Genetics, revised edition. Karen Vipond
are said to be homozygous for that trait.
Whether an individual is said to be homozygous or heterozygous for a particular trait indicates whether they carry the same or different alleles within that gene. This can be described as the individual’s genotype. A person’s genotype is the genetic make-up for a particular trait. The term phenotype is used to describe the expression of the gene (or paired alleles) for the same trait (Table 2.1).
Table 2.1 Genotypes and phenotypes for freckles
| Genotype | Classification | Phenotype | |
| Allele 1 | Allele 2 | ||
| freckles | freckles | homozygous | has freckles |
| freckles | no freckles | heterozygous | has freckles |
| no freckles | freckles | heterozygous | has freckles |
| no freckles | no freckles | homozygous | no freckles |
Allelic relationships
Dominant alleles are phenotypically expressed in both heterozygotes and homozygotes. Recessive alleles are only expressed if the alleles are both in a recessive form (homozygous recessive).
Upper and lower case letters are used to represent dominant and recessive alleles. Upper case letters are used to represent a dominant allele and lower case for a recessive allele. If the letter ‘F’ was chosen to represent the gene for freckles, then ‘F’ would represent the dominant allele and ‘f’ would represent the recessive allele. An individual who is heterozygous for the freckles gene would be represented as an ‘Ff’ genotype. A homozygous dominant genotype would be ‘FF’, while a homozygous recessive would be ‘ff’.
Any letter can be chosen to represent different allelic traits. However, it is good practice to choose a letter that has a different form in upper case compared with lower case. For example, A and a, B and b would be good to use but avoid C and c. This helps when drawing out inheritance patterns as different forms can be visually recognised as dominant or recessive, and it avoids errors due to poor handwriting.
Examples of Mendelian traits
Cleft chin
A cleft chin is due to a dominant allele (Figure 2.2). A person without a cleft chin has two recessive alleles for no cleft in their chin.
Figure 2.2 A cleft chin
Ear lobes
Free-hanging ear lobes, as in Figure 2.3, is a dominant trait.
Figure 2.3 Free-hanging ear lobes
Attached ear lobes, as in Figure 2.4, is a recessive trait.
Figure 2.4 Attached ear lobes
Tongue rolling
The ability to form a U shape with the tongue is a dominant trait (Figure 2.5).
Figure 2.5 Tongue rolling
Widow’s peak
When the hairline forms a V shape on the forehead, it forms a widow’s peak (Figure 2.6). This is a dominant trait. Any individual who does not have a widow’s peak is homozygous recessive for a straight hairline.
Figure 2.6 Widow’s peak
Dimples
Having dimples is due to a dominant gene (Figure 2.7). Individuals who do not have dimples when they smile have two recessive genes.
Figure 2.7 A cheek with dimples
Hitchhiker’s thumb
The ability to bend the thumb forward is due to a dominant gene (Figure 2.8). Individuals who have straight thumbs have two recessive alleles.
Figure 2.8 Hitchhiker’s thumb
Carriers
A carrier refers to an individual who ‘carries’ a recessive allele for a particular trait but does not express that trait due to the presence of a dominant allele. Carriers are heterozygous, in that the recessive allele is present but is not expressed. An individual who carries a dominant and recessive allele for the freckles gene (heterozygous) has freckles but is also a carrier of the ‘no-freckles’ allele.
| ACTIVITY 2.1 |
a. Which of the following would be a possible abbreviation for a genotype?
AB Cd Ee fg
b. Do the letters AA describe a heterozygous individual or a homozygous individual?
c. How many alleles for one trait are normally found in the genotype of an individual : 1, 2 or 3?
3. Principle of Segregation
During gamete formation alleles separate so that the gametes contain only one allele of each pair. Allele pairs are restored again after fertilisation.
All the nucleated cells in the body, except for the germ cells (sperm and ova), contain 46 chromosomes. These chromosomes consist of 22 paired autosomes and two sex chromosomes. Mendel’s experiments were only on the traits carried by the autosomes of the plants and, therefore, the principles that he postulated apply to the 22 paired autosomes in humans (see Chapter 4 for sex-linked inheritance).
Somatic cells have the full set of paired chromosomes and are diploid (two copies of each chromosome). Germ cells have only half that amount (haploid) as none of the chromosomes are paired. The separation of the chromosomal pairs occurs during meiosis, leading to the formation of a haploid gamete. When fertilisation occurs between a sperm and an ovum to produce a zygote, the two sets of unpaired chromosomes unite to form a diploid zygote. Alleles combine in the offspring (see Figure 2.9).
Figure 2.9 Meiosis
Working out the different allele combinations in the offspring is straightforward with single gene inheritance. The union of gametes that carry identical alleles will only produce a homozygous genotype (see Figure 2.10).