Transcript Lecture 13
Unit 3: Genetics • • • • The Cell Cycle + DNA structure/function Mitosis and Meiosis Mendelian Genetics (aka - fun with Punnett squares) DNA replication Yesterday’s Exit Ticket • Create and complete two testcross Punnet squares: (assume G=green and g=yellow) gg x Gg G gg x GG g g Gg gg g Gg gg ½ green ½ yellow G G g Gg Gg g Gg Gg all green • Why homozygous recessive for testcross? –Clear and easy determination of unknown’s genotype: 1:1 = heterozygote; all dominant = homozygote Today’s Agenda: • Mendel and multiple characters •Exceptions to Mendel • Sex-linked traits • Gene linkage How To Punnet Squares https://www.youtube.com/watch?v=Y1PCwxUDTl8 14-8 2. Fig. Probability and genetic outcomes What about multiple characters? Are they inherited together or separately? For the purposes of example, consider the following two characters: 1. Seed color: 1 Generation • FPossible phenotypes = Yellow YyRr OR green Hypothesis of • Yellow is dominant to greenHypothesis of dependent assortment Predictions independent assortment Predicted 2. Seed shape: Sperm offspring of Sperm or • FPossible / YR / yr = Round OR/ wrinkled 2 generation phenotypes YR / Yr / yR / YR • Round is dominant to wrinkled YYRR YYRr YyRR 1 2 1 1 2 1 1/ 2 4 1 4 1/ 4 YYRR YyRr 4 1/ 4 YyRr Yr Eggs yr YyRr 3/ 4 yyrr 1/ 4 YYRr YYrr YyRr Yyrr YyRR YyRr yyRR yyRr YyRr Yyrr yyRr yyrr yR 1/ 4 Phenotypic ratio 3:1 1/ 4 yr 9/ 16 3/ 16 3/ 16 1/ 16 Phenotypic ratio 9:3:3:1 RESULTS yr YR Eggs 1/ 2 1 4 14-8 2. Fig. Probability and genetic outcomes What about multiple characters? Are they inherited together or separately? YYRR P Generation Gametes yyrr YR F1 Generation EXPERIMENT yr YyRr Hypothesis of dependent assortment Predictions Predicted offspring of F2 generation Hypothesis of independent assortment Important Sperm Sperm or Vocab Note: / yR / yr YR / yr cross deals /with YR /one Yr gene A MONOHYBRID / YR e.g. Aa x Aa YYRR YYRr YyRR YyRr / YR 1/ 2 1 1 2 1 1 1 4 4 1 4 1 4 4 2 YYRR Eggs YyRr 1/ 4 Yr YYRr YYrr Yyrr YyRr A DIHYBRID cross deals with two genes YyRr yyrr / yR e.g. AaBb x AaBb YyRR YyRr yyRR 1/ 2 Eggs yr 1 3/ 4 4 yyRr 1/ 4 Phenotypic ratio 3:1 1/ 4 yr YyRr 9/ 16 3/ 16 Yyrr yyRr 3/ 16 yyrr 1/ 16 Phenotypic ratio 9:3:3:1 RESULTS Fig. 14-8 P Generation YYRR Gametes yyrr YR EXPERIMENT yr F1 Generation Suppose that two F1 individuals are crossed. Consider two mutually exclusive hypotheses about inheritance: 1. Strict dependent assortment = inherited allele ALWAYS preserved in the gametes an individual produces combinations are 2. Independent assortment = all possible combinations of inherited alleles of different genes are equally likely in an individual’s gametes 14-8 2. Fig. Probability and genetic outcomes YYRR P Generation Gametes yyrr YR F1 Generation EXPERIMENT yr YyRr Hypothesis of independent assortment Hypothesis of dependent assortment Sperm or 1/ 4 Sperm 1/ 2 YR 1/ 2 1/ 2 1/ 4 yR 1/ 4 yr YR YYRR YYRr YyRR YyRr YYRr YYrr YyRr Yyrr YyRR YyRr yyRR yyRr YyRr Yyrr yyRr yyrr YR YyRr YYRR Eggs 1/ 2 Yr yr 1/ 4 Predicted offspring of F2 generation YR 1/ 4 1/ 4 Yr Eggs yr YyRr 3/ 4 yyrr 1/ 4 yR 1/ 4 Phenotypic ratio 3:1 1/ 4 yr 9/ 16 3/ 16 3/ 16 1/ 16 Phenotypic ratio 9:3:3:1 RESULTS 315 108 101 32 Phenotypic ratio approximately 9:3:3:1 14-8 2. Fig. Probability and genetic outcomes YYRR P Generation Gametes yyrr YR F1 Generation EXPERIMENT yr YyRr Hypothesis of independent assortment Hypothesis of dependent assortment Sperm or 1/ 4 Sperm 1/ 2 YR 1/ 2 1/ 2 1/ 4 yR 1/ 4 yr YR YYRR YYRr YyRR YyRr YYRr YYrr YyRr Yyrr YyRR YyRr yyRR yyRr YyRr Yyrr yyRr yyrr YR YyRr YYRR Eggs 1/ 2 Yr yr 1/ 4 Predicted offspring of F2 generation YR 1/ 4 1/ 4 Yr Eggs yr YyRr 3/ 4 yyrr 1/ 4 yR 1/ 4 Phenotypic ratio 3:1 1/ 4 yr 9/ 16 3/ 16 3/ 16 1/ 16 Phenotypic ratio 9:3:3:1 RESULTS 315 108 101 32 Phenotypic ratio approximately 9:3:3:1 Fig. 15-2b F1 Generation: 2 possible arrangements of 0.5 mm chromosomes All F1 plants produce yellow-round seeds (YyRr) R r LAW OF SEGREGATION The two alleles for each gene separate during gamete formation. LAW OF INDEPENDENT ASSORTMENT Alleles of genes on nonhomologous chromosomes assort independently during gamete formation. R y y r Y Y Meiosis r R r R Y y Metaphase I Y y 1 1 r R r R Y y Anaphase I Y y r R Metaphase II R r 2 2 Gametes y Y Y R R 1 4 r 1 YR 3 4 yr Y Y y r y Y y Y r r 14 Yr y y R R 14 yR 3 2. Probability and genetic outcomes • Mendel’s “law” of independent assortment = alleles for each character segregate independently during gamete formation • Given what YOU know about the relationship between genes and chromosomes (which Mendel did NOT), when would this “law” be violated? 