In your experiments with Drosophila, you found that the wildtype allele that codes for grey adult insects is sometimes mutated, with mutants having a black colour. Similarly, you find that your population has alleles that code for vestigial wings instead of the wildtype normal wings. (Both mutations are recessive.) You want to know whether the two genes for body colour and wings are linked. In your preliminary crosses you have established a large number of individuals that you require for testcrosses.
9a. Which two genotypes will you use for the testcross? (Hint: One of the two genotypes should be a heterozygous dihybrid.) Use the common wildtype notation (e.g., ar+ tg) rather than the allele notation (VvZZ). For simplification, disregard the gender. (2)
9b. Indicate the two phenotypes of your testcross. (1)
9c. Indicate the genotypes of the progeny (next generation) of the testcross by drawing a Punnett square. (3)
Assuming a total number of 4400 flies for the progeny generation, what are your expectations for the ratio of genotypes if
9d. the two genes are linked? (1)
9e. the two genes are not linked? (1)
9f. Finally, if the two genes are not linked according to your results, would you expect that they lie on different chromosomes? Explain your answer. (2)
Testcross in Drosophila: An Analysis
In this experiment involving Drosophila (fruit flies), we are examining the potential linkage between two genetic traits: body color and wing type. Both traits are controlled by recessive alleles, where the wildtype allele results in grey body color (denoted as C+) and normal wings (denoted as W+), while mutants exhibit black color (c) and vestigial wings (w).
9a. Testcross Genotypes
To determine whether the two genes are linked, we will perform a testcross using the following genotypes:
1. Heterozygous Dihybrid: CcWw (carrying both dominant and recessive alleles for both traits)
2. Homozygous Recessive: ccww (showing the mutant phenotypes for both traits)
9b. Testcross Phenotypes
The two phenotypes of the testcross are:
1. Grey body with normal wings (C+W+)
2. Black body with vestigial wings (ccww)
9c. Punnett Square for the Testcross
To visualize the potential progeny genotypes, we can draw a Punnett square. The gametes from the heterozygous dihybrid (CcWw) can be CW, Cw, cW, and cw, while those from the homozygous recessive (ccww) will only be cw.
| CW | Cw | cW | cw
-------------------------------------------
cw | CcWw | Ccw | ccWw | ccww
Progeny Genotypes from the Testcross
Based on the Punnett square, the expected genotypes of the progeny are:
1. CcWw (grey body, normal wings)
2. Ccw (grey body, vestigial wings)
3. ccWw (black body, normal wings)
4. ccww (black body, vestigial wings)
9d. Expectations if Genes are Linked
If the two genes are linked, we would expect to see a higher proportion of parental phenotypes than recombinant phenotypes among the progeny. Assuming no recombination occurs, the expected ratio of genotypes would be:
- Grey body with normal wings (CcWw): 2200
- Grey body with vestigial wings (Ccw): 2200
- Black body with normal wings (ccWw): 0
- Black body with vestigial wings (ccww): 0
Thus, the expected ratio of genotypes would be:
- Grey Normal: Grey Vestigial: Black Normal: Black Vestigial = 2200:2200:0:0
9e. Expectations if Genes are Not Linked
If the two genes are not linked, they assort independently according to Mendel's law of independent assortment. Therefore, we would expect a 1:1:1:1 ratio:
- Grey body with normal wings (CcWw): 1100
- Grey body with vestigial wings (Ccw): 1100
- Black body with normal wings (ccWw): 1100
- Black body with vestigial wings (ccww): 1100
Thus, the expected ratio of genotypes would be:
- Grey Normal: Grey Vestigial: Black Normal: Black Vestigial = 1100:1100:1100:1100
9f. Implications of Independent Assortment
If the two genes are found to be not linked based on our results, it suggests that they assort independently during meiosis. This does not necessarily imply they are on different chromosomes; rather, they could be on the same chromosome but far enough apart that recombination occurs frequently between them.
Conclusion
This experiment provides insight into genetic linkage in Drosophila and highlights how understanding genotype ratios can lead to conclusions about gene placement and inheritance patterns. Future studies could investigate further the physical locations of these loci on the chromosomes to better understand their relationship.