Cambridge International A Level Biology Revision Guide

Chapter 16: Inherited change
Katydids
Oblong-winged katydids are normally green and well
camouflaged. Occasionally, a bright pink katydid
occurs (Figure 16.1).
The pink colour is caused by an allele (variety) of
the gene that determines body colour. It has always
been thought that the allele causing the pink colour
was recessive, but breeding experiments in 2013
showed – surprisingly – that this is a dominant allele.
A katydid that has one allele for pink colour and one
for green colour is pink. As you read this chapter, think
about how breeding experiments with katydids could
have determined whether the pink colour is caused by
a dominant allele or a recessive allele.
Figure 16.1 A rare pink katydid, Amblycorypha oblongifolia.
You have already learned that the nuclei of eukaryotic
cells contain chromosomes and that the number of
chromosomes is characteristic of the species. Look back to
Chapter 5 to remind yourself about the structure
of chromosomes.
A photograph such as Figure 16.2 is called a
karyogram. Karyograms are prepared by cutting out
individual chromosomes from a picture like Figure 5.2
and rearranging them.
Homologous chromosomes
Figure 16.2 shows the chromosomes from Figure 5.2
(page 95) rearranged into numbered pairs, and
Figure 16.3 is a diagram of the chromosomes.
365
1 2 3 4 5 6 7 8
9 10 11 12 13 14 15 16
17 18 19 20 21 22 X Y
Figure 16.3 Diagram showing banding patterns of human
chromosomes when stained. Green areas represent those regions
that stain with ultraviolet fluorescence staining; orange areas are
variable bands. Note that the number of genes is greater than the
number of stained bands. Only one chromosome of each pair is
shown, except for the sex chromosomes, which are both shown.
Figure 16.2 Karyogram of a human male. There are 22
homologous pairs of non-sex chromosomes (autosomes). The
sex chromosomes (X, female; Y, male) are placed separately.
QUESTION
16.1 Look at Figures 16.2 and 16.3 and try to decide why
the numbered chromosomes are arranged in the
particular order shown.