IB Biology genetics

1. 10.2 Dihybrid Crosses
Topic 10 Genetics

2. 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.

3. 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.

4. 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.

5. 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

6. 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

7. 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)

8. 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.

9. 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.

10. 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.

11. Crossing Over
Ref: Year 12 Biology Biozone

12. 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.