3.2 Applications and skills

3.2.4 Punnett grids and pedigree charts

  • A Punnett grid shows the possible allele combinations that result in the offspring from parents with known genotypes.
  • Pedigree charts are used to trace the inheritance pattern of a genetic condition through a family.
  • At the end of this page, you should be able to:
    • draw a Punnett grid for a cross involving one gene
    • analyse pedigree charts to deduce patterns of genetic disease
    • use Punnett grids (or pedigree charts) to compare predicted and actual outcomes of genetic crosses using real data.

Skill: Using Punnett grids

Worked example 1
The yellow pea allele is dominant over the green pea allele. Determine the ratios of genotypes and phenotypes resulting from a cross between a heterozygous plant that produces yellow peas and a plant that produces green peas.

Step 1: Choose a letter to represent the alleles.

Yellow is dominant, so Y = yellow, and y = green.

Step 2: Determine the genotypes of the P generation.

            Yellow pea is heterozygous, so Yy. Green pea is recessive, so yy.

Step 3: Draw a 2x2 Punnett grid. Place the parents on the grid as follows: 

 

Gametes from one parent

Y

y

Gametes from one parent

 y

 

 

 y

 

 Step 4: Pair the gametes in the boxes representing the F1 generation.

 

Gametes from one parent

Y

y

Gametes from one parent

 y

 Yy

yellow

 yy

green

 y

 Yy

yellow

 yy

green

 Step 5: Summarise the genotypes and phenotypes of the offspring.

Genotypes:    50% of the F1 will be Yy (heterozygous).

50% of the F1 will be yy (homozygous recessive).

Phenotypes:  The F1 ratio is 1:1 yellow to green peas.

Worked example 2
In guinea pigs, black coat colour is dominant to white. Two black guinea pigs were mated. In the first year, they produced 7 offspring with black coats. Over the course of several years these parents produced an additional 14 offspring with black coats and 6 offspring with white coats.

i. Deduce the genotypes of the parents.
ii. Explain why the observed ratio of black to white guinea pigs is different from the expected ratio.

Answer

i. Since both parents have black coats, and they are producing at least some white offspring, they must both be heterozygous, Bb.

 

Gametes from one parent

B

b

Gametes from one parent

 B

 BB

Black

 Bb

Black

 b

 Bb

Black

 bb

white

If either parent were homozygous for the trait, there would be no white coats in the F1.

 

Gametes from one parent

B

B

Gametes from one parent

 B

 BB

Black

 BB

Black

 b

 Bb

Black

 Bb

Black

 

ii. When both parents are heterozygous, the expected phenotype ratio in the F1 is 3:1 (black coats:white coats). For all 28 individuals in the F1, then, we expect there to be 7 white coats and 21 black coats. This is very close to the observed results (6 white: 22 black).

The predicted ratios are based on probabilities – we expect that the observed ratios will converge on the predicted ratios as the clutch size grows. This seems to be the case since the parents produced only black-coated offspring in the first year when the total number of offspring was only 7. We might have deduced that at least one parent was homozygous dominant if we did not have the additional information about subsequent offspring.

Skill: Pedigree charts

For all pedigree charts:

  • Males are represented as squares, and females as circles.
  • Matings are horizontal lines connecting circles to squares.
  • Vertical lines indicate offspring.
  • Individuals affected by the disease are black.
  • Individuals unaffected by the disease are white.

Figure 3.2.4a – Common symbols used in pedigree chartsFigure 3.2.4a – Common symbols used in pedigree charts

Skill: Deducing patterns of inheritance from pedigree charts

The figure below shows the inheritance pattern of a disease over three generations in one family.

Figure 3.2.4b – Inheritance of a disease over three generationsFigure 3.2.4b – Inheritance of a disease over three generations

1. List the sex and disease status of offspring in the F1 generation.

Answer

  • 1 affected male
  • 1 unaffected male
  • 2 affected females
  • 1 unaffected female

2. Determine if the allele disease is dominant or recessive, autosomal or sex-linked. Explain.

Answer

The disease is autosomal dominant because there are affected individuals in every generation and every affected individual has at least one affected parent. It is not sex-linked because males and females are affected equally.

3. Describe one disease that shows this inheritance pattern.

Answer

Huntington’s disease is a rare brain disorder that shows an autosomal dominant pattern of inheritance.

4. Deduce the genotypes of the P generation.

Answer

Mother: Hh Father: hh

*All unaffected individuals have the genotype hh. All affected individuals have the genotype Hh.

Figure 3.2.4c – Pedigree chartsFigure 3.2.4c – Pedigree charts
Pedigree charts showing different patterns of inheritance.

Figure 3.2.4d – Genetic counsellingFigure 3.2.4d – Genetic counselling

Science and social responsibility

Are there ethical implications to genetic counselling? Are families stigmatised by diagnoses of mutation and cancer risk?

Figure 3.2.4e – PunnettFigure 3.2.4e – Punnett
Reginald Crundall Punnett (1875–1967), inventor of the Punnett square, was in university when Mendel’s laws were rediscovered.

Did you know?

Mendel did not have knowledge of genes or chromosomes. He did not use Punnett grids to determine probabilities, but counted thousands of peas and based his prediction of the existence of ‘inheritable factors’ on his observed ratios of phenotypes.

Figure 3.2.4f – Mendelian ratiosFigure 3.2.4f – Mendelian ratios
Mendel’s experimental results.

Try it!

Draw a 2x2 Punnett square showing:

  • the F2 generation from worked example 1
  • how different human conditions you learned about in 2.1.4 are inherited.

Course link

HL students should be able to draw Punnett squares for two factor crosses. 10.2.2

Figure 3.2.4g – Dihybrid crossFigure 3.2.4g – Dihybrid cross