Unit 3: Genetics The Cell Cycle + DNA structure/function

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 gg x GG ½ green all green ½ yellow Why homozygous recessive for testcross? Clear and easy determination of unknown’s genotype: 1:1 = heterozygote; all dominant = homozygote G g Gg gg G g Gg

Today’s Agenda: Mendel and multiple characters Exceptions to Mendel
Sex-linked traits Gene linkage 3

How To Punnet Squares

2. Probability and genetic outcomes
Fig. 14-8 What about multiple characters? Are they inherited together or separately? For the purposes of example, consider the following two characters: 1. Seed color: Possible phenotypes = Yellow OR green Yellow is dominant to green 2. Seed shape: Possible phenotypes = Round OR wrinkled Round is dominant to wrinkled F1 Generation YyRr Hypothesis of dependent assortment Hypothesis of independent assortment Predictions Predicted offspring of F2 generation Sperm Sperm or 1/2 YR 1/2 yr 1/4 YR 1/4 Yr 1/4 yR 1/4 yr 1/4 YR Figure 14.8 Do the alleles for one character assort into gametes dependently or independently of the alleles for a different character? YYRR YYRr YyRR YyRr 1/2 YR YYRR YyRr 1/4 Yr Eggs YYRr YYrr YyRr Yyrr Eggs 1/2 yr YyRr yyrr 1/4 yR YyRR YyRr yyRR yyRr 3/4 1/4 1/4 yr Phenotypic ratio 3:1 YyRr Yyrr yyRr yyrr 9/16 3/16 3/16 1/16 Phenotypic ratio 9:3:3:1 RESULTS 5

Fig. 14-8 What about multiple characters? Are they inherited together or separately? YYRR yyrr P Generation EXPERIMENT Gametes YR  yr F1 Generation YyRr Hypothesis of dependent assortment Hypothesis of independent assortment Predictions Important Vocab Note: A MONOHYBRID cross deals with one gene e.g. Aa x Aa A DIHYBRID cross deals with two genes e.g. AaBb x AaBb Predicted offspring of F2 generation Sperm Sperm or 1/2 YR 1/2 yr 1/4 YR 1/4 Yr 1/4 yR 1/4 yr 1/4 YR Figure 14.8 Do the alleles for one character assort into gametes dependently or independently of the alleles for a different character? YYRR YYRr YyRR YyRr 1/2 YR YYRR YyRr 1/4 Yr Eggs YYRr YYrr YyRr Yyrr Eggs 1/2 yr YyRr yyrr 1/4 yR YyRR YyRr yyRR yyRr 3/4 1/4 1/4 yr Phenotypic ratio 3:1 YyRr Yyrr yyRr yyrr 9/16 3/16 3/16 1/16 Phenotypic ratio 9:3:3:1 RESULTS 6

Fig. 14-8 P Generation YYRR yyrr EXPERIMENT Gametes YR  yr F1 Generation Suppose that two F1 individuals are crossed. Consider two mutually exclusive hypotheses about inheritance: 1. Strict dependent assortment = inherited allele combinations are ALWAYS preserved in the gametes an individual produces 2. Independent assortment = all possible combinations of inherited alleles of different genes are equally likely in an individual’s gametes Figure 14.8 Do the alleles for one character assort into gametes dependently or independently of the alleles for a different character? 7

Phenotypic ratio approximately 9:3:3:1
2. Probability and genetic outcomes Fig. 14-8 P Generation YYRR yyrr EXPERIMENT Gametes YR  yr F1 Generation YyRr Hypothesis of dependent assortment Hypothesis of independent assortment Sperm or 1/4 YR 1/4 Yr 1/4 yR 1/4 yr Sperm 1/2 YR 1/2 yr 1/4 YR Predicted offspring of F2 generation YYRR YYRr YyRR YyRr 1/2 YR YYRR YyRr 1/4 Yr Eggs YYRr YYrr YyRr Yyrr Eggs Figure 14.8 Do the alleles for one character assort into gametes dependently or independently of the alleles for a different character? 1/2 yr YyRr yyrr 1/4 yR YyRR YyRr yyRR yyRr 3/4 1/4 1/4 yr Phenotypic ratio 3:1 YyRr Yyrr yyRr yyrr 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 8

