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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