Cambridge International A Level Biology Revision Guide

Cambridge International A Level Biology
400
In Chapter 16, you met a number of examples of
discontinuous variation showing the large effect of
the different alleles of a single gene. The inheritance of
sickle cell anaemia and haemophilia are examples of
discontinuous variation in humans. Flower colour in
snapdragons, stem colour of tomato plants and feather
colour of chickens are examples from other organisms.
From these examples, you can see that dominance and
gene interaction tend to reduce phenotypic variation.
Two of the typical effects of the inheritance of
continuous variation, namely the small effects of the
different alleles of one gene on the phenotype and the
additive effect of different genes on the same phenotypic
character, can be seen in a hypothetical example of the
inheritance of an organism’s height.
Suppose that the height of an organism is controlled by
two unlinked (that is, on different chromosomes) genes:
A/a and B/b. The recessive alleles of both genes (a and b)
each contribute x cm to the height of the organism. The
dominant alleles (A and B) each add 2x cm.
Since the effect of such genes is additive, the
homozygote recessive (aabb) is therefore potentially 4x cm
tall and the homozygote dominant (AABB) is potentially
8x cm tall. The other genotypes will fall between
these extremes.
Parental phenotypes 4x cm tall 8x cm tall
Parental genotypes aabb AABB
Gametes ab AB
Offspring genotypes
Offspring phenotypes
all AaBb
all 6x cm tall
Interbreeding these potentially 6x cm tall offspring
gives all possible genotypes and phenotypes among the
16 possibilities.
Parental
phenotypes
Parental
genotypes
6x cm tall
AaBb
6x cm tall
AaBb
Gametes AB or Ab or aB or ab AB or Ab or aB or ab
in equal proportions
continued ...
Offspring genotypes and phenotypes:
Gametes from one parent
Gametes
from
other
parent
Number of offspring
6
5
4
3
2
1
0
AB
Ab
aB
ab
AB Ab aB ab
AABB AABb AaBB AaBb
8x cm 7x cm 7x cm 6x cm
AABb AAbb AaBb Aabb
7x cm 6x cm 6x cm 5x cm
AaBB AaBb aaBB aaBb
7x cm 6x cm 6x cm 5x cm
AaBb Aabb aaBb aabb
6x cm 5x cm 5x cm 4x cm
The number of offspring and their potential heights
according to their genotypes are summarised in the
histogram in Figure 17.3. These results fall approximately
on a normal distribution curve.
4x 5x 6x 7x 8x
Potential height from genotype / cm
Figure 17.3 The additive effect of alleles.
These hypothetical results come from assuming that
two unlinked genes, each with two alleles, contribute
to the height of the organism. Think about what would
happen to the quantitative character if more genes, each
with an additive effect, were involved (polygenes). The
genes may have more than two alleles. Suppose that all the
genes affecting height are on different chromosomes: the
number of discrete height classes increases as more genes
are involved and the differences between these classes get
less. Even if two or more of the genes are linked on the
same chromosome (Chapter 16, page 383), potentially
reducing the number of classes of offspring and increasing
the difference between them, crossing over in meiosis
(page 384) will restore the variation. The differences
between different classes will be further smoothed out by
environmental effects, as discussed in the next section.