Hypothesis of Hypothesis of independent dependent assortment
2. Probability and genetic outcomes Fig. 14-8 P Generation YYRR yyrr EXPERIMENT Gametes YR  yr F1 Generation YyRr Hypothesis of dependent assortment Hypothesis of independent assortment Sperm or 1/4 YR 1/4 Yr 1/4 yR 1/4 yr Sperm 1/2 YR 1/2 yr 1/4 YR Predicted offspring of F2 generation YYRR YYRr YyRR YyRr 1/2 YR YYRR YyRr 1/4 Yr Eggs YYRr YYrr YyRr Yyrr Eggs Figure 14.8 Do the alleles for one character assort into gametes dependently or independently of the alleles for a different character? 1/2 yr YyRr yyrr 1/4 yR YyRR YyRr yyRR yyRr 3/4 1/4 1/4 yr Phenotypic ratio 3:1 YyRr Yyrr yyRr yyrr 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 9

The two alleles for each gene separate during gamete formation.
Fig. 15-2b F1 Generation: 2 possible arrangements of chromosomes All F1 plants produce yellow-round seeds (YyRr) 0.5 mm R R LAW OF INDEPENDENT ASSORTMENT Alleles of genes on nonhomologous chromosomes assort independently during gamete formation. y y r r LAW OF SEGREGATION The two alleles for each gene separate during gamete formation. Y Y Meiosis R r r R Metaphase I Y y Y y 1 1 R r r R Anaphase I Y y Y y Figure 15.2 The chromosomal basis of Mendel’s laws R r Metaphase II r R 2 2 Y y Y y Y y Y Y Y y y y Gametes R R r r r r R R 1 4 YR 1 4 yr 1 4 Yr 1 4 yR 3 3 10

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? 11

R Y r y Y R y r 2. Probability and genetic outcomes
Figure 14.4 Alleles, alternative versions of a gene y r 12

Today’s Agenda: Mendel and multiple characters Exceptions to Mendel
Sex-linked traits Gene linkage 13

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) 14

To learn more about all of these, take EBIO 2070!
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: 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! 15

For now, the simplest exceptions:
Genes on sex chromosomes Gene linkage X Y 16

A complete single set of human chromosomes includes:
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) 17

X Y 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 Figure 15.5 Human sex chromosomes Only the ends of the Y chromosome have regions that are homologous with the X chromosome Fig. 15-5 18

Other forms of chromosomal sex determination in the animal kingdom…
76 + ZW 76 + ZZ Fig. 15-6c Figure 15.6 Some chromosomal systems of sex determination The Z-W system 19

What consequences might sex chromosomes have for patterns of inheritance and gene expression?
22 pairs of chromosomes + 22 pairs of chromosomes + Figure 15.6 Some chromosomal systems of sex determination X Y X X 20

Who determines the sex of our offspring?
Diploid Parent Cell Gametes XX XY X X X Y Dad determines a child’s sex! 21

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 Gametes XAXa XAY XA Xa XA Y 22

Patterns of inheritance in mammals (and other XY systems)
from female parent from male parent allele on Y chromosome (“Y-linked”) typically not possessed by females passed on ONLY to sons with probability 1 Dad is who determines a child’s sex Diploid Parent Cell Gametes XX XYA X X X YA 23

Patterns of gene expression in mammals (and other XY systems)
expression in females expression in males dominant X-linked allele yes recessive X-linked allele ONLY if present with other recessive allele Y-linked allele never present (never expressed) Male-pattern baldness SRY gene: Testes formation 24

Santhi’s Story how-are-athletes-gender-tested.html Santhi Soundarajan won the silver medal in the 800-meter race at the 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. From India Speakequal.com 25

Patterns of gene expression in mammals (and other XY systems)
expression in females expression in males dominant X-linked allele yes recessive X-linked allele ONLY if present with other recessive allele Y-linked allele never present (never expressed) 26

(Drosophila melanogaster)
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 27

Figure 15.3 Morgan’s first mutant: Who would have thought that fly eyes could be so interesting and important??? Fig. 15-3 28

All offspring had red eyes
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? 29

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 Figure 15.3 Morgan’s first mutant: Who would have thought that fly eyes could be so interesting and important??? 30

The eye color gene is on an autosome
The best explanation for the pattern of inheritance seen in the F2 generation is: The eye color gene is on an autosome The eye color gene is sex-linked, on the X chromosome The eye color gene is sex-linked, on the Y chromosome There is not enough information to discriminate between hypotheses (a) through (c) 31

CONCLUSION Genes on chromosomes b) Original discoveries P X X
Fig. 15-4c b) Original discoveries CONCLUSION P R r X X Generation  X Y r r Sperm Eggs All females XRXr All males XRY F1 R R Generation R r R Figure 15.4 In a cross between a wild-type female fruit fly and a mutant white-eyed male, what color eyes will the F1 and F2 offspring have? Sperm Eggs R R F2 R Generation r r r r R 32

