Why does linkage between loci tend to decrease the amount of variability among offspring compared to the case where loci are unlinked?

Answer 1

A loci is a certain place on a chromosome like where a certain gene may exist. When two genes are on the same chromosome they are said to be "linked genes". This means that when certain traits are passed on they tend to get passed on together.

As an example lets pretend that the genes for eye color and hair color are linked. Humans have two copies of every gene (except for sex-linked genes in males but this is an exception and another question altogether), one copy from their mother and one from their father. Let's simplify things a bit and think of the eye color trait and the hair color trait to be determined by a single gene (things are of course more complicated than this but it makes for a better example). A mother has a chromosome for black hair and black eyes and a chromosome for yellow hair and yellow eyes. When she has a child one of either of the chromosomes will go to the child and since the genes are linked then that means they will either have black hair/eyes or yellow hair/eyes (let's leave the father out of this for now and just assume whatever traits he passes on are recessive to either the black or yellow traits). They will not be able to have a combination of the two since the traits are passed on a single chromosome (this is not exactly true, but let's make life simpler and go over this at the end). If the loci were unlinked by being on a separate chromosome then the traits would be passed separately and you would be able to get children that have black eyes/hair, black eyes and yellow hair, yellow hair and black eyes, and yellow eyes/hair. Now in real life you probably would see a mixture of those traits (well if there were such things as yellow or black eyes) due to something known as chromosomal linkage. This is just when some of the DNA on a chromosome gets transfered to the other chromosome when they cross over before meiosis. You would still see black eyes/hair and yellow eyes/hair predominately among the children but depending on how close the genes are on the chromosome this will be much more likely than any of the mixed traits.

So linked loci decrease the variability in offspring by only allowing two phenotypes for two different traits. Unlinked loci however will show offspring with four different phenotypes for two different traits.