply them together to get the final likelihood. 1) probability of getting dominant phenotype for each trait separately: P(dominant phenotype A) = probability of Aa from this cross = 1/2

2) Probability of getting dominant phenotype for all the traits P(dominant phenotype for all traits in F1) = ½ x 1 x 1 x ¾ x ¾ x ½ = 9/64 General tips when dealing with probabilities 1. ___________________________________________________________________ 2. __________________________________________________________________

Before doing any math, ask yourself what you're trying to figure out. Are you using the word "and" or "or" (e.g., are you using the multiplication rule or the sum rule)? Break it down into smaller steps. Don't try to solve it all at once. Deal with it piece by piece to make it more manageable.

________________________ may complicate or compromise meiosis when we have a ________________________ (i.e., an individual with two different chromosomal arrangements). The two homologous chromosomes have problems aligning and forming the _________________. This can lead to problems in _______________. In the case of inversions, for example, with a heterokaryotype consisting of an inversion-bearing and a non-inversion-bearing chromosome, ______________________. If a recombination event occurs within the inversion, ________________.

Chromosomal rearrangements heterokaryotype synaptonemal complex gamete production the chromosomes form a loop structure to accommodate the difference between them half of the resulting gametes will be inviable

Prophase

Chromosomes, consisting of two sister chromatids, condense. The sister chromatids are held together along their length via the action of cohesin proteins. The cohesin proteins attach to the chromosomes during 6 S-phase and persist until the chromatids are separated. As the chromosomes condense, if they become entangled or crossed, the enzyme topoisomerase cleaves one chromosome and passes the other through to resolve the tangle, then re-seals the break.

Anaphase

Contraction of the microtubules creates tension that stimulates the enzyme separase to cleave the cohesin proteins connecting the sister chromatids. The sister chromatids then separate and are pulled to opposite poles of the cell. Upon separation, each chromatid becomes a chromosome and there cease to be any sister chromatids in the cell.

_____________________________________ describes systems in which an individual's environment, not their genotype, determines their sex. There are a number of different environmental factors that influence sex determination in different species. For example, in turtles, _______________________________________________. At low temperatures, sexual development tends to take the "___________" path, while at higher temperatures, more eggs develop into __________. The effect of temperature can vary between species. For example, in alligators, _____________________________________, whereas intermediate temperatures ______________________________.

Environmental sex determination the temperature at which eggs are incubated determines what sex they will develop into male females the extreme temperatures (hot or cold) result in males lead to females

_________________________ is similar to chromosomal sex determination in that genetic factors - genes - are responsible for determining an individual's biological sex. However, the difference is that ____________________________________________. In cases where many genes are involved, the sex-determining genes __________________________________. ________________________________________ will determine their biological sex, rather than a combination of sex chromosomes. This mode of sex determination occurs in _______________________________.

Genic sex determination those genes are not located on observable sex chromosomes can be scattered around the genome The specific combination of alleles that an individual inherits at these loci many species of fish

__________________________ are rare sexual chimeras that can best be seen in _____________________________________________. Gynandromorphs contain __________________________________________________________. This makes the organism itself _____________________________. The mechanism by which this occurs is best understood in butterflies, where it can be (though not always) caused by __________________________________________.

Gynandromorphs birds and butterflies (because males and females are visibly different) cells that possess different combinations of sex chromosomes - some cells have sex chromosomes that make them female, while others have sex chromosomes that make them male partially male and partially female *spontaneous mitotic nondisjunction of sex chromosomes during development*

*Dosage Compensation* *Q:* Previously, we talked about the importance of gene dosage, and the (sometimes devastating) consequences of dosage imbalances that arise, for example, in individuals with trisomy of specific autosomes. In particular, only a small number of autosomal trisomies are tolerated in humans. So, why doesn't this apply to sex chromosomes? That is, if females have two X chromosomes, and males have one, how can the difference in dosage of those ~800 X-linked genes be tolerated? *A:* __________________________________________________________________________________________________________________________________________________________________________________________________________________________

In species that use chromosomal systems for sex determination, specific mechanisms have evolved to compensate for differences in the number of sex chromosomes found in males and females. This is called *dosage compensation*.

Other sex chromosome abnormalities involve individuals that have "extra" sex chromosomes. Males who inherit one or more extra X chromosomes (so have genotype XXY, XXXY, or XXYY) have ____________________. This is _____________________________, and is characterized by traits such as _____________________________________. Nonetheless, these individuals ________________________, providing further support for the idea that __________________________________.

Klinefelter Syndrome the most common male sex chromosome disorder small testes, reduced facial hair, and sterility have many male sexual characteristics a single Y chromosome, even with multiple X chromosomes, is sufficient for development of the male phenotype

There is a specific region on the Y chromosome that is responsible for "turning on" testicular development. This is known as ________________________________________. This region encodes a __________________________________________________________. When an SRY transgene is inserted into an XX embryo, even without the other Y chromosome genes, ________________________________________________. This shows that SRY can serve as a _______________________________.

SRY - the sex determining region on the Y chromosome transcription factor that is necessary for an individual to become biologically male and is sufficient for male development it results in the embryo developing the male phenotype, including functional male internal and external genitalia and gametes "switch" to initiate the typical male development pathway during *embryogenesis*

Telophase

The chromosomes arrive at the nuclear pole and the nuclear envelope reforms forming two distinct nuclei, each containing a full complement of chromosomes. The chromosomes decondense back into diffuse chromatin.

Metaphase

The microtubules move the chromosomes to the central plane of the cells by pulling on the attached chromatids until they reach the center of the cell. This central plane is also known as the metaphase plate or the equator, and at metaphase the chromosomes are all aligned there. When the chromosomes are aligned in the center, there is approximately equal tension "pulling" on each chromosome from the two poles of the cell. This tension sends a signal to the cell that all of the chromosomes are attached to both poles, and it is safe to proceed through mitosis.

Prometaphase

The nuclear envelope disintegrates. Microtubules begin to grow out from two centrioles that migrate to opposite poles of the cell. The microtubules grow and retract until they encounter a kinetochore, a protein structure located on the centromere of a condensed chromosome. There are two opposing kinetochores on each side of the centromere, each attaching to a sister chromatid. Attachment of a microtubule from either pole of the cell ensures that the sister chromatids will be segregated to one of the two resulting daughter cells.

