1. Chromosomal Basis of Inheritance Featuring fruit fly: Drosophila Melanogaster

2. Basic Terms/Information about Drosophila My diploid number is 2N = 8 Mutants are non wild-type traits Wild-type (+) refers to the most common phenotype in fruit flies. It’s usually dominant (but not necessarily)

3. Sex-Linkage or (X-linked) XX XY In fruit flies, (R) is the dominant wild-type gene for red eyes, and (r) is the recessive, mutant gene for white eyes. The gene is found on the “X” chromosome only. This is considered X- linked. What would be the phenotype of this female fly? Rr These are the X and Y chromosomes of a male fly. How is the Y chromosome different from the X? Does the gene for eye color exist on the “Y” chromosome? Why or why not? What would be the phenotype of this male fly? r The Y chromosome is shorter than the X No, because the gene for eye color is found on the longer segment of the X chromosome. This part is missing on the Y Red eye female White eye male

4. Sex-Linkage or (X-linked) When genes are sex-linked, we include the X and Y as part of their genotype. For example, the allele for red eye is not “R” but is written as X R. How would you write the allele for white eye? X r

5. Writing X-linked Genotypes What is the possible genotype(s) for the eye color of this fly if it is a female? What is the possible genotype for the eye color of this fly if it is male? Answer the above questions again for this fly. X R X R or X R X r X R Y XrXrXrXr XrYXrY

6. Sample Problems Example 1: What is the F1 genotypic and phenotypic ratio of a female true-breeding wild-type fly for red eyes crossed with a white-eyed male? X XRXR XRXR XrXr Y XRXrXRXr XRXrXRXr XRYXRYXRYXRY Genotypic Ratio X R X r :X R Y 2:2 reduced to 1:1 Phenotypic Ratio Red-eye female:Red-eye male 2:2 reduced to 1:1

7. Sample Problems Example 2: What would the genotypes and phenotypes be of the F2 generation? X XRXR XrXr XRXR Y XRXRXRXR XRXrXRXr XRYXRYXrYXrY Genotypic Ratio X R X R : X R Xr : X R Y : X r Y 1 : 1 : 1 : 1 Phenotypic Ratio Red-eye female : Red-eye male : White-eyed male 2 : 1 : 1

8. X-linked disorders Definition: diseases or disorders whose genes are found on the X-chromosome, but not on the Y. Ex: hemophelia (X h ), color blindness (X b ), muscular distrophy (X m ) If the disorder is recessive, more males than females will tend to have the disorder. Why?

9. Take, for example, colorblindness (X b ) If you have a normal female, what is her possible genotype(s)? _____, or _____ If you have a colorblind female, what is her genotype? ______ If you have a colorblind male, what is his genotype? ______ How many colorblind genes do males need to inherit to be colorblind? _____ Females? _____ Who does the male inherit the colorblind gene from? _____________________________________ XBXBXBXB XBXbXBXb XbXbXbXb XbYXbY 12 His mother, who donates the X chromosome

10. Other traits and alleles of Drosophila melanogaster Wild Type Traits (+) Mutant Traits Gray body = G + Black body = g Normal wings = N + Shriveled, vestigial wings = n Body-color and wing-type are NOT located on the sex chromosome, so they are considered autosomal Things to think about independently….How would you confirm or test that these mutant traits are recessive?

11. Example 3 In flies, grey bodies (G + ) and normal-wing size (N + ) are dominant to black bodies (g) and small wing size (n). Predict a cross between G + gN + n and ggnn.

12. Predicted Cross G + gN + n x ggnn G+NG+NG+nG+ngN + gn 25% G + gN + n25% G + gnn 25% ggN + n25% ggnn Surprising Results!

13. Actual Results 8.5% 41.5% 41.5% 41.5% 41.5% Why did this happen???

14. Linked Genes The genes for body color and wing size are linked, meaning they are found on the same chromosome. They will most likely be inherited together and will not undergo Mendel’s Law of. segregates cross over segregates the linked genes Independent Assortment unless G+ g nN+ G+ g nN+ Homologous Chromosomes

15. What are recombinants? Recombinants are offspring that have different phenotypes from those of the parents. Let’s look back at our original cross X

16. 8.5% 41.5% 41.5% 41.5% 41.5% X A B C D Which offspring (A-D) from this cross are the recombinants?

17. How do we determine if two genes are linked or if two genes are located on different chromosomes? Answer Calculate the recombination frequency! If the frequency is less than 50%, it is assumed that the two genes are on same chromosomes.

18. 206 185 944 965 X A B C D Calculate the recombination frequency for this cross! Total Flies: 965 + 206 + 185 + 944 = 2500 flies Total number of recombinants: 206 + 185 = 391 Recombination Frequency = recombinants / total flies = 391/2500 =.156 = 16%

19. Gene Mapping Genes are mapped on a chromosome based upon the recombination frequency. For ex. the distance between the genes for body color and wing type is therefore 16 “map-units” apart (16% frequency) Black Body Small wings Grey Body Normal wings

20. Using recombination frequencies, create a linkage map for the following a - c: between genes b and a = 10.5%; between genes c and a = 48%; between genes c and b = 37.5% 48% ac 10.5%37.5% b