CONCLUSION Genes on chromosomes b) Original discoveries P X X
Fig. 15-4c b) Original discoveries CONCLUSION P R r X X Generation  X Y r r Sperm Eggs All females XRXr All males XRY F1 R R Generation R r R Figure 15.4 In a cross between a wild-type female fruit fly and a mutant white-eyed male, what color eyes will the F1 and F2 offspring have? Sperm Eggs R R Females all red: ½ XRXr ½ XRXR Males half red (XRY) and half white (XrY) F2 R Generation r r r r R 33

For now, the simplest exceptions:
Genes on sex chromosomes Gene linkage X Y 34

Gene Linkage and Fruit Flies

Fig. 15-2b All F1 plants produce yellow-round seeds (YyRr) R LAW OF INDEPENDENT ASSORTMENT Alleles of genes on nonhomologous chromosomes assort independently during gamete formation. y r Y Meiosis r R Metaphase I Y y 1 r R Anaphase I Y y Figure 15.2 The chromosomal basis of Mendel’s laws Metaphase II r R 2 Y y Y Y y y r r R R 1 4 Yr 1 4 yR 3 36

Phenotypic ratio 3:1 Phenotypic ratio 9:3:3:1
Fig. 14-8  In crosses involving two characters, sometimes you get outcomes that were intermediate between these two hypotheses. F1 Generation YyRr Hypothesis of dependent assortment Hypothesis of independent assortment Sperm or 1/4 YR 1/4 Yr 1/4 yR 1/4 yr Sperm 1/2 YR 1/2 yr 1/4 YR Predicted offspring of F2 generation YYRR YYRr YyRR YyRr 1/2 YR YYRR YyRr 1/4 Yr Eggs YYRr YYrr YyRr Yyrr Eggs Figure 14.8 Do the alleles for one character assort into gametes dependently or independently of the alleles for a different character? 1/2 yr YyRr yyrr 1/4 yR YyRR YyRr yyRR yyRr 3/4 1/4 1/4 yr Phenotypic ratio 3:1 YyRr Yyrr yyRr yyrr 9/16 3/16 3/16 1/16 Phenotypic ratio 9:3:3:1 37

Example Morgan crossed flies to study the characters of body color and wing size  Genes for both are located on autosomes 38

? F1 generation b+ b+ vg+ vg+ b b vg vg P Generation (homozygous)
Fig P Generation (homozygous) EXPERIMENT Double mutant (black body, vestigial wings) Wild type (gray body, normal wings)  b+ b+ vg+ vg+ b b vg vg F1 generation Figure 15.9 How does linkage between two genes affect inheritance of characters? ? 39

x Observed (approx.): 8 : 2 : 2 : 4 b+ b vg+ vg b+ b vg+ vg 3 : 1
Fig. 14-8 F1 dihybrid (wild type phenotype) x F1 dihybrid (wild type phenotype) b+ b vg+ vg b+ b vg+ vg Hypothesis of dependent assortment Hypothesis of independent assortment Figure 14.8 Do the alleles for one character assort into gametes dependently or independently of the alleles for a different character? b+b?v+v? bbvv b+b?v+v? b+b?vv bbv+v? bbvv 3 : 1 9 : : : 1 Observed (approx.): : : : 4 40

Each chromosome has hundreds or thousands of genes
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 41

Recombination of Linked Genes: Crossing Over
Sources of genetic variation crossing over Recombination of Linked Genes: Crossing Over Prophase I of meiosis Nonsister chromatids held together during synapsis Pair of homologs Chiasma Centromere Figure The results of crossing over during meiosis Anaphase I  Crossing over during Prophase I of meiosis is the mechanism for recombining alleles 42

b+ vg+ b vg  Parents in testcross b vg b vg b+ vg+ b vg Most or
Gene linkage Fig. 15-UN1 b+ vg+ b vg  Parents in testcross b vg b vg b+ vg+ b vg Most offspring or b vg b vg 43

965 944 Black- vestigial 206 Gray- vestigial 185 Black- normal
3. Gene linkage Fig b Recombinant chromosomes b+ vg+ b vg b+ vg b vg+ Eggs Testcross offspring 965 Wild type (gray-normal) 944 Black- vestigial 206 Gray- vestigial 185 Black- normal b vg b+ vg+ b vg b+ vg b vg+ Figure Chromosomal basis for recombination of linked genes b vg b vg b vg b vg Sperm Parental-type offspring Recombinant offspring 5 : 5 : : 44

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. What genotypes are possible for the mother of the colorblind man? What are the chances that the first child from this marriage will be a color-blind boy? Of the girls produced by these parents, what proportion can be expected to be color-blind? Of all the children (sex unspecified) of these parents, what proportion can be expected to have normal color vision? 45

Unit 3: Genetics The Cell Cycle + DNA structure/function

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