Module 4: Inheritance Patterns of Single Genes and Gene Interactions

Gene action

A gene makes a protein and the protein produces the phenotype

Gene to protein to phenotype

Genes give instructions to make proteins and the proteins are what actually produce the observable traits

What does “Wild type” mean?

means the standard, normal, or most common version of a gene, protein, or trait found in a natural population.

Alleles and phenotype

Different alleles make different proteins which lead to different traits

Molecular basis of dominance

Dominance depends on how well the protein made by an allele functions

Haplosufficient

One working copy of a gene makes enough functional protein to produce the normal (wild-type) trait.

So even if the other allele is weak or broken, the phenotype still looks normal. Remember these phenotypes require 40 catalytic units to look a normal

Haplosufficiency example

A heterozygote appears normal because one allele makes enough enzyme

Haploinsufficient

One working copy of a gene is NOT enough to make the normal (wild-type) phenotype.

So if you lose one functional allele, the phenotype already looks mutant.

Haploinsufficiency example

A heterozygote shows a mutant phenotype because enzyme levels are too low. Gene T
T1 allele (wild type) produces 10 enzyme units
T2 allele (mutant) produces 5 enzyme units
Wild-type phenotype requires 20 enzyme units we only have 15 so this is clearly not enough

Loss of function mutation

A mutation that reduces or eliminates gene activity. by causing a little to be less active, these also tend to be recessive and may be haploinsufficient

• makes little or no functional protein.

Why loss of function is recessive

One normal allele usually produces enough functional protein. The normal allele still produces enough protein to meet the threshold needed for the wild-type phenotype. Because the phenotype looks normal, the mutation is hidden in the heterozygote, which makes it recessive.

Gain of function mutation

A mutation that increases activity or creates a new function. An allele that is more active, are haplosufficient

Gain of function dominance

Gain of function mutations are usually dominant

Why gain of function is dominant

The mutant allele actively changes the phenotype

Incomplete dominance

The heterozygote does not look like either parent.
Instead, it looks in between the two parental phenotypes.

“In between” = intermediate, not mixed and not both at once.

Incomplete dominance example

Flowering time in pea plants is intermediate in heterozygotes All F1 plants are heterozygous.

Instead of flowering early or late, they flower: At a time between early and late

This is the intermediate phenotype.

• Early parent → early flowering

• Late parent → late flowering

• F1 heterozygote → medium flowering time

Codominance

Both alleles in a heterozygote are fully and equally expressed.

Nothing is blended.
Nothing is hidden.
Both show up clearly.

Codominance example

IA and IB blood type alleles are both expressed

Allelic series

A single gene has more than two alleles in the population, and each allele has a different effect on the phenotype.

You still only inherit two alleles per individual, but many versions of that gene exist overall.

Allelic series example

Rabbit coat color is controlled by multiple C gene alleles, many different skin and fur color end up showing

Tyrosinase

An enzyme involved in melanin production

Full color allele

Produces full pigment when tyrosinase is functional

Albino allele

Produces no pigment due to inactive tyrosinase

Penetrance

The percentage of individuals with a genotype who show the phenotype

Penetrance example

The genotype is having the switch installed. The phenotype is the light actually turning on. Now imagine 10 houses all have the switch.

• In 10 houses, only 6 lights turn on
• In 4 houses, the light does not turn on

Even though all houses have the switch, not all show the light. So the penetrance is: 6 out of 10 = 60 percent

nonpenetrance

Some individuals with the genotype do not show the phenotype

nonpenetrant example

The genotype is having the switch installed. The phenotype is the light actually turning on. Now imagine 10 houses all have the switch.

• In 10 houses, only 6 lights turn on
• In 4 houses, the light does not turn on

the 40 percent is the nonpenetrant that doesn’t turn the light on

Incomplete Penetrance

when traits are occasionally nonpenetrant

Variable expressivity

The same genotype produces different degrees of a phenotype

Gene environment interaction

Environmental factors influence how a genotype is expressed. In the slides, plant height depends on:

• genotype

• food availability

Different genotypes respond differently to the same environment.

Polydactyly example

An autosomal dominant trait that shows incomplete penetrance (remember the 40 percent of lights not turning on in the house example).

Penetrance calculation

Number showing the trait divided by total with genotype times 100

Retinoblastoma penetrance

Only seventy five percent of carriers develop the disease

Retinoblastoma expressivity

The disease may affect one eye or both eyes

Gene environment interaction model

Phenotype is influenced by both genotype and environment

Norm of reaction

A graph showing how phenotype changes across environments for a genotype

Norm of reaction meaning

Different genotypes respond differently to environmental conditions

Pleiotropy

One gene affects multiple traits

Pleiotropy example

Sickle cell disease affects blood shape pain organs and lifespan

Genetic dissection

Using mutants to determine the order of steps in a biochemical pathway

Epistasis

gene interactions where alleles at one gene influenced the function of alleles at another gene

Why epistatic ratios differ

Genes do not act independently here unlike where mendels classic dihybrid ratio 9:3:3:1 showed that each gene acts independently and each directly affects the phenotype here the epistatic ratios look different because those assumptions are violated

Complementary epistasis

Two different genes are both required to produce the normal phenotype.

If either gene is not working, the normal phenotype does not appear.

So the genes “complement” each other — they must both function. For example, a mutation in either or both genes can produce the mutant phenotype

Duplicate epistasis

Either of the two genes can produce the normal phenotype by itself. You only get the mutant phenotype when BOTH genes are nonfunctional, so the genes “duplicate” each other’s function.

• One working copy of gene A OR gene B is enough

• Both genes must fail to see the mutant trait

Recessive epistasis

A recessive allele masks the expression of another gene

Dominant epistasis

A dominant allele masks the expression of another gene

Dominant suppression

The dominant allele at one gene suppresses the expression of the dominant allele at the second gene

Complementation test

Determines whether mutations are in the same gene

Failure to complement

No wild type recovery means mutations are in the same gene

Complementation group

A set of mutants affecting the same gene