Chapter 7

Question 1

calculating recombination frequency

Answer 1

recombinant frequency= number of progenty/total number of progeny (100)

Question 2

Coupling

Answer 2

wild-type alleles are found on one chromosome and mutant alleles are found the other.

Question 3

Repulsion

Answer 3

Each chromosome contains one wild-type and one muthant allele.

Question 4

independent assortment

Answer 4

the genes maybe located on different chromosomes; in this case they exhibit independent assortment, and combine randomly when gametes are formed.

Question 5

Linkage

Answer 5

genes can be completely linked- meaning that they are on the same chromosome and lie so close together that crossing over between them is rare.

Question 6

incomplete linkage

Answer 6

the genes are physically linked on the same chromosome which prevents independent assortment. However the occasional crossovers break up the linage and allow the genes to recombine.

Question 7

Interchromosomal recombination

Answer 7

takes place between genes located on different chromosomes. It arises from independent assortment. produces 50% nonrecombinant gametes and 50% recombinant gametes.

Question 8

Intrachromosomal recombination

Answer 8

takes place between genes located ont eh same chromosome. This recombination arises from crossing over. Frequently produces less than 50% recombinant genes.

Question 9

concept check

Answer 9

A genetic map provides the order of the genes on a chromosome and the approximate distances from one gene to another based on recombination frequencies. In genetic maps, 1% recombination equals 1 map unit. Double crossovers between two genes go undetected ; so map distances between distant genes tend to underestimate genetic distances.

Question 10

constructing a genetic map with the use of two-point testcrosses

Answer 10

a testcross between two genes is called a two-point testcross.

Question 11

a three-point testcross can be used to map three linked genes

Answer 11

a more efficent mapping technique is a testcross for 3 genes- a three point cross.

Question 12

Steps in determing gene order in a three-point cross

Answer 12

1. identify the nonrecombinant progeny( two most- numerous phenotypes.
2. identify the double-crossover progeny (two least-numerous phenotypes)
3. compare the phenotype of double-crossover progeny with the phenotype of nonrecombinant progeny. They should be alike in two characteristics and differ in one.
4. The characteristic that differs between the double crossover and th nonrecombinant progeny is encoded by the middle gene.