As cells grow and divide, the genome must duplicate as well, as it is critical that each cell receives _________________________________. Mitosis is __________________________. To ensure that each daughter cell receives the correct number of chromosomes, __________________________. The cell cycle is a highly orchestrated process that ________________________________, each of which has a specific function that contributes to the ultimate goal of producing cells with the correct nuclear content.

a complete copy of the genome during cell division the process by which a normal eukaryotic nucleus divides mitosis can only occur at the culmination of the cell cycle can be divided into distinct stages

Pedigrees are __________________________________________________. It is unethical and impossible to plan human crosses in the same way as is done for model organisms such as pea plants or Drosophila. Instead, geneticists analyze the data presented in pedigrees of pre-existing families to understand the genetics of human traits. Pedigrees follow a convention for presenting data. When reading a pedigree, ____________________________________________________________________________________.

a graphical method for presenting the data from human mating events females are always depicted as circles and males as squares; affected individuals' symbols are shaded in, while unaffected individuals' symbols are not. In pedigree analysis, characteristic patterns are observed for dominant and recessive traits (i.e., traits that exhibit dominant inheritance display characteristic patterns that are distinct from recessive traits)

While the Punnett square represents a convenient method of predicting the results of monohybrid or dihybrid crosses, _____________________________________. Instead, we can __________________________________________.

it quickly becomes cumbersome when you wish to analyze more than two traits use two simple rules of probability to determine the outcomes of more complex genetic crosses

Alternatively, _________________________________ can also prevent activation of the male differentiation pathway. Some XY individuals have a mutation in the _____________________________________ (i.e., *androgen insensitivity syndrome*). These individuals may produce __________________________________________________________.

mutations in any one of a number of genes that function after SRY activation autosomal androgen receptor that responds to testosterone a high level of testosterone, but their cells cannot respond to the hormone, and so instead they develop as female

Many duplicate genes in fact become ______________ as they pick up mutations that make them functionless, but occasionally _________________. For example, opsins are __________________. Different opsins are configured in subtly different ways to be sensitive to different wavelengths of light. Over time, _______________________________.

pseudogenes these duplicates evolve new functions related to that of the original gene the photoreceptor proteins in your eye that transduce light into neuronal signals that are sent to the brain for processing the original opsin gene has undergone duplication events and these duplicates diversified in function

More recently, the discovery that fetal DNA can be found circulating in the mother's bloodstream has allowed _____________________________________. By taking a blood sample from the mother, DNA sequencing can be performed to determine if the fetus possess any chromosomal abnormalities.

the development of less invasive methods of prenatal genetic testing such as those developed by MATERNIT21

Meiosis and gamete formation Meiosis shares many similarities with mitosis, but there are also some key differences between the two processes. In mitosis, ___________________________________________. The goal of meiosis is quite different; ___________________________________. Therefore, they are genetically distinct from the diploid parent cell. Prior to meiosis, the cell proceeds through the cell cycle, growing and preparing for division. The interphase part of the cell cycle is ____________________________________. The nuclear divisions that occur during meiosis are broken down into the same stages as those described for mitosis. However, there are ______________________________. Below are the key aspects of meiosis that are distinct from mitosis. The discussion that follows is based on meiosis in human cells, which start with __________________________________________________.

the goal is to create identical daughter cells with the same number of chromosomes here the goal is to take diploid cells and form haploid ones that contain half the number of chromosomes exactly the same as that which precedes mitosis and includes the G1, S and G2 phases two rounds of division in meiosis, thus there are two prophases, two metaphases, and so on 46 chromosomes (23 homologous pairs) and produce gametes containing 23 chromosomes each at the end of meiosis

The probability (p-value) associated with a chi-square value is _________________________________________________. This p-value can be determined by looking up your _______________________________________________. But to look up the p-value you must first determine the degrees of freedom (DF). The degree of freedom represents____________________________. In the SMA example, we have a total of 228 children and two phenotypic classes (affected and unaffected). The expected data is therefore free to vary in one way only. As soon as we know the number of affected children and the total number of children, we know the number of unaffected children, and vice versa (e.g., given that the expected number of affected children is 57, then the number of unaffected children must be 171(228-57)). Therefore, in this example we have one degree of freedom. A quick method rule of thumb for determining degrees of freedom is: ___________________________________.

the probability that the null hypothesis is consistent with the observations, even with the observed deviation from the expected results calculated χ2 value in a table of critical values the numbers of ways in which the expected classes are free to vary # of phenotypic classes -1 = Degrees of freedom

Meiosis I The goal of meiosis I is _____________________________. Thus, there is a reduction in ploidy (the number of sets of chromosomes in a cell) during meiosis I from diploid to haploid.

to separate the 23 pairs of homologous chromosomes and ensure that the resulting two daughter cells receive one copy of each chromosome (one homolog, containing two sister chromatids)

Prior to the 1900 re-discovery of Mendel's 1866 publication, the prevailing theory of heredity was _______________________. This seems intuitively appealing: two tall people will generally produce tall children, just as two short people will produce short children, and a tall person paired with a short one will produce children somewhere in the middle. At around the same time as Mendel was doing his work, however, people realized ___________________. Think of a bell curve distribution of a trait, like height. The only way in which the tallest size category could be preserved in the next generation is through breeding between the tallest people (and, similarly, for the other end of the distribution: the shortest extreme can only be preserved if the shortest people only breed with each other). And yet, on average, a tall person will breed with someone rather shorter than they, leading, under blending inheritance, to children who are shorter than the tall parent. If this pattern is repeated over a number of generations, ________________________________________________.

*blending inheritance*, in which offspring are a mix, or blend, of their parents that blending inheritance inevitably eliminates genetic variation blending inheritance will lead to convergence on the mean - the bell curve will become narrower and narrower until there is no variation left

In addition to the monohybrid cross, Mendel also performed ________________________________________________________________________________. By carefully analyzing the results from many different dihybrid crosses, Mendel deduced the ___________________________________________________. The second law states that _________________________________________________. Note, this law concerns ________________________________, in contrast to the law of segregation, which concerns _______________________.

*dihybrid crosses* between peas that differed in two characteristics, e.g., a cross between a plant with yellow, round peas and a plant with green, wrinkled peas (so the peas differ in both color and shape) second fundamental law of inheritance - the law of independent assortment genes responsible for different characteristics (pea color or pea shape, etc.) sort independently of one another during gamete formation the sorting of alleles for *different* genes the sorting of alleles for *the same* gene

In mammals, biological sex is initially determined by the sex chromosome content of an individual. Females have two copies of the X chromosome, while males have one X and one Y. The X chromosome is large, containing approximately ________________________, while the Y chromosome is much smaller (both in physical size and gene content), with ______________________. Sex-linked traits (either X-linked and Y-linked traits) in humans _______________________________________________ (check out the "Pedigree Analysis Tutorial" to read more about how you can distinguish sex-linked traits from autosomal ones).

800 protein-coding genes only ~75 protein-coding genes show unique inheritance patterns that can be detected in pedigree analysis

Females who inherit one or more extra X chromosomes (so have genotype XXX or XXXX) have __________________. ________________________ does _________________________. Some individuals with the disorder __________, but this is not true of all Poly-X individuals. While little effect is observed for physical features, the probability that a poly-X female will exhibit ___________________________with increasing number of X chromosomes, underscoring the importance of ________. Gene expression and protein levels are finely tuned, so *the more extra X's an individual has, the more negative consequences are observed,* _________________________________.

Poly-X syndrome Poly-X syndrome not lead to any particular physical features are sterile intellectual disabilities increases gene dosage likely because X-inactivation is not complete (more on this below)

Contraction of the ___________ in anaphase I separates the homologous chromosomes, apportioning one of each chromosome to the resulting daughter cells. At the end of meiosis I, _____________________.

microtubules each daughter cell has 23 chromosomes and 46 chromatids

Cytokinesis

often (but not always) occurs after telophase.

