Bio 220 Unit 4 L21

Where are genes located on chromosomes?

At loci

Do genes move during meiosis?

No they do not. Genes are located in the DNA of chromosomes. The chromosomes are what’s moving and the genes are the passengers on the ride.

What independently assorts genes or chromosomes?

Chromosomes

Recombination

Crossing over between loci (genes) creates recombinant chromosomes. Occurs during prophase I. This is not a mutation but a nonparental combination.

What is complete linkage?

This is when alleles are linked together and no crossing over occurs. This creates only two combinations of gametes.

What is a single crossover?

This is when only one crossover between sister chromatids takes place. It produces four gametes.

What is the golden rule?

Recombinants are the combinations that are NOT present in the parent.

What explains the exceptions to independent assortment?

Crossing over. Done at loci.

As distance increases, what happens to recombination frequency?

It increases. The farther apart genes are, the more they behave like unlinked genes.

50% recombination and its behavior

The maximum recombination frequency (RF) is 50%. Once that is reached, you can no longer tell whether genes are on the same chromosome or if they are unlinked (independent assortment). 50% does not prove that genes are on different chromosomes. They could be different chromosomes but they also could just be very far apart on the same chromosome.

Can recombination occur on circular chromosomes?

Yes, but mapping it becomes more complicated because there are two different paths around a circle. Linear chromosomes make gene mapping predictable.

/ vs ; for gene notation

A / means the genes are on homologous chromosomes while ; means the alleles are on different chromosomes.
/ = linked
;= unlinked

Why can’t recombination exceed 50%?

Every crossover affects only two chromatids leaving two parental chromatids unchanged. At maximum 50% of parental and 50% of recombinant can exist.

What is a tester?

Homozygous recessive at all loci of interest

Frequency of recombinant gametes vs parental gametes

Recombinant gametes of linked genes are always less frequent than parental but in unlinked genes they will happen at the same frequency as the parent.

What is a chiasma

This is the physical manifestation of crossing over. The x shaped structure that occurs from sister chromatids during crossing over.

Phase

Combination of alleles that are on the same chromosome.

Cis phase

Two dominant alleles are together or two recessive alleles are together. On the same chromosome! This is for linked alleles only.
Ex: AB/ ab

Trans phase

One locus is dominant and the other is recessive. On the same chromosome! This is for linked alleles only.

Ex: Ab/aB

Linkage maps

Tell the distance between genes of interest

Map unit

The physical distance between genes that results in 1% RF.

How to calculate RF

(from table)
# recombinants/ total number of both parent and recombinant offspring

Hot spots vs cold spots on RF diagram

Hot spots are places where recombination happens very often while cold spots are where it happens very infrequently.

Haplotype

Linear stretch of DNA that is inherited as a chunk. These are cold spots.