Chapter 4

Question 1

What is linkage?

Answer 1

• When alleles of 2 different genes do NOT assort independently
• Genes are relatively close together on the SAME chromosome

Question 2

Observations by Thomas Hunt Morgan

Answer 2

• Mendel’s 2nd law does not hold true for the genes in the cross
• Even though F1 genotype appears same for both crosses, get different ratios in test cross
• Higher phenotypic classes in test cross progeny are the classes of the P0 gen

Question 3

If there are no crossovers between linked genes:

Answer 3

only parental gametes will be made

Question 4

How to look for recombination in a diploid organism

Answer 4

• Must use true breeding P0
• F1 gen will be heterozygous
• F1 gametes will either be parent or recombinant
• use test cross to determine genotypes of gametes

Question 5

Frequency of recombinants is directly related to:

Answer 5

distance between genes. the further apart, the higher chance of crossover

Question 6

Three point testcrosses can determine:

Answer 6

1. gene order

2. relative distance of all three genes

Question 7

Why do the smaller distances not add up to the larger distance?

Answer 7

Double crossovers. The 2 smallest categories in a 3-factor cross will be the double crossovers. Re-calculate R.F for genes furthest apart, but add in the double crossover categories twice

Question 8

Smaller distances are more accurate than larger because:

Answer 8

• genes that are far apart are more likely to have double crossovers between them.
• Double crossovers look like parents; therefore you underestimate the number of recombinants

Question 9

The pairwise combination that is parental in the DCO class are:

Answer 9

the two outside genes

Question 10

If two genes are ~50 m.u apart or greater, they will:

Answer 10

appear unlinked