Another type of environmental sex determination uses __________________ as the "deciding factor" in determining sex. For example, in clownfish, sex is based on social hierarchy, and can change when an individual's social circumstances change. All clownfish are born ________, and only the dominant individual in any group will be _________. The ________________________ is usually the largest and most dominant of the _________, and only ____________________. If the female dies or is removed, ________________________________________________.

social environment male female second-in-command male males he will mate with the female this large, dominant male will switch sex, becoming female, and the largest, most dominant males of the remaining members of the group will become her breeding partner

Evolution of Sex Determination When we look at the different sex determination strategies that have evolved in vertebrates, it is clear that __________________________________________________. Even within a *single group*, such as turtles or amphibians, _____________________________.

there are many different sex-determining mechanisms, and they can evolve very rapidly multiple sex determination mechanisms have evolved in even closely related species

Clearly this model of blending inheritance could not account for all the variation observed in the natural world, or even in the human population. What was needed was a non-blending, particulate model of inheritance. Why particulate? ____________________________________________. Thus, in Mendel's famous monohybrid cross (i.e., a cross focusing on the inheritance of a trait encoded at a single locus) between a dominant homozygote, AA, and a recessive homozygote, aa, we see only one phenotype (the one encoded by the dominant allele) in the resulting offspring, all of them Aa. But, if we cross the offspring among themselves (i.e., Aa x Aa), we recover all three genotypes and both phenotypes. Because of the particulate nature of the gene, we did not lose variation in the first generation, even if it was not apparent.

Because particles cannot and do not blend; they remain discrete

The second seminal conclusion that Mendel drew from his study of monohybrid crosses is known as ________________________. This law states that ______________________________________. This law explains the 3:1 ratio of round to wrinkled peas observed in the F2 progeny of the monohybrid cross. When two heterozygote F1 plants are crossed, 50% of the gametes from each plant contain the W allele and 50% contain the w allele. These alleles then combine randomly during fertilization to produce the F2 population. In this F2, all possible allele combinations are equally likely to occur, and three out the four possible allele combinations (WW, wW, Ww) produce round peas - only the fourth combination (ww) produces wrinkled peas. One way to lay out such a cross is a grid-based Punnett Square.

Mendel's first law of inheritance: the law of segregation each of the two alleles present in a diploid organism separate in equal proportions into gametes

__________________ occurs when sister chromatids fail to separate during anaphase. The result is that one cell receives an extra chromosome (2n+1), known as a trisomy and the other daughter cell lacks a chromosome (2n-1), known as a monosomy. These aneuploid cells will then proceed through further rounds of mitosis, resulting in ____________________.

Mitotic non-disjunction clones of related cells that are all either monosomic or trisomic

Mistakes in X-Y chromosome pairing can still happen despite the existence of _______________________________. Unlike most autosomal aneuploidies, sex chromosome abnormalities ______________________. Because the Y chromosome contains few genes, having _____________________________________. Additionally, cells that have more than one X chromosome ___________________________, so that any "extra" X chromosomes ____________________________.

PARs leading to sex chromosome non-disjunction are relatively well tolerated in humans an "extra" copy of it does not lead to the extreme imbalances in gene dosage that are observed when extra autosomes (especially large ones with lots of genes) are present "turn off" all but one of them, in a process known as *X inactivation* don't cause major dosage imbalances. (For more information, see "Dosage compensation" below)

During meiosis I, homologous chromosomes pair so that they can be properly segregated into gametes. ___________________________________, it is nonetheless necessary for them to pair up during meiosis. This ensures they ______________________________________________________.

Though the X and Y chromosomes are not very similar too can segregate properly such that each gamete receives either an X or a Y chromosome

Many sex chromosome abnormalities are known by specific names. Individuals who have only one X chromosome (and no other sex chromosome) have a disorder known as ____________________________. Their sex chromosome genotype is denoted as "__________" to indicate that they have only one X chromosome. Individuals with ___________________________________. These individuals are not fertile, but they do survive, and _______________________. However, individuals with no X chromosomes are never observed. This indicates that _______________________________________________.

Turner syndrome XO Turner syndrome typically develop with short stature, distinctive facial features, and fail to undergo normal sexual development at puberty emerging IVF therapies are even allowing them to have children at least one X chromosome is required for human development, and two for fertility in females

If SRY is present and functional,______________________________. Even though the SRY transcription factor ____________________________________. SRY causes the ___________________________________, which secretes (1) ______________________________, and (2) __________________________________________. Testosterone (and other androgens) cause the Wolffian duct to ___________________________________________________________________________________________.

a male-specifying transcription factor is made is only produced at a low level for a few days, it initiates a cascade of developmental events that leads to male sexual differentiation bipotential gonad to develop into the testis (1) *anti-Mullerian hormone*, causing the *Mullerian ducts* to degenerate (2) the *androgen hormone testosterone*, which promotes male characteristics differentiate into the epididymis, vas deferens, seminal vesicles, and ejaculatory duct, and for other male reproductive organs like the prostate and penis to form

Centromere is ______________________________________________. Centromeres and telomeres are comprised of _______________________________________________. This tightly compacted DNA is _________________________________________________. In contrast, the chromosomal regions that encode actively transcribed genes are comprised of relaxed, open DNA known as _________________.

a region of DNA that connects the two arms of a chromosome densely packed DNA, which is known as heterochromatin inaccessible to the transcriptional machinery, and as such centromeres and telomeres do not contain actively transcribed genes euchromatin

Pregnant women can undergo genetic testing to determine if their fetus contains a genetic abnormality such as trisomy 21. This can be done via ___________________________, a procedure for obtaining __________________. Amniotic fluid _______________________________.

amniocentesis a sample of amniotic fluid from the pregnant women the substance that fills the amniotic sac and surrounds the developing fetus- contains fetal cells that can be used for genetic testing

Errors in Mitosis and Meiosis Recall that _________________ are those that contain something other than the normal number of chromosomes per cell (46 in humans). ___________________________________________________________________________________________. For example, ________________________________________________. The failure of chromosome to separate properly during mitosis or meiosis is known as _____________. The consequences of nondisjunction are ________________________.

aneuploid cells Aneuploidy results from improper segregation of chromosomes during mitosis or meiosis if opposing microtubules fail to attach to the kinetochore of a chromosome, this results in a failure for them to segregate nondisjunction different depending on whether it occurs during mitosis, meiosis I or II

Karyotypes may also vary through chromosomal rearrangements, which are typically caused by ___________________________. Segments may be duplicated, deleted, inverted (i.e., flipped in their direction), or translocated (to different locations, even on different chromosomes). A _________________________ is a special case in which two chromosomes are combined into one with relatively little loss of genetic material. Human chromosome 2 is the product of a ________________________. When we compare genomes among relatives, it becomes clear that _____________________________________________________________________________________________________.

attempted repair following DNA damage Robertsonian translocation Robertsonian translocation between two smaller great ape chromosomes genomic segments get shuffled extensively among chromosomes over evolutionary time, suggesting that translocation events have played an important role in the shaping of genomes

Much like the law of segregation, the law of independent assortment is _____________________________________________. Each pair of homologous chromosomes _______________________________________________________________. The order in which the chromosomes pair up along the metaphase plate during meiosis I ___________________________________________________________. Since there are 23 pairs of chromosomes in a human cell, and these chromosomes all align independently of one another, _____________________________________________________________________________. Note, the law of independent assortment ______________________________________. The close proximity of genes results in _________________________________________. We will learn more about this limitation of the law of independent assortment in future lectures on linkage and genetic mapping.

