Gen 1

How does X-ray diffraction work?

1. A crystal has its molecules locked in position (that is, they're not floating freely in solution) and are oriented in the same way (facing the same direction) 2. Blasting x-rays at such a crystal, results in x rays being diffracted and detected on a photographic plate (how the x ray is diffracted is entirely dependent on the structure of the crystals)

What were Mendel's Five Conclusions?

1. A hereditary factor (gene) is necessary for producing seed shape & other traits examined 2. Each plant has a pair of these hereditary factors for a trait 3. Each factor has two forms (alleles) 4. One allele is dominant, and the other is recessive 5. Two alleles separate during gamete formation and segregate with equal probability into the gametes

What are two differences in meiosis between animals and plants?

1. Although the cellular events that produce reproductive cells in plants and animals differ in the number of cell divisions 2. In animals, the diploid phase is multicellular (haploid phase is unicellular); in plants, both the diploid and haploid phases are multicellular.

What are the similarities of mitosis and meiosis?

1. Both are preceded by an interphase stage that includes G1, S and G2. 2. In both, chromosomes contract and become visible 3. In both, chromosomes are moved to spindle poles 4. Both are accompanied by cell division 5. In diploid cells, homologous chromosomes are present in both

What are the three elements of a chromosome and describe them?

1. Centromere: attachment point for microtubules - filaments responsible for moving chromosomes in cell division a. it's a sequence required for attachment of the mitotic spindle b. it appears as a constricted region. c. Before cell division, a multiprotein complex called the kinetochore assembles onto it. d. Later, spindle microtubules attach to the kinetochore. e. Chromosomes that lack a centromere or kinetochore cannot be drawn into the newly formed nuclei (such chromosomes are lost). f. Chromosomes are submetacentric, metacentric, telocentric & acrocentric 2. Telomere: tips/ends of a whole linear chromosome. Telomeres protect and stabilize the ends: if a chromosome breaks, producing new ends, the chromosome is degraded at the newly broken ends. Research shows that telomeres also participate in limiting cell division and play important roles in aging & cancer. a. telomeres also serve as a method for counting how many cell divisions a chromosome has been through 3. Origin of replication: sites where DNA synthesis begins

Describe how the monkey mtDNA disease was cured, in which monkey A had defective mitochondria and monkey B had healthy mitochondria and monkey C had normal sperm?

1. Chromosomes from monkey A are removed and inserted into monkey B's egg. 2. Chromosomes from monkey B are removed 3. Sperm from monkey C is fertilized with the egg from monkey B 4. The fertilized egg is then inserted into womb of surrogate mother where a healthy embryo develops 5. Baby monkey is born with three genetic parents

What are the sources of genetic variation?

1. Crossing Over 2. Independent Assortment

What are the five stages of prophase in meiosis I, and what happens in each?

1. Leptotene: chromosomes contract & become visible 2. Zygotene: chromosomes continue to condense, and homologous chromosomes pair up to begin synapsis (a very close pairing association). Each homologous pair of synapsed chromosomes consists of four chromatids called a bivalent/tetrad. 3. Pachytene: chromosomes become shorter and thicker, and a three-part synaptonemal complex develops between homologous chromosomes. The function of the synaptonemal complex is unclear, but cells that lack such complexes have chromosomes that cannot properly separate. Crossing over takes place here, whereby homologous chromosomes exchange genetic information --> genetic variation. Crossing over is essential for proper alignment and separation of homologous chromosomes. 4. Diplotene: centromeres of the paired chromosomes move apart, and the two homologs remain attached at each chiasma, which is the result of crossing over 5. Diakinesis: nuclear membrane/envelope breaks down and mitotic spindle forms

What are the differences between mitosis and meiosis?

