Biology · Principles of Inheritance and Variation · NEET
It tells you the number of DIFFERENT types of gametes an organism can make. Write it as 2^n (2 multiplied by itself n times). The number '2' is fixed because meiosis puts only one of the two alleles of a pair into each gamete. The power 'n' is the number of gene pairs that are heterozygous (mixed, like Aa). So 2^n = number of gamete types.
No. This is the most common mistake. 'n' is NOT the total number of genes. 'n' is only the number of HETEROZYGOUS gene pairs. A heterozygous pair has two different alleles (Aa, Bb). If a pair is homozygous (AA or aa), it makes only ONE kind of gamete for that gene, so it does not add to n. Count only the mixed pairs.
AaBb has two heterozygous pairs: Aa and Bb. So n = 2, and 2^n = 2^2 = 4. The four gamete types are AB, Ab, aB and ab. This matches Mendel's dihybrid cross, where each parent forms 4 kinds of gametes and this gives the 9:3:3:1 ratio.
No. A homozygous pair like AA can only give the allele A to every gamete, and aa can only give a. There is no choice, so they produce just one type. Only heterozygous pairs (Aa) give a choice of two alleles. So skip AA and aa completely when you count n.
Look at each pair. AA is homozygous, so it does NOT count. Bb is heterozygous, count it. Cc is heterozygous, count it. So n = 2 (only Bb and Cc). Number of gamete types = 2^2 = 4. The genotype AABbCc has three genes but only two heterozygous pairs, so the answer is 4, not 8.
Because in a diploid organism every gene has exactly TWO alleles sitting on a pair of homologous chromosomes. During meiosis (segregation), the two homologous chromosomes separate, so each gamete gets only ONE allele from each pair. Two choices per heterozygous pair means the base is always 2. This is Mendel's Law of Segregation in action.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
It quickly gives the number of different gamete types, the number of genotype classes, and helps predict cross results. For gametes: n = number of heterozygous gene pairs, so gamete types = 2^n.
All three pairs are heterozygous, so n = 3. Gamete types = 2^3 = 8. This is why a trihybrid cross is more complex than a dihybrid cross.
Yes. It assumes the genes are on different chromosomes and assort independently. If genes are linked (on the same chromosome), fewer gamete types than 2^n are made, so linkage reduces recombinant gametes.
In AaBb x AaBb, each parent makes 2^2 = 4 gamete types. Combining 4 x 4 = 16 boxes in the Punnett square gives the 9:3:3:1 phenotype ratio. So the gamete count directly builds the ratio.
Yes. NEET often asks how many gamete types or genotypes a genotype gives, and questions on dihybrid/trihybrid crosses. Knowing to count only heterozygous pairs saves time and avoids the common trap.