based on the physical behavior of chromosomes during meiosis aligns independently of each other during metaphase I determines how the homologs will separate during anaphase, and what combinations of alleles will be present in each cell during meiosis II this process produces new combinations of alleles and thus generates extensive genetic diversity in the gametes does not apply to genes that are located very close to one another on the same chromosome linkage whereby alleles at loci that are close to each other on a chromosome tend to be inherited together (i.e., they do not assort independently)

We will talk about two different types of gynandromorphs: _________________ and __________________. __________________________________________ are those that are half male and half female, with the right half of the body displaying ____________________ and the left half of the body displaying ______________________. ______________________ predominantly display __________________________________, with _________________________________________. The difference between bilateral and mosaic phenotypes is due to __________________________________________ (with ___________________________________ leading to bilateral gynandromorphs).

bilateral mosaic Bilateral gynandromorphs male characteristics female characteristics *(or vice versa)* Mosaic gynandromorphs the phenotype associated with one sex patches of cells having characteristics of the other differences in the *timing* of sex chromosome nondisjunction *very early* non-disjunction

Non-disjunction (cont.) As we have seen, trisomy 21 is caused by non-disjunction in either Meiosis I or Meiosis II. In Meiosis I non-disjunction, _____________________. In Meiosis II non-disjunction, ___________________________________. Recombination could _______________________________________________________.

both homologous chromosomes are involved in the potential trisomy both chromosomes involved will be from just one of the homologous chromosomes (former sister chromatids) potentially interfere with identifying which of these cases we are dealing with, but because recombination does not involve the centromere and the heterochromatic region nearby, it's possible to recognize a recombinant chromosome's parent of origin by inspection of the region around the centromere

How is this possible? The X and Y chromosomes are not very similar in size or gene content, making it difficult to understand how they could be recognized as "homologous." However, _______________________________________________. Both contain sequences known as ______________________________________________________. In other words, the PARs are homologous, and therefore ______________________________. Because at least one cross-over event is required at pairing, the PAR _______________________________________ (we'll return to talk more about recombination rates in later lectures).

both the X and Y do contain specific regions that have similar sequences and can therefore be recognized as homologous *pseudoautosomal regions*, or PARs. These are specific regions at the end of both the X and Y chromosomes that have sequences that are similar enough to pair up and cross over during meiosis facilitate X-Y chromosome pairing during meiosis in males has the highest recombination rate (number of cross-overs per unit of DNA) in the genome

During mitosis, the chromosomes are physically moved around the cell ________________________________________________________________________________________________.

by the action of microtubules, long fibers composed of the protein tubulin, that expand or contract to ensure that the chromosomes are correctly partitioned into two resulting daughter cells

However, maintaining multiple genes of the same gene family in close proximity can have its costs. Unequal crossing over between the red and green opsin genes _________________. These genes are 98% identical in their sequence, which can lead __________. For example, a red opsin gene on one homolog may erroneously align with a green opsin gene on the other homolog. ____________________________________________________________ Since the opsin genes are on the X chromosome, when a male inherits an X chromosome that is missing one of the opsin genes, red-green color blindness results. Females are sometimes affected by red-green colorblindness, but not as often as males since they have two X chromosomes, meaning that __________________________. We will learn more about inheritance of sex-linked genes such as the red and green opsins in Unit 2.

can result in color blindness to misalignment during prophase I If crossing over then occurs between these misaligned homologs, this will result in deletion or duplication of opsin genes on each chromosome. a chromosome carrying a normal complement of opsin genes will mask the effect of a problem chromosome

The consequences of meiotic non-disjunction differ depending on whether ______________________. Nondisjunction during meiosis I results ____________________. Consequently, ___________________________________________. These aberrant daughter cells then proceed through meiosis II, producing two gametes that lack this chromosome and two that contain two copies (instead of only one). Thus, non-disjunction in meiosis I _____________________. When these abnormal gametes combine with normal gametes, they produce aneuploid offspring that are _______________________________.

chromosomes or chromatids fail to separate during the first or second division from a failure of homologous chromosomes to separate both homologs of a pair segregate into a single daughter cell and the other daughter cell does not receive a copy of this chromosome results in all four gametes being aneuploid trisomic (from the gamete that contained both homologs) or monosomic (from the gamete that was lacking the chromosome)

Most animals are __________, but ______________ is common in plants. Polyploidy can create ___________________________________________. In aneuploidy, _______________________________. In polyploids, ____________________________________.

diploid polyploidy developmental and genetic problems, but in general these are less severe than those encountered in aneuploidy dosage imbalance leads to uneven numbers of gene products, disrupting homeostasis as discussed previously with Trisomy 21 there is typically no such imbalance, because the copy number of each locus is the same

A third chromosomal sex determination mechanism has ____________________. In these organisms, the sex chromosomes are called ___________ to differentiate them from X and Y because ___________________________________. Females are _______________ they have __________________________. Males are _______________________________. In contrast, in mammals, males (XY) are ________________, and females (XX) ___________.

evolved independently in birds and butterflies Z and W their sex determination is like the mammalian system, but backwards heterogametic two different sex chromosomes, one Z, and one W homogametic and have the sex chromosome genotype ZZ heterogametic homogametic

How do gynandromorphs work? Let's start with a ZW individual, which you would expect to develop with exclusively ___________________________. If the W chromatids fail to separate during the first cell division in the zygote, __________________________________________ (female), while the other would be ______ (male). The _______ cell could then give rise to the "male" half of the organism, while the _________ cell could give rise to the "female" half of the organism. When _______________________________________, a mosaic gynandromorph can be produced. If we start with ________________________________, most of the cells would develop the female phenotype, but if __________________________________ it (and its descendent cells) will display the male phenotype instead. The __________________________________, the *more cells will be affected* and the ________________________________________________________.

female characteristics (remember, in butterflies and birds, the female is heterogametic) one of the daughter cells would then be ZWW ZO ZO ZWW this nondisjunction happens later in development an initial ZW individual the W chromosome is "lost" by a cell later in development, earlier nondisjunction happens greater the extent of the mosaicism

Interestingly, molecular biologists have identified the molecular details underlying _______________. We thus have a mechanistic understanding of the mode of action (e.g., recessivity) identified by Mendel. For example, the white flower mutation is caused by _________________. _______________________________________________________________________________________________________________________. This is a loss-of-function mutation, so it is not surprising that the white petal form is ____________________. Having one working pigment pathway is ________________________________________________.

four of Mendel's seven mutants a gene involved in the production of pigment A single base pair mutation at the margin of an exon and an intron causes the intron to be incorrectly spliced out, such that the resulting frameshift in the reading frame causes the protein to be terminated prematurely *recessive* (loss of function) enough to ensure the production of purple flower, an instance of *haplosufficiency*

A fourth chromosomal sex determination mechanism is known as _____________________________________, and it is used in some insect groups (e.g., bees, ants and wasps). In these insects, females are ___________, and produced by________________________, while males are ______________, and produced by __________________. ____________________________ is a form of _____________________________.

haplo-diplo sex determination diploid "standard" sexual reproduction (sperm + egg à offspring) haploid *parthenogenesis* Parthenogenesis asexual reproduction in which an unfertilized egg develops into a new individual

Throughout interphase, chromosomes are ____________________________. The contents of the nucleus resemble an entangled mass of spaghetti! You can imagine how difficult it would be to untangle this mess and ensure that one copy of each and every chromosome is correctly apportioned to each daughter cell. Instead of attempting the impossible, _________________________________________________, which are easily manipulated. It is these condensed chromosomes that are visualized in a karyotype.

