MBG*2040: Extensions and Modifications of Basic Principles (Chapter 5)

incomplete dominance

BB, Bb, and bb all differ phenotypically; Bb is intermediate between homozygous phenotypes.

codominance

BB, Bb, and bb all differ phenotypically, but Bb exhibits phenotypes of both homozygotes.

dominance/allelic series

Series describing the dominance hierarchy of multiple alleles.

• Null allele = nonfunctional

• Hypomorphic allele = partial function

wildtype allele

TYPES OF ALLELES ARE BASED ON PHENOTYPES.

A functional enzyme or other protein is produced.

Often dominant over loss of function allele; half as much protein is synthesized yet this is often sufficient to achieve the phenotype.

• Sometimes used to refer to the most common phenotype (or genotype) found in a natural population.

loss of function allele

TYPES OF ALLELES ARE BASED ON PHENOTYPES.

An enzyme or other protein is no longer being produced, is produced at lower levels, or in nonfunctional.

• e.g. ii blood

haplosufficiency

Often the wildtype allele is dominant over the loss of function allele.

Half as much protein is synthesized yet this is often sufficient to achieve the wildtype phenotype (half the functioning allele but functions as expected).

• Diploid is more functional than haploid because 2n has a “backup” if only one is required in order to function

gain of function mutation

Mutation in which the mutant allele produced a protein that has increased (detrimental) function (commonly dominant alleles).

• e.g. Huntington’s Disease

haploinsufficient

Dominant allele can be a loss of function allele.

In the heterozygote, half as much protein is synthesized and this is not sufficient for a normal phenotype (biochemical pathway failing).

• e.g. Tailless cats (Manx)

recessive lethal alleles

Essential genes, when mutated, lead to a lethal phenotype.

Appears in both dominant forms and recessive forms.

• Can be dominant allele → essentially a single gene that is dominant for one thing (colour) and recessive for something else (lethality)

dominant lethal genes.

Lethal genes that can be expressed in both the heterozygote and the homozygote.

• e.g. Huntington Disease: Bb lethal, BB lethal, bb not lethal.

recessive lethal genes

Lethal genes that are only expressed in the homozygote.

• e.g. Tay Sachs: tsts lethal, TSts not lethal, TSTS not lethal.

penetrance

The proportion of individual organisms having a particular genotype that express the expected phenotype - variation in the population.

• Could have another gene affecting it

• e.g. Polydactyly (extra fingers and toes) is not fully penetrant.

expressivity

The degree to which a phenotype is expressed (mild to severe); variation in the individual.

norm of reaction

The range of phenotypes expressed by a single genotype under different environmental conditions.

phenocopy

A change in phenotype arising from environmental factors that mimic the effects of a mutation in a gene (copies genotype).

Mendal’s Law of Independent Assortment

The inheritance pattern of one trait will not affect the inheritance pattern of another trait.

complementation

When two strains of an organism with different homozygous recessive mutations that produce the same phenotype produce offspring of the wildtype phenotype when mated or crossed. Will only occur if the mutations are in different genes. The other gene supplies the wildtype allele to “complement” the mutated allele. Will not occur if the mutations are in the same gene.

heterogeneous trait

A mutation in any one of a number of genes can give rise to the same phenotype.

epistasis

The masking of the expression of one gene by another. No new phenotypes are produced. The “epistatic gene” does the masking while the “hypostatic gene” is masked.

recessive epistasis

F2 phenotypic ratio = 9:3:4.

Homozygous recessives at one gene pair mask expression from the other gene (only when homozygous recessive at the correct gene pair).

A-bb and aabb have the same phenotype.

dominant epistasis

F2 phenotypic ratio = 12:3:1.

One dominant allele at one gene masks expression from the other gene.

A-B- and A-bb have the same phenotype.

pleiotropy

A single gene can be responsible for a number of distinct and seemingly unrelated phenotypic effects; single mutation in single gene that has multiple different phenotypes.

Why? All cells have the same genes (the inherited mutation is in all of your cells).

• Sickle Cell Disease: respiratory problems, sickled cells, chronic infections, joint pain, enlarged spleen, stroke, etc.

• Cystic Fibrosis: mucus clogs the lungs and leads to infections; mucus obstructs the pancreatic ducts creates digestion problems → two different organs affected by mutation in single gene

inbreeding depression

The reduced survival and fertility of offspring of related individuals. Inbred lines of experimental species are often less vigorous than hybrid lines.

heterosis (hybrid vigor)

When two different inbred lines are crossed (complementation), the hybrids are heterozygous for many genes.

Hardy Weinberg Principle

Predicting genotypes through allele frequencies in a population. Only correct in the absence of evolutionary influences (the following)!

1. Nonrandom mating - if individuals are selecting a mate with a specific trait, the desired trait will be more prevalent, which skews the equilibrium.

2. Unequal survival - if (for example) homozygotes don’t survive as well, they won’t be alive as long to reproduce, which skews the equilibrium to heterozygotes.

3. Population and subdivision - if a habitat is suddenly divided, allele frequencies change on both sides.

4. Migration - new population = new/different allele frequencies while the original population loses some.

dosage compensation

A way of equalizing gene expression in the face of different gene dosage → e.g. in humans, we want two copies of each gene, not more or less.

In females (XX), only one X chromosome is expressed while the other is inactivated so that at the gene expression and protein levels, males (XY, only one X chromosome) and females are the same.

X-inactivation

Random inactivation of one female X chromosome. If a cell contains more than two X chromosomes, all but one of them are inactive. A 50/50 decision that happens in embryo → even if one chromosome has harmful traits, it can stay; you can have one X chromosome in half of your cells and the other X chromosome in the other half. Any new cell produced from these cells makes new 50/50 decision on which X to inactivate.

Therefore, females are functionally hemizygous for X-linked genes at the cellular level.

Barr Body

An inactivated X chromosome.

genetic mosaics

Females that are heterozygous for X-linked traits.