BIOL 2500 - Topic 5 (part 3)

C-gene system for mammalian coat colour

Polygenic with multiple alleles

C-gene allelic series dominance

C > C^ch > C^h > c

C coat colour

It is the dominant WT colour

C^ch coat colour

1.) Homozygotes produces a chindilla phenotype

2.) It has a reduced coat colour, due to reduced production of tyrosine

C^h coat colour

1.) Produces a himalayan phenotype

2.) The resulting protein is temperature sensitive, such that it works better at cool temps, therefore more pigment is produced away from the core and towards the extremeties

c coat colour

Homozygotes produces an albino phenotype due to the lack of enzymatic activity (i.e. null mutation)

Pure breeding chinchilla x himalayan (C^ch x C^h)

1.) They are codominant to each other, such that heterozygotes have a chinchilla body core but then himalayan extremities

2.) Therefore resulting in a 1:2:1 ratio

Lethal alleles

Mutations in essential genes that causes death if it is homozygous

Recessive lethal alleles

They are rare, as they are usually hidden by the dominant phenotypes as carriers, which is what allows them to persist for long periods of time

Lethal alleles in animals

It is detected due to a distortion in Mendelian segregation proportions, such that you expect a 3:1 ratio, but instead see 100% of the dominant phenotype

Lethal alleles in plants

It is detected due to embryos dying or gametophytic lethals that fail to produce gametes

Lethal allele example

1.) The agouti gene, which are essential in mice

2.) It is lethal because there is a deletion in the gene that is needed for embryonic development

Agouti gene F1 and F2

1.) F1: if you cross a homozygous WT brown mouse with a yellow mouse, we get a 1:1 ratio (AA:AA^Y)

2.) F2: if you cross two yellow mice, you get a 2:1 phenotypic ratio, rather than 3:1 (because the homozygous yellow is lethal)

Lethal alleles in humans

1.) Tay-Sachs disease

2.) Huntington’s disease

Huntington disease

1.) A neuromuscular disorder, caused by a mutation on the end of chromosome 4

2.) It is lethal in homozygous dominant and heterozygous genotypes

Huntington’s disease symptoms

It has delayed age onset, such that the symptoms are not seen until adulthood, therefore the affected individuals still have a chance to reproduce and pass on the trait to their offspring