in an uncoiled and relaxed state, which makes it very difficult to distinguish individual chromosomes DNA undergoes extreme condensation during the beginning of mitosis, producing discrete and discernible chromosomes

In evolution, this expansion and contraction of the number of similar genes along a chromosome has been an important source of novelty. Suppose a mutational event resulted in the duplication of a gene, such that the two copies of the gene sit beside each other on a chromosome. Unequal crossing has now _______________________________. Once there are superfluous copies of a gene present, _________________________. If there is only a single copy of a gene present, and ______________________________. But once there is a second copy, ______________________.

increased the number of copies of the gene in the gene family divergent evolution is possible it performs a vital function, then *purifying selection* prohibits evolutionary divergence (i.e., all mutations affecting its function will be selectively eliminated) we have redundancy in the system, and one of the two is free to evolve, unconstrained by *purifying selection*

Meiotic non-disjunction of chromosome 21 results in Down syndrome, a disorder in which ____________________. You may be wondering why it's relatively common to observe individuals with Down syndrome but you never hear of individuals with trisomy 1, 2 or 3. _________________________________. The symptoms of Down syndrome, such as intellectual disability and facial abnormalities, are due to_______________________________. Wild type cells have ______________________________________. However, in the case of trisomy 21, these cells have an extra copy of chr21 and thus ________________________________________ highly sensitive to the relative amounts of gene products, this increased expression ________________________.

individuals have three copies of Chr21 This is because of the importance of maintaining a balance of gene dosage from all chromosomes an imbalance in the amounts of gene products produced from chromosome 21 two copies of each chromosome and as such the cell is considered 'balanced,' meaning that the correct amount of mRNA and protein is produced from each gene there is increased expression from all the genes present on this chromosome. Since many cellular processes are disrupts many essential cellular functions

In humans, early development of the future reproductive organs of both males and females is ____________________________________. During the ____________________________________, both sexes develop an *undifferentiated (or bipotential) gonad*, which will produce the gametes, and accessory organs known as ______________________________. If the SRY gene is absent (or nonfunctional), the embryo __________________________________. __________________________________ will give rise to the female reproductive organs - fallopian tubes, uterus, cervix, and upper vagina, while the _______________________ will degenerate.

initiated in the exact same way first six weeks of development the Mullerian and Wolffian ducts "defaults" to develop female reproductive traits The Mullerian duct Wolffian duct

So far, we have discussed aneuploidy in which just a single chromosome is involved in a non-disjunction event. What if the event does not affect just a single chromosome, but all of them? (For example, systemic failure of the spindle would affect all chromosomes equally). The result in this case ______________________________________________________ A single set is: _______________ Two sets are: _________________ Three: __________ Four: _________

is polyploidy, extra sets of chromosomes (i.e., entire extra genomes). Haploid Diploid Triploid Tetraploid

In Drosophila melanogaster (every geneticist's favorite fruit fly!), sex is also determined by an individual's chromosome content; but in flies, it is not the presence or absence of a Y that matters, instead ________________________________. Though most Drosophila males ________________________________. In a "normal" diploid female, there are ______________________________________. Most often, diploid males have _______________________. Variations on these ratios are possible because fruit flies ____________________________________.

it is the ratio between X chromosomes and autosomes do contain a Y chromosome, sex is determined by the ratio of X chromosomes to haploid sets of autosomes (e.g., each X chromosome counts as 1, and each haploid set of autosomes also counts as 1) 2 X chromosomes and 2 sets of autosomes (one homologous pair per chromosome) for an *X:A ratio of 1:1* one X, one Y, and 2 sets of autosomes, for an *X:A ratio of 1:2, or 0.5* can survive as triploid, producing other ratios that can lead to intersex, meta-female, and meta-male phenotypes

Key to Mendel's discoveries was a specific type of cross known as the ___________________ - a cross between two individuals that __________________________. In other words, homozygous individuals contain two identical alleles at the loci that determine the trait. In the case of pea shape, if one parent has genotype WW (round) and the other has genotype ww (wrinkled), each of these parents would be considered homozygous for their respective trait, producing only a single type of gamete. The true-breeding round parent will only produce W-containing gametes while the true-breeding wrinkled parent will only produce w-containing gametes.

monohybrid cross differ by a single trait and are homozygous (or true-breeding) for that trait

This last fact might seem trivial, but was a major stroke of luck. It is difficult to identify ______________________________________________________. While pathological variants are often monogenic because ______________________________ (Cystic Fibrosis, for example, is caused by mutations at a single locus), most traits are determined ______________. Even traits that are popularly supposed to be monogenic are often the result of multiple loci. Eye color, to take a famous example, is determined by several genes, with two having the main effects. The claim that two blue-eyed people can only have blue-eyed children is bogus. Eye color is thus a polygenic trait, encoded by multiple loci. Often, traits encoded by one or very few loci are termed _______________________. Those encoded by several (or more) loci are complex or quantitative. We'll be looking at these in detail in Module 2.

non-pathological genetic variation in our own species that is monogenic they involve the failure of a key component of a pathway by more than one gene simple or Mendelian, because it is feasible to work out their Mendelian basis

It's important to note that rejecting the null hypothesis is ____________________________________________________. Many more experiments are required to fully understand the factor(s) responsible for the observed deviations, and it would be _____________________________________________________. Therefore, when performing chi-square calculations, the results _____________________________________________________________________.

not equivalent to accepting the alternative hypothesis - it simply lends support to the alternative hypothesis woefully premature to accept an alternative hypothesis on the basis of this single experiment lead you to either reject the null hypothesis (and thus seek out alternative explanations for why the data do not conform to your expectations) or fail to reject the null hypothesis, suggesting that chance is a sufficient explanation for why your results may not have perfectly matched your expectation

Though gynandromorphy has been observed in birds, butterflies, and other sexually dimorphic species, it is unlikely to occur in humans or other mammals because ___________________________________________________. In addition, it should be noted that in many gynandromorphs, the mechanism has not been confirmed experimentally; and there may be other common causes (e.g., __________________________________________________) of gynandromorphy.

of the mechanisms that control the development of mammalian sex characteristics (e.g., the role of circulating hormones) *polyspermy*, where two sperm simultaneously fertilize a single egg

The final sex chromosome abnormality we will discuss is one that _____________________________. Males with one or more extra Y's (e.g., genotype XYY) have _____________, and _________________________. They may ________________________ but these effects are not as obvious as those caused by extra or missing X chromosomes.

often goes undiagnosed, because individuals who have it rarely exhibit any significant observable anomalies Poly-Y syndrome only mild phenotypic effects are found in these individuals have acne, be somewhat taller than average, and an increased risk of learning disabilities,

Thus far, we have been focusing on one specific sex determination mechanism: X-Y chromosomal sex determination, which is observed in (most) mammals. Other animal groups, though, use different sex determination mechanisms, including _________________________________.

other chromosomal sex determination systems, genic sex determination, and environmental sex determination

Despite their lack of dosage problems, _________________________, and many are sterile. Whether or not they are sterile depends on the __________________________, but the usual problem is associated with _________________________. If a species is triploid, how can the three homologous chromosomes consistently pair up? Things get even more complicated if there are non-sister chromatid crossing over events among all three homologous chromosomes. These meiotic problems may lead to _______________________. For the animal or plant breeder, however, this sterility may be an advantage. The _________________________________________________________.