1. Mitosis: single nuclear division accompanied by a single cell division--> usually two new cells a. Meiosis: two divisions --> usually four new cells 2. Mitosis: after mitosis, chromosome number in newly formed cells is the same as that in the original cell a. meiosis: chromosome number is reduced by half as a consequence of anaphase I 3. Mitosis: genetically identical cells are produced a. meiosis: genetically variable cells are produced, resulting from crossing over in prophase I and the random separation of maternal and paternal chromosomes in anaphase I 4. Pairing of homologous chromosomes only takes place in meiosis 5. Meiosis I & Mitosis also differ in chromosomal behavior in metaphase and anaphase a. Metaphase I: homologous pairs of chromosomes line up on the metaphase plate b. Mitotic metaphase & metaphase II: individual chromosomes (sister chromatids) line up on the metaphase plate c. Anaphase I: paired chromosomes separate and migrate toward opposite spindle poles, each chromosome possessing two chromatids attached at the centromere d. Mitotic anaphase & anaphase II: sister chromatids separate and each chromosome that moves toward a spindle pole is unreplicated

What are two differences between spermatogenesis and oogenesis?

1. Spermatogenesis produces four sperm, while oogenesis produces one ovum 2. Formation of sperm is continuous, while formation of ovum takes place discontinuously

What is the meiotic basis for the principle of segregation? Explain assuming we're working with two alleles of a genotype found on different homologous chromosomes?

1. The two alleles of a genotype are found on different but homologous chromosomes. One chromosome from each homologous pair is inherited from the father, and the other from the mother. 2. In S phase of meiotic interphase, each chromosome replicates producing two copies of each allele (one from each chromatid). 3. The homologous chromosomes separate in anaphase I -->two alleles are separated, serving as the basis for the principle of segregation. 4. In anaphase II, the two chromatids of each replicated chromosome separate --> each gamete resulting from meiosis carries only a single allele at each locus. 5. If crossing over has taken place in prophase I of meiosis, then the two chromatids of each replicated chromosome are no longer identical, and the segregation of different alleles takes place during anaphase I & anaphase II

How did Hershey and Chase prepare their experiment?

1. They used radioactive isotopes of phosphorus & sulfur as markers (DNA contains phosphorus, not sulfur so they used 32P to follow phage DNA, and 35S to follow phage protein). 2. They grew one batch of E. coli in a medium containing the sulfur; and another in one with phosphorus. Both E coli were infected with the T2 phage. They also infected several batches of unlabeled E coli both sulfur, and other batches with phosphorus 3. Then, they placed the E coli into separate blenders and sheared off the protein coats. They then separated out the protein coats and cultured the infected bacterial cells. 4. They found that the progeny phages grown with P were radiolabeled and that those grown with S were not --> DNA, not protein, from infecting phages had been passed onto the progeny

What happens to the male and female gametes when the plant "flowers?"

1. When the plant flowers, the stamens open and release pollen grains. 2. Pollen then lands on a flower's stigma - a sticky platform that sits atop a long stalk called the style, which has the ovary at the base of it: if a pollen grain germinates, it grows a tube down the style into the ovary. 3. The two sperm cells pass down this tube and enter the embryo sac. 4: One of the sperm cells fertilizes the egg cell, producing a diploid zygote that develops into an embryo. 5: The other sperm cell fuses with two nuclei enclosed in a single cell, producing a triploid endosperm, which stores food that the embryonic plant will use at a later time. These two fertilization events are termed double fertilization.

What are the three characteristics of dominance?

1. dominance is a result of gene interactions at the same locus; in other words, dominance is an allelic interaction 2. dominance does not alter the way in which genes are inherited; it only influences the way in which genes are expressed in a phenotype o so, the allelic interaction that characterizes dominance is therefore an interaction between products of genes 3. Often, the recessive allele is a mutant version that encodes a non-functional protein and the dominant allele is the "normal" copy o non-functional can result from early stop codon, ∆AA sequence, etc. o oftentimes having just one copy (being heterozygous) is good enough because a functional protein is still produced

How does epistasis differ from dominance in two ways?

1. epistasis is similar to dominance, except dominance entails the masking of genes at the same locus (allelic genes). In epistasis, the gene that does the masking is called an epistatic gene, and the masked gene is the hypostatic gene. 2. Epistatic genes can be dominant or recessive

What are the two pathways once a spermatogonium is made?

