Beyond Dominant and Recessive

GeneticsChapter 5Extensions and Exceptions to Mendels Laws

Exceptions to MendelMendel studied simple dominant/recessive traits, but many inherited traits are not as simplistic.

If methods of allele transmission do not occur in the proportions that Punnett squares predict, the nature of the gene may be more complex.Polygenic TraitsMany traits are controlled by more than one gene

For example, eye color is not just B=brown, bb=blue but is controlled by approximately 10 different genes working together.

Lethal AllelesLethal = deadly before birth (miscarriage) OR deadly before reproductive age.Huntingtons = non lethal to population even though its lethal to the individualSometimes inheriting two dominant alleles is lethal

HH= deadHh=hairlesshh=hairy

Multiple AllelesSome genes have more than two alleles (instead of A and a, perhaps A, B, d, f) Example: PKU (lacks enzyme to break down phenylalanine)There are more than 300 possible alleles for this gene. Depending on which you have, you may be phenotypically normal, severe PKU, moderate PKU, or mild PKU.

Incomplete DominanceThe alleles are neither dominant nor recessive. Homozygous genotypes each have their own trait, and the heterozygous genotype has a different or intermediate trait.

Example: snapdragons

CodominanceBoth alleles are expressed in a heterozygote

Example: BB chickens have black feathers, WW chickens have white feathers, and BW chickens have both black and white feathers.

Example: ABO blood typeBW

BWBWBW

BBW

W

=

+EpistasisOne gene masks the effects of another gene (do not confuse with dominant and recessive)

Usually the two genes affect related structures: Example: a recessive hairless gene (hh) will cover up the effects of the hair color gene in mice since you cant see hair color if there is no hair!

PenetranceRefers to what % of individuals that have the genotype will have the phenotype.

Huntingtons = 100% penetrant because everyone who has the genotype is affected

Polydactyly = incompletely penetrant because about 20% of those with the genotype have normal fingers/toes

ExpressivitySeverity or extent of a phenotype

Again with polydactyly some who have the genotype have only an extra tip of a toe or finger; some will have many full extras. It is variably expressive.

PleiotropyOne gene = many effects can be difficult to trace

Can be Mendelian but it is a disorder with many symptoms/controls several functions/has more than one effect

Ex: Porphyria variegata the missing enzyme causes porphyrin to accumulate in many body systems causing many effects

PhenocopyThis is an environmentally-caused trait that appears to be inherited/mimics an inherited trait

Ex: Limb birth defect caused by the drug thalidomide, but it mimics the inherited defect phocomelia

Genetic HeterogeneityWhen several genes produce the same phenotype (redundancy)

Ex: 132 genes can cause hearing loss. If mom is deaf by two recessive alleles on gene 3 and dad is deaf by two recessive alleles on gene 17, the child will not necessarily be deaf.