normal genetics ratios Flashcards
12:3:1
dominant epistasis
ie dominant A masks expression of B (A is epistatic of b) B only expresses when aa present
9:3:4
recessive epistasis
ie recessive alleles (aa) mask teh phenotypic expression of another locus. Therefore the other locus can only be expressed when Aa or AA is present
mouse example-homozygous albino is epistatic to aguoti and black
9:7
complementary gene action.
ie If two genes are involved in a specific pathway and functional products from both are required for expression, then one recessive allelic pair at either allelic pair would result in the mutant phenotype.
Example: Flower color in sweet pea
9:6:1
additive gene action
example-complete dominance at both gene pairs creates a new phenotype.
e.g Fruit shape in summer squash
Gene pair ‘A’ sphere shape dominant over long.
Gene pair ‘B’ sphere shape dominant overlong.
Interaction at ‘AB’ when present together, form disc-shaped fruit
9 disc
6 sphere
1 long
15:1
If a dominant allele of both gene loci produces the same phenotype without cumulative effect
example
Complete dominance at both gene pairs, but either gene when dominant, epistatic to the other.
Seed capsule of shepherd’s purse (Caps ell a bursa-pestoris)
Gene pair ‘A’: Triangular shape dominant over ovoid
Gene pair ‘B’: Triangular shape dominant over ovoid (double recessive)
define epistasis
Epistasis - the interaction between two or more genes to control a single phenotype
13:3
dominant+recessive interaction
Sometimes the dominant alleles of one gene locus (A) in homozygous and heterogygous (AA, Aa) condition and homozygous recessive alleles bb of another gene locus (B) produce the same phenotype. The F2 ratio will become 13: 3.The genotype AABB, AaBB, AAbb, Aabb and aabb produce one type of phenotype and genotype aaBb, aaBB will produce another type of phenotype.
example:
Complete dominance at both gene pairs, but one gene when dominant epistatic to the other, and the second gene when homozygous recessive, epistatic to the first.
Feather Colour of Fowl
Gene pair ‘A’: colour inhibition is dominant to colour appearance.
Gene pair ‘B’: colour is dominant to white.
Interaction:
Dominant colour inhibition prevents colour even when colour is present, colour gene, when homozygous recessive prevents colour even when dominant inhibitor is absent.
1:2:1
incomplete dominance
e.g colour plants or codominance e.gABO
e.g snapdragon colour. We would expect red or white flowers in this generation, depending upon which allele is dominant. But, the F1 plants produced pink flowers.
2:1
recessive lethal alleles
e.g Manx
(birth phenotype ratio)
9:3:3:1
no gene interaction
the expected mendelian ratio of a dihybrid cross
