Two Drosophila flies were mated that had normal (transparent, long) wings. In th

Question

Two Drosophila flies were mated that had normal (transparent, long) wings. In the progeny two new phenotypes appeared: dusky wings (having a semi-opaque appearance) and clipped wings (square end). The progeny were as follows:

                         Females                 224     transparent long

                                                       73      dusky long

                         Males                      115     transparent long

                                                       111     transparent clipped

                                                       35      dusky long

                                                       39      dusky clipped

Provide a genetic explanation for these results, show the genotypes of parents and all of the progeny classes under your model.

Does a chi-square analysis support your hypothesis?

Explanation / Answer

a. The given data shows that there are approximately equal numbers of males (300) and females (297), the males fall into four phenotypic classes, but the females constitute only two. This fact should immediately suggest some kind of sex-linked inheritance. When we study the data, we also see that the long and clipped phenotypes are segregating in both males and females, but only males have the dusky phenotype. This observation suggests that the inheritance of wing transparency differs from the inheritance of wing shape. First, long and clipped are found in a 3:1 ratio in both males and females. This ratio can be explained if the parents are both heterozygous for an autosomal gene; we can represent them as L/l, where L stands for long and l stands for clipped.

Having done this partial analysis, we see that it is only the wing transparency inheritance that is associated with sex. The most obvious possibility is that the alleles for transparent (D) and dusky (d) are on the X chromosome, because gene location on this chromosome gives inheritance patterns correlated with sex. In this situation, the parental female must be the one sheltering the d allele, because, if the male had the d, he would have been dusky, whereas we were told that he had transparent wings. Therefore, the female parent would be D/d and the male D. Let’s see if this suggestion works: if it is true, all female progeny would inherit the D allele from their father, so all would be transparent winged. This was observed. Half the sons would be D (transparent) and half d (dusky), which was also observed.

So, overall, we can represent the female parent as D/d;L/l and the male parent as D/D;L/l. Then the females progeny would be transparent long (3/4) and transparent clipped (1/4) and

male progeny would be transparent long (3/8), transparent clipped (1/8), dusky long (3/8), dusky clipped (1/8).

Notice that using one of the female progeny as a parent would not serve our needs: we cannot say from observing the phenotype of any one of these females what her genotype is. A female with transparent wings could be D/D or D/d, and one with long wings could be L/L or L/l.  The Chi Square statistic is commonly used for testing relationships between categorical variables. The null hypothesis of the Chi-Square test is that no relationship exists on the categorical variables in the population; they are independent.

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