9.8.3 Gene Mapping Using Recombination Frequencies

Question 1

note

Answer 1

• Thomas Hunt Morgan found that crosses between linked genes produce recombination, although less frequently than the recombinations of independent assortment.
• For linked genes, the more distant the genes are from each other, the greater the likelihood of crossing over during meiosis.
• The technique of gene mapping uses recombination frequencies to determine where genes are located on chromosomes.
- In Morgan’s experiment, the parental phenotypes are the offspring that have an appearance identical to one of the parents. The recombinant phenotypes are the offspring with an appearance that differs from either of the parents.
- Review: When heterozygous (BbVv) individuals were test crossed, four phenotypes were observed in the F1. The parental phenotypes (gray-normal and black-vestigial) comprised 80% of the offspring. The recombinant phenotypes (gray/vestigial and black/normal) comprised 20% of the offspring.
- A higher number of parental vs. recombinant phenotypes indicates linkage. But why do linked genes produce any recombinants?
- Review: During prophase I of meiosis, replicated
homologous chromosomes pair, a process called synapsis. Crossing over between chromosomes involves the breaking off and exchange of portions of the chromosomes. Chiasmata are the X-shaped regions where the exchange takes place. When the chromosomes switch parts, the allelic composition
for certain genes can also change.
- Question: What are the odds that two particular genes will cross over?
- Answer: The closer two genes are together on the
chromosome, the less likely crossing over will occur between the two.
- Sturtevant determined that genes on a chromosome could be mapped based on the relative frequency of crossover events that occur during meiosis. A map unit is calculated by dividing the number of recombinants by the total number of offspring times 100. For example, two genes on a particular chromosome control body color and wing type. Recombination between the two genes occurred in 17% of the offspring. The two genes are 17 map units apart.
- Another gene on the chromosome controls eye color. The gene is mapped to each of the other two genes. The gene for eye color is 9 map units away from the gene for body color and 9.5 map units away from the gene for wing type. The gene for eye color must be located in between the other two genes. The final map is shown on the left.

Question 2

Crosses involving linked genes should not result in recombinant phenotypes.

Answer 2

• false

Question 3

If a gray-bodied and normal-winged fruit fly is mated with one that is black-bodied with vestigial wings, which one of the following represents the phenotype of a recombinant offspring?

Answer 3

• Black body & normal wings

Question 4

An individual of the genotype ST / st is test crossed to an st / st individual. In 80% of the meioses, no chiasmata form between the linked genes. 20% of the time, one chiasma forms between the genes. What proportion of the progeny will be Sstt?

Answer 4

• 5%

Question 5

During which phase of gamete formation does crossing over occur?

Answer 5

• Prophase of meiosis I

Question 6

An organism that is heterozygous at four gene loci is test crossed; AaBbCcDd × aabbccdd. The gametes of the heterozygotes are as follows:
ABcd	 	405
AbCd	 	14
ABcD	 	12
aBCD	 	89
abCD	 	399
Abcd	 	81
ABCd	 	120
AbcD	 	110
Which genes are linked?

Answer 6

• All genes are linked.