Extensions of Mendelism Flashcards
Incomplete Dominance
Heterozygote is intermediate between homozygous phenotypes
Codominance
Heterozygotes display both phenotypes
i.e. blood type, red and white flower
Allelic Series
Describes dominance hierarchy of multiple alleles
Blood Type
I(A) encodes a transferase which adds acetylgalactosamine
I(B) encodes a transferase which adds galactose
i encodes a non functional transferase
Haplosufficiency
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
Haploinsufficient
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)
Recessive Lethal Alleles
Need 2 copies of allele to kill you (MM or mm but not both)
i.e. mm- normal
Mm- no tail
MM- dead
Lethal Alleles
Dominant lethal genes expressed in both heterozygote and homozygote
Recessive lethal genes expressed only in homozygote
Mutations and Proteins
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)
Penetrance
How many people display the genotype
i.e. 70% penetrance= 7/10 people who have the genotype display the phenotype
Expressivity
How severe the phenotype is expressed
Environmental Factors and Phenotypic Expression
Age
Sex
Temperature
Chemicals
Norm of Reaction
Range of phenotypes expressed by a single genotype under different environmental conditions
Temperature and Rabbit Fur Colour
High body temperatures denatures protein (results in white fur)
At cooler temperatures, extremities have lower body temperatures, so protein is active (black at extremities)
Phenocopy
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
Eye Colour
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
Complementation
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)
Heterogeneous Trait
Mutation in any one of a number of genes can give rise to the same phenotype
Epistasis
Masking of expression of one gene by another (no new phenotypes are produced)
Epistatic gene does the masking
Hypostatic gene is masked
Recessive Epistasis
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)
Dominant Epistasis
One dominant allele at one gene masks expression from other gene
A/- B/- and A/-b/b have same phenotype (A/- masks B gene)
Molecular Mechanisms of Recessive Epistasis
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/-)
Summary of F2 Ratios
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
Pleiotropy
Single gene responsible for a number of distinct and seemingly unrelated phenotypic effects
i.e. cystic fibrosis
Inbreeding
Inbred lines of experimental species are often less vigorous than hybrid lines
Increases frequency of homozygotes, decreases frequency of heterozygotes
Heterosis
The 2 different inbred lines are crossed, hybrids are heterozygous for many genes
Heterozygotes display heterosis or hybrid vigor
Hardy Weinberg Principle
Valid when the following are absent: nonrandom mating, unequal survival, population subdivision, migration
p^2 + 2pq +q^2 =1
