Inheritance!

[a7018567]

Hi, im studying for a genetics module and stuck with where to start with this question...
Iknow its dihybrid inheritance but cant seem to make it fit the rules ive learned????

Help would be appreciated...

Two domestic rats with brown fur and brown eyes mate a number of times over a period of time. The offspring do not match the parents... The offspring are:

8 with white fur and blue eyes
14 with white fur and brown eyes
26 with brown fur and brown eyes
12 with brown fur and blue eyes

Using knowledge of genetics give the likely genotype of the parents and theoretical distribution of the phenotypes of their offspring. You should include a punnet square.

[Jesse2504]

First of all you have to realise that there are two phenotypical features being discussed, eye color and fur color.

From the given numbers of the rats offspring, you will notice that the highest portion of them were Brown eyes Brown fur. This is most likely due to dominant alleles for these phenotypes, lets call the eye alleles E,e and the fur alleles F,f.

Both parents have Brown eyes and fur but, their offspring were not the same, this rules out EEFF for parent genotype as it would only give rise to Brown eyes and fur. Since full genetic variation was seen in at least one of the offspring the parents genotype must be EeFf.

When we cross these we get the following punnet square:


You can see the following theoretical ratios:

1 : Blue eyes, White fur -- ffee
3 : Brown eyes, White fur -- ffEe, ffEE
3 : Blue eyes, Brown fur -- Ffee. FFee
9 : Brown eyes, Brown fur -- FfEe, FFEe, FfEE, FFEE

Our given offspring ratios are

8 : Blue eyes, White fur
14 : Brown eyes, White fur
12 : Blue eyes, Brown fur
26 : Brown eyes, Brown fur

As these ratios very roughly match (due to small number of offspring), this supports the parent theoretical genotype.