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).
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