1. it can go through additional rounds of mitosis 2. alternatively, a spermatogonium can initiate meiosis and enter into prophase I and become a primary spermatocyte that is still diploid since the homologous chromosomes have not separated yet. After the primary spermatocyte completes meiosis I, two new haploid cells (called secondary spermatocytes) are espoused. The secondary spermatocytes undergo meiosis II, where each produce two haploid spermatids (so, one primary spermatocyte creates four spermatids) that develop into sperm.

What are three genetic consequences of the cell cycle?

1. it produces two cells with identical genetic information, and they're identical because DNA synthesis during their S phase created an exact copy of each DNA molecule--> two identical sister chromatids, and M phase ensured that one of the two sister chromatids from each replicated chromosome passed into each new cell 2. each of the new cells produced contains a full complement of chromosomes: there is no net reduction or increase in chromosome number 3. each cell also contains approximately half of the cytoplasm and organelle content of the original parent cell, but no precise mechanism analogous to mitosis ensures that organelles are evenly divided --> not all cells resulting from the cell cycle are identical to each other in respect to cytoplasmic content

What three events must take place for a cell to reproduce successfully?

1. its genetic information must be copied 2. its copies of genetic information must be separated from each other 3. it must divide

What are the genetic consequences of meiosis?

1. meiosis comprises two cell divisions--> each cell creates four cells 2. chromosome number is reduced by half --> cells produced by meiosis are haploid 3. cells produced by meiosis are genetically different from each other and from the parent cell

Describe the two processes in sexual reproduction?

1. meiosis: creates gametes, in which the number of chromosomes is reduced by half 2. fertilization: two haploid gametes fuse and restore the number of chromosomes to its original diploid value

Describe how the female eggs are made in plants?

1. the female part of the flower, the ovary, contains diploid cells called megasporocytes, each of which undergoes meiosis to produce four megaspores, only one of which survives. 2. The nucleus of the surviving megaspore divides thrice mitotically to produce eight haploid nuclei that make up the female gametophyte called the embryo sac. 3: Division of the cytoplasm then produces separate cells, one of which becomes an egg

Describe how the male gametophytes are made in plants:

1. the stamen (male part of the flower) contains diploid reproductive cells called microsporocytes, each of which undergoes meiosis to produce four haploid microspores 2.: each microspore divides mitotically to produce an immature pollen grain consisting of two haploid nuclei 3: one of these nuclei, called the tube nucleus, direct the growth of a pollen tube 4: the other, called the generative nucleus, divides mitotically to produce two sperm cells. The pollen grain including its two haploid nuclei is called the male gametophyte

Definition of genotype:

A complete set of genes that an organism possesses

What did Hershey and Chase contribute to DNA and what did they use?

A second piece of evidence that DNA was genetic material resulted from a study of the T2 virus, which is a bacteriophage that infects E coli. •

Definition of allele:

Alternative forms of genes are called alleles, which are located a specific place (a locus) on a chromosome.

Describe Anaphase I:

Anaphase I is marked by the separation of homologous chromosomes: the two chromosomes of a homologous pair are pulled toward opposite poles Although the homologous chromosomes separate, the sister chromatids remain attached and travel together

When do segregation and independent assortment happen?

Both happen in anaphase I. Note that if crossing over takes place, then segregation and independent assortment also occur in anaphase II. If no crossing over takes place, then the only biological basis for both segregation and independent assortment is just anaphase I.

What are the two causes of incomplete penetrance and expressivity?

Both incomplete penetrance & expressivity are due to the effects of other genes & to environmental factors that can alter or completely suppress the effect of a particular gene. For example, a gene may encode an enzyme that results in a phenotype only within a temperature range: at a non-optimal temperature, the enzyme doesn't function --> protein is not expressed --> allele encoding such an enzyme is therefore penetrant only within a particular temperature range

What is crossing over?

CO is the exchange of genetic material between nonsister chromatids (which are ones that differ from homologous chromosomes)

How do crossing over and random distribution of chromosomes differ?

CO shuffles alleles on the same chromosome into new combinations. Random distribution of maternal and paternal chromosomes shuffles alleles on different chromosomes into new combinations. Together, CO and Random Sep. are capable of producing a tremendous amount of genetic variation among the cells resulting from meiosis.

What is TMV?

