Genetics 3000 Extensions of Mendelian Inheritance

Gene =

the sequence of DNA that produces either a protein or functional RNA molecule

Incomplete or Partial Dominance is when …

neither allele for a trait is completely dominant to the other and they mix to form intermediates (like a flower ranging in shades of pink as opposed to being just red or just white), so the heterozygote produces the intermediate trait

Example of incomplete dominance

skin color in eggplants, a cross between purple and white (PP x pp) = violet (Pp)

Incomplete and Codominance produce …

3 unique phenotypes instead of the normal 2 (genotypes and phenotypes are 1:2:1 instead of phenotypes being 3:1)

Codominance of alleles is when …

both alleles for a gene are expressed equally, think of a purple flower being crossed with a white flower and the heterozygotes have purple and white speckled petals (the third phenotype is NOT an intermediate)

Example of codominance

Human MN blood (gene codes for antigens placed on the surface of red blood cells

There can be more than …

2 alleles for one gene in the population, but each diploid organism will have only two alleles

Human ABO blood has …

3 alleles (A, B, and O), this gene is unique because A and B are dominant to O, but they are codominant for AB

For lethal alleles, a homozygous individual would …

die because of the amount of gene product produced, usually seen in genes essential for development

Lethal alleles can behave in a …

dominant or recessive manner, often results in two phenotypes even though 3 genotypes are produced, this is because the third is homozygous for the lethal alleles and they die so that trait is never seen among live organisms

Huntington’s disease is a …

dominant lethal allele, most who have Huntington’s disease are heterozygous for the trait because being homozygous for the affected allele is most often not compatible with life

Penetrance is when we have assumed …

that all organisms with a given genotype will produce the expected phenotype, called complete penetrance/100% penetrance

In ducks, them being mallard, dusky, or restricted is an example of …

complete penetrance when they show the phenotype associated with the genotype and express it indefinitely

Penetrance is a value from 0-1 where the …

higher the value, the higher the penetrance, incomplete penetrance has a value less than 1 while complete should = 1

Expressivity is the degree to which …

a trait is expressed, variable expressivity can be observed across individuals with the same variant genotype

Multiple factors can affect penetrance and expressivity such as …

age, heterogeneity(allelic looks at one gene and its possible alleles while genetic looks at different genes), and environment

Example of the environment affecting penetrance or expressivity

some people do not get fulfilling nutrition meaning they won’t express all of their traits that require nutrition to sustain such as their height since we need nutrients to grow

Penetrance vs Expressivity chart

Pleiotrophy is when a single …

gene product has multiple phenotypic effects, a protein may be used in multiple systems so when something goes wrong in its function the all those systems will be affected

Example of pleiotrophy

Marfan System (believed that Abraham Lincoln had this condition), is a single gene mutation that causes longer extremities, vision problems, flexible joints, and heart defects because of a defect in the production of the fibrillin-1 protein

Multiple genes can …

influence the phenotype of one characteristic, development is a good example because multiple organs need different proteins that don’t all come from the same gene

Genes that interacts have different …

locations (loci) on the chromosome

Epistasis occurs …

when one gene product masks expression of another

Dominant epistasis is when only …

one allele is needed to mask other genes, this creates a 12:3:1 ratio instead of the normal 9:3:3:1

Recessive epistasis is when …

two alleles are needed to mask other genes, creates a 9:3:4 ratio

Example of epistasis

coat color in Labradors, if they have at least one allele for pigment deposition, the dog will be black or chocolate(black = dominant) because this allows the pigment to be put into the hairs, if the dog has two recessive alleles for the pigment deposition gene then the lab will be yellow regardless of what color they produce for the color gene,

Sex chromosome complement can influence other genes means …

that the phenotypes of genes on autosomes can be affected by the biological sex (hormone profiles alter appearance) this is called sex-influenced inheritance

Example of Sex-influenced inheritance

beards in goats, for males the allele for beards is dominant but for females the allele is recessive, so males only need one copy to have a beard while females need two

Sex-limited inheritance is when a characteristic that’s encoded by an autosomal gene is …

present in only one biological sex, the same genotype in the opposite sex presents a different phenotype

Example of sex-limited inheritance

cock-feathering (HH and Hh = hen-feathered) hh in females = hen-feathered, but hh in males = cock-feathered

Mitochondrial inheritance is where …

the DNA for mitochondria only comes from the maternal parent, this is because the egg is larger and doesn’t need to spend energy moving around like sperm, on a pedigree, every single offspring and the mother will carry the trait, but sons won’t pass it down to their children

Genomic imprinting is the …

selective expression of one allele of a gene, the parent of origin affects the phenotype, the allele on the opposite chromosome is silenced, the imprinted gene is the one that is silenced, something is added to the imprinted gene when gametes are made

Multiple diseases are the result of imprinting deficits like …

Angelman syndrome, Prader-Willi syndrome, or Beckwith-Wiedemann syndrome

Genomic imprinting is a black hole because …

only mammals do this and we aren’t sure why

The external environment can affect …

phenotypes, genotypes may produce different phenotypes based on the external environment, such as temperature sensitive alleles

During development, the physical temperature of the embryo can affect …

which genes are expressed, darker fur color for example in Himalayan rabbits and cats is a result of the cooler parts of the body producing the darker color