1. The Chromosomal Basis of Inheritance Chapter 15

2. The Chromosomal Theory of Inheritance chromosomes and genes are present as pairs in diploid cells homologous chromosomes separate during meiosis fertilization restores chromosome number to diploid condition The chromosomal theory of inheritance claims it is the chromosome that segregates and assorts independently.

4. Morgan Chose fruit flies Why are fruit flies good organisms to do genetic studies on? Prolific breeders ◦ New generation every 2 weeks ◦ Hundreds of offspring Only 4 chromosomes Traits easy to see

5. Morgan and Linked Traits

7. Morgan and sex linkage Crossed red eye female with white male F1 showed all red eye offspring Crossed red eye offspring from F1 Noticed that all males in F2 were white eyed, females were red eyed Determined that eye color is sex linked

8. Show a Punnett square of a cross between a white eyed male with a carrier female fly

9. Sex Linked Traits Eye color is linked on the X chromosome Females carry 2 copies of the gene while males have only one copy If mutant allele is recessive, white eyed female has the trait on both X’s White eyed male cannot hide the trait since he has only one X

10. Who is responsible when a boy inherits a sex linked condition? the mother A carrier of a condition is heterozygous for the trait, so only females can carry sex linked traits

11. In flies gray body(b+) is dominant to black body (b), and long wings (vg+) are dominant to vestigial wings (vg). What is the expected phenotypic ratio from a cross between a heterozygous gray, heterozygous normal winged fly with a black vestigial fly? 1:1:1:1 ratio But what he got was…..

13. What explains the non-Mendelian results? There was no independent assortment! The traits are linked on the same chromosome. If the traits are linked, then all of the offspring should resemble the parental types, black vestigial or gray normal. How did we get the recombinant traits of black, normal and gray vestigial?

16. Linkage maps Sturtevant Map shows sequence of genes on a chromosome, not precise location Genes farther apart have a greater recombination frequency

17. Using Recombination Frequency to Map Traits Recombination frequency between black and vestigial traits is 17% Recombination frequency between cn and black is 9% Recombination between cn and vg is 9.5% Map these traits on the chromosome

19. Determine the order of genes along a chromosome based on the following recombination frequencies: A - B 8% A - C 28% A - D 25% B - C 20% B - D 33% DABC

22. X Inactivation in Females How does an organism compensate for the fact that some individuals have a double dosage of sex-linked genes, while others have only one? X inactivation, the inactive X condenses into a Barr body The inactivation is random and approximately half the X’s from Mom and half the X’s from Dad will inactivate

23. The calico cat is the result of multiple alleles on the X chromosome combines with X inactivation. One allele causes orange fur and the other causes black fur. The male cat will be either orange or black. Only a heterozygous female will be calico with regions that are orange and regions that are black based on the X inactivation.

26. Nondisjunction of sex chromosomes Klinefelter Syndrome XXY ◦ Male, sterile, some female secondary sex characteristics Turner Syndrome XO ◦ Female, short stature