I often hear casual mention of this or that gene “skipping generations.” Is this really possible? Can genes skip generations? As posed, the answer to this question is “no.” Genes do not disappear and then reappear in later generations. But the expression or manifestation of genes — traits — can skip generations under some circumstances.
First, a quick lesson on genetics. If you already have a passing familiarity with how inheritance works, you may want to just skim the next bit. Genes, or “loci,” (singular: locus) are regions of DNA, but not the DNA sequence at the region. (The word “gene” is sometimes used to mean other things, but this is the definition I’ll be using for this discussion.) The actual sequence of DNA at the locus is called an “allele.” A gene or locus is where the DNA is found that produces a particular trait, and the allele at the locus determines the nature of the trait. For example, there are genes that control finger length. You might have an allele at that locus that gives you long fingers or an allele that gives you short fingers. At a locus that controls eye color you could have an allele that gives you blue eyes or an allele that gives you green eyes. (Eye color is actually controlled by many different genes, but I hope this gives you the idea.)
Typical humans have two copies of each chromosome, and therefore have two copies of each gene. The alleles at these loci may be two identical copies, or two different versions. When you have two different alleles for the same trait, they have to decide which one gets expressed. Some alleles are dominant and some alleles are recessive. If a dominant allele is present, then the trait that the allele codes for will be expressed, regardless of what the other one is. If a recessive allele is present, it will not be expressed if there is also a dominant allele present. For a recessive trait to be expressed, there need to be two copies of it. Take our earlobes, for example. The dominant allele produces free earlobes, and the recessive allele produces attached earlobes (see picture below). If you have two dominant alleles, you will have free earlobes. If you have one dominant and one recessive allele, you will also have free earlobes, because the presence of just one dominant allele will always result in the expression of that trait. If you have two recessive alleles, you will have attached earlobes.
This is true for all genes except those that are located on the sex chromosomes. The X and Y chromosomes have different genes on them. Human females, who have two X chromosomes, have two copies of each gene on the X chromosome. Human males, who have one X and one Y, have only one copy of all of the genes on the X chromosome, and one copy of all of the genes on the Y chromosome. When there is a recessive allele on a chromosome that there is not a second version of (i.e. the X and Y chromosomes in males), it will be expressed even though there is only one copy of it, because there is no other allele to be dominant over it.
For people with two X chromosomes, one is inherited from each of her parents. Her mother, who has two X chromosomes herself, gives one of her two X’s at random. From her father, she will inherit the only X chromosome he has. For people with one X and one Y, the X always comes from the mother (who only has X’s to give) and the Y always comes from the father. This has some very particular implications for inheritance.
If a man has a particular allele that is located on the Y chromosome (a “Y-linked” trait), he will pass it on to his sons 100% of the time, because sons always get their Y chromosome from their father. If he has a particular trait that is located on the X chromosome, he will never pass it on to his sons. He will have a 100% chance of passing the allele on to his daughters, and they will express it or not based on the normal rules of allele dominance.
If a woman has a particular trait that is located on one of her X chromosomes (an “X-linked” trait), there is a 50% chance that it will be passed on to either a son or a daughter. If the son inherits the trait, he will always express it, because he only has one X chromosome. If a daughter inherits the trait, she will express it or not based on the normal rules of allele dominance.
Here is where the generation skipping comes in. Consider this family:
Our first generation people are Bob and Sue. Bob has a recessive allele on his X chromosome, shown in blue, and Sue does not. Because Bob only has one X chromosome, this recessive allele is expressed. When they have children, their son, Fred, will inherit an X chromosome only from his mother, so he does not inherit his father’s recessive allele. Their daughter, Jill, inherits one X from her father, which carries the recessive allele, and one X from her mother that does not have the allele. Jill will not express this trait because it is recessive.
Fred marries Jean, who does not carry the recessive allele. None of their children will inherit the recessive allele because neither of their parents had it.
Jill marries Kyle, who does not have the recessive allele. Half of their sons will inherit the recessive allele and express the trait. Half of their daughters will inherit the allele but will not express it.
So who in this family expresses the recessive allele? Only Bob and one half of Jill and Kyle’s sons. The trait skipped Fred and Jill’s generation, although Jill carried an allele for it.
I have written further on this topic here.
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