polyploids often face significant biological problems specifics of their chromosome complement homologous pairing in Prophase I of Meiosis I full sterility or to severely compromised gamete production domesticated banana (3n = 33), being sterile, does not produce the hard, bulky seeds that are made by its diploid ancestor and is accordingly a better crop fruit

Looking up 14.61 with one degree of freedom in the chi square critical values table gives a p-value of < 0.001. This p-value, or probability value, tells us how likely the observed deviation from the expected values is _________________. If the p-value is high (p>0.05), then we conclude that ___________________________________________. If the p-value is low, ____________________________________. It is convention in science to use the 0.05 (5%) probability level as the cutoff value. This means that if the p-value is greater than 5% then you would fail to reject the null hypothesis. In the SMA example, our results show that there is a less than 0.1% chance that we would have observed these results on the basis of chance alone given our 3:1 null hypothesis. In this instance, we reject the null hypothesis, as the p-value is less than the 0.05 cut-off level.

simply due to chance chance alone could be responsible for producing deviations from the expected results then this tells us that the deviations are so great that it is likely that some factor other than chance is responsible for the observed results

The homologous pairs align at the metaphase plate during metaphase I. The order in which the chromosome align is completely random. This has important consequences, _____________________________________________. This independent assortment of chromosomes is ________________________.

since as the chromosomes segregate during anaphase I, each daughter cell receives a unique combination of maternal and paternal chromosomes the second mechanism by which genetic diversity is created during meiosis I

The ends of the linear chromosomes are known as _______________; these act as _____________________________. Because of the _______________________________________, the replication of the two DNA strands proceeds in two very different ways. The ___________ is copied continuously, but the ______________ is copied in segments (Okazaki fragments) which are then ligated together, with each segment being ________________________________. This process creates a problem at the ends of chromosomes (i.e., the ends of DNA molecules) because ________________________________. As a result, each round of DNA replication will _____________________________________________. Telomeres help prevent this from happening. An enzyme, ___________________, extends the template strands of the lagging strands beyond their actual ends by _____________________________________. The RNA-priming system can then _____________________________________________________________________________________________________.

telomeres protective caps that prevent DNA degradation directional processing of DNA polymerase leading strand lagging strand independently initiated by an RNA primer the fragment-initiating RNA primer is unlikely to anneal to the template DNA of the lagging strand right at the very end of the strand in principle lead to the loss of DNA sequence from the end of the chromosome telomerase adding telomere-specific repeat sequences prime the synthesis of DNA from within this added material, ensuring that the full length of the original template strand is copied

Even though there are very few genes on the Y chromosome, it nonetheless contains the genes required for male reproductive traits, such as those needed for _____________________________. In mammals, the presence or absence of _______________________________ individuals who possess a Y chromosome (with exceptions noted below) will develop with _____________________________________________________________.

testicular development and sperm production the Y chromosome (or really, the genes on it) is largely responsible for determining one's biological sex male sexual organs, while those who do not possess a Y chromosome will develop with female sexual organs

Given this, it is clear that the Y chromosome, and SRY specifically, is key in mammalian sex determination. However, there are some situations in which an individual can possess a Y chromosome and still be anatomically female. One possibility is __________________________________________________. In the absence of a functional SRY transcription factor, ________________________________________________________________. If the mutation is not a complete loss-of-function in SRY, it is possible _______________________________________________________. Mutations on SRY are responsible for ___________ of cases in which a genetically XY individual develops as female.

that SRY is either deleted or mutated so that it no longer functions the male-development "switch" is not activated, and the individual will follow the path to female sex organ differentiation for intermediate or variable phenotypes to emerge; for example, an individual may possess some male and some female sex characteristics (an intersex individual) 10-15%

When you cross two true-breeding parents, you designate them as _______________________. When crossed, the parents (round, WW, and wrinkled, ww) produce ____________________________________. In this cross 100% of the F1 offspring have genotype Ww and round seeds - the wrinkled phenotype is not observed at all. The results of this cross inform us which phenotype, round or wrinkled, is dominant. In this instance, round is dominant as this is the phenotype displayed in the heterozygous F1 offspring. When these F1 plants are crossed with each other they produce the second filial (F2) generation. Here we see the reappearance of the wrinkled phenotype at a ratio of 3 round to 1 wrinkled. Thus, it was from studying many versions of this monohybrid cross that Mendel deduced the concept of dominance. The phenotype that is displayed in the heterozygote is denoted as dominant and the phenotype that 'disappears' is considered recessive.

the P0, or parental, generation the F1 (First Filial) generation

The second rule is _______________________________________________. For example, let's return to the monohybrid cross between round (WW) and wrinkled (ww) pea, but this time we wish to know: What is the probability of producing a round pea in the F2 generation? Since round is the dominant phenotype there are multiple genotypes (WW or wW or Ww) that will produce a round pea and we must account for this. Therefore, in this situation we utilize the addition rule, in addition to the multiplication rule: P(WW) = P(W) x p(W) = 1/2 x 1/2 = 1/4 P(Ww) = P(W) x p(w) = 1/2 x 1/2 = 1/4 P(wW) = P(w) x p(W) = 1/2 x 1/2 = 1/4 P(Round) = 1/4 + 1/4 + 1/4 = 3/4 The above examples illustrate the application of the product and sum rules in a relatively simple cross, however the same principles can be used just as easily to calculate the probability of a particular outcome from a more complex cross involving many different characteristics. A helpful way to determine whether you should apply the addition or multiplication rule when solving these types of problems is to look for the key words "AND" and "OR." "AND" signifies independent events occurring together and thus the multiplication rule should be used. "OR" signifies two or more mutually exclusive events occurring and thus the addition rule is applicable.

the addition, or sum, rule, which states that the probability of any one of *two or more mutually exclusive events* occurring is the sum of the probabilities of the individual events occurring

A new cycle begins after a cell has divided to produce two daughter cells. Let's follow the cell cycle in one of the daughter cells. The first stage of the cell cycle is Gap 1 (G1). During this phase, __________________________________________________. After ____________________, the cell is held at the ________________________. Checkpoints are vital for _______________________________________________________________________________________________. At the G1/S checkpoint, _______________________________________________. After passing through the G1/S checkpoint, cells enter S-phase of the cell cycle 5 (the "S" stands for DNA synthesis). _____________________________ S-phase. At the end of S-phase, each chromosome consists of two identical sister chromatids. Importantly, even though the amount of DNA in the nucleus has doubled during S-phase, __________________________. At the end of S-phase, the cell enters a second gap phase, G2. ____________________________________. The G2/M checkpoint follows the G2 stage. Here the cell ____________________________ may have occurred during replication. If the cell is unable to repair damaged DNA, or if it detects unreplicated DNA, _______________________. The three stages just described (G1, S, G2) are collectively referred to as ______________, which is an extended period of growth prior to the cell division. After the cell has completed interphase, it is ready to enter the ______________ phase of the cell cycle.

the cell grows and synthesizes all the components necessary for DNA replication ~10 hours in G1 G1/S checkpoint quality control and help prevent the production of aberrant, aneuploid cells; cells without sufficient materials will not proceed past specific checkpoints the cell evaluates whether there are sufficient enzymes and components for DNA replication The entire genome is replicated during there are still only 46 chromosomes, only now each consists of two identical sister chromatids Specific cellular machinery required for division are synthesized during this phase evaluates whether the DNA has been replicated successfully and corrects any errors that it will remain held in the G2/M checkpoint, unable to proceed to mitosis interphase M (mitosis)

