Heredity

XIST

(X inactive specific transcript) A part of the chromosome contains several genes involved in the inactivation process --Inactivation of an X chromosome involves modification of the DNA and histones, including attachment of methyl groups to DNA nucleotides. The two regions, one on each X chromosome, associate briefly with each other in each cell in an early stage of embryonic development. Then XIST becomes active only on the chromosome that will become the Barr body. Multiple copies of the RNA product of this gene apparently attach to the X chromosome on which they are made, eventually almost covering it. Interaction of this RNA with the chromosome initiates X inactivation, and the RNA products of the nearby genes help to regulate the process.

Meiosis I

-During this division the chromosome number is reduced by half -The chromosomes never duplicate in Meiosis I and II -First division, homologous chromosomes separate creates haploid cells (n) - each replicated chromosome is from either parent. 23 chromosomes in each cell

Inheritance of X linked genes

-Many Y linked genes are related to sex determination -X linked genes follow a specific pattern of inheritance --For a recessive X-linked trait to be expressed, A female needs two copies of the allele (homozygous) A male needs only one copy of the allele (hemizygous) Some disorders caused by recessive alleles on the X chromosome in humans: -Color blindness (mostly X linked) -Duchenne muscular dystrophy -Hemophilia

Prophase I

-Nucleus and nucleolus disappear, Spindle fibers form, Chromosomes condense synapsis: The DNA breaks are closed up so that each broken end is joined to the corresponding end of the nonsister chromatid. Thus, a paternal chromatid is joined to a piece of maternal chromatid beyond the cross point, vice versa. homologous chromosomes come together to form a tetrad crossing over: The DNA molecules of nonsister chromatids are broken by proteins and are rejoined to each other. crossing over must occur with a nonsister chromatid because the homologs remain attached. Sister chromatids are still held together by sister chromatid cohesion, even though some of the DNA may no longer be attached to its original chromosomal DNA. Segments of non sister chromatids break off and reattach to the other chromatid. -Causes genetic recombination chiasmata: The X shaped regions where crossovers have occurred.

Prophase II

-Same as prophase in mitosis -Nucleus and nucleolus disappear -Chromosomes condense -Spindles form

The chromosomal basis of sex

-Short segments at the end of the Y chromosomes are homologous with the X, allowing the two to behave like homologs during meiosis in males -In mammals, a gene on the Y chromosome called SRY (sex-determining region on the Y) is responsible for development of the testes in an embryo -Sex linked gene: A gene that is located on either sex chromosome --Y linked genes: Genes on the Y chromosome --X linked genes: GEnes on the X chromosome; the human X chromosome contains about 1,100 genes

1. Crossing over and synapsis in prophase I: a closer look

1. After interphase, the chromosomes have been duplicated, and sister chromatids are held together by proteins called cohesins. Each pair of homologs associate along their length. The DNA molecules of two nonsister chromatids are broken at precisely corresponding points. The chromatin of chromosomes start to condense 2. A zipper-like protein complex, the synaptonemal complex, begins to form, attaching one homolog to the other. The chromatin continues to condense. 3. The synaptonemal complex is fully formed; the two homologs are said to be in synapsis. During synapsis, the DNA breaks are closed up when each broken end is joined to the corresponding segment of the nonsister chromatid, producing crossovers 4. After the synaptonemal complex disassembles, the homologs move slightly apart from each other but remain attached because of sister chromatid cohesion, even though some of the DNA may no longer be attached to its original chromosome. The points of attachment where crossovers have occurred show up as chiasmata. The chromosomes continue to condense as they move toward the metaphase plate.

What is the difference between a character and a trait?

A character is a heritable feature that varies among individuals, such as flower color. A trait is a variant for a character, such as purple or white color for flowers

Nondisjunction

A mishap in which members of a pair of homologous chromosomes do not move apart properly during meiosis I or sister chromatids fail to separate during meiosis II. One gamete receives two of the same copy of chromosome and another gamete receives no copy.

