Biology Forums - Study Force

Explain why a characteristic can jump a generation

There are many reasons why a characteristic can jump one (or more) generations.
1) Sex- linked recessive traits sometimes jump some generations. For example: a man (generation I) with a sex-linked recessive trait, may pass the recessive gene to his daughter (generation II). Because the gene is recessive, the daughter won't have the trait. If his daughter makes a son (generation III) and passes him the recessive gene, then her son will have the trait. As you see, the recessive trait "jumped" a generation.

2) Many genes have penetrance (http://en.wikipedia.org/wiki/Penetrance) that is lower than 100%. This means, that the gene (or genes) responsible for a trait, will not always express the trait. For example, a person may have a gene responsible for a disease, while his daughter/son with the same gene is not expressing the disease (and vise-versa a parent may not express the disease, and the daughter/son with the same gene may express it)

3) Autosomal recessive genes are sometimes responsible for jumping generations, because two recessive alleles are needed in order for the trait to be expressed. This is more rare however, because it requires that the descendant with the recessive gene will mate with a person with the same recessive gene.

There might be other reasons. If I think of anything else, I'll respond again.