Another example of gene duplicate diversification is _________________________________. Again, a process of ancestral duplication has permitted ______________________________. For example, there is a switch over in the globin subunits comprising hemoglobin around the time of birth, as the baby shifts from acquiring its oxygen from its mother's bloodstream across the placenta to acquiring it from the atmosphere via the lung.

the evolution of the globin genes, which are involved in transport and storage of oxygen the evolutionary fine-tuning of particular globins for particular functions

Chromosome 21 has ___________________________________. During meiosis, there will be non-disjunction of chromosomes other than 21 - errors such as nondisjunction occur more or less randomly, so all chromosomes are affected with approximately equal frequency. However, the additional copy of these larger chromosomes, which contain more genes, ______________________________________.

the fewest number of protein-coding genes of all the chromosomes and this explains why trisomy 21 is relatively common, but trisomy 1 is never observed upsets the normal equilibrium in cells so much that the resulting embryos are inviable

Gregor Mendel is considered the father of genetics due to his groundbreaking work in which he used peas as a model system. Through careful analysis of seven characteristics of the pea plant, Mendel discovered ___________________________, which we will discuss in further detail in Lecture 8.

the fundamental laws of inheritance

Geneticists use a statistical test known as _______________________________________ to evaluate the role of chance in producing deviations between expected and observed values. This test can be used _______________________________________. The goodness of fit test is calculated using the following equation: _______________________________________________ (i.e., the chi-square value is equal to the sum of the differences between the observed data (O) and the expected value (E), squared, and divided by the expected value.)

the goodness of fit chi-square test to detect deviations that are great enough to suggest that the fundamental laws of Mendelian inheritance are not being met X^2 = SIGMA(O-E)^2/E

During prophase I _____________________________________. While paired, non-identical sister chromatids (one from each homolog) _________________________. Crossing over (or recombination) is an essential feature of meiosis I. Cohesins are _____________________________________________. _______________ is one mechanism by which genetic diversity is generated during meiosis, as it produces novel combinations of alleles.

the homologous chromosomes pair up, gene by gene, along their entire lengths, forming the synaptonemal complex exchange genetic information in the process of crossing over at chiasmata (singular: chiasma) deposited at the sites where recombination occurs and help to stabilize the homologous pairs as they are moved to the metaphase plate Crossing over

Probability refers to _________________________________________. The probability of an event occurring is the number of times an event takes place, divided by the number of all possible outcomes. For example, the probability of rolling the number 2 on a six-faced die is 1/6 (i.e., one out of the six possible outcomes is a 2). The probability of rolling the number 1 is also 1/6, because there are six faces on the die and each has an equal probability of occurring (provided that the die is fair!). Mendel's laws of inheritance allow us to utilize the rules of probability when predicting the outcomes of genetic crosses, as we will see below.

the likelihood of a particular event occurring

The first rule of probability that we will utilize to predict the outcome of crosses is ____________________. This rule states that the probability of *two independent events* occurring together is _____________________________________. For example, in a monohybrid cross between a round (WW) and wrinkled (ww) pea, the probability of obtaining a wrinkled pea (ww) in the F2 generation is the probability of a "w" gamete from one parent and a "w" gamete from the other parent coming together. Each heterozygote parent produces 1/2 W and 1/2 w gametes, therefore, the probability of getting a "w" gamete from either parent is 1/2, and the probability of getting a "w" from both parents (ww offspring) is 1/2 multiplied by 1/2, which equals 1/4.

the multiplication, or product, rule the product of their independent probabilities

In contrast, in C. elegans (our favorite nematode worm!), it's __________________ that determines an individual's sex. ______ individuals (the most common sex chromosome genotype) are ______________ (individuals producing _________________), and _____ individuals (with only ______________) are ________. There are ______________________________.

the number of X chromosomes XX hermaphrodites both eggs and sperm XO a single X chromosome male no Y chromosomes (and no females)

Mitosis is ________________________. During mitosis, identical sister chromatids are ____________________. Although mitosis is a continuous process, it can be divided into five stages: prophase, prometaphase, metaphase, anaphase and telophase. Cytokinesis, the division of cytoplasm, is the final stage of M phase, ____________________________.

the phase of the cell cycle during which the nucleus and all of the chromosomes divide separated into individual chromosomes and then partitioned into two daughter cells although it is not itself part of mitosis

If the chromosome sets are all derived from the same species, ______________________________. If the chromosome sets are derived from different species (through inter-specific hybridization), __________________. In many organisms, cell volume is ____________________________________. The increase in chromosome number in polyploidy is thus often associated with ___________________________________. Breeders have used this effect to __________________________________________________________.

the result is an *autopolyploid* the result is an *allopolyploid* correlated with nuclear volume, which in turn is determined by genome size (i.e., ploidy number) an increase in cell size, and many polyploids are physically larger than diploids produce plants with larger leaves, flowers, fruits, and seeds. As a result, many familiar domesticated plants are polyploids

Mendel was both smart and lucky. He chose both the right system and the right traits. By careful manipulation with hand pollination, he could ensure that he only mated between plants with traits he wanted to cross. As for the traits themselves, ______________________. Further, ________________________________.

these he inherited from generations of farmers and gardeners who had identified the mutants (e.g., wrinkled versus round peas; purple versus white flowers) and maintained them as homozygous stocks the traits were discrete, easy to score, and each individual trait was determined by a single locus (monogenic)

Another important consideration is the statistical power of the test we are running. Imagine a case of a biased coin that produces, on a coin flip, Heads 80% of the time. We flip the coin ten times, and get 8 Heads. We run a goodness of fit chi-square test of the null hypothesis that the coin is unbiased (i.e., the expectation is 5 Heads, 5 Tails). We have one degree of freedom (if we know the total, 10, and the number of Heads, 8, then we know the number of Tails, 2), and we have a chi square value of 3.6. Looking this up in the table, we find that p > 0.05, meaning that we cannot reject the fair-coin null hypothesis. Note, critically, ____________________________________. The message here: ________________________________________________________________________________________________________________________________________________________________________________________________________.

this does not mean that the coin is fair; rather, we just don't have enough data to reject the null just because you've failed to reject the null hypothesis, does not mean that the process on which the null is predicated - in this case, a fair coin - is actually operating. We have to be concerned about the statistical power of our test. Let's run the same test again, but with 100 coin flips, resulting in 80 Heads and 20 Tails. Now the chi square value - same null hypothesis of a fair 50/50 coin - is 36, and p << 0.005. We reject the null hypothesis of a fair coin, because we had the statistical power - sufficient sample size - to do so

*Sex Chromosome Evolution* In the long-term, this could be problematic. Recombination along a chromosome is important for it ______________________________________. This process suggests that the Y chromosome will ______________________________________________________. (Yes, think about that for a moment!) However, comparison of Y chromosome size across mammals suggests that Y degradation has ______________-. Further study has revealed that Y chromosome extinction is unlikely thanks to an interesting feature of the Y chromosome: ___________________________________________. These palindromic regions allow the Y chromosome to _______________________________________________, and "saving" the Y chromosome from its eventual degradation and demise. Phew!

