Question

Mosquitoes can have bigger or smaller body sizes. Two parents that have medium-sized bodies have offspring. One of their offspring has a small body, and one has a large body. How did the offspring end up with traits that are different from the traits of their parents? How did the gene combinations result in these traits?

Answer 1

Traits like body size are set by the combination of genes an offspring inherits plus environmental conditions. Two “medium” parents can produce both smaller and larger offspring because their gametes can shuffle the alleles they carry so an offspring ends up with a different combination of size‑influencing alleles.

How that happens (step by step)
- Many size traits are polygenic — controlled by several genes that add up to make the final size. Each gene has alleles that push size a bit larger or a bit smaller.
- If both parents are “medium,” they often carry a mixture of alleles for larger and for smaller size (they’re heterozygous at several loci).
- During meiosis the parents’ alleles segregate and assort independently, and fertilization combines one gamete from each parent. That produces offspring genotypes that can concentrate more “large” alleles or more “small” alleles than the parents had on average.
- Offspring that inherit many alleles that increase size become large; those that inherit many alleles that decrease size become small. Offspring that inherit an intermediate mix are medium.

Simple illustrative example
- Suppose three genes each have a “big” allele (B) and a “small” allele (b). Parents are each Bb Bb Bb (medium).
- Gametes from each parent can carry different mixes of B and b. Occasionally an offspring will receive B at all three genes (BBB = large) or b at all three (bbb = small) even though both parents looked medium.

Other contributing factors
- Environment (nutrition, larval density, temperature) can push genetically similar individuals to different sizes.
- Dominance relationships, epistasis, or new mutations can also change outcomes, but the main reason is segregation/recombination of the parents’ alleles plus environmental effects.

In short: the parents carried both “big” and “small” alleles. Meiosis and fertilization produced offspring with new allele combinations, so some offspring ended up genetically biased toward smaller or larger body size and therefore had different phenotypes than their medium-sized parents.