A blood type

A positive: has Antigen A, rh blood factor. Antibodies attack B. I^AI^ARr , I^AI^ARR , I^AIRr , I^AIRR Can donate to A+ and AB+ A negative: has Antigen A. Antibodies attack B and Rh I^AI^Arr , I^AIrr Can donate to A-, AB-, A+, AB+

Alternation of generations

A second life cycle that plants and some species of algae exhibit, including both diploid and haploid stages that are multicellular. Two generations: Sporophyte and gametophyte -Sporophyte is diploid, haploid is gametophyte

AB blood type

AB positive: Has Antigens A+B and rH factor. No antibodies. I^AI^BRr , I^AI^BRR Can donate to AB+, universal reciever AB negative: Has A+B antigens. Antibodies attack Rh. I^AI^Brr Can donate to AB+, AB-

Mendel's first concept

Alternative versions of genes account for variations in inherited characters.

Metaphase I

At metaphase 1 of meiosis, tetrads are positioned at the metaphase plate, rather than individual chromosomes, as in the metaphase of mitosis. (not single file) -Shorter phase Independent assortment: Chromosomes from different parents separate randomly causing genetic recombination

Telophase I

At the end of the first meiotic division, the daughter cells are haploid -Cytokinesis occurs and two haploid daughter cells are formed

B blood type

B positive: has Antigen A, Rh factor. Antibodies attack A. I^BI^BRr , I^BI^BRR , I^BIRr , I^BIRR Can donate to B+ and AB+ B negative: has Antigen B. Antibodies attack A and Rh I^BI^Brr , I^BIrr Can donate to B-, AB-, B+, and AB+

X inactivation in female mammals

Barr body: The object that the inactive X chromosome in each female condenses into. Females show a Barr body because almost all of one X chromosome in female mammals becomes inactivated during early embryonic development. As a result, the cells of females and males have the same effective dose of most X linked genes. -If a female is heterozygous for a particular gene located on the X chromosome, she will be a mosaic for that character

Blood groups

Blood transfusions must match blood type Mixing of foreign blood = clumping = death Rh factor: protein found in RBCs (Rh+= has protein, Rh-= no protein)

Codominance and multiple allele

Codominance: the two alleles each affect the phenotype in separate, distinguishable ways. The two phenotypes are not intermediate, which distinguishes codominance from incomplete dominance. Rather, both phenotypes are exhibited by heterozygotes, since both molecules are present. (Codominance=two capital letters, incomplete dominance = one capital, one lowercase letter) -Co dominance has two dominant alleles EX: red hair x white hairs = roan horses Multiple alleles: Gene has 2+ alleles EX: Human ABO blood groups Alleles = IA, IB, i IA and IB are codominant Antigens: Tags on blood (A, B) recessive i does not have an antigen Rh factors: RR genotype = Rh+ phenotype Rr genotype = Rh+ phenotype rr genotype = Rh- genotype

Crossing over

Crossing over produces chromosomes with new combinations of maternal and paternal alleles. At metaphase II, chromosomes that contain one or more recombinant chromatids can be oriented in two alternative, nonequivalent ways with respect to other chromosomes because the sister chromatids are no longer identical. The different possible arrangements, and therefore diversity, of nonidentical sister chromatids during meiosis II thus increases the number of genetic types of daughter cells that can result from meiosis.

Pedigree

Diagram that shows the relationship between parents/offspring across 2+ generations

Precursor cells

Diploid, cells have homologous chromosomes - one copy from each parent -Chromosomes copied during interphase (S) - creates identical sister chromatids - At G2, there are 92 chromatids but 46 chromosomes

Map units:

Distances between genes can be expressed as map units; one map unit represents a 1% recombination frequency. Relative distance and order, not precise locations of genes -the farther apart two genes are, the higher the probability that a crossover will occur between them and therefore the higher the recombination frequency -Genes that are far apart on the same chromosome can have a recombination frequency near 50% -Such genes are physically linked, but genetically unlinked, and behave as if found on different chromosomes

Dominant allele

Dominant alleles are not necessarily more common than recessive alleles in the gene pool. An allele is called dominant because it is seen in the phenotype, not because it somehow subdues a recessive allele. Alleles are simply variations in a gene's nucleotide sequence. When a dominant allele coexists with a recessive allele in a heterozygote, they do not actually interact at all. It is in the pathway from genotype to phenotype that dominance and recessiveness come into play.

Alterations in chromosome structure

Down syndrome: Down syndrome is usually the result of an extra chromosome 21 from the egg, so that each body cell has a total of 47 chromosomes.

Why is a particular trait recessive?

