14 dihybrid cross

Dihybrid Inheritance

• A cross between two true-breeding parents that possess different forms (alleles) of two genes is called a dihybrid cross.

Mendel’s laws

We saw in monohybridcrosses that the allelesof a gene exist as pairsbut when gametes areformed, the membersof each pair pass intodifferent gametes(Mendel’s first law: theprinciple ofsegregation)

Mendel’s second law , the principle ofindependent assortment, means that duringgamete formation the two alleles of a genesegregate into different gametes independentlyof the two alleles of any other gene.

Dihybrid crosses – continued.

Recombination

In a dihybrid cross, two of the F2 phenotypes

resemble the original (P) parents and twodisplay new combinations of the characteristics.This is called recombination and individualspossessing them are called recombinants

Fruit flies:

P = pointed abdomen, p= rounded abdomenW = normal winged, w = vestigial winged

Cross a homozygous pointed abdomen, longwinged fly with a homozygous roundedabdomen, vestigial winged fly

Dihybrid Cross examples

PPWW x ppwwGametes all PW all pw

F1 genotype all PpWw

2nd cross F1 x F1

PpWw PpWw

Gametes PW or Pw or pW or pw

F1 x F1

PW Pw pW pw

PW PPWW PPWw PpWW PpWw

Pw PPWw PPww PpWw Ppww

pW PpWW PpWw ppWW ppWw

pw PpWw Ppww ppWw ppww

Usual phenotypic ratio of 9:3:3:1 observed

Which phenotypes are recombinants?

• Torrance (Moth cover), page 76 – answer questions 1 and 2

Dihybrid backcross (testcross)

A useful method in studies of dihybridinheritance is the backcross - this is where theF1 from a normal dihybrid cross are backcrossed

(testcrossed) with double recessive individuals.

e.g. another fruit fly cross:

W = normal wings, w = vestigial wings,Y = yellow knees, y= grey kneesBackcross: WwYy x wwyyGametes: WY, Wy, wY, wy all wy

F2 genotypes: WwYy Wwyy wwYy wwyy

F2 phenotypes normal normal vestig. vestig.wing wing wing wingyellow yellow yellow greyknees knees knees knees

Ratio: 1 : 1 : 1 : 1

• This F2 generation consists of 50% parental genotype and 50% recombinant genotypes because the two genes involved are on separate chromosomes and show independent assortment.

Linkage and Crossing overIf each chromosome consisted of just one genethen independent assortment would happen inall dihybrid crosses.Each chromosome is, however, made up of ahuge number of different genes and these don’tbehave independently of one another.When a cross involves two different geneslocated on the same chromosome, the twogenes are transmitted together, i.e. theyare linked

If genes are completely linked, then a backcrosswould fail to produce any recombinantse.g. Moth book page 74, figure 14.6 – all

offspring are the phenotype of one of the parents

See also Frog book page 98, figure 14.6

Separation of linked genes

Straightforward transmission of linked genestogether doesn’t always happen.This is because during meiosis, crossing over canoccur between chromatids at chiasmata,allowing genes that were previously linked onthe same chromosome to recombine in differentcombinations, generating recombinants

Copy the cross in fig. 14.7 on p75 (moth book)

Frequency of recombination

Since chiasmata can occur at any point along achoromosome, the further apart genes are on achromosome, the more crossing over (andtherefore recombination) can take place.

The distance between two genes on a chromosomeis therefore indicated by the number, or percentage,of recombinants in the F2 generation resulting from

a testcross.

e.g.

The frequency of chiasmata formation between A and C is greater than between A and B and is much greater than between B and C

The recombination of linked genes is yet another source of variation provided by meiosis.

Recombination frequency

Recombination frequency or Crossover Value (COV) is calculated as follows:

No. of F2 recombinants

COV = x100Total No. F2 offspring

Chromosome MapsThe frequency of recombination is used to

calculate the positions of genes on chromosomes to produce chromosome maps.

A COV of 1% represents 1 unit of distance on a chromosome

The position of a gene on a chromosome is it’s locus (plural = loci)

(See also Fig 14 page 100 Torrance Blue Book)

e.g. Drosophila:

GeneGene Phenotypic expressionPhenotypic expression

T/tT/t Thick or thin legThick or thin legL/lL/l Long or vestigial wingLong or vestigial wingB/bB/b Presence of bristlesPresence of bristlesS/sS/s Straight or curved wingStraight or curved wing

Gene pair in crossGene pair in cross % frequency of % frequency of recombinationrecombination

T/t x L/lT/t x L/l 1212T/t x B/bT/t x B/b 44T/t x S/sT/t x S/s 2121L/l x B/bL/l x B/b 1616L/l x S/sL/l x S/s 99B/b x S/sB/b x S/s 2525