Question

Consider a gene that has two alleles and shows complete dominance. When two heterozygotes for this gene breed, they have a 25% chance of producing a homozygous recessive offspring. The next time they breed, what are the chances that they will once again have a homozygous recessive progeny?
A. 0%
B. 25%
C. 75%
D. 100%

Answer 1

Hint: Draw a monohybrid cross to know the chance of producing homozygous recessive offspring. Use the genotypic monohybrid ratio is 1:2:1 (Homozygous dominant: heterozygous dominant: Homozygous recessive) to get the frequency of Homozygous recessive offspring.

Complete Step by Step Answer:
Two alleles of a gene are completely dominant, that is when a dominant and recessive allele is present (heterozygous condition) dominant allele is expressed. When two homozygous alleles are bred, it produces all heterozygous offspring. These offspring are then bred amongst themselves. This produces half heterozygous, 1/4th homozygous dominant and 1/4th homozygous recessive. That is we get 25% of the offspring homozygous recessive.

According to Mendel’s Principles of inheritance, the phenotypic ratio of F2 generation is 3:1 and the genotypic ratio is 1:2:1. Hence 1 out of 4 offspring are homozygous recessive.
When the heterozygous offspring are bred again, they will produce the same type of offspring in the same amount (or percentage). In this case, the two situations are independent of each other. The outcome of the first event does not depend on the other. Hence, the chances of getting homozygous recessive are 1/4th of the total offspring, that is 25% of the offspring.

Hence, The correct option is B.

Note: Here they want to study an independent event. We can also get the frequency of any dependent events. If you want to check the frequency of having a homozygous recessive offspring and another homozygous recessive offspring, the frequency of both must be multiplied. This can be identified by the use of the words ‘AND’ or ‘OR’.