Contrat & Singer worked with tobacco mosaic virus (TMV), a virus that infects and causes disease in tobacco plants • TMV possesses a single RNA molecule surrounded by a helical arrangement of protein molecules

What are multiple alleles?

For some loci, more than two alleles are present within a group of organisms Although there may be more than two alleles within a group of organisms, the genotype of each individual diploid organism still consists of only two alleles. the inheritance characteristic encoded by multiple alleles is no different from the inheritance characteristics encoded by two alleles, except that a greater variety of genotypes & phenotypes are possible • SEE BELOW one allele, M, produces the wild-type mallard pattern; MR produces a restricted pattern; md produces a dusky pattern. o restricted is dominant over mallard & dusky, and mallard is dominant over dusky: MR>M>md o the six genotypes possible with these three alleles and their resulting phenotypes are: MRMR restricted MRM restricted MRmd restricted MM mallard Mmd mallard mdmd dusky • In general, the number of phenotypes possible is [n(n+1)/2] where n is the number of different alleles at a locus • Mendel's principle of segregation still holds • ABO blood type is also a manifestation of multiple alleles, where IA>i, IB?i, IA=IB o both IA & IB are dominant over i, and are codominant to each other o type O people possess anti-A & anti-B antibodies; type A possess anti-B antibodies; type B possess anti-A antibodies

Definition of a gene:

Genes are inherited factors (regions of DNA) that determine a characteristic

What is the human mtDNA genome size, and how does it compare to nuclear DNA?

Human mitochondrial genome contains 15k DNAs encoding 37 genes (as opposed to the nuclear genome that contains 3 billion nucleotides & 20,00 genes

Describe cohesin's and shugosin's behavior during anaphase I

In anaphase I, cohesin along the chromosome arms is broken, allowing the two homologs to separate. However, cohesin at the centromere is guarded by the shugoshin protein. Because the shugosin protects the centromere, the centromeric cohesin stays intact and prevents the separation of the two sister chromatids during anaphase I.

Describe Anaphase II:

In anaphase II, the kinetochores of sister chromatids separate. Then the chromatids are pulled to opposite poles. Each chromatid is now a distinct chromosome.

What does separase do?

In anaphase of mitosis, cohesin is along the entire length of the chromosome is broken down by separase, allowing the sister chromatids to separate

Describe Telophase II

In telophase II, chromosomes arrive at spindle poles. Then, a nuclear envelope re-forms around the chromosomes and the cytoplasm divides in cytokinesis. The chromosomes relax and are no longer visible under the microscope.

Down syndrome (trisomy 21) results when an individual has three copies of chromosome 21. Do you think this is due to defects in mitosis or meiosis?

Meiosis: a zygote ended up with three chromosomes. Specifically anaphase: two homologs were supposed to go in different directions, but they both went to one side of the gamete. So, the zygote created from the fusion of this gamete with another gamete (which has one copy of chromosome 21) resulted in a trisomeric zygote.

How did Mendel find incomplete dominance?

Mendel found that when he crossed two homozygous varieties that differed in flowering time by an average of 20 days, the length of time taken by the F1 plants to flower was intermediate between those of the two parents (hence incomplete dominance).

Did Mendel assume that genes were located in the nucleus?

Mendel's principles of segregation & independent assortment assume that genes are located in the nucleus of the cell. For most genetic characteristics, this assumption is valid, however not always

Describe metaphase I:

Metaphase I is initiated when homologous pairs of chromosomes align along the metaphase plate: a microtubule from one pole attaches to one chromosome of a homologous pair, and a microtubule from the other pole attaches to the other member of the pair

Describe Metaphase II:

Metaphase II is similar to mitotic metaphase because the individual chromosomes line up at the metaphase plate, with the sister chromatids facing opposite poles.

