Ch 7:Linkage, Recombination, and Eukaryotic gene mappingt4

Recall

principle of segregation. Alleles separate during meiosis

Independent assortments leads to

4 types of gametes

Completely Linked genes

linked genes that do not exhibit crossing over. Only 2 kinds of gametes

What are recombinant gametes with linked genes?

Crossing over has occured, so we get a new recombination of two alleles. Half with have unchanged chromosome an dhalf will have a recombinant chromosome. There are now 4 types of gametes

When recombinant

2/4 gametes are recombinant.

Genetic mapping based on recombination frequencies between genes.

Specifically means using recombination frequencies between genes. Genes that are further apart (less closely linked) will have a higher recombination frequency than genes closer together

Recombination frequency is measured in

Map units (M.U.) or centiMorgans (cM)

Complete linkage leads to

nonrecombinant gametes and nonrecombinant progeny

Nonrecombinant

An offspring whose combination of traits has not changed from the parental generation.

Crossing over between linked genes

Does lead to recombinant gametes and recombinant progeny

Any time we have 1 crossover

Creates 2 recombinant gametes

Complete linkage compared with independent assortment

heteroz x homozygous recessive= test cross.

Heterozygous x Homozygous recessive

Test cross

coupling (cis) configuration

one homolog contains both wild-type alleles, the other homolog contains both mutant alleles

repulsion (trans) configuration

One wild-type and one mutant allele on each chromosome.

recombinant frequency

Number of recombinant types / total number of offspring x100

Three point test cross

A test cross where one parent is heterozygous for 3 genes. Used in linkage analysis and gene mapping. Allows us to know the order of genes on a chromosome. The order of three geens can be established in the F1

Steps in determining gene order in a three-point cross

1- Identify the nonrecombinant progeny (two most numerousphenotypes)

2- Identify the double-crossover progeny (two least numerousphenotypes).

3. Compare the phenotypes of double-crossover progeny with thephenotypes of nonrecombinant progeny. They should be alike in twocharacteristics and differ in one

.4. The characteristic that differs between the double crossover

independent assortment

alleles at one locus sort independently from alleles at another locus. shuffles different chromosomes

Recombination

alleles sort into new combinations

Crossing over

Linked genes ability to become recombinant

The further genes are on a chromosome

higher level of crossing oveer

For single crossovers, the frequency of recombinant gametes is half the frequency of crossing over because

each crossover takes place between only two of the four chromatids of a homologous

Calculating Recombination Frequency

Recombination frequency = (No. recombinant progeny/Total no. of progeny) x 100%

Genetic mapping based on recombination frequencies between Genes

Genetic maps determined by recombinant frequency; genes further apart have higher recombination frequencies than genes closer together• RF is measured in map units (M.U.)or centiMorgans (cM) -> they're the same thing• Genetic maps are different thanPhysical maps (which are measuredin basepairs)

the following test cross produces the progeny shown ( AaBbx aaBb= 10AaBb, 40 aaBb, and 10 aabb). Were the A and B alleles in the Aa Bb parent in coupling or repulsion?

Repulsion

4 Chromosomes coem together in a Tetrad during crossing over but

only two chromatids exchange genetic info

How does a genetic map differ from a physical map?

Genetic maps are based on rates of recombination. Physical maps are based on physical distances.

Constructing a genetic map:

Determine the gene order. Determine the location of crossovers.

Somatic-cell hybridization

Fusion of somatic cells of different types.

in situ hybridization

A technique using nucleic acid hybridization with a labeled probe to detect the location of a specific mRNA in an intact organism.

Somatic-cell hybridization is used to

determine which chromosome contains a gene of interest