Skipping Generations, Part 2

A few weeks ago, I wrote about traits skipping generations. Be sure to read that entry to get caught up. Genes that are located on the X-chromosome sometimes, but not always, lead to traits skipping generations. But this is not the only way traits can skip generations. Skipping generations can also occur with genes that are not located on the sex chromosomes. The rules of inheritance are such that, given the right combination of alleles in parents, traits can sometimes skip a generation.

When sexual reproduction occurs, the offspring gets a random combination of alleles from their parents. Each parent has two copies of each gene, and one of those copies from each parent is given to each offspring. There are four possible alleles to choose from — two from each parent — and each offspring gets one at random from each parent. This is exactly the same way it works for the sex chromosomes.

Here is one way a recessive trait can skip a generation:

In genetics, it is the convention for dominant alleles to be expressed with a capital letter, and recessive alleles with a lower-case letter.

The parents in our first generation are Sam and Jess. Sam has two copies of the dominant allele (“AA”) — which causes him to express the dominant trait — and Jess has two copies of the recessive allele (“aa”) — which causes her to express the recessive trait. It doesn’t matter what the trait is. It could be attached vs free earlobes, widow’s peak, knuckle hair, or any other trait that follows the basic rules of inheritance. (There are traits that follow slightly different rules, but I’m not going to talk about them right now.)

When they have children, every child will get an “A” from Sam and an “a” from Jess, because Sam has only “A’s” to give, and Jess has only “a’s” to give. All of the children will express the dominant trait, although they all carry (but do not express) the recessive allele. Unlike with x-linked traits, the expression of these traits is not related to sex.

Ann grows up and marries Luke, who is also “Aa.”

There are four possibilities for the children:

(1) They can inherit Ann’s “A “and Luke’s “A” (“AA”).

(2) They can inherit Ann’s “A” and Luke’s “a” (“Aa”).

(3) They can Inherit Ann’s “a” and Luke’s “A” (“aA”).

(4) They can inherit Ann’s “a” and Luke’s “a” (“aa”).

Options (2) and (3) result in the same combination of alleles — “Aa” — even though the source of the alleles are from different parents. That is, it doesn’t matter whether the “A” comes from the father or the mother. It will always be dominant over the “a.”

In the three generations of this family, the recessive trait is only expressed in one individual in generation 1 (Jess), and in one person in generation 3 (Tim). Nobody in generation 2 expressed the recessive trait, even though people in generation 2 carried the recessive allele.

As I said before, these types of traits only sometimes skip generations, but do not always skip generations. By changing the genetics of the example above only slightly, the scenario will play out very differently. If Sam is “Aa” instead of “AA,” half of his children with Jess will be “aa” and the other half will be “Aa.” The recessive trait will be expressed in the second generation and there will be no skipping of generations.

Sometimes generation skipping can happen with dominant traits, too, although by a slightly different mechanism. Recessive alleles can be carried by individuals who do not express the trait, making it easy for the recessive traits to be silent. The only way for a dominant trait to skip generations is if the dominant allele is not passed on in a family, and it is then added in a subsequent generation.

For example, Jim (Aa) and Jill (aa) have children. There is a 50% chance that any child they have will inherit an “a” from both parents. If they have two children, there is a 25% chance that both of them will be “aa.”

Their daughter, Liz, grows up and marries Bill, who is also “aa.” Since neither of them has any “A” alleles, all of their children will be “aa.”

Their son, Rob, grows up and marries Kim, who is “AA.” All of their children will be “Aa.”

Between the first and second generations, the “A” allele was removed from the family lineage. It was not until Kim that the allele was introduced back into the gene pool. The dominant trait skipped generation 2 entirely. Again, this is not because of something to do with the trait itself — the various allele combinations of everyone interacted in certain ways to make the trait skip a generation.

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