T13: gene interactions

what does 3:1 phenotypic ratio tell u

one allele is completely dominant

what does 1:2:1 phenotypic ration tell you

incomplete dominance or co-dominance

what does 2:1 phenotypic ratio tell you?

recessive lethality

what is the purpose of complementation test

crossing two pure breeding mutant strains to see if the mutuation occurs on the same gene or different genes

phenotypic ratio dihybrid cross trick

9 both dominant: 3 one reccessive: 3 one recessive: 1 recessive for both

• only works for unlinked, non-lethal genes

2 genes affecting one trait by seperate pathways

• affects 2 genes

• two spereate pathways that leads to wildtype phenotype

• if one is blocked, only other pathway is expressed and wildtype colour is not achieved

complementary gene action

both genes act on the same pathway

• one homozygous mutant causes pathway to shut down, no pgiment is produced, so the wildtype is not expressed and only gets the other pathway colour

• both genes mutant alleles affect the same pathway in the same way so 3:3:1 have same phenotype → to combine to make 7

• 9:7 ratio

epistasis

expression of one gene masks the expression completely of a different gene

recessive epistasis

• when one gene s homozygous recessive and its phenotype masks the other genes phenotype

• two genes encode enzymes in the same pathway but the intermediate compound can be observed, the earlier acting gene tends to be the epistatic one

why does the complementation tests only work for recessive mutations?

the mutant is dominant, we dont see the wildtype phenotype when we try to do the complementation test

dominant gene interaction

when both dominant alleles are present, they interact to produce a new phenotype

duplicate gene action

when one of the two genes has a dominant wildtype, the offsprings are wildtpye

• 15:1

• only needs one dominant to be wildtype

recessive suppressor

recrssive disorder is suppressed when s/s (homo recessive)

• 13:1

dominant suppressor

13:3

• for a disorder, a dominat suppressor allele can inhibit expression of a dominant mutation

synthetic lethality

two recessive mutations can have mild phenotypes when one gene is homo recessive, but when both are homo mutant, the offspring wont survive

• ratio is out of 15 not 16 since the double mutant offsprings die

mammal coat colour genes

mammals share a number of genes that affect coat colouration