Describe the variegation experiment:

Mirabilis jalapa plants He found that leaves of shoots of one variety of the plant were variegated (meaning that had both green & white splotches. He also found that some branches of the variegated strain had all green leaves, and others had all white leaves Each branch produced flowers, so Correns was able to cross flowers from variegated, green & white branches in all combinations The seeds from the green branches gave rise to green progeny, no matter whether the pollen was from a green, white or variegated branch. Similalry, flowers on white branches always produced white progeny. Flowers on variegated branches gave rise to green, white & variegated progeny in no particular ratio So, Correns crosses show cytoplasmic variegation in these plants: the phenotypes of the offpsring were the same as their maternal parent (the seed plant) and not influenced by the offspring of the paternal plant (pollen plant). It turns out that variegation is caused by a defective cpDNA gene, which results in a failure to produce green pigment chlorophyll. Cells that contain normal chloroplasts are from green branches only, cells that contain abnormal chloroplasts are from white branches, and cells that contain a mixture of normal & abnormal chloroplasts are variegated. In the flowers from variegated branches, the random segregation of chloroplasts during oogenesis produces some egg cells with only abnormal-white causing cpDNA, some with only normal-green causing cpDNA and some cells have a mixture.

How is cytokinesis unequal in oogenesis?

Most of the cytoplasm is allocated to one of the two haploid cells (called the secondary oocyte). The smaller cell, which also contains half the number of chromosomes but only a small part of the cytoplasm is called the first polar body, which may or may not divide further. The secondary oocyte completes meiosis II, again producing a larger cell (ovum - mature female gamete) that acquires most of the cytoplasm and a smaller cell (second polar body) Only the ovum can be fertilized. The second polar body disintegrates.

How do plants have a complex life cycle?

Most plants have a complex life cycle that includes two distinct structures/generations: 1. multicellular diploid sporophyte 2. multicellular haploid gametophyte these two generations alternate: sporophyte produces haploid spores via. meiosis, while gametophyte produces haploid gametes via. mitosis

Are the sister chromatids identical after crossing over?

No

Why do cytoplasmically inherited characteristics frequently exhibit extensive phenotypic variation?

No mechanism analogous to mitosis or meiosis ensures that cytoplasmic genes are evenly distributed in cellular division --> different cells & individual offspring will contain various proportions of cytoplasmic genes there are thousands of mitochondria in a cell, and each mitochondrion contains 2-10 copies of mtDNA. some of the mtDNA may be wild type, and the other may be mutant. In cell division, it's completely random as to whether the new cell will get the mutant mtDNA

Does prophase II occur in all cells?

No. Only occurs in cells - that during telophase I - had their spindle broken down, their chromosomes relaxed, and their nuclear envelope re-formed. During interkinesis, some of the cells have chromosomes that remain condensed. These cells that forgo prophase II go straight into metaphase II following interkinesis

Is the phenotype inherited?

No; only the genotype is

What is epistasis?

One gene interaction masks/hides the effect of another gene at a different locus

Look at this example of dominant epistasis

P Cross white with green --> white F1: cross white F2: 12 white: 3 yellow: 1 green

What happens to cohesin and shugosin during meiosis II?

Shugoshin is degraded-->at the end of metaphase II, the centromeric cohesin breaks down, allowing the sister chromatids to separate in anaphase II just as they do in mitotic anaphase

Are the sex organs located in the same flower?

Sometimes

Describe Telophase I:

Telophase I is marked by the chromosomes arriving at the spindle poles.

What did Hershey and Chase conclude?

The Hershey-Chase experiment determined that DNA, not protein, is the genetic material of bacteriophages

Why do homologous chromosomes separate during anaphase I, while chromatids separate during anaphase II?

The forms of cohesin differ

What happens between meiosis I and meiosis II?

The period between Meiosis I and Meiosis II is called interkinesis, whereby: 1) the nuclear membrane re-forms around the chromosomes clustered at each pole 2) spindle breaks down 3) chromosomes relax.

What happens to the primary oocyte?

They complete meiosis I

Are RNAs are also connected by phosphodiester linkages to form 5' & 3' ends?

Yes

Are gene interactions frequent?

Yes

Are negative charges of the phosphate groups are frequently neutralized by the association of positively charged proteins, metals or other molecules?

Yes

Can organisms have crossing over even if they lack a synaptonemal complex?

Yes

Does oogenesis also involve sex cells either undergoing repeated rounds of mitosis or entering into meiosis?

Yes

Is crossing over the basis of recombination?

