What is the reproductive mechanism that allows for an offspring to inherit a trait that is expressed in neither parent?

Answer 1

Each parent has two versions (called alleles) of every gene, although only one is expressed. During the formation of gametes (sperm or egg), the two alleles separate and each gamete gets only one of the two alleles that a parent could contribute.

So, let's pretend that both parents have brown eyes, and that the allele W is responsible for brown eyes. Let's also suppose that w (lower case) is responsible for blue eyes. Remember...both parents have two alleles, but only one is expressed, with the capital W being dominant over lower case w, so Ww will have brown eyes even though the blue eye gene is present.

The mother, who has brown eyes will have the alleles Ww, so can produce eggs that have either a W or a w. Each egg is equally likely to contain either a W or a w, so about 1/2 of the eggs produced will have a W, the other half a w.

The father, who has brown eyes will have the alleles Ww, so can produce sperm that have either a W or a w. Each sperm is equally likely to contain either a W or w, so about 1/2 of the sperm will contain a W, the other half a w.

The progeny of these parents will have one of the four possible allele combinations:

• Father contributes a W, and mother contributes W - offspring will have WW, resulting in brown eyes.
• Mother contributes a W, and father contributes a w - offspring will have Ww, resulting in brown eyes.
• Father contributes a W, and mother contributes a w - offspring will have Ww, resulting in brown eyes.
• Both father and mother contribute w - offspring will have ww, resulting in blue eyes.

Note that if the father's sperm contained a W, and the mother's egg contained a W, the progeny will have brown eyes. Also, note that if the father's sperm contained a W and the mother's egg contained a w, the progeny will have brown eyes, because W is dominant over w. The same can be said if the mother contributes a W and the father a w. Three of four possible combinations will result in the progeny inheriting a W from one or both parents, so will have brown eyes, thus, there is a 3/4 probability that the progeny from such a combination will have brown eyes.

Also, note that there is a 1/4 probability that an offspring will inherit ww, because each parent contributed the recessive w gene. This ww offspring will have blue eyes even though both parents have brown eyes!

The Principle of Segregation of alleles in the formation of gametes makes it possible for each gamete to be equally likely to contain either allele.

Answer 2

This can occur through a genetic phenomenon known as recombination, where during meiosis, genetic material from each parent is shuffled and exchanged, leading to unique combinations of genes in the offspring. Additionally, new mutations can arise spontaneously in the germ cells of parents, leading to novel traits in their offspring that were not present in either parent.