Genetics exam 3

epistasis

happens when one gene (epistatic gene masks or interferes with the expression of another gene

no gene interaction

9:3:3:1

9: (both dominant alleles present)

3: Single Mutant

3: Single Mutant

1: Doubel Mutant

Duplicate (redundant) gene action

Dominant allele of either gene can work to produce the phenotype

15:1

15: WT

1: mutant

Dominant (additive) gene action

Dominant allele of either gene can contribute to an intermediate phenotype

9:6:1

9: WT

6: intermediate

1: mutant

Complementary gene action

phenotype requires dominant allele of both genes

9:7

9: WT

7: mutan

Recessive Epistasis

recessive allele of one gene hides the effects of an allele of another gene

9:3:4

9: no mutant

3: single mutant

4: 3 single mutant, and 1 double mutant that looks like single mutant

Dominant Epistasis

dominant allele of one gene hids the effects of an allele of another gene

12:3:1

12: 9 no mutant and 3 singe mutant that looks like no mutant

3: single mutant

1: double mutant

Dominant Suppression

dominant allele of one gene suppress the effect of the other gene

13:3

13: 9 no mutant, 3 single mutant, and 1 double mutant

3: single mutant

Genetic complementation

biological pathway is restored

DIFFERENT GENES ARE MUTATED IN DIFFERENT PARENTS

Genetic non-complementation

biological pathway is not restored

SAME GENE IS MUTATED IN BOTH PARENTS

Unlinked genes

random distribution of all offspring

equal amounts parental and recombinant offspring

Linked genes

genes have significantly more parental offspring than recombinant offspring

Chi-square formula

X²= summation (observed - expected)²/e

Expected ratios (if not linked)

Classic cross → 9:3:3:1

Autosomal test cross → 1:1:1:1

Classic x-linked (males only) → 1:1:1:1

interpreting X² values

X² calc < table value

• genes are NOT linked

X² calc > table value

• genes linked

Linkage mapping steps

1. determine gene interaction

• start by looking at our phenotypes of our F2 generation fo a dihybrid cross

• look if there are variations too the 16 part ratio, to see if there is gene interaction, tells you what expected ratios should be

2. determine linkage

• First, look at the ratio after a test cross

  • If equal parental & recombinant, then they are unlinked

  • If significantly more parental than recombinant than they are linked 

• Then perform a  X² test using expected assortment frequencies

  • X² > table → genes are linked

X² < table → genes are not linked

chromosomal interference

phenomenon where crossover in one region increases the liklihood of a second crossover nearby

causes

• Mechanical stress on chromatids due to chasma

• presence of enzymes that are involved in breaking and rejoining strands

Biological influence

Homogametic organisms (girls) experience higher recomb. frequencies than heterogamous organisms

when does the epistatic gene act?

Biochemical pathway

• it acts first

regulatory pathway

• it acts second

Non-random mating (inbreeding)

consistent increase in homozygous genotypes and a decrease in heterozygous genotypes

emergent property

new properties arise from the collective action of many individual components

Classic mendelian expression models

one band present in every cell, no pattern

imprinting

opposite ends of the chromosome are linked

A linked with 1 and B linked with 2

X-inactivation

whole chromosome is gone

A linked with 2 and B linked with 1

co-dominance

both alleles active in all cells