Chapter 6 Gene Interaction

Question 1

Why are some mutant alleles recessive and others are dominant?

Answer 1

Usually due to amount of gene product (protein) needed for proper biological function

Question 2

What is incomplete dominance?

Answer 2

• Phenotype of heterozygote is intermediate between two homozygotes (red + white = pink)

Question 3

What is codominance?

Answer 3

• Heterozygote has phenotype of both homozygotes (i.e blood type AB)

Question 4

Interactions of multiple genes

Answer 4

1. Many genes are usually involved in the presentation of a phenotype
2. These genes are often in a pathway with 1 gene turning on the next
3. Mutate any of these genes in the pathway results in a mutant phenotype

Question 5

Beadle and Tatum experiment observations

Answer 5

• Found numerous mutant strains that were arginine auxotrophs
• Each mutation behaved as a single gene
• Mapped the mutations relative to other genes and found that they map to three different loci

Question 6

What does complement mean?

Answer 6

• Alleles are on different genes

Question 7

What does fail to complement mean?

Answer 7

• Alleles are on same gene

Question 8

What is the approach that reveals interacting genes for a particular biological property?

Answer 8

1. Obtain many single-gene mutants and test for dominance
2. Test the mutant for allelism (are they at one or several loci?
3. Combine the mutants in pairs to form double mutants to see if the genes interact

Question 9

Gene interaction is inferred from the phenotype of the double mutant:

Answer 9

If the genes interact, then the phenotype differs from the single combination of both single-gene mutant phenotypes

Question 10

No Interaction (Double mutant)

Answer 10

When the heterozygotes are crossed and produce a 9:3:3:1 ratio because the genes act independently at the cellular level, i.e one pathway fails (black) still produces the other pathway (orange)

Question 11

Same pathway

Answer 11

9:7 ratio is possible if the double mutant has the same phenotype as the two single mutants. Identical phenotypes of the single and double mutants suggest each mutant allele controls a different step in the same pathway. Will have phenotype if homozygous for recessive mutant allele of either or both genes.

Question 12

What is epistasis?

Answer 12

When the phenotype of a mutant allele masks the phenotype of the mutant allele of another gene

Question 13

Recessive Epistasis

Answer 13

Whenever the recessive allele is present it masks the phenotype of the other allele resulting in a 9:3:4 ratio
ex:
w+/- ; m+/- 9 blue
w+/- ; m/m 3 magenta
w/w ; m+/- and w/w ; m/m 4 white

Question 14

Dominant epistasis

Answer 14

Whenever the dominant allele is present it masks the phenotype of the other possible phenotypes resulting in a 12:3:1 ratio
ex:
D/- ; W/- (9) and d/d ; W/- (3) = 12 white with spots
D/- ; w/w 3 dark red
d/d ; w/w 1 light red

Question 15

Suppressor mutations

Answer 15

Cause the mutant phenotype of another mutation not to be expressed, resulting in the wild-type phenotype (13:3)
ex:
9 T- ; S- , 3 T- ; ss , 1 tt ; ss = 13 wild type
3 tt ; S- disease

Question 16

Synthetic mutantions

Answer 16

• Mutations in two different genes do not individually cause a phenotype, but together cause a mutation (synthetic)
• Due to redundancy (15:1)
  ex:
  9 A-/B- + 3 A-/bb + 3 aa/B- = 15 wildtype
  1 aa/bb mutant

Question 17

What is penetrance?

Answer 17

The percent of individuals with a mutation will show the phenotype

Question 18

What is expressivity?

Answer 18

Differing levels to which a phenotype is expressed i.e how light or dark colour of eyes

Question 19

Why would individuals with the same mutation not show exactly the same phenotype?

Answer 19

1. Environment
2. Other genes (genetic background)
3. Subtlety of mutant phenotype -> missed classification/human error