Genetics, revised edition. Karen Vipond
Figure 2.10 Single gene inheritance
For example, a mother who is homozygous recessive for straight hairline (not a widow’s peak) and a father who is also homozygous recessive for this trait will only produce an offspring who is also homozygous recessive and will also have a straight hairline (see Figure 2.11).
Figure 2.11 A homozygous recessive genotype ‘a’ represents the recessive gene for straight hairline. The dominant gene ‘A’ for widow’s peak is not present.
The union of gametes carrying different alleles for the same gene will produce an offspring with a heterozygous genotype (see Figure 2.12).
Figure 2.12 A heterozygous genotype
There are in fact six basic types of mating for single gene inheritance (Table 2.2). The examples that follow (Figures 2.13i–vi) are for ear lobe shapes, although these examples apply to all single gene recessive and dominant traits. Some individuals have ‘free’ ear lobes while others have elongated attachment of the lobe to the neck. Both ‘free’ ear lobes and ‘attached’ ear lobes are determined by different alleles of the same gene. The allele for free ear lobes is dominant over the allele for attached lobes. The letter chosen to represent the alleles in this instance is ‘E’ (E for ear lobes).‘E’ represents the dominant ‘free’ lobe allele and ‘e’ represents the recessive ‘attached’ lobe.
Figure 2.13i
Figure 2.13ii
Figure 2.13iii
Figure 2.13iv
Figure 2.13v
Figure 2.13vi
Table 2.2 Summary of the six basic types of single gene inheritance
| Number | Parents | Genotypes | Phenotypes |
| i | EE x EE | 100% EE | 100% free lobes |
| ii | EE x Ee | 50% EE, 50% Ee | 100% free lobes |
| iii | EE x ee | 100% Ee | 100% free lobes |
| iv | Ee x Ee | 25% EE, 50% Ee, 25% ee | 75% free lobes, 25% attached lobes |
| v | Ee x ee | 50% Ee, 50% ee | 50% free lobes 50% attached lobes |
| vi | ee x ee | 100% ee | 100% attached lobes |
Punnet square
An alternative method for working out the possible genotype of an offspring is the Punnet square. The Punnet square helps to visualise the segregation of alleles and the possible combinations within the offspring.
Drawing a Punnet square is quite straightforward once you realise that the parents’ alleles segregate to form a gamete. The main framework consists of a grid composed of four perpendicular lines (see Figure 2.14i).
Figure 2.14i Punnet square
The genotype of one parent is then written across the top of the grid and the genotype of the other parent is written down the left-hand side of the grid. It makes no difference which parent’s genotype is written at the top or the side (see Figure 2.14ii).
Figure 2.14ii As alleles segregate to form a gamete, only one letter is inserted in each box at this stage
By copying the column and row letters in each square, the possible combinations within the offspring can be worked out (see Figure 2.14iii).
Figure 2.14iii Punnet squares can be used to work out the possible genotype of offspring
| ACTIVITY 2.2 |
a. By drawing a Punnet square, what possible genotypes could the offspring of a Bb father and a bb mother have?
b. In the mating of two Bb individuals, what percentage of the offspring would have the same genotype as the parents? What percentage would have the same phenotype?
The maximum number of possibilities for a single gene inheritance is four (corresponding to the four squares in the Punnet square). However, these four possible outcomes can only contribute to a maximum of two phenotypes. In some situations there can only be one possible genotype and phenotype shared by all the offspring. For example, if one parent is homozygous dominant for a particular trait (GG) and the other parent is homozygous recessive (gg) the only possible outcome is for a heterozygous offspring (Gg).
The ability to form a U shape with the tongue is a dominant trait in humans. Consider the dominant allele being represented by the letter T and the recessive allele by the letter t. If two tongue rollers who were both heterozygous for this trait (Tt) had a child, what is the chance that the child would also be a tongue roller?
To work out this problem, a Punnet square needs to be drawn with the parents’ genotypes inserted on the top and side of the square, and the possible offspring combinations inserted into the square (see Figure 2.15).
The results show:
• one homozygous dominant offspring (a tongue roller);
• two heterozygous offspring (tongue rollers);
• one homozygous recessive offspring (a non-tongue roller).
Figure 2.15 Punnet square for tongue rollers
As three out of the four outcomes are tongue rollers, the chance of having a tongue rolling child is 75 per cent.
| ACTIVITY 2.3 |
Albinism is a condition that results in the lack of melanin pigmentation in skin. Individuals with this condition also lack pigmentation in both hair and the irises of the eyes. It is a recessive disorder and the condition only affects individuals if they have two recessive