Each gene is a sequence of nucleotides at a locus, along a particular chromosome. The DNA at that locus, however, can vary slightly in its nucleotide sequence, which can affect the function of the encoded protein and thus an inherited character of an organism. The dominant allele and recessive allele are two DNA sequence variations possible at the locus on an organism's chromosomes. The dominant allele sequence allows synthesis of its trait, and the recessive allele sequence does not. Alleles: Alternative versions of genes

independent assortment of chromosomes

Each pair may orient with either its maternal or paternal homolog closer to a given pole - its orientation is random. Thus, there is a 50% chance that a particular daughter cell of meiosis I will get the maternal chromosome of a certain homologous pair and a 50% chance that it will get the paternal chromosome, increasing diversity.

Inheritance of organic genes

Extranuclear genes: (cytoplasmic genes) Found in organelles in the cytoplasm. Examples include Mitochondria, as well as chloroplasts, and other plant plastids, which carry small circular DNA molecules -Extranuclear genes are inherited maternally because the zygote's cytoplasm comes from the egg -Some defects in mitochondrial genes prevent cells from making enough ATP and result in diseases that affect the muscular and nervous systems

Genomic imprinting

For a few mammalian traits, the phenotype depends on which parent passed along the alleles for those traits. Offspring only. get dad's allele, dad's allele silences mom's allele -Genomic imprinting involves the silencing of certain genes depending on which parent passes them on -Most imprinting genes are autosomes -It seems that imprinting is the result of methylation (addition of -CH3 groups) of cysteine nucleotides -Genomic imprinting may affect only a small fraction of mammalian genes -Most imprinting genesis are critical for embryonic development

1. Mendel's second concept

For each character, an organism inherits two versions (alleles) of a gene, one from each parent. Each somatic cell in a diploid organism has two sets of chromosomes, one set inherited from each parent. Thus, a genetic locus is actually represented twice in a diploid cell, once on each homolog of a specific pair of chromosomes. The two alleles at a particular locus may differ in the F1 generation or be identical.

Linked genes

Genes that are located on the same chromosome that tend to be inherited together. These genes do not assort independently. Nonparental phenotypes are also produced in the testcross, suggesting that the two traits could be separated sometimes. This involves genetic recombination, the production of offspring with combinations of traits differing from either parent

Mapping the distance between genes using recombination data: scientific inquiry

Genetic map: An ordered list of the genetic loci along a particular chromosome Linkage map: A genetic map of a chromosome based on recombination frequencies

Gene

Hereditary units that is coded information that parents pass on to their offspring asexual reproduction: A single individual is the sole parent and passes copies of all its genes to its offspring. sexual reproduction: Two parents give rise to offspring that have unique combinations of genes

15. Alles of homologous chromosomes

Homologous chromosomes each have a given gene at the same locus. Because genes vary so much, homologous chromosomes may have alleles that are the same or different.

Anaphase I

Homologous chromosomes separate and move towards the poles Sister chromatids remain attached at their centromeres

Mendel's third concept

If two alleles at a locus differ, then the dominant allele determines the organism's appearance. The recessive allele has no noticeable effect on the organism's appearance.

Parental combination

If two genes are linked on the same chromosome these genes will be transmitted as a unit and will not sort independently. -Parental types: Offspring with a phenotype matching one of the parental (P) phenotype --A 50% frequency of recombination is observed for any two genes on different chromosomes However, during meiosis, crossing over occurs between homologous chromosomes (Prophase of meiosis I), and the linked genes can become "unlinked." In general, the farther two genes are from each other along the chromosome, the more often they will come "unlinked."

Explain how incomplete dominance is different from complete dominance and give an example of incomplete dominance.

In complete dominance, the phenotype of the heterozygote and the dominant homozygote are indistinguishable. EX: YY or Yy In incomplete dominance, neither allele is completely dominant, and the F1 hybrids have a phenotype somewhere between those of two parental varieties. An example of incomplete dominance is the cross between red and white snapdragons. All the F1 hybrids have pink flowers. This third, intermediate phenotype results from the flowers of the heterozygotes having less red pigment than the red heterozygotes.

Describe what you think is medically important to know about the behavior of recessive alleles.

