Question

A population contains an equal number of individuals with the genotype AA and Aa. The proportion of alleles A & a in their gametes will be:
A) 3:1
B) 1:1
C) 1:2
D) Can't be determined

Answer 1

The Hardy-Weinberg theorem characterises the distributions in populations of genotype frequencies that do not evolve, and is, therefore, the basic null model for population genetics. An organism is considered heterozygous because there are two separate alleles of a gene. Heterozygous genes, including Aa and Bb, are represented by an upper case and a lower-case letter.

Complete answer: The genotype of an individual is the combination of alleles at a given genetic locus found in that individual. If two alleles exist at locus A (A and a in a population, then AA, Aa, and aa are the possible genotypes in that population. Homozygotes (i.e., they have two copies of the same allele) are individuals with genotypes AA and aa. Heterozygotes (i.e., they have two different alleles at the A locus) are individuals with genotype Aa. The allele found in that homozygote is said to be dominant if the heterozygote is phenotypically identical to one of the homozygotes, and the allele found in the other homozygote is recessive. With the parent organism in the population to be AA and Aa, we obtain gametes from these parents as:
From AA: A and A, and from Aa: A and a and a. So, the composition of 'A 'and 'a' in them is 3:1 out of four formed gametes, as 3 gametes are of 'A' allele while 1 is of ‘a’ allele.
$AA$ → Gamete$AA$ $AA$
$Aa$ → Gamete$AA$ $aa$
$A=6$
$a=2$
$\frac{A}{a}$ = $\frac{6}{3}$ = $\frac{3}{1}$
Hence, the correct answer is option A.

Note: Alleles are defined as the alternative forms of a gene. A gene comprises two alleles. The allele that expresses itself is the dominant alleles, whereas an allele that does not express itself is said to be recessive.