Question

Gene linkage. Study the fly experiment with b and vg. How would you figure out the expected frequencies of genotypes and phenotypes? Which are the recombinant phenotypes, and which are parental? How do you know if the loci are linked? What is the recombination frequency of unlinked loci? Which of Mendel’s laws is violated by gene linkage?

Linkage maps: What determines whether two loci are linked? How were the fruit fly linkage maps constructed? Could the whole human genome be shown on one linkage map? If not, how many maps would it take?

Sex-linked genes

Contrast X vs. Y chromosome. How are alleles on X & Y inherited differently from alleles on autosomes? Study the two examples of X-linked inheritance – from mother and from father.

Epigenetic inheritance: imprinting. Prader-Willi vs. Angelman: the same chromosomal deletion has different effects when inherited from father or mother.

Organelle genes. Why is a disorder such as Leber hereditary optic neuropathy not inherited in a Mendelian fashion?

Answer 1

Gene linkage:

If the loci are close to each other, they might stick together when crossing over occurs. So the existence of one gene might very well mean that the other gene exists too. In other words, the probability of crossing over is proportional to the distance between loci.

How would you figure out the expected frequencies of genotypes and phenotypes?

Punnett square, at any given locus, you have an equal chance of passing on mom's allele or dad’s allele.

Which are the recombinant phenotypes, and which are parental? • How do you know if the loci are linked?

When we cross b+b+vg+vg+ and bb vg vg ( parental phenotypes), the recombinant phenotype is b+b vg+vg. If the loci are linked, the real frequency of recombinant phenotype is less than 50%. The recombinant frequency is proportional to distance.

a.First parent ­ Double mutant: bad wings/black body

b.Second Parent ­ Heterozygous: looks normal (wild-type) but carries genes from mutant-type

c.If it has a grey body (wild-type) it usually has normal wings, if it has a black body it usually has bad wings.

What is the recombination frequency of unlinked loci?

Unlinked genes have 50% recombination frequency: they show independent assortment.

Which of Mendel’s laws is violated?

The law of independent assortment.

Linkage maps•

What determines whether two loci are linked?

The distance, or the relative location of the two loci.

How was the fruit fly linkage map constructed?

In fruit flies, the recombination frequency (recombinants/total offsprings) is 17%, massively lower than the expected frequency of 50%. And the recombination frequency is proportional to distance.

Could the whole human genome be shown on one linkage map? If not, how many maps would it take?

No, only the loci on one chromosome. Because if two genes are on different chromosomes, they are independent. They do not link to each other at all. For a human, maybe 23.

Sex­linked genes.•

Contrast X vs. Y chromosome.Y: 23 Mbp, 78 genes? X: 153 Mbp, 900­1200 genes.

How are alleles on X & Y inherited differently from alleles on autosomes?

Study the two examples of X­linked inheritance – from mother and from father.

X-­linked recessive genes:

From mother: mother with one allele on X will pass it on to half of her children.

From father: always pass on to daughter.

Autosomal inheritance of allele: Inheritance of autosomal allele doesn't depend on the gender of an individual, they inherit equally in both male and female.