2. Probability and genetic outcomes R Y r y YR yr Today’s Agenda: • Mendel and multiple characters • Exceptions to Mendel • Sex-linked traits • Gene linkage If only it were all so simple… The view provided by (my simplified presentation of) Mendel’s pea experiments: • one gene one character (e.g., flower color gene color of flower) • one allele one phenotype (e.g., P allele purple flower) • two alleles of each gene, one completely dominant, the other recessive (e.g., P dominant to p) 3. Extending the Mendelian model • Patterns of inheritance different from those discussed so far can be caused in many ways. Just to name a few: a) b) c) d) e) Lack of complete dominance by one allele A gene has more than two alleles A gene produces multiple phenotypes Multiple genes affect a single phenotype Environmental circumstances affect the phenotype To learn more about all of these, take EBIO 2070! For now, the simplest exceptions: 1. Genes on sex chromosomes 2. Gene linkage X Y REMINDER: A complete single set of human chromosomes includes: • 22 autosomes (non-sex chromosomes) • 1 sex chromosome (diploid cells have 44 autosomes and 2 sex chromosomes) Humans and many other species have chromosomal sex determination In the human system, females have two “X” chromosomes, males have one “X” and one “Y” X Y Fig. 15-5 Other forms of chromosomal sex determination in the animal kingdom… 76 + ZW The Z-W system 76 + ZZ Fig. 15-6c What consequences might sex chromosomes have for patterns of inheritance and gene expression? 22 pairs of chromosomes + 22 pairs of chromosomes + X X Y X Who determines the sex of our offspring? Diploid Parent Cell Gametes XX X XY X X Y Dad determines a child’s sex! Patterns of inheritance in mammals (and other XY systems) from female parent from male parent allele on X chromosome (“X-linked”) passed on to either sons or daughters with probability ½ passed on ONLY to daughters with probability 1 Diploid Parent Cell XAXa XAY Gametes XA Xa XA Y Patterns of inheritance in mammals (and other XY systems) allele on Y chromosome (“Y-linked”) from female parent from male parent typically not possessed by females passed on ONLY to sons with probability 1 Diploid Parent Cell Gametes Dad is who determines a child’s sex XX X X X XYA YA Patterns of gene expression in mammals (and other XY systems) expression in females expression in males dominant Xlinked allele yes yes recessive Xlinked allele ONLY if present with other recessive allele yes never present (never expressed) yes Y-linked allele Male-pattern baldness SRY gene: Testes formation Santhi’s Story http://www.ibnlive.com/videos/28851/ how-are-athletes-gender-tested.html Santhi Soundarajan won the silver medal in the 800-meter race at the 2006 Asian Games in Doha, Qatar. Following her silver medal performance, she was stripped of her medal. Santhi has female genitalia but her genotype is XY. Speakequal.com Patterns of gene expression in mammals (and other XY systems) expression in females expression in males dominant Xlinked allele yes yes recessive Xlinked allele ONLY if present with other recessive allele yes never present (never expressed) yes Y-linked allele Genes on chromosomes b) Sex-linked traits • Breeding fruit flies (Drosophila melanogaster) – Rapid breeders – Males = XY; Females = XX – For Drosphila: recessive alleles = “mutant” (b) dominant alleles = “wild type” (b+) News.wisc.edu Fig. 15-3 One of Morgan’s experiments (think back to Mendel’s peas): • Character: eye color • Phenotypes: red or white P Generation (true breeding) F1 Generation All offspring had red eyes Is the allele for white eyes