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.)

sam and jess

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.”

ann and luke

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.”

jim and jill

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.”

liz and bill

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

rob and kim

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.

Have a topic you want me to cover? Let me know in the comments on on twitter @CGEppig.


About Christopher Eppig, Ph.D.

I have a Ph.D. in biology and a passion for sharing my knowledge and understanding of the natural world with anyone who will listen. At a time where science is permeating public life more than ever, it is especially important that the public understand what science is, and how its findings intersect with their own lives. In addition to the more practical benefits of scientific literacy, I believe strongly that understanding the natural world enriches peoples lives. The man behind the curtain is not me — it is the real world, which we can discover through science, and it is beautiful. Let me show it to you.  Follow me on twitter @CGEppig. View all posts by Christopher Eppig, Ph.D.

4 responses to “Skipping Generations, Part 2

  • Adam

    What about the dominant trait- cleft chin. If neither mother or father has a cleft chin but their baby does, what does that mean? Both parents were homozygous recessive carriers and there was a spontaneous mutation in the offspring? Or, something else…?

    • Christopher Eppig, Ph.D.

      Hi Adam, thanks for reading.
      If neither parent has a trait, but their baby does, that is a pretty good sign that the trait is recessive, and that both parents were heterozygous for that trait.
      There are a couple of other possibilities, though. The first, as you point out, is that it could be a spontaneous mutation. Mutations at any given point in the genetic code are pretty rare, but everyone carries new mutations. Most of these new mutations are “silent,” meaning that they do not result in a different phenotype.
      Another possibility is that the trait is not controlled by a single gene. Things like height and eye color, for example, have many genes that contribute to final expression of these traits. This can get very complicated very quickly, but the basic idea is that a lot of different genes can combine to produce a variety of effects.

  • Adam

    Thank you Christopher, for the insightful reply.

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