Lecture 2ab

Question 1

Why is Mendel’s observation of independent assortment observed?

Answer 1

Independent assortment is only true if genes are on different chromosomes or if on the same chromosome but recombination occurs frequently.

Question 2

What does it mean when two genes are linked?

Answer 2

The loci of those genes are on the same chromosome so the alleles on one homolog are linked by the DNA between them

Question 3

What breaks the law of independent assortment and how?

Answer 3

Linked genes: TH Morgan’s fruit flies experiment demonstrated that parental allele combinations are over represented and non-parental allele combinations are under represented. They do not follow Mendel’s F1 dihybrid 9:3:3:1 ratio. Morgan conducted test cross between F1 and homozygous recessive parent; He observed very little % recombinant gametes when Mendel would have predicted 1:1:1:1 ratio of gametes

Question 4

What is recombination?

Answer 4

The breakage of two DNA molecules at the same position and their rejoining in two reciprocal recombinant recombinations

Question 5

If genes are on the same chromosome, do you ever get non-parental types?

Answer 5

Yes through recombination. When genes are closer together on the same chromosome, they do no assort independently but produce a recombinant frequency of less than 50%

Question 6

When does recombination happen?

Answer 6

When homologous chromosomes line up in the middle of the cell during meiotic metaphase I

Question 7

Does recombination occur during two chromatid phase or four chromatid phase? Why?

Answer 7

Four chromatid phase: a dihybrid cross would result in four different genotypes, two parental and two recombinant. Recombination during two chromatid phase would result in two different genotypes, no parental.

Question 8

Describe multiple cross overs for trihybrid between two chromatids, three chromatids, and four chromatids.

Answer 8

Two chromatids: Odd number cross overs results in recombinant genotypes. Even number of cross overs results in parental genotypes.

Two cross overs between three chromatids: observe 3 recombinant genotypes, 1 parental

Two cross overs between four chromatids: observe 4 recombinant genotypes, 0 parental

Question 9

What is the general rule for predicting the total number of phenotypes and genotypes in n-hybrid?

Answer 9

3^n = number genotypes
2^n = number phenotypes
n = # genes of interest