Theoretical genetics - Dihybrid crosses and

epistasis

Theoretical genetics

Learning objectives Success criteriaUnderstand the basics of

theoretical geneticsDescribe the interactions

between loci (epistasis). (Production of genetic diagrams is not required)

Predict phenotypic ratios in problems involving epistasis

Terminology: Dihybrid crosses involve two genes which

control two characteristics. There are complications of these patterns as illustrated in the calculations that follow.

Unlinked genes are found on different chromosomes and can be segregated by random assortment of meiosis/ metaphase II

Linked genes are found on the same chromosome and are normally inherited together unless crossing over my occur

Dihybrid crossesThe example is based on Mendel's pea crosses

and begins with 'true' breeding or homozygous plants for two unlinked genes.

The phenotype which is used includes the colour and the texture of the seeds of the pea plant.

R = Round r = wrinkledY = yellow y = green

Round is dominant to wrinkled Yellow is dominant to green

RrYy x RrYy

How did they work out their expected numbers?

EpistasisThis is where one gene masks or suppresses the

expression of another. They may workAntagonisticallyComplementary

Antagonistically :Prevents expression of another allele at a

different locusCrosses involving epistatic genes don’t result in

the expected phenotypic ratios (9:3:3:1)The ratio depends whether the epistatic allele is

dominant or recessive

Recessive epistatic alleles (Ratio 9:3:4)If the epistatic allele is recessive then 2 copies will mask

the expression of the other gene.

TASKMouse coat colour is controlled by two pairs of alleles: B and CB = black coat colour, b = brown coat colourC = pigment production, c = no pigment productionTherefore, if a mouse has cc, it will be an albino, if it

has Cc or CC it will be black or brown.

Work out the F1 and F2 generation of the following cross

Black x albinoCCBB ccbb

Mouse coat colour is controlled by two pairs of alleles: B and CB = black coat colour, b = brown coat colourC = pigment production, c = no pigment productionTherefore, if a mouse has cc, it will be an albino, if it has Cc or CC it

will be black or brown.

Black

Mouse coat colour is controlled by two pairs of alleles: B and CB = black coat colour, b = brown coat colourC = pigment production, c = no pigment productionTherefore, if a mouse has cc, it will be an albino, if it has Cc or CC it will be

black or brown.

Dominant epistatic alleles (Ratio 12:3:1)If the epistatic allele is dominant then 1 copies

will mask the expression of the other gene.TaskHair type in Dillybopper beetles is controlled by 2

genes : Hair (H - bald, h – hair) and type ( S – straight, s – curly). The offspring of the following cross are shown in the table. Explain the phenotypic ratios

Homozygous bald (HHSS) x Homozygous curly (hhss)Phenotypes of F2 generation

Bald Straight hair Curly hair36 9 3

Draw the dihybrid cross for F1 and F2 generations, what is the ratio in the F2 generation?

F2 HS Hs hS hsHS HHSS HHSs HhSS HhSsHs HHSs HHss HhSs HhsshS HhSS HhSs hhSS hhSshs HhSs Hhss hhSs hhss

Parental phenotype Bald CurlyParental genotype HHSS x hhss

Gametes HS hs

F1 generation HhSs all bald

Parental genotypes HhSs x HhSs

Phenotypic ratios 12 : 3 : 1Bald : Straight : Curly

The hair gene has a dominant epistatic allele, which means having one copy will result in a bald phenotype that masks the type of hair gene

Complementary epistatic genes

The alleles are complemeting each other