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Week 2 Genetics

by: Emma Notetaker

Week 2 Genetics CELL 2050

Marketplace > Tulane University > CELL > CELL 2050 > Week 2 Genetics
Emma Notetaker
GPA 3.975

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Week 2 of notes
Dr. Meadows
Class Notes
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This 6 page Class Notes was uploaded by Emma Notetaker on Friday September 9, 2016. The Class Notes belongs to CELL 2050 at Tulane University taught by Dr. Meadows in Fall 2016. Since its upload, it has received 8 views. For similar materials see Genetics in CELL at Tulane University.


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Date Created: 09/09/16
Saturday, September 3, 2016 Week 2 Heredity • Gregor Mendel: • proper experimental model • experimental approach and mathematically analyzed results studied easily differentiated characteristics • • pea plant - analyzed seed color, shape, seed coat color, pod shape and color, flower position and stem length • gene: inherited factor (region on DNA) that helps determine characteristic • allele: one of 2+ forms of a gene • locus: specific place on a chromosome occupied by an allele genotype: set of alleles in an organism • • heterozygote: organism with 2 different alleles at a locus • homozygote: organism with 2 of the same alleles at a locus • phenotype (trait): appearance/manifestation of a characteristic • characteristic/character: attribute/feature • monohybrid cross: cross between 2 parents that differ in a single characteristic one character is encoded by 2 genetic factors • • 2 alleles separate when gametes are formed • concept of dominant and recessive traits • 2 alleles separate with equal probability into the gametes • Mendel’s method: removed anthers of flowers to prevent self-fertilization and dusted stigma with pollen from a different plant P generation: homozygous round male with homozygous wrinkled female seeds • • they were crossed • F1 (filial) seeds were all round • let f1 self-fertilize • 3/4 of F2 were round, 1/4 were wrinkled (3:1 ratio) • reciprocal cross: homozygous round female with homozygous wrinkled male (same results as above experiment) • traits of parent cells DO NOT BLEND • principle of segregation (Mendel’s 1st law): each individual diploid organism possessed 2 alleles for any particular characteristic. These 2 alleles segregate when gametes are formed, and each gamete gets 1 allele • dominance: only the trait encoded by dominant allele is observed - recessive is not seen 2 genetic alleles separate when gametes are formed with equal probability • • gametes are haploid • Sutton: chromosomal theory of heredity - genes are located on chromosomes (meiosis) • genetic crosses in relation to meiosis: • 2 alleles of genotype Rr located on homologous chromosomes which replicate in S phase • in prophase I of meiosis, cross over MAY take place in anaphase I, homologous chromosomes separate • • two chromatids of each chromosome separate in anaphase II • if no crossing over, identical • if crossing over, different alleles segregate in anaphase II • Punnet square used to determine results of genetic cross 1 Saturday, September 3, 2016 • backcross: crossing F1 with parent generation • multiplication rule: probability of 2+ independent events taking place together is calculated by multiplying independent probabilities • independent: outcome of one doesn’t affect outcome of the other • keyword: AND • used to predict the GENETIC ratio of progeny • addition rule: probability of and one of two or more mutually exclusive events calculated my adding the events • keywords: either, or • used to predict PHENOTYPIC ratio of progeny (phenotype results from different genotypic backgrounds) • testcross of a tall plant (can be TT or Tt): cross with homozygous recessive • if genotype TT, all plants will be tall • if Tt, half will be tall and half will be short • inheritance of an allele from the male is independent of an allele contributed by the female • dihybrid crosses: examine 2 traits at a time • reveal the principle of independent assortment to meiosis • gametes on different chromosomes will sort independently multiplication rule applies here • • broken into 2 monohybrid crosses • independent assortment: alleles at different loci separate independently of one another • independent assortment occurs in anaphase I • relating meiosis to independent assortment: • cell contains 2 pairs of homologous chromosomes • in anaphase I, each pair separates independently, which leads to multiple combinations • genes located on different pairs of chromosomes assort independently, which produces different combinations of alleles in the gametes after separating in anaphase II • probability and dihybrid crosses • 2 round yellow seeds, both heterozygotes in both traits broken down into 2 monohybrid crosses • • look at expected proportions for each character • these proportions are then combined using the branch method (multiplication rule) • multiply each proportion from shape by each proportion of color - this is the total proportion • example: AaBbccDdEe x AaBbBcddEe, which proportion is aabbccddee? • Aa x Aa —> aa is 1/4 • Bb x Bb —> bb is 1/4 • cc x Cc —> cc is 1/2 • Dd x dd —> dd is 1/2 • Ee x Ee —> ee is 1/4 —> 1/4*1/4*1/2*1/2*1/4 = 1/256 • • dihybrid testcross: cross with homozygous recessive for BOTH traits • chi-square goodness of fit: indicates the probability that the difference between the observed and expected values is due to chance • X = ∑(observed - expected) 2 expected • cutoff value is .05 • P < .05: NOT due to chance • P > .05: chance is responsible 2 Saturday, September 3, 2016 • n: number of expected phenotypes • expected values obtained by multiplying expected proportions by the total chi square value is then calculated • Sex Determination and Sex-Linked Characteristics • sex determination: mechanism by which sex is establishes • sex: sexual phenotype • sexual reproduction: alternates between haploid and diploid states in most eukaryotes • meiosis produces haploid gamets • fertilization produces a diploid zygote • most organisms have 2 sexual phenotypes, male and female • gamete size different in each sex • sex determination mechanisms: • monoecious: both male and female reproductive structures are in the same organism (hermaphroditism) - plants • dioecious: either male or female structures in one organisms (humans) • chromosomal sex determination systems: sex chromosomes and autosomes