BIOL 2500 - Topic 6 (part 3)

Crossover occurs between __________ chromatids

Nonsister

What helped explain crossing over between non-sister chromatids?

Tetrad analysis

How many gametes do we get when crossing over occurs between two homologs??? *ASK J*

2 kinds of gametes

What tetrad analysis has shown us about crossing over

Multiple crossovers can occur along a homologous pair, such that two or more chromatids can exchange genetic material (i.e. any of the non sister chromatids can crossover)

Simple crossovers

When two linked genes cross over (AB x ab)

Double crossovers

When three linked genes cross over at the same time (ABC x abc)

Importance of recombination frequency

It is the key to chromosomal mapping, as it tells us if genes are near or far from each other

Recombination frequency role in chromosomal mapping

1.) Genes that are farther apart on the same chromosome have a higher chance of a crossover event, allowing for the formation of recombinant gametes and a recombination frequency of more than 50%

2.) Genes that are closer together are inherited together, because they have a lower chance of a crossover event, resulting in a recombination frequency of less than 50%

Complete linkage

When the genes are side by side, resulting in a recombination frequency of zero

Incomplete linkage

Recombination that occurs between homologs that produces a mixture of parental and recombinant gametes, such that the genes may be close together, but they are not always inherited together like in complete linkage

Common incomplete linkage results

1.) It results in 4 genetically different gametes, 2 parental and 2 recombinants

2.) However, the typical 9:3:3:1 ratio is not observed, as the frequency of the parental gametes are different from the recombinant gametes

Frequency of the parental genes

The two have the same frequency, which is equal to >50%

Frequency of recombinant gametes

The two have the same frequency, which is equal to <50%

What does it mean when the frequency of parental gametes is 100%?

It means that no crossing over occurs at all

Linked genes do not assort _________

Independently

Linked genes are always…

They are always syntenic and located near one another on the chromosome

Syntenic genes that are far apart

They are not linked, as they assort independently due to crossing over of homologous alleles

Genetic linkage expected gamete frequencies

It leads to the production of different expected gamete frequencies, where we expect a greater number of parental gametes than recombinant gametes

Crossing over is less/more likely to…

It is less likely to occur between linked genes that are closer together and more likely to occur between those apart

Frequency of crossing over proportions

It is roughly proportionate to the distance between genes