Yes

Do reciprocal crosses differ with respect to mtDNA?

Yes (they're usually passed from their mother).

How is oogenesis discontinuous?

a. Oogenesis begins before birth when oogonia initiate meiosis and give rise to primary oocytes b. Meiosis is then arrested in prophase I, so a female born with primary oocytes is arrested in prophase I. This period can last for 30-40 years. c. Before ovulation, rising hormone levels stimulate one or more of the oocytes to recommence meiosis. d. The first division of meiosis is completed, and a secondary oocyte is ovulated from the ovary. e. In humans and many other species, the second division of meiosis is delayed until contact with sperm f. Once sperm penetrates the outer layer of the secondary oocyte, the second meiotic division takes place, the second polar body is extruded from the egg, and the nuclei of the sperm and newly formed ovum fuse creating the zygote

What happens in telophase of mitosis?

after the chromatids have separated, each is considered a separate chromosome; telophase is marked by the arrival of the chromosomes of spindle poles; nuclear membrane/envelope re-forms around each set of chromosomes, producing two separate nuclei within the cell; condensed chromosomes relax and lengthen and disappear from microscopic view

What happens in metaphase of mitosis?

chromosomes become arranged on a single plane, called the metaphase plate, between the two centrosomes, both of which are at opposite poles of the cell with microtubules radiating outward and meeting in the middle of the cell, and which center at the spindle poles; a spindle-assembly checkpoint ensures that each chromosome is aligned on the metaphase plate and attached to spindle fibers from opposite poles; passage through the spindle-assembly checkpoint depends on the tension that the kinetochore generates as the spindle fibers pull on the two conjoined chromatids on opposite sides: if a microtubule attaches to one chromatid but not the other, then the cell cannot progress to the next stage of the cell cycle. Cells that are defective in this checkpoint end up with abnormal pairs of chromosomes.

What happens in prophase of mitosis?

chromosomes condense, becoming visible under the microscope; because such chromosomes were duplicated in the preceding S phase, each chromosome has two chromatids attached at the centromere; the mitotic spindle - organized array of microtubules that moves the chromosomes in mitosis - forms by growing out from a pair of centrosomes that migrate to opposite poles; note that the spindle is outside the nucleus

What happens in anaphase of mitosis?

connection between sister chromatids breaks and the sister chromatids separate, becoming individual chromosomes that move toward opposite spindle poles; chromosome movement is due to the disassembly of tubulin molecules at both the kinetochore/+ end and the spindle/- end of the spindle fiber. In this stage, motor proteins called molecular motors disassemble tubulin molecules from the spindle and generate forces that pull the chromosomes toward the spindle pole

What are the results of mitosis?

copies of the cells chromosomes (sister chromatids) separate and cells undergo division, resulting in the two new cells having a complete set of genetic information for each of the resulting cells

What happens in prometaphase of mitosis?

disintegration of the nuclear membrane; spindle microtubules enter the nucleus; tubulin molecules are added to and removed from the microtubules causing them to undergo repeated cycles of growth and shrinkage; microtubules are stabilized when they encounter a kinetochore; each chromosome attaches to microtubules from opposite spindle poles and for each chromosome, a microtubule from one of the centrosomes anchors to the kinetochore of one of the sister chromatids; a microtubule from the opposite centrosome then attaches to the other sister chromatid, anchoring the chromosome to both of the centrosomes--> bi-orientation

Define the principle of segregation:

each individual diploid organism possesses two alleles for any particular characteristic, one inherited from the maternal parent; the other, from the paternal parent. These two alleles separate when gametes are formed, and one allele goes into each gamete. The two alleles segregate into gametes in equal proportions.

