Genetics Workshop

THE DIHYBRID CROSS

The nuclei of two individuals of the same imaginary species of insect are shown below. Each individual has 2n=4, or two pairs of homologous chromosomes. Thus, the individuals are diploid. Gene loci are shown for:

1) The gene coding for the antenna trait, where the allele "A" gives an individual with antennae. This allele is dominant over the recessive allele "a", where the homozygous individual will have no antenna.

2) The gene coding for the eye size trait, where "B" gives an individual with big eyes. This is dominant over the "b" allele, where the homozygous individual will have small eyes.

Parent types

An organism undergoes meiosis to halve its genetic makeup in preparation for combining its genes with that of another organism. Thus, meiosis is the formation of gametes. Each gamete contains one copy of each gene, ie. each trait must be accounted for in the gamete. Gametes are haploid.

Possible gametes


The first filial generation (F1) is formed by the combination of the parental gametes. From the possible gametes given above, there is only one type of offspring possible. Thus all offspring from the parental cross will resemble the individual given below.

F1 generation


By inbreeding the F1 generation, we get the second filial generation (F2). In a dihybrid cross where there is no epistasis or linkage, we expect to obtain the 9:3:3:1 ratio to be shown in the phenotypes of the F2.

F1xF1
Possible gametes


As this can get quite confusing, the most methodical way of proceeding with the cross is to use a punnet square, where all possible combinations of F1 parental gametes are considered. The results from the punnet square will show all possible genotypes and phenotypes resulting from the cross, and will also indicate the proportions in which these will occur.

Punnet Square for F1xF1

Gametes, F1 Parent1

 

AB

Ab

aB

ab

Gametes, F1 Parent 2

AB

AABB

AABb

AaBB

AaBb

 

Ab

AABb

AAbb

AaBb

Aabb

 

aB

AaBB

AaBb

aaBB

aaBb

 

ab

AaBb

Aabb

aaBb

aabb

Sorting these into genotypes and phenotypes gives us the following offspring. The numbers indicate the expected proportion of that type of offspring to arise from the cross.

 Phenotype: Antennae, big eyes  9  Genotype:  AABB  1
       AABb  2
       AaBB  2
       AaBb  4
 Phenotype: Antennae, small eyes  3 Genotype:  AAbb  1
       Aabb  2
 Phenotype: No antennae, big eyes  3  Genotype:  aaBB  1
       aaBb  2
 Phenotype: No antennae, small eyes  1  Genotype:  aabb  1

This ratio of 9:3:3:1 combines both the Principle of Segregation and the Principle of Independent Assortment The former because each allele of each gene will occur in the gametes with equal probability. The latter as alleles of the antenna trait will occur in gametes independently of the alleles of the eye size trait.

Note that the 9:3:3:1 dihybrid ratio will be observed when:

  • the two loci assort independently
  • there is complete dominance
  • there is no epistasis
  • there are no lethal genes


Another way of testing whether two different genes (each with two alleles) are inherited independently is to perform a test-cross. This involves crossing a dihybrid with an individual that is known to be homozygous recessive for the loci under study. If the conditions listed above are satisfied, this will give offspring in a ratio of 1:1:1:1 in the phenotypes.

Test cross
Possible gametes
Possible offspring

Here, when the conditions for the 9:3:3:1 ratio have been satisfied, the ratio of these offspring will be 1:1:1:1.

Other References

Knox et al., pages 160-161.

Lecture notes and Web lecture notes for Lecture 31.