Gene Linkage, recombinants & Chromosome Mapping in Eukaryotes

Question 1

What are Linked Genes?

Answer 1

• Genes located near each other on the same chromosome tend to be inherited together in genetic crosses and are such deemed to be genetically linked – linked genes.
• When you follow linked genes in breeding experiments, the results differ to that in Mendel’s law of independent assortment.

Question 2

Describe Morgan’s Drosophila experiment:.

Answer 2

• Determine whether gene for body colour and wing size are genetically linked.
• Crossed true breeding black vestigial-winged (b b vg vg) X true breeding grey body normal wings (b+ b+ vg+ vg+). - heterozygous grey body normal wing F1 (b b+ vg vg+).
• Crossed F1 X homozygous recessive males ((b+ b+ vg+ vg+). - F2
• The testcross male’s sperm only contributes recessive alleles – reveals genotype of the eggs.
• F2 ratio is 1:1:0:0 (wild type: black vestigial: grey vestigial: black normal)
• Conclude:
o Since most offspring had P-gen phenotype – genes for body and wing size are genetically linked on the same chromosome.
o However, production of a small number of offspring with non-parental phenotype indicates that some mechanism occasionally breaks the linkage. (recombination).

Question 3

What are Recombinants? What determines the frequency? When does it occur?

Answer 3

• Offspring that show variations that differ from both parents are said to be recombinant types/recombinants.
• If half of the offspring are recombinants they are said to have a frequency of recombination of 50%.
• Recombination is the reason why gene linkage mechanism is broken and why some offspring show phenotypic differences to both parents.
• In meiosis crossing over occurs in prophase I – this is the swapping over of genetic material between non-sister chromatids.

Question 4

What are the effects of single and double crossing overs?

Answer 4

• Single crossing overs result in recombinants however, double crossing overs reduces apparent recombination and can show linkage between distant genes.

Question 5

What is Gene mapping?

Answer 5

• Morgan suggested that recombination frequencies could be used as a quantitative measure of genetic distance between 2 genes.
• Genetic mapping is based on the frequency of crossovers between genes.
• Crossovers can’t be directly scored.

Question 6

What are Chiasma(ta)? What is their role in recombination?

Answer 6

• Those with no or even number of chiasmata between a pair of genes(A-B) = 0% recombination.
• Those with odd number of chiasmata between 2 genes (A-B) = 50% recombination.
• RF (recombination frequency) depends on relative proportion of these possibilities.
• No matter how far apart two genes may be – the maximum RF between 2 genes is 50%
• A length of a chromosome that has an average of 1 chiasma is said to be 50 map units long or cM (centiMorgans).

Question 7

What is the Frequency of recombination for cis and trans heterozygotes?

Answer 7

• The same.

Question 8

What are the 2 configurations of 2 linked genes alleles in heterozygous genotypes?

Answer 8

• Heterozygous genotype for two linked genes can have alleles in either two configurations:
o Trans/repulsion = mutant alleles on opposite chromosomes e.g. Ab/aB
o Cis/coupling = mutant alleles on the same chromosome e.g. AB/ab
.

Question 9

What is a Three-point test cross? What is it used for? What can it detect?

Answer 9

• Maps three different genes.
• Double crossovers are detectable in recombination experiments that employ three point crosses.
• Two genes are checked for RF at a time which allows distance between the three genes to be mapped.

Question 10

What are the Key assumptions and exceptions you need to consider when using the three-point cross?

Answer 10

• ‘Probability of recombination is approx. equal along the length of a chromosome’.
• Clearly not the case in some species – meiosis study used to improve randomness.
• If multiple recombinations occur they can involve more than two chromatids – they behave independently.
o Longer regions are more likely to have multiple crossovers.

Question 11

What is RFLP?

Answer 11

• Restriction Fragment Length Polymorphism.

Question 12

• If genes are not linked all progeny should be in _________ frequency.

Answer 12

same.

Question 13

• If linked some progeny should be in ____________ numbers.

Answer 13

higher or lower.