http://biology.clc.uc.edu/Courses/bio105/sex-link.htm MUST use this site!

Mendel NOT: Linked Genes AP Bio Mendel NOT: Linked Genes

The dihybrid crosses we previously did assumed the genes were on different pairs of chromosomes. Now, we want to look at an example where the genes involved are on the same chromosome.

Doesn’t seem any different so far…

All F1’s are Abab But note: not AaBb, this IS different

If you were to cross two F1’s… what ratio would we expect? 9 : 3: 3: 1

Actually, this F1 cross reveals: 3: 1 This is a “Mendel NOT”

These genes are Linked” The only possible gamete combinations are: AB and ab (like a monohybrid cross) not AB Ab aB ab (like a dihybrid cross) b/c the genes are on the SAME chromosome

These genes are Linked”

HOWEVER… The only possible gametes are AB and ab… BUT, wierdly, when scored… a few offspring did illustrate the unexpected: A_bb and aaB_

This is due to the fact that occasionally, during synapsis in meiosis I, while the homologous chromosomes are paired up, sister chromatids from the homologous chromosomes exchange equal segments Crossing over

NOTE: the unexpected is known as a RECOMBINANT THIS could account for the small number of the unexpected A_bb and aaB_ NOTE: the unexpected is known as a RECOMBINANT Crossing over

T. H. Morgan found that the further apart two genes are on a chromosome, the more likely there is to be crossing over between those two genes.

The more likely there is to be crossing over between The more likely there is to be crossing over between those two genes… those are the RECOMBINANTS! (they look different from either parent)

That means that the more likely a RECOMBINANT is shown, the further apart it is on the gene.

When crossing Over happens, a RECOMBINANT is shown