Is the following a description of incomplete penetrance or expressivity: some polydactylics have extra digits that are fully functional, while others just have extra skin

expressivity

What influences phenotype?

genotype and environment

Describe the checkpoints of the cell cycle:

i. G1 (10 hours in a typical dividing mammalian cell ): cell grows, proteins necessary for cell division are synthesized, lasts several hours. Cell may exit the cycle in response to regulatory signals and go into G0, during which cell is stable and remains the same size. The cell can remain in G0 indefinitely or re-enter G1. Many cells are never pulled out of the cycle to G1. ii. G1/S checkpoint: holds cell in G1 until the cell has all the enzymes necessary for DNA replication. Passage of this checkpoint allows division. iii. S (9 hours): each chromosome duplicates. Although passage of the G1/S checkpoint commits the cell to divide, it will not enter m phase unless the chromosomes duplicate (a mutation/drug that blocks DNA synthesis inhibits the cell from entering m phase). Before S phase, each chromosome is unreplicated; after S phase, each chromosome consists of two sister chromatids). iv. G2 (4hrs): essential biochemical reactions occur to prepare the cell for dividing v. G2/M checkpoint: lets the cell enter m phase only if the cell's DNA is undamaged and completely replicated: unreplicated or damaged DNA inhibits the activation of proteins needed for mitosis to start. Passage through this checkpoint allows the cell to enter M phase.

What happens in cytokinesis?

in many cells, this phase is simultaneous with telophase; cytoplasm divides; cell wall forms in plants

Is the following a description of incomplete penetrance or expressivity: we examined 42 people having an allele for polydactyly (dominant allele) and found out that only 38 were polydactylic, the penetrance is 38/42 = 90%

incomplete penetrance

Definition of phenotype:

manifestation or appearance of a characteristic and may refer to a physical, biochemical or behavioral characteristic

What gives rise to gene interactions?

o frequently, genes exhibit independent assortment but don't act independently of their phenotypic expression; instead, the effects of genes at one locus depend on the presence of genes at other loci. Such a relationship/interaction between the effects of genes at different loci (genes that are not allelic) is termed gene interaction. With gene interaction, products of genes at different loci combine to produce new phenotypes that are not predictable from the single-locus effects alone. the interaction of genes tend be observed with respect to two loci, but interactions occurring among more than two genes is also common

Look at the two examples of recessive epistasis:

o one locus determines the type of pigment produced by epithelial cells (dominant allele B codes for black, while recessive allele b codes for brown). o the other locus affect the deposition of the pigment in the hair shaft (dominant allele E codes for dark pigment - black or brown - to be deposited, while recessive allele prevents the deposition of dark pigment causing the hair to be yellow). the genotype ee at this locus masks the expression of black or brown alleles at the first locus so, B_E black bb E_ brown B_ee yellow bbee yellow so, crossing black BBEE with a yellow bbee --> BbEe (black). Crossing them yields: • 9/16 B_E black • 3/16 bbE_ brown • 3/16 B_ee yellow • 1/16 bbee yellow o 9:3:4 phenotypic ratio o we say that e is epistatic to B and b, because e masks the expression of the alleles for black & brown pigments, and alleles B & b are hypostatic to e. o we say that e is a recessive epistatic allele, because two copies of e must be present to mask the expression black & brown pigments Above is another example of a recessive epistasis (the Bombay allele) o this gene masks the expression of alleles at the ABO locus o the alleles at the ABO locus encode antigens on RBCs, and these antigens consist of short chains of carbohydrates embedded in the membranes of RBCs o the difference between the A & B antigens is a function of chemical differences in the terminal sugar of the chain. o IA & IB alleles actually encode different enzymes, which add sugars designated A or B to the ends of the carbohydrate chains o The common substrate for these enzymes is H o The enzyme encoded by the i allele either adds no sugar to H or no functional enzyme is specified o In most people, a dominant allele H at the H locus encodes an enzyme that makes H, but people with the Bombay phenotype are homozygous for a recessive mutation (h) that encodes a defective enzyme, which is incapable of making H, and because H is not produced, no ABO antigens are synthesized --> the expression of the alleles at the ABO locus depends on the genotype at the H locus H_IAIA,H_IAi H present A H_IBIB,H_IBi H present B H_IAIB H present AB H_ii H present O hhIAIA,hhIAi,hhIBIB, hhIAIB & hhii H not presentO o the alleles at the ABO locus are hypostatic to the recessive h allele o O blood type can arise in one of two ways 1. from failure to add a terminal sugar to compound H 2. from failure to produce compound H o It turns out that a gene that has an effect on an early step in a biochemical pathway will be epistatic to downstream genes

What is penetrance?

penetrance is defined as the percentage of individual organisms having a particular phenotype that express the expected phenotype.

