• http://biology.clc.uc.edu/Courses/bio105/sex-link.htm

• MUST use this site!

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”

• 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

HOWEVER…• The only possible

gametes are AB and ab…

• BUT, wierdly, when scored… a few offspring

did illustrate theunexpected:

A_bb and aaB_

Crossing over

• 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

• THIS could account for the small number of the unexpected A_bb and aaB_

•NOTE: the unexpected is known as a RECOMBINANTRECOMBINANT

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 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