Extensions of Mendelism

Question 1

Incomplete Dominance

Answer 1

Heterozygote is intermediate between homozygous phenotypes

Question 2

Codominance

Answer 2

Heterozygotes display both phenotypes

i.e. blood type, red and white flower

Question 3

Allelic Series

Answer 3

Describes dominance hierarchy of multiple alleles

Question 4

Blood Type

Answer 4

I(A) encodes a transferase which adds acetylgalactosamine

I(B) encodes a transferase which adds galactose

i encodes a non functional transferase

Question 5

Haplosufficiency

Answer 5

Often the wild type allele is dominant over the loss of function allele

Half as much protein is synthesized (only one copy of wild type allele), but that is enough to achieve wild type phenotype

Question 6

Haploinsufficient

Answer 6

Dominant allele can be a loss of function allele

Half as much protein synthesized, but not enough for normal phenotype (need 2 good alleles to have normal phenotype)

Question 7

Recessive Lethal Alleles

Answer 7

Need 2 copies of allele to kill you (MM or mm but not both)

i.e. mm- normal
Mm- no tail
MM- dead

Question 8

Lethal Alleles

Answer 8

Dominant lethal genes expressed in both heterozygote and homozygote

Recessive lethal genes expressed only in homozygote

Question 9

Mutations and Proteins

Answer 9

Amporphic LOF- produces non functional protein (wild type phenotype)

Hypomorphic LOF- produces partially functionally polypeptide (wild type phenotype)

Dominant negative- produces protein that interferes with wild type, mutant phenotype (dominant)

Question 10

Penetrance

Answer 10

How many people display the genotype

i.e. 70% penetrance= 7/10 people who have the genotype display the phenotype

Question 11

Expressivity

Answer 11

How severe the phenotype is expressed

Question 12

Environmental Factors and Phenotypic Expression

Answer 12

Age
Sex
Temperature
Chemicals

Question 13

Norm of Reaction

Answer 13

Range of phenotypes expressed by a single genotype under different environmental conditions

Question 14

Temperature and Rabbit Fur Colour

Answer 14

High body temperatures denatures protein (results in white fur)

At cooler temperatures, extremities have lower body temperatures, so protein is active (black at extremities)

Question 15

Phenocopy

Answer 15

Change in phenotype arising from environmental factors that mimic the effects of a mutation in a gene

i.e. exposure to a chemical copies the phenotype for a disease

Thalidomide phenocopies phocomelia, making it seem like pregnant women exposed to thalidomide were giving birth to children with phocomelia

Question 16

Eye Colour

Answer 16

A enzyme changes blue pigment to green pigment

B enzyme changes green pigment to brown pigment, or blue to gray

bb (non functional B enzyme)- A enzyme present, so you get green eyes

aa (non functional A enzyme)= Don’t have A enzyme to get to green eyes, but B enzyme functional so you get gray eyes

Question 17

Complementation

Answer 17

Parental are homozygous recessive in 2 different genes, but have same phenotype

Can only occur if mutations occur in 2 different genes

i.e. aaBB x AAbb (both deaf)
AaBb offspring (hearing)

Question 18

Heterogeneous Trait

Answer 18

Mutation in any one of a number of genes can give rise to the same phenotype

Question 19

Epistasis

Answer 19

Masking of expression of one gene by another (no new phenotypes are produced)

Epistatic gene does the masking

Hypostatic gene is masked

Question 20

Recessive Epistasis

Answer 20

Homozygous recessives at one gene pair mask expression from other gene

A/- b/b and a/a/ b/b have same phenotype (bb masks A gene)

Question 21

Dominant Epistasis

Answer 21

One dominant allele at one gene masks expression from other gene

A/- B/- and A/-b/b have same phenotype (A/- masks B gene)

Question 22

Molecular Mechanisms of Recessive Epistasis

Answer 22

Colourless–C/- allele—> black pigment—A/- allele–> agouti pattern

If mouse has c/c at C gene, then mice can’t form black pigment, and are albino

Genotype at A locus determine if phenotype is black (a/a) or agouti (A/-)

Question 23

Summary of F2 Ratios

Answer 23

Monohybrid:

3: 1 Complete dominance
1: 2:1 Incomplete/ co dominance
2: 1 Recessive lethal

Dihybrid:

9: 3:3:1- complete dominance
9: 3:4 Recessive epistasis
9: 7 Complementation
12: 3:1- Dominant epistasis

Question 24

Pleiotropy

Answer 24

Single gene responsible for a number of distinct and seemingly unrelated phenotypic effects

i.e. cystic fibrosis

Question 25

Inbreeding

Answer 25

Inbred lines of experimental species are often less vigorous than hybrid lines

Increases frequency of homozygotes, decreases frequency of heterozygotes

Question 26

Heterosis

Answer 26

The 2 different inbred lines are crossed, hybrids are heterozygous for many genes

Heterozygotes display heterosis or hybrid vigor

Question 27

Hardy Weinberg Principle

Answer 27

Valid when the following are absent: nonrandom mating, unequal survival, population subdivision, migration

p^2 + 2pq +q^2 =1