to repair DNA damage that occurs and gain genetic material from its partner chromosome degrade (i.e., Y chromosome degeneration) and eventually become extinct over time plateaued sequences on the Y chromosome are often palindromic - they read the same forwards and backwards recombine with itself, repairing DNA damage using the other copy as a template

Meiosis II The goal of meiosis II is ________________________. Meiosis II often appears similar to mitosis (with half the chromosomes of a diploid cell), except _______________________. After the chromosomes have aligned at the metaphase plate in metaphase II, the sister chromatids are ________________________. After their separation, the sister chromatids are now considered individual chromosomes. Meiosis I produced two daughter cells and each of these cells undergoes meiosis II, producing four haploid gametes. In females, _________________________________. In contrast, ___________________________________ in males.

to separate the sister chromatids that the sister chromatids are no longer identical (due to recombination) segregated to opposite poles during anaphase II only one of these gametes develops into a functional egg, the other three eggs form polar bodies, which are degraded all four haploid gametes form functional sperm

In contrast, non-disjunction in meiosis II results in _______________________________. Here, the cell proceeds normally through meiosis I, and the homologous chromosomes segregate evenly into the two daughter cells. Non-disjunction in meiosis II results in ______________________________________. One daughter cell therefore __________________________. Non-disjunction typically ____________________________________, producing two normal haploid gametes. Therefore, non-disjunction in meiosis II produces two aneuploid and two haploid gametes. When the abnormal gametes combine with normal gametes, _________________________________________________.

two normal and two aneuploid gametes a failure of sister chromatids to separate in one of the cells during anaphase II receives both sister chromatids, and the other does not receive a copy of this chromosome only occurs in one of the two products of meiosis II, while the other cell proceeds normally through meiosis they produce aneuploid offspring that are trisomic (from the gamete that contained both copies of the chromosome) or monosomic (from the gamete that was lacking the chromosome)

Another source of errors in meiosis stems from _____________________________________________. Crossing over occurs during _____________________. Errors sometimes arise from unequal crossing over, in which tandemly duplicated genes misalign during prophase I: genes on one chromosome align not with their direct homologs on the other chromosome but with _______________________. This can result in ___________________________________. These duplicated genes are considered to be ___________________________________.

unequal crossing over between homologous chromosomes prophase and is essential for stabilizing the homologous pairs as they move through meiosis I related nearby genes of similar sequence duplications and deletions that affect the number of copies of these genes inherited by each gamete members of the same gene family

The results of a monohybrid or dihybrid cross can be visualized __________________________________________. In a Punnett square, you write out the possible gametes produced by one parent in rows, and the possible gametes produced by the other parent in columns, and "combine" the alleles found in each gamete in the corresponding cell in the grid. You can then evaluate the genotypes that result to predict the phenotypes of the offspring. Regardless of the traits under investigation, any monohybrid dominant-recessive cross will produce a characteristic _______________________________. Similarly, any dominant-recessive dihybrid cross produces a characteristic ___________________________________________________________. This genotypic ratio is determined by _________________________________.

using a Punnett square - a shorthand method in which you can use a grid to predict the results of a cross 1:2:1 genotypic ratio in the F2 progeny, and a corresponding 3:1 phenotypic ratio 9:3:3:1 phenotypic ratio amongst the F2 progeny as a direct consequence of the dominance relationships between alleles and the genotypic ratio in the offspring the independent assortment of genes

A karyotype is a __________________________________________. Like the majority of mammals, humans are diploid (2n), meaning they contain two copies of each chromosome. During fertilization, the egg contributes one complete set of chromosomes (n), as 4 does the sperm (n). A single cell therefore has _______________________________________________. Homologous chromosomes possess the same genes, in the same order, though they may possess different alleles or versions of those genes. Normal human karyotypes contain 46 chromosomes. Deviations from this number are known as _____________________________________. Karyotypes with 45 chromosomes are _______________________________ - for one pair of homologs, only one is found (instead of the normal two). Karyotypes with 47 chromosomes are ___________________________________ - for one pair of homologs, a third copy is found (instead of the normal two).

visual representation of the chromosomes in an individual's genome 23 pairs of chromosomes, and the two chromosomes that comprise each pair are referred to as homologous chromosomes, or homologs. aneuploid karyotypes monosomic, because they are "missing" one chromosome trisomic, because they have an "extra" chromosome

*Sex Chromosome Evolution* The question then becomes how sex chromosomes can evolve, quickly and repeatedly, in different evolutionary lineages. Here we will focus on a model that has been developed to understand the evolution of sex chromosomes (X and Y) in mammals, which have been studied in some detail. In this model, we start with the assumption that sex determination _________________________________________________________. The model first proposes that approximately ___________________________________________, triggering the male pathway. Second, over time, mutations at other loci on that ________________________________________________________________. Third, these male-specific genes are under selective pressure to ______________________________________. Over time, this allows the two proto sex chromosomes to _______________________________________.

was originally not chromosomal (and may not even have been genetic at all; perhaps it was environmental) 250 million years ago, a mutation in an autosomal gene resulted in a gene with SRY-like function "proto-Y" chromosome occur, and genes that benefit males (such as those that are involved in sperm production) accumulate on that chromosome because they are guaranteed to be in male individuals and never in females not recombine with the "non-male" chromosome further differentiate - in sequence, gene content and size

In lecture, we look at an example analyzing whether embryos that are homozygous for the SMA mutation _____________________________________. Our sample population is 228 children, whose parents (both mothers and fathers) are all heterozygous for the SMA mutation. The observed results are the number of children with SMA (32) and the number of unaffected children (196). Observed data: • Sample size: 228 children • Children with SMA: 32 • Children unaffected by SMA: 196 The expected results are calculated based on the null hypothesis. In this instance the null hypothesis is that ___________________________________. Based on this, we would expect there to be ¾ x 228 = 171 unaffected and ¼ x 228 = 57 affected children. Expected data: • Sample size: 228 children • Expected ratio of phenotypes based on heterozygous parents: o ¾ children with SMA o ¼ children unaffected by SMA • Expected values: o Children with SMA: ¾ times sample size = 228 o Children unaffected by SMA: ¼ times sample size = 57 (Note: a chi-square test must always be applied to numbers of progeny, rather than fractions or percentages. Therefore, we multiplied the expected phenotypic ratio by the total number of offspring in order to calculate the expected number of individuals in each phenotypic class.) We then plug the observed and expected values into the chi-square formula to derive the chi-square value, which in this instance is 14.61. Now that we have the chi-square value, we need to check the probability of this outcome.

were dying in utero and thus underrepresented in the children of heterozygous parents *the observed numbers of individuals with or without SMA does not differ from the expected phenotypic ratio (i.e., 3 unaffected :1 affected) more than can be explained by chance alone*

These diverse mechanisms for chromosomal sex determination show that, _____________________________________________, and importantly, that these systems _____________________.

while they may have some similarities, there are many ways to determine sex genetically have evolved independently over time

The human genome is a massive repository of information and is comprised of ___________________________ of DNA arranged into chromosomes. DNA is further organized _________________________________. In the human genome there are ___________________________________. The unique structure of chromosomes facilitates ____________________.

~ 3 billion base pairs into functional units known as genes ~20,000 genes that encode for proteins and a similar number that encode for functional RNAs the packing, the organization, and the transport of DNA