Question

1 For any gene with a dominant allele ‘ R’ and recessive allele ‘r ‘ (a)...

1 For any gene with a dominant allele ‘ R’ and recessive allele ‘r ‘
(a) Show the genotype of the offspring from a cross between RR and Rr
(b) What proportions of the offspring are expected to be homozygous dominant, homozygous
recessive and heterozygous?

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Answer #1

1a. The genotype of the offspring can be obtained by crossing RR and Rr with the help of punnet square.
Each allele contribute to the genotype of the offspring.

R R
R RR RR
r Rr Rr

So the genotypes of the offspring can be RR or Rr.


1b.
We know that R is dominant and r is recessive.
RR and rr are homozygous.
Rr is heterozygous.
Because R is dominant over r, RR will be homozygous dominant. Similarly rr will be homozygous recessive.
Therefore the proportion is 2:2 that is homozygous dominant : heterozygous.
One of the parent has only R and R there is no presence of r which is a recessive allele hence recessive dominant will not be obtained in this case.

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