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4.4
I
1
2
2
3
II
1
4
5
?
III
1
4.4
3
2
4
Figure 5
Pedigree chart that shows a family with the
trait of shortsightedness. The allele for shortsightedness (S ) is dominant over the allele
for normal vision (s).
Multiple Alleles
For each of the traits studied by Mendel, there were only two possible alleles.
The dominant allele controlled the trait. It is possible, however, to have more
than two different alleles for one gene. In fact, there are many genes with multiple alleles.
Geneticists who study the tiny fruit fly called Drosophila melanogaster
(Figure 1) have noted that many different eye colours are possible. The red, or
wild type, is the most common, but apricot, honey, and white colours also exist.
Although a fruit fly can have only two different alleles for eye colour at any one
time, more than two alleles are possible. A fruit fly may have an allele for wildtype eyes and another for white. Its prospective mate may have an allele for
apricot-coloured eyes and another for honey-coloured eyes. The dominance
hierarchy is as follows: wild type is dominant over apricot, which is dominant
over honey, which is dominant over white. In the case of multiple alleles, it is no
longer appropriate to use uppercase and lowercase letters. Capital letters and
superscript letters or numbers are used to express the different alleles and their
combinations. Blood types are often denoted using superscripts. For example,
blood type A, if homozygous, contains two A alleles, IaIa.
The dominance hierarchy and symbols for eye colour in Drosophila are
shown in Table 1. For simplicity, the capital letter E is used for the eye colour
gene and superscript numbers used to indicate the dominance hierarchy for the
allele.
Table 1: Dominance Hierarchy and Symbols for Eye Colour in Drosophila
Phenotype
Genotypes
wild type
E 1E 1, E 1E 2, E 1E 3, E 1E 4
apricot, honey, white
apricot
E 2E 2,
honey, white
honey
E 3E 3, E 3E 4
white
E 4E 4
E 2E 3,
(a)
(b)
Dominant over
E 2E 4
white
Figure 1
(a) Drosophila melanogaster, the fruit fly, is
widely used for genetics studies.
(b) Wild type, or red, is the most common eye
colour. The wild-type allele is the dominant
allele.
Sample Problem
What is the phenotypic ratio of the offspring from the mating of the following
Drosophila?
E1E 4 (wild-type eye colour) × E2E3 (apricot eye colour)
Genes and Heredity
143