Chapter 15 The Chromosomal Basis of Inheritance 15

Chapter 15 The Chromosomal Basis of Inheritance 15. 1 Mendelian inheritance has its physical basis in the behavior of chromosomes I. Chromosome theory of inheritance A. Chromosomes undergo segregation & independent assortment 1. Where alleles located & where did they come from? II. Morgan’s experimental evidence: Scientific inquiry A. Morgan’s choice of experimental organism 1. The fruit fly: Drosophila melanogaster a. Advantages 1. Reproduce quickly & in large #s 2. Only have 4 pairs of chromosomes a. 3 pairs of Autosomes & 1 pair of sex (XX or XY) b. Terminology 1. Wild type a. Phenotype most commonly observed in natural pop. b. + designation w+ = red eyes

2. Mutant phenotypes a. Phenotypes that are alternatives to wild type 1. Mutation of wild type b. No designation w = white eyes

B. Correlating behavior of a gene’s alleles with behavior of a chromosome pair 1. Morgan’s experiment a. Red eyed female (w+) X white eyed male (w) 1. F 1 all red eyed b. F 1 Red female X red male 1. 3 red 1 white

a. Only males had white 1. Eye color linked to sex chromosome a. Supports chromosome theory of inheritance

15. 2 Sex linked genes exhibit unique patterns of inheritance I. The chromosomal basis of sex A. XX or XY for most 1. Humans a. 2 month embryo sex determination begins 1. SRY Sex determining Region of Y a. No SRY= female II. Inheritance of sex-linked genes A. Males pass only to daughters B. Females pass to both daughters and sons C. Examples 1. Male patter baldness 2. Color blindness 3. Duchenne Muscular Dystrophy 4. Hemophilia

III. X inactivation in female mammals A. Barr body 1. In females 1 X chromosome becomes inactive & condenses into a Barr body a. Prevents over production of proteins 2. Which X becomes inactive? a. Random & independent in each embryonic cell 1. Results in 1/2 the cells with maternal X active and 1/2 the cells with paternal X active a. Will be passed on during mitosis 3. Affects a. 1/2 the cells will express paternal genes & 1/2 will express maternal genes 1. Male patter baldness 2. Tortoiseshell cats

4. Reactivated in cells giving rise to eggs

15. 3 Linked Genes I. Intro A. Mendelian inheritance 1. Law of segregation 2. Law of independent assortment RRNN x rrnn Rr. Nn x rrnn B. Chromosome theory of inheritance 1. Supported by Morgan Rr. Nn x Rr. Nn

C. Morgan’s experiments 1. After hundreds of crosses discovered many mutants a. Performed 2 trait crosses & test crosses to study b. Conclusion: did not always get expected ratio D. Inheritance reality 1. We do not always see the expected ratio for offspring a. Why? b. However 1. Morgan did observe a small pattern a. A greater proportion of individuals exhibited parental phenotypes than would be expected given independent assortment b. Observed that certain traits tend to be inherited together 1. Concluded that some traits tend to be inherited together because they are located on the same chromosomes linked traits

II. How linkage affects inheritance A. Morgan’s experiment (part deux)

1. Reality check a. Most offspring have parental phenotypes 1. Linked genes b. However, once again not exactly what was expected 1. Some individuals have nonparental phenotypes 2. Some mechanism must be breaking the linked genes apart a. What? III. Genetic recombination & linkage A. Recombination of unlinked genes: Independent assortment of chromosomes 1. Results from random orientation of homologous chromosomes in metaphase I

2. Expect to see a 50% frequency of recombination a. A 1 : 1 : 1 ratio B. Recombination of linked genes: crossing over 1. Morgan’s results a. 83% parental type; 17% recombinant b. Results from crossing over during prophase I

IV. Mapping the distance between genes using recombination data A. Genetic map 1. An ordered list of genetic loci along a particular chromosome a. Shows relative locations of genes along a chromosome 1. More precisely the linear order but not physical location b. Allows us to better predict which genes will be linked & which will not B. The process 1. Based on the direct relationship between the frequency of crossing over (% of recombination) & distance between alleles a. % = map units 2. Find the greatest # of map units between 2 loci & place on map a. This will be either end of map 3. Place the rest in the map at appropriate distances

4. Examples a. Example 1 1. Genes A & B are 8 map units apart 2. Genes B & C are 10 map units apart 3. Genes A & C are 2 map units apart What is the order of these genes? b. Example 2 1. The distance between Black body & purple eye alleles is 6 map units 2. The distance between purple eye & vestigial wing alleles is 12. 5 map units 3. The distance between black body & vestigial wing alleles is 18. 5 map units Where are the alleles located on the chromosome?

C. The point of genetic maps 1. The closer 2 genes are the greater the chance of linkage between them 15. 4 Alterations of chromosome number or structure causes some genetic disorders 15. 5 Some inheritance patterns are exceptions to the standard chromosome theory I. Genomic imprinting A. Variations on phenotype depending on maternal or paternal inheritance B. Occurs during gamete formation 1. One allele of a gene is silenced a. Results in zygote expressing only 1 of the alleles 2. Imprinting different in sperm & egg C. Transmitted to all body cells