10.2 Applications and skills

10.2.2 Predicting ratios and identifying recombinants

  • Until now, all of the two-trait crosses that you have seen have included the typical 9:3:3:1 ratio.
  • You should be able to use Punnett squares to calculate ratios of genotypes and phenotypes from other types of crosses.
  • You should also be able to use a chi-square test, and identify recombinant genotypes from crosses of linked genes.

Skill: Draw a Punnett square

In rabbits, grey hair (G) is dominant to white hair (g) and black eyes (B)are dominant to red eyes (b). The genes are unlinked.

1. Draw a Punnett square for the parental cross: Ggbb x ggBb.

 

Gb

Gb

gb

gb

gB

GgBb

GgBb

ggBb

ggBb

gB

GgBb

GgBb

ggBb

ggBb

gb

Ggbb

Ggbb

ggbb

ggbb

gb

Ggbb

Ggbb

ggbb

ggbb

*Note: For a dihybrid cross, you need a 4x4 table

2. Identify the ratio of genotypes and phenotypes in the F1.

Ratio

1

1

1

1

Genotype

GgBb

Ggbb

ggBb

ggbb

Phenotype

Grey hair, black eyes

Grey hair, white eyes

White hair, black eyes

White hair, red eyes

Skill: Identify recombinants and linked genes

In Drosophila, mutant vestigial wings and ebony body are recessive to the wild-type wing shape and body colour. The genes are linked.

Figure 10.2.2a – Drosophila wild type and black body, vestigial wing mutantsFigure 10.2.2a – Drosophila wild type and black body, vestigial wing mutants

A male fruitfly with vestigial wings (vg) and black body (b) was mated with a true-breeding wild-type female. The F1 was then mated to produce the following F2.

Phenotypes

Number of progeny

Males

Females

Total

Wild type

23

34

57

Black body

5

9

14

Vestigial wings

4

7

11

Black body, vestigial wings

2

4

6

 

 

 

88

1. Outline the evidence that the genes for vestigial wings and black body are not sex-linked.

The genes are not sex-linked – females display the mutant traits in the F2.

2. List the parental and F1 genotypes, assuming autosomal linkage.

Parental genotypes

Male         x          Female

vg  b        x          vg+ b+

vg    b                   vg+ b+

 

F1 genotype (male and female)

vg  b

vg+ b+

*Notice that linked genes are written as vertical pairs, rather than horizontal pairs for unlinked genes.

3. Draw a Punnett square to show the predicted Mendelian ratio of genotypes and phenotypes in the F2.

F1 genotype

vg+ b+

vg+ b

vg b+

vg b

vg+ b+

vg+ b+

vg+ b+ 

vg+   b

vg+ b+

vg   b+

vg+ b+

vg    b

vg+ b+

vg+ b

vg+ b+

vg+  b

vg+ b

vg+ b

vg  b+

vg+ b

vg   b

vg+ b

vg b+

 vg+ b+

vg   b+ 

vg+  b

vg  b+

 vg  b+

vg  b+

vg    b

vg b+

vg b

vg+ b+

vg   b+

vg+  b

vg    b

vg  b+

vg  b+

vg    b

vg   b

 

Predicted phenotype and genotype ratio: 9:3:3:1

9: wild-type wings and body shape (vg+b+/vg+b+)

3: wild-type wings, black body, (vg+ b/vg+ b)

3: vestigial wings, wild-type body (vg b+/vg b+)

1: vestigial wings, black body (vg b/vg b)

4. Identify the recombinant genotypes.

Recombinant genotypes are non-parental genotypes.  The parents are both homozygous for both traits. The recombinant genotypes are:

vg+ b     ,    vg b+

vg+ b          vg b+

5. Perform a chi-square test and discuss the results.

x2 = 4.29

df = 3

The critical value at p = 0.05 for three degrees of freedom is 7.81. Since our x2 value is smaller than the critical value, we cannot reject the null hypothesis that genes are unlinked. However, the sample size may be too small to make a conclusion about gene linkage.

Figure 10.2.3b – White bunnyFigure 10.2.2b – White bunny
Recessive phenotypes are not always less frequent.

Science and social responsibility (Aim 8)

Experiments with Drosophila are discouraged by the IB’s animal experimentation policy. There are many good Drosophila simulation labs available online.

Figure 10.2.3c – Foil methodFigure 10.2.2c – Foil method

Concept help

Having trouble figuring out the combination of alleles to place on your Punnett square? Try the FOIL method.