In the case of disorders classified as recessive, heterozygotes typically have the normal phenotype because one type of the normal allele produces a sufficient amount of the specific protein. Thus, a recessively inherited disorder shows up only in the homozygous individuals who inherit a recessive allele from both parents. Although phenotypically normal with regard to the disorder, heterozygotes may transfer the recessive allele to their offspring and thus are carriers.

Chromosome theory of inheritance

It explains that mendelian genes have specific loci along chromosomes, and it is the chromosomes that undergo segregation and independent assortment. Wild type: Normal phenotypes that are common in populations -Traits alternate to the wild type are mutant phenotypes

Meiosis

It reduces the number of sets of chromosomes from two in the parent cell to one in each gamete, counterbalancing the doubling that occurs at fertilization. As a result, each human sperm and egg cell is a haploid. -plants have a life cycle that involves spores, which form as a result of meiosis, so these spores are haploid. Notice also that both haploid and diploid cells can divide by mitosis. However, meiosis always begins with cells that are diploid, and as a result of meiosis, daughter cells are formed that are always haploid. These cells can be gametes (in animals) or spores (in plants). Gametes: Reproductive cells, transmit genes from one generation to the next male gamete: Sperm female gamete: Eggs

The chromosomal basis of Mendel's laws

Law of independent assortment: each resultant gamete has one long chromosome and one short chromosome. During metaphase one of meiosis, the homologous chromosomes line up with each other on the metaphase plate on different sides, which causes the diversity of gametes during metaphase I Law of segregation: Each resultant gamete has one of each homologous chromosome pairs (50% of gametes have dad's allele, 50% of gametes have mom's allele for every single gene during anaphase I)

Mendel's fourth concept

Law of segregation, The two alleles for an heritable character segregate during gamete formation and end up in different gametes. Thus, an egg or sperm cell gets only one of the two alleles that are present in the diploid cells of the organism making the gamete. In terms of chromosomes, this segregation corresponds with the distribution of copies of the two members of a pair of homologous chromosomes to different gametes in meiosis. If an organism has two identical alleles for a particular character, then that allele is present in all gametes. Because it is only the allele that can be passed onto the offspring when the parent self pollinates, the offspring always look the same as their parent in regard to that characteristic. If different alleles are present, as in the F1 hybrids, then 50% of the gametes receive the dominant allele and 50% receive the recessive allele. Genotypic ratio: 1:2:1 Homo dominant, heterozygous, homo recessive for 2x2 punnett square List traits with each number for bigger punnet square Phenotypic ratio: 3:1

a. In meiosis, what follows the duplication of chromosomes? b. How many daughter cells are formed at the conclusion of meiosis? c. Describe the chromosomes of the daughter cells.

Meiosis I and Meiosis II Four daughter cells They have half (one set, rather than two) as many chromosomes as the parent cell.

By what process are the damaged cells in a wound replaced? By what process are eggs formed? By what process does a zygote develop into a multicellular organism? In which process are identical daughter cells produced? Which process reduces the chromosome number of daughter cells?

Mitosis Meiosis Mitosis Mitosis Meiosis

A comparison of mitosis and meiosis

Mitosis: -Enables multicellular animal to arise from a single cell; produces cells for growth and repair -1 DNA réplication -One division, including prophase, prometaphase, metaphase, anaphase, and telophase. -2 identical daughter cells -Chromosome number: two Meiosis: -Produces gametes; reduces chromosome sets by half and introduces genetic variability among the gametes -1 DNA replication -Two divisions, each including prophase, metaphase, anaphase, and telophase -4 different daughter cells -Chromosome number: Four, each haploid (n)

O blood type

O positive: has Rh factor. Antibodies attack A+B iiRr, iiRR Can donate to A+, B+, AB+, O+ O negative: Has nothing. Antibodies attack A+B and Rh iirr Universal donor

Recombinant types/recombinants

Offspring with nonparental phenotypes (new combination of traits). The production of offspring with combinations of traits differing from either parent, the process during which linked genes become unlinked. This abundance of genetic variation is the raw material upon which natural selection works

two normally occurring exceptions to mendelian genetics

One exception involves genes located on the nucleus, and the other involves genes located outside the nucleus In both cases, the sex of the paent contributing an allele is a factor of the pattern of inheritance

Generations

P generation: The mating of two true breeding varieties (homozygous) F1 generation: The hybrid offspring of two true breeding varieties F2 generation: The generation produced when the f1 hybrids self pollinate

Recombinant chromosomes

Recombinant chromosomes are individual chromosomes that carry genes from two different parents. In meiosis in humans, an average of one to three crossover events occur per chromosome pair, depending on the size of the chromosomes and the position of their centromeres.