What is a bacteriophage?

phage reproduces by attaching itself to the outer wall of a bacterial cell and injects its DNA into it causing replication resulting in the synthesis of a phage protein. The phage then becomes encapsulated within the proteins, producing progeny phages that lyse open the cell and escape

What is cohesin?

protein molecule that holds chromatids together from S to early mitosis At the beginning of meiosis, the meiosis-specific protein is found along the entire length of a chromosome's arms. The cohesin also acts on the chromosome arms of homologs at the chiasmata, tethering two homologs together at their ends.

What is independent assortment?

random separation of homologous chromosomes This principle states that alleles at different loci separate independently of one another. Mendel performed Dihydrid crosses to discover this principle. This principle is really an extension of the principle of segregation. Recall that the principle of segregation states that the two alleles of a locus separate when the gametes are formed; the principle of independent assortment states that when these two alleles separate, their separation is independent of the separation of alleles at other loci.

What is the definition of a chromosome?

single DNA molecule that has a specific set of genetic elements on it. Different chromosomes (1,2,etc). have different sets of genes on them. There are a couple thousand of genes on a chromosome, so the number of different genes on chromosomes vary, as chromosomes vary in size.

What are the immediate products of plant meiosis? Are they also called gametes?

spores, (not gametes) which undergo one or more mitotic divisions to produce gametes

What is a sporophyte? How is the flower related to the sporophyte?

sporophyte is the vegetative part of the plant and the gametophyte consists of only a few haploid cells within the sporophyte the flower, which is part of the sporophyte, contains the reproductive structures

What is the primary structure of DNA?

string of nucleotides joined together by phosphodiester linkages

When does crossing over happen?

takes place in prophase I In some organisms it's initiated in zygotene, before the synaptonemal complex develops, and isn't completed until the end of prophase I In other organisms, it's initiated after the synaptonemal complex forms

Where does spermatogenesis take place?

testes, where the diploid primordial germ cells divide mitotically to produce spermatogonia diploid cells

What is expressivity?

the degree to which a trait is expressed

What are three similarities in meiosis between animals and plants?

the number of haploid gametes produced is the same the relative size of the final products is the same the overall result is the same: meiosis creates haploid, genetically variable cells that then fuse during fertilization to produce diploid progeny.

What is spermatogenesis?

the production of gametes in a male animal

What is the secondary structure of DNA?

the secondary structure is its 3D configuration (which is its fundamental 3D structure).

Describe the experiments on TMV:

• Contrat found that, after separating the RNA & protein of TMV, he could remix the RNA and protein of different strains of TMV and obtain intact, infectious viral particles • Contrat & Singer created hybrid viruses by mixing RNA and protein of different strains of TMV. When these hybrid viruses infected tobacco leaves, new viral progeny were produced o these progeny were identical to the strain from which the RNA had been isolated --> RNA carries TMV's genetic information • Grier & Schramm demonstrated that RNA isolated from TMV is sufficient to infect tobacco plants and direct the production of new TMV particles.

Describe how the pepper color observations

• Here, two loci interact to produce a single characteristic (fruit color) • We get the classic dihybrid ratio for a cross between two heterozygotes for two loci • In this case, the Y locus & C locus interact to produce a single phenotype: pepper colo r • Color in peppers results from relative amounts of red & yellow carotenoids (compounds synthesized in a complex biochemical pathway). o The Y locus encodes one enzyme (first step in the pathway), followed by the C locus encoding for a different enzyme (last step in the pathway). When different loci influence different steps in a common biological pathway, gene interaction often arises because the product of one enzyme affects the substrate of another enzyme

Describe what the Avery lab showed:

• The Avery Lab isolated and partially purified the transforming substance o they showed that it had a chemical composition closely matching to DNA o Proteases trypsin, chymotrypsin, and ribonuclease had no effect on the transforming substance o they showed that the transforming substance precipitated at about the same length as purified DNA o they showed that the transforming substance absorbed UV light at the same wavelengths as DNA