dominant or recessive? Then, cross the F1 offspring with each other, and what does the F2 generation look like? 3:1 ratio of red : white 2:1:1 ratio of red female : red male : white male The best explanation for the pattern of inheritance seen in the F2 generation is: a)The eye color gene is on an autosome b)The eye color gene is sex-linked, on the X chromosome c)The eye color gene is sex-linked, on the Y chromosome d)There is not enough information to discriminate between hypotheses (a) through (c) b) Original discoveries Genes on chromosomes Fig. 15-4c CONCLUSION P Generation R X X X Y r r r Sperm Eggs F1 Generation R R R All females XRXr All males XRY r R Sperm Eggs F2 Generation R R R r r r R r b) Original discoveries Genes on chromosomes Fig. 15-4c CONCLUSION P Generation R X X X Y r r r Sperm Eggs F1 Generation R R R All females XRXr All males XRY r R Sperm Eggs F2 Generation R R R r r r R r Females all red: ½ XRXr ½ XRXR Males half red (XRY) and half white (XrY) For now, the simplest exceptions: 1. Genes on sex chromosomes 2. Gene linkage X Y Gene Linkage and Fruit Flies https://www.youtube.com/watch?v=-_UcDhzjOio Fig. 15-2b All F1 plants produce yellow-round seeds (YyRr) LAW OF INDEPENDENT ASSORTMENT Alleles of genes on nonhomologous chromosomes assort independently during gamete formation. R y r Y Meiosis r R Y y Metaphase I 1 r R Y y Anaphase I Metaphase II R r 2 y Y Y Y r r 14 Yr y y R R 14 yR 3 Fig. 14-8 In crosses involving two characters, sometimes you get outcomes that were intermediate between these two hypotheses. F1 Generation YyRr Hypothesis of independent assortment Hypothesis of dependent assortment Sperm or 1/ 4 Sperm 1/ 2 YR 1/ 2 1/ 2 1/ 4 yR 1/ 4 yr YR YYRR YYRr YyRR YyRr YYRr YYrr YyRr Yyrr YyRR YyRr yyRR yyRr YyRr Yyrr yyRr yyrr YR YYRR Eggs 1/ 2 Yr yr 1/ 4 Predicted offspring of F2 generation YR 1/ 4 YyRr 1/ 4 Yr Eggs yr YyRr 3/ 4 yyrr 1/ 4 yR 1/ 4 Phenotypic ratio 3:1 1/ 4 yr 9/ 16 3/ 16 3/ 16 1/ 16 Phenotypic ratio 9:3:3:1 Example • Morgan crossed flies to study the characters of body color and wing size Genes for both are located on autosomes Fig. 15-9-1 EXPERIMENT P Generation (homozygous) Wild type (gray body, normal wings) b+ b+ vg+ vg+ Double mutant (black body, vestigial wings) b b vg vg F1 generation ? Fig. 14-8 F1 dihybrid (wild type phenotype) F1 dihybrid (wild type phenotype) x b+ b vg+ vg b+ b vg+ vg Hypothesis of dependent assortment Hypothesis of independent assortment b+b?v+v? bbvv b+b?v+v? 3 : 1 Observed (approx.): 9 : 8 : b+b?vv bbv+v? 3 : 3 2 : 2 : 4 bbvv : 1 Why would some genes be inherited neither completely together nor completely independently? Gene linkage • Each chromosome has hundreds or thousands of genes • Genes located on the same chromosome that tend to be inherited together are called linked genes • Occasional crossing over leads to occasional, but not common, recombinant chromosomes crossing over Sources of genetic variation Recombination of Linked Genes: Crossing Over Prophase I of meiosis Pair of homologs Nonsister chromatids held together during synapsis Chiasma Centromere Anaphase I Crossing over during Prophase I of meiosis is the mechanism for recombining alleles Fig. 15-UN1 Gene linkage b+ vg+ Parents in testcross Most offspring b vg b vg b vg b+ vg+ b vg or b vg b vg 3. Gene linkage Recombinant chromosomes Fig. 15-10b b+ vg+ b vg b+ vg b vg+ 944 Wild type Black(gray-normal) vestigial 206 Grayvestigial 185 Blacknormal Eggs Testcross offspring 965 b+ vg+ b vg b+ vg b vg+ b vg b vg b vg b vg Recombinant offspring Parental-type offspring 5 : 5 : 1 : 1 b vg Sperm Today’s Exit Ticket An x-linked recessive allele b produces red-green color blindness in humans. A normal-sighted woman whose father was color-blind marries a color-blind man. 1.What genotypes are possible for the mother of the colorblind man? 1.What are the chances that the first child from this marriage will be a color-blind boy? 1.Of the girls produced by these parents, what proportion can be expected to be colorblind? 1.Of all the children (sex unspecified) of these parents, what proportion can be expected to have normal color vision?