Aneuploidy

Results from the fertilization of gametes in which nondisjunction occurred. Offspring with this condition have an abnormal number of a particular chromosome -Monosomic: Fertilization involving a gamete that has no copy of a particular chromosome will lead to the missing chromosome in a zygote (so that the cell has 2n-1 chromosomes.); that aneuploid zygote is said to be mosomic for that chromosome. -Trisomic: If a chromosome is present in triplicate in the zygote (so that the cell has 2n+1 chromosomes), the aneuploid zygote is said to be trisomic for that chromosome. Polyploidy: A condition in which an organism has more than two complete sets of chromosomes -Triploidy: (3n) Three sets of chromosomes -Tetraploidy: (4n) four sets of chromosomes Polyploidy is common in plants, but not animals Polyploids are more normal in appearance than aneuploids

Anaphase II

Same as anaphase in mitosis Sister chromatids pulled apart

Metaphase II

Same as metaphase in mitosis Chromosomes (not tetrads) line up on equator

Telophase II

Same as telophase in mitosis Nuclear membrane reform Cytokinesis occurs Four haploid daughter cells are produced

Chromosomes

Sex chromosome: X and Y chromosomes (only one pair) Autosome: Genes on a chromosome that do not have a counterpart X or Y chromosome. (22 pairs) locus: A gene's specific location along the length of a chromosome Homologous chromosomes: The two chromosomes of a pair have the same length, centromere position, and staining pattern. -Each locus (positions of a gene) is in the same position on the homologs. alleles: Different versions of genes at corresponding loci. For example, one chromosome might have an allele for freckles, but the homologous chromosome may have an allele for the absence of freckles at the same locus. Tetrad: two chromosomes or four chromatids (sister and non-sister chromatids). Form between two chromosomes, it doesn't have to be all. Homologous chromosomes (one from each parent) must be crossing in order to form a tetrad

1. Interphase

Similar to mitosis in interphase -Chromosomes (DNA) replicate in the S phase -Each duplicated chromosomes consist of two -identical sister chromatids attached at their centromeres -Centriole pairs also replicate

Meiosis II

Sister chromatids separate -Second division - skips interphase sister chromatids separate; four haploid cells form (n). 23 chromosomes in each cell -No interphase II, No DNA replication, Meiosis II is essentially the same as mitosis

spermatogenesis vs oogenesis

Spermatogenesis: 4 cell made Oogenesis: 1 cell made

Amniocentesis and chorionic villus sampling (CVS)

Steps in amniocentesis -A sample of amniotic fluid can be taken starting the 15th or 16th week of pregnancy. -Genetic and biochemical tests are performed on the amniotic fluid and fetal cells in it -If indicated, fetal cells from either amniocentesis or CVS can be cultured and then karyotyped to check for large scale chromosomal abnormalities Steps in chorionic villus sampling -A sample of chorionic villus tissue can be taken as early as the 10th or 11th week of pregnancy -Genetic and biochemical tests are performed on the fetal cells -If indicated, fetal cells from either amniocentesis or CVS can be cultured and then karyotyped to check for large scale chromosomal abnormalities

Systems of sex determination

The X-Y system In mammals, the sex of an offspring depends on whether the sperm cell contains an X chromosome or Y b. The X-0 System In grasshoppers, cockroaches, and some other insects, there is only one type of sex chromosome, the X. Femaes are XX; males only have one sex chromosome (X0). Sex of the offspring is determined by whether the sperm cell contains an X chromosome or no sex chromosomee c. The Z-W system In birds, some fish, and some insects, the sex chromosome present in the egg (not the sperm) differ, and thus determine the sex of offspring. The sex chromosomes are designated at Z or W. Females are ZW and males are ZZ. d. The haplo diploid system There are no sex chromosomes in most species of bees and ants. Females develop from fertilized eggs and are thus diploid. Males develop from unfertilized eggs and are haploid, they have no fathers

Polygenic inheritance:

