Chromosomal inheritance & linkage. Exceptions to Mendel s Rules

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1 Overhead If a cell is 2n = 6, then how many different chromosomal arrangements at Metaphase I (not including mirror images) could it have? show 5. 5

2 Probability What is the probability that a dividing cell will have ONE PARTICULAR chromosomal arrangement? 1/4 One over the # of possible chromosomal arrangements.

3 Chromosomal inheritance & linkage Exceptions to Mendel s Rules

4 Recall Mendel & Meiosis Principle of Segregation The alleles of a gene separate into gametes Mendel demonstrated this via his monohybrid cross Principle of Independent Assortment Alleles for seed color assort into gametes independently of alleles for seed shape Mendel demonstrated this via his dihybrid cross

5 Chromosomal theory of Inheritance Mendel s observations were formalized by Sutton (grasshopper chromosomes) and Boveri. Mendels observations can be explained by independent alignment & separation of homologous chromosomes during Meiosis I Do these observations confirm that genes are located ON chromosomes? Recall that mitosis and meiosis are almost ALWAYS followed by cytokinesis

6 Segregation of alleles Alleles for pea shape segregate during meiosis How many alleles for pea shape are carried in each gamete? R r

7 Independent Assortment R Y RrYy r y RrYy 1/4 RY 1/4 ry 1/4 Ry 1/4 ry

8 Assortment is random Sorting of alleles for different traits is truly random, because sorting of the chromosomes they lie on is random Ex: probability of getting 2 PAT chromosomes in same daughter cell = probability of getting 1 MAT & 1 PAT chromosome in same daughter cell.

9 Drosophila (fruit fly) Another model organism, studied by T. H. Morgan ~ 1908 Small Easy to keep and propagate ~ 10 day reproductive cycle ~ 100 offspring per mating Much like peas!

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11 Morgan found Mutants Morgan had no obvious variable phenotypes, so he hunted for his own. Wild type: most common phenotype Mutation: change in an allele of a gene (may lead to change in phenotype) Mutant: individuals with traits attributable to mutations First mutant = White-eyed males

12 Crosses M x F 1 F F 2 What happened when Morgan crossed the F1 offspring? 3:1 phenotype ratio, BUT ALL F Red 1/2 M Red, 1/2 M White 3:1 Phenotype DEPENDS on sex of offspring. How? 1/2 M 1/2 M All F

13 M x P F Crosses M x F 1 F F 2 From the top: So far, only males have white eyes. Can we generate a hypothesis about white eyed phenotype & sex? Predictions? Experiment? 3:1 1/2 M 1/2 M All F

14 More Crosses! P M x F 1 F F 1 red-eyed Female x white-eyed Male Half of the females have white eyes! F M F 2 Both males & females can have white eyes, BUT we still have to explain how sex matters Now what kind of cross can we do?*

15 Reciprocal Cross P, True-breeding red P, True-breeding red M x F M x F F 1 F 1 M F M F Predictions?

16 Sex Chromosomes Nettie Stevens, working on beetles, discovers chromosomal asymmetry Adult females: 20 large Adult males: 19 large, 1 puny (Y) Y & X form tetrads at metaphase plate (no crossing-over, but behave as homologs) Gametes differ too: All eggs: 10 (including X) Sperm: 9 + Y OR 10 (including X)

17 Male spermatogenic cells 50% of sperm get an X chromosome & 50% of sperm get a Y chromosome. Clearly differ in size and shape Do you think X and Y chromosomes differ in content (i.e. do they contain the same genes)?*

18 Sex determination This system explains sex determination (and ~ equal sex ratios) in beetles, Drosophila, and humans, among others. Egg (X) + sperm (Y) = male (XY) Egg (X) + sperm (X) = female (XX)* Morgan realized that it also explains patterns of eye color! Red/white gene must lie on X chromosome

19 More Crosses! P, True-breeding red P, True-breeding red M x F M x F X r Y X R X R X R Y X r X r F 1 F 1 X R X r F X R Y M X R X r F X r Y M

20 Punnett Squares Same as before How many different alleles can male contribute? How many can female contribute?

21 Linkage Sex-linkage: Gene for a trait lies on sex chromosomes X-linkage: Only on X chromosome Y-linkage: Only on Y chromosome X R X R or XY Q or whatever Autosomal linkage: Genes physically reside on same autosome DEg deg D, E & G are linked on same autosome

22 Clicker Q If genes lie on chromosomes, and chromosomes assort independently, then what type of assortment should we expect from two genes that lie on the SAME chromosome? 1. Dependent assortment 2. Independent assortment

23 Body color & eye color X rg /X Rg r G r G R g X G = grey body; wild type X g = yellow body; mutant X R = red eyes; wild type X r = white eyes; mutant R g X rg X Rg Only 2 gamete types

24 Predictions X rg /X Rg X rg /Y What phenotypic ratios do we expect in male offspring from this cross? 1:1; white,gray : Red,yellow What genotypic ratios do we expect? 1:1 X rg /Y : X Rg /Y

25 Results rg Rg rg RG

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27 Recombination disrupts linkage Crossing over shuffles linked genes! Usually occurs one to multiple times during tetrad formation Same experiments with other X-linked genes produce variable numbers of recombinant genotypes Why?

28 Physical distance and recombinant phenotypes Cross-over can occur anywhere, but is more likely to occur between two genes when they are far apart

29 A. H. Sturtevant (Morgan s student), realized that frequency of recombinant offspring correlates with distance Creates centimorgan (cm) as unit of measure; If 1/100 offspring show recombinant phenotype, then distance separating genes is 1 cm. Genetic mapping

30 Two powerful tools Determine relative positions of genes on same chromosome Determine average distance separating those genes