GENETICS

 

3. (8 marks) In rat coat colour, two independently assorting genes A and R interact as follows:
A-R- Grey aaR- Black A-rr Yellow aarr Cream
The phenotypes are only observed in the presence of the dominant allele of a third independently assorting gene C. The recessive genotype cc for this gene causes albinism regardless of the genotype at the A and R genes. Four different homozygous albino strains are each crossed with a true-breeding Grey strain. They produce Grey F1 rats. The F1 rats in turn produce large litters of F2 rats in the proportion shown below:
Albino Line 1: 9 Grey : 3 Black : 4 Albino
Albino Line 2: 3 Grey : 1 Albino
Albino Line 3: 9 Grey : 3 Yellow : 4 Albino
Albino Line 4: 27 Grey: 9 Yellow: 9 Black : 3 Cream : 16 Albino
For each cross state the genotype of each albino line. Give reasons for your conclusion, being sure to explain the observed F2 phenotypic ratios.
3) (6 marks) In penguins, alleles of many genes cause blindness. Blindness alleles can be either dominant or recessive. You have two true-breeding strains of blind penguins and a true breeding normal penguin strain. You make a series of crosses:
Cross 1: females from strain 1 and true-breeding normal penguins give rise to blind male penguins and normal female penguins.
Crosses 2 and 3: females (Cross 2) or males (Cross 3) from strain 2 with true-breeding normal penguins give rise to only normal penguins.
Cross 4: males from strain 1 and females from strain 2 give an F1 of only blind penguins.
Cross 5: F1 litter mates from cross 4 give rise to F2 litters where all male penguins are blind while 3/8 of female penguins are normal and 5/8 of female penguins are blind.
a) How is blindness inherited in strain 1 and in strain 2? Support your answer.
b) Using appropriate symbols, indicate the genotypes of the two blind strains and the genotypes of the F1 litter mates from cross 4 and the F2 progeny described in cross 5.

 

 

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