The effect of 2 or more genes acting upon a single phenotypic character. Quantitative variation usually indicates polygenic inheritance. EX: Skin color, height

epistasis

The phenotypic expression of a gene at one locus alters that of a gene at the second locus. -For a lab to have brown fur, its genotype would be bb. A second gene determines whether or not pigment will be deposited in the hair. The dominant allele, symboled by E, results in the disposition of either black or brown pigment, depending on the genotype of the first locus. But if the Lab is homozygous recessive at the second locus (ee) then the coat is yellow, regardless of the genotype at the black/brown locus. In this case, the gene for pigment disposition (E/e) is said to be epistatic to the gene that codes for black or brown pigment (B/b). -EX: The offspring of purple and white-flowering plants all produce purple flowers, but still are able to form gametes for white-flowering offspring.

Addition rule

The probability that any one of two or more mutually exclusive events will occur is calculated by adding their individual probabilities. EX: chances of throwing a die that will land on 4 or 5? ⅙ + ⅙ = ⅓ Picture: segregation of alleles and fertilization as chance events

random fertilization

The random nature of fertilization adds to the genetic variation arising from meiosis. In humans, each male and female gamete represents one of about 8.4 million possible chromosome combinations due to independent assortment. The fusion of a male gamete with a female gamete during fertilization will produce a zygote with any of about 70 trillion diploid combinations. Factoring in the variation brought about by crossing over, the number of possibilities is diverse.

Multiplication rule

To determine the probability of one event and the other occurring, we multiply the probability of one event by the probability of another event. EX: probability of throwing two sixes, ⅙ x ⅙ = 1/36 EX: probability of having 5 boys in a row ½ x ½ x ½ x ½ x ½ = 1/32 EX: if cross AaBbCC x AABbcc, probability of offspring with AaBbCc is ½ x ½ x 1 = ¼ independent event: A phenomenon where each trial is independent of every other trial

Mendel's law of independent assortment

Two or more genes assort independently - that is, each pair of alleles segregates independently of any other pair of alleles - during gamete formation. -Genes are packaged into gametes in all possible allelic combinations, as long as each gamete has one allele for each gene. An F1 plant will produce four classes of gametes in equal quantities: YR, Yr, yR, and yr. -Independent assortment occurs between metaphase and the formation of daughter cells. -EX: Color is separate from shape

Testcros\

Used to determine if dominant trait is unknown (homozygous or heterozygous) by crossing with recessive parental cross: The true breeding between parents (PP X pp) monohybrid cross: The cross between heterozygotes being followed by a cross (Pp X Pp) dihybrid cross: A cross between F1 dihybrids (Pp X Pp)

pleiotropy

When genes have multiple phenotypic effects. In humans, pleiotropy alleles are responsible for the multiple symptoms associated with hereditary diseases, such as cystic fibrosis and sickle cell disease.

Aneuploidy of sex chromosomes

XXY: Male, Have male sex organs, but their testes are small and produce little or no sperm. May have taller than average height, less muscle mass, and enlarged breast tissue. May also have learning disabilities. XXX: Female, Generally healthy, but often are slightly taller than average. X0: Female,At risk for learning disabilities. Usually sterile because their sex organs do not mature. it is the only known viable monosomy in humans XYY: Male, Generally healthy, but often are slightly taller than average.

human inherited conditions

a. albinism b. Tay-Sachs disease c. cystic fibrosis d. sickle-cell disease e. achondroplasia

karyotype

chromosomes arranged in pairs, starting with the longest chromosome. They are prepared from isolating somatic cells, which are treated with a drug to simulate mitosis and then grow in a culture for several days. Cells are arrested when the chromosomes are most condensed. An image of the chromosomes is displayed on a computing monitor, and digitaling software is used to arrange them in order according to their appearance. -It can be used to screen for defective chromosomes or abnormal numbers of chromosomes.

What type of reproduction is self fertilization

sexual

14. Null hypothesis

the hypothesis that there is no significant difference between specified populations, any observed difference being due to sampling or experimental error.

Chromosomal basis for recombination of linked genes

the parental female can produce only one type of gamete but the phenotypically identical F1 female can produce four types of gametes. The genes are genetically linked on the same chromosome. The male's sperm contributes only recessive alleles, so the phenotype of the offspring reflects the genotype of the female's eggs.