autosome: non-sex chromosome • • O: ansence of sex chromosome • XX-XO system: • XX: female (homogametic) • XO: male (heterogametic • example: grasshoppers during meiosis, 1 gamete receives an X and the other gets nothing • • XX-XY system: • XX: female (homogametic) • XY: male (heterogametic) • mammals • 1:1 sex ratio produced in F1 generation X and Y chromosome pair during meiosis even though they are NOT homologous • • genes located on each are different • homologous only at pseudoautosomal regions (which are needed for XY chromosome pairing in meiosis in the male • this is the way they pair - similar at these regions even though a large size different • primary pseudoautosomal region on top secondary pseudoautosomal region on the bottom • • ZZ-ZW system: flipped from our’s (male is homo, female is hetero) • ZZ: male (homogametic) • ZW: female (heterogametic) • birds, snakes, butterflies, some amphibians, fish • haplodiploidy system: haploid set: male • • diploid set: female • bees, wasps, ants • gametes of the heterogametic sex have different sex chromosomes, while gametes of homogametic sex have the same sex chromosome • **sex is determined by individual genes EVEN in chromosomal systems** 3 Saturday, September 3, 2016 • genic sex-determining system: • no sex chromosomes, only sex-determining genes plants, fungi, protozoans and fish • • environmental sex determination: • environmental factor examples: • limpet position in the stack • larva that settles onto unoccupied substrate develops into a female • this female produces chemicals that attract other larvae these new larvae settle on top of female and become males, which mate with • original female • eventually, the males on top switch sex to become female • these females attract new larvae, which settle on top and are male • temperature change in turtles/alligators dictates which sex they are • sex determination in drosophilia melanogaster (XX-XY) X:A ratio • • X = X chromosomes • A = haploid sets of autosomes) • ignore Y • this ratio can help determine the sex of the fly • table 4.2 ex: XXYYY sex chromosomes and two sets of autosomes —> 2:2 ratio (2X, 2A) —> • female • sex determination in humans XX-XY: • SRY gene on y chromosome determines maleness • y-linked (ONLY on y) • this gene turns on enzyme to eliminate female part as well - females are “default” because they lack y gene • located on the short arm (top) • experiment: scientists put SRY gene on X chromosome in mice • this gave mouse a male phenotype (even though mouse was XX) • androgen insensitivity syndrome: caused by defective androgen receptor • female externally but internalized testes • genotype XY • testes produce testosterone BUT testosterone receptor (androgen) is defective • —> so, female characteristics form even with Y chromosome present • sex not just determined by SRY (other genes too) • abnormal sex chromosome numbers: • Turner syndrome: XO (1/3000 female births) • missing a chromosome • broad chests, sterile, neck folds • Klinefelter syndrome: XXY or XXXY or XXXXY or XXYY (1/1000 male births) • always multiple X with Y chromosomes - always male • Poly-X: many more X chromosomes (1/1000 female births) • the more X chromosomes, the worse the condition is • YY: not viable - die as embryos (lethal, need at least one X) • role of sex chromosomes: • x chromosome has genetic info essential for both sexes (NEED at least one x to survive) • y chromosome contains male-determining gene • one y even with many x’s will produce male 4 Saturday, September 3, 2016 • absence of y = female • genes affecting fertility are located on x and y chromosomes female needs 2 x to be fertile usually • • additional x copies may upset normal development in both males and females • **+ means wild type/normal • x-linked characteristics • x-linked white eye in drosophila • Thomas Hunt Morgan - came up with idea of sex inheritance experiment: are white eyes in fruit flies inherited as autosomal recessive trait? • • crossed red eyed X+X+ female with white eyed male (XwY) • got red-eyed males and females (suggested autosomal trait) • reciprocal cross: white-eyed female (XwXw) with red-eyed male (X+Y) • red-eyed female and white-eyed male (XwY) • males more likely to get white eyes because the mutation is on the X chromosome x-linked color blindness in humans (red-green) • • example: hemophilia is x-linked. A woman with hemophilia mates with a man with normal blood clotting. • probability of their child having hemophilia is 1/2 (female child will be normal, male child will have hemophilia) • dosage compensation: amount of protein produced usually a function of the number of gene • • because males only have one x, this is a problem for balancing protein production when compared to females (they would have less protein available) • balanced by dosage compensation: equalization of gene expression/protein amount produced by x-linked genes • fruit flies: males double expression of x • humans: females inactivate one x • Lyon hypothesis: female cells inactivate one x chromosome randomly • mosaic: some cells express genes from one x while others express genes from the other x • barr body: inactive x • regardless of how many x’s, only ONE will be active • if you have more X’s, more will be inactive Barr bodies so that only one remains functional • Xist: RNA molecule that aids in x-inactivation • random X inactivation: leads to mosaicism of fur color in cats (tortoiseshell/calico) • random inactivation of gold cells and black cells (create random clusters of cells) • Y-Linked characteristics: • evolution of y chromosome (lost DNA over time) • used to be that we only had autosomal chromosomes • mutation of gene on one chromosome causes maleness • mutations at other genes affect male characteristics • suppression of crossing over keeps genes for male traits linked to the male- determining gene • over time, lack of crossing over between x and y leads to a degeneration of the y • only in males • all male offspring will have trait (y-linked markers on DNA used to trace ancestry) • important for sex determination in SRY • Z-linked characteristics: 5 Saturday, September 3, 2016 • same as x-linked, but male and female inheritance is reversed • females more likely to get the characteristics because they are Z-W • common misconceptions: characteristics in which male and females differ is NOT sex linked • • character found more frequently in one sex is NOT sex linked 6


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