10.2 Dihybrid Crosses

Topic 10 Genetics

Dihybrid Crosses 10.2.1 Calculate and predict the genotypic and phenotypic

ratio of offspring of dihybrid crosses involving unlinked autosomal genes.

10.2.2 Distinguish between autosomes and sex chromosomes.

10.2.3 Explain how crossing over between non-sister chromatids of a homologous pair in prophase I can result in an exchange of alleles.

Dihybrid Crosses The basic principle applied to solving dihybrid problems

are the same as for monohybrid. Steps:

determine the genotypes of the parents. work out the different pairs of alleles for each parent. work out the alleles present in the gametes produced by the

parents draw up the punnett square. do the crosses. count the number of different genotypes. work out the phenotypes of the different genotypes.

Dihybrid Cross Problem A fly with pink eyes and short wings was crossed with a

pure breeding one that had red eyes and long wings. Assuming that red eyes and long wings are dominant, determine the genotype and phenotype ratios in the F1 and F2 generations.

Solution Let:

R = red eyes and r = pink eyes L = long wing and l = short wings

Parents are: pink eyes and short wings: rrll gametes: rl Red eyes and long wings: RRLL gametes: RL

RRLL x rrll

F1 rl rl

RL RrLl RrLl

RL RrLl RrLl

All the offspring are RrLl

Red eyes and long wings.

Heterozygous

Solution If the F1 is allowed to interbreed: RrLl x RrLl Gametes formed are:

RL and Rl and rL and rl

F2 RL Rl rL rl

RL RRLL RRLl RrLL RrLl

Rl RRLl RRll RrLl Rrll

rL RrLL RrLl rrLL rrLl

rl RrLl Rrll rrLl rrll

You get:

9 red eyes, long wings

3 red eyes, short wings

3 pink eyes, long wings

1 pink eyes, short wings

Solution In this cross the parents were:

red eyes, long wings and pink eyes, short wings.

The recombinants are: red eyes, short wings pink eyes, long wings

The ratios observed are:

• 9 red eyes, long wings (same as parent genotype)

• 3 red eyes, short wings (recombinant)

• 3 pink eyes, long wings (recombinant)

• 1 pink eyes, short wings (same as parent genotype)

Ratio in a Dihybrid Cross When two pure breeding individuals are crossed, the F1

will all have the same genotype and phenotype. They will all be heterozygous. When allowed to interbreed, the F2 will have a ratio of:

9 : 3 : 3 : 1 This is the same ratio that Gregor Mendel found during

the 19th century in his studies of pea plants.

The chi-squared Test The chi-squared test can be used in analysing monohybrid

and dihybrid crosses. The chi-squared test can be used to establish whether an

observed ratio differs significantly from the expected one. The larger the calculated value of chi-squared, the greater

is the difference between the observed and the expected results.

Crossing Over In Prophase I, homologous chromosomes, each consisting

of two identical chromatids, lie adjacent to each other – pair up. This is called a synapsis.

The pair of chromosomes is referred to as a bivalent. At this stage corresponding sections of non-sister

chromatids may touch (cross over). This point is called a chiasma (chiasmata – plural).

Sections of the chromosomes are swapped between the non-sister chromatids.

This produces recombinant chromosomes. This process is called Crossing-over. Crossing over increases the genetic variability of the

offspring by altering the combination of genes on the gametes formed.

Crossing Over

Ref: Year 12 Biology Biozone

IBO guide: 10.2.1 Calculate and predict the genotypic and phenotypic

ratio of offspring of dihybrid crosses involving unlinked autosomal genes.

10.2.2 Distinguish between autosomes and sex chromosomes.

10.2.3 Explain how crossing over between non-sister chromatids of a homologous pair in prophase I can result in an exchange of alleles.