1.1 Chromosomes are structures found in the nucleus of
most cells. They consist of long strands of a substance
called deoxyribonucleic acid (DNA).
1.2 A section of DNA that has the genetic code for
making a particular protein is called a gene.
1.2.1 The proteins can either be...
18.104.22.168 ....structural proteins such as the ones found in
muscles and hair.
22.214.171.124 ...enzymes, such as proteases and other
2.1 Individuals differ in all sorts of ways, even when they
are offspring of the same parents. These differences
are called variation.
2.2 Most characteristics, such as height, are
determined by several genes working together.
They are also influenced by environmental
2.2.1 These factors are...
126.96.36.