Question

The data come from an alien species of fly
whose genetics is identical to that of earth flies.

As part of your assignment, you must confirm your hypothesized
modes of inheritance by applying a chi-square analysis.

NOTE ABOUT DEGREES OF FREEDOM: In situations where the observed
counts and the expected counts for a category are both 0, that
category does not figure into the chi-square calculation, so that
the degrees of freedom is reduced by one in such situations.

The possible modes of inheritance are:
(i)   Autosomal recessive;
(ii) Homozygous lethal (dominant);
(iii) Sex-linked recessive.
The Sex-linked recessive mode can only occur in females parents.

Counts of phenotype pairs for the F1 generation give you clues
as to the mode of inheritance of each phenotype. Good luck!

ONE PHENOTYPE PER PARENT, TWO PHENOTYPES TOTAL

For the Tratagorian Kumbus Fly, that lives on the planet Zeon,
parental matings occur only among flies in which each parent has
a disease phenotype at one locus and is wild-type at the other locus.


Problem 1. A female with Rheingoldenbach bristle mates with a male with Flonasean bristle,
producing the following counts:

F1 Generation
Males
WT-WT   Disease-WT   WT-Disease   Disease-Disease
0   253       0       248

Females
WT-WT   Disease-WT   WT-Disease   Disease-Disease
248   0       243       0

Answer 1

We hypothesize here that the possible mode of inheritance is Sex linked recessive.

Degree of freedom (df) is calculated as: number of categories -1 = 4 -1 =3

The four categories being WT:WT, disease:WT, WT:disease, disease:disease

Hence, Degree of freedom =3

Also, here the observed counts and expected counts in two categories are 0. In these situtations, degrees of freedom are reduced by 1.

So, Degree of freedom is now 3-1-1 = 1

Using the degree of freedom table for chi square test, we can say that we accept the hypothesis for degree of freedom 1. Hence, the mode of inheritance here is Sex linked recessive.