Cambridge International A Level Biology Revision Guide

Cambridge International A Level Biology
410
For thousands of years, people have tried to ‘improve’
their cattle. Desired features include docility (making
the animal easier to control), fast growth rates and high
milk yields. Increases in these characteristics have been
achieved by selective breeding. Individuals showing one or
more of these desired features to a larger degree than other
individuals are chosen for breeding. Some of the alleles
conferring these features are passed on to the individuals’
offspring. Again, the ‘best’ animals from this generation
are chosen for breeding. Over many generations, alleles
conferring the desired characteristics increase in
frequency, while those conferring characteristics not
desired by the breeder decrease in frequency. In many
cases, such ‘disadvantageous’ alleles are lost entirely.
Such selective breeding of dairy cattle presents the
breeder with problems. The animals are large and take
time to reach maturity. The gestation period is long and
the number of offspring produced is small. A bull cannot
be assessed for milk production since this a sex-limited
trait (note that this is not the same as sex-linked). Instead,
the performance of the bull’s female offspring is looked
at to see whether or not to use the bull in further crosses.
This is called progeny testing and is a measure of the bull’s
value to the breeder.
It is important to realise that selective breeders have to
consider the whole genotype of an organism, not just the
genes affecting the desired trait, such as increased milk
yield. Within each organism’s genotype are all the alleles
of genes that adapt it to its particular environment. These
genes are called background genes.
Suppose that the chosen parents come from the same
environment and are from varieties that have already
undergone some artificial selection. It is likely that such
parents share a large number of alleles of background genes,
so the offspring will be adapted for the same environment.
But suppose instead that one of the chosen parents
comes from a different part of the world. The offspring will
inherit appropriate alleles from only one parent. It may
show the trait being selected for, but it may not be welladapted
to its environment.
Crop improvement
The same problem is seen when a cross is made between
a cultivated plant and a related wild species. Although
most species will not breed with a different species, some
can be interbred to give fertile offspring. Such species are
often those that do not normally come into contact with one
another, because they live in different habitats or areas. The
wild parent will have alleles that are not wanted and which
have probably been selected out of the cultivated parent.
Farmers have been growing cereal crops for thousands
of years. There is evidence that wheat was being grown in
the so-called ‘fertile crescent’ – land that was watered by
the rivers Nile, Tigris and Euphrates – at least 10 000 years
ago (Figure 17.18). In South and Central America, maize
was being farmed at least 7000 years ago.
Black Sea
Mediterranean Sea
Lower Egypt
Upper Egypt
Nile
Assyria
Mesopotamia
Phoenicia
Red
Sea
Euphrates
Tigris
Caspian Sea
Persian
Gulf
Figure 17.18 Wheat was first farmed in the ‘fertile crescent’
(shown in green) around 10 000 years ago.
It was not until the 20th century that we really
understood how we can affect the characteristics of crop
plants by artificial selection and selective breeding. But,
although these early farmers knew nothing of genes
and inheritance, they did realise that characteristics
were passed on from parents to offspring. The farmers
picked out the best plants that grew in one year, allowing
them to breed and produce the grain for the next
year. Over thousands of years, this has brought about
great changes in the cultivated varieties of crop plants,
compared with their wild ancestors.
Today, selective breeding continues to be the main
method by which new varieties of crop plants are
produced. In some cases, however, gene technology is
being used to alter or add genes into a species in order to
change its characteristics.
Most modern varieties of wheat belong to the species
Triticum aestivum. Selective breeding has produced many
different varieties of wheat. Much of it is grown to produce
grains rich in gluten, which makes them good for making
bread flour. For making other food products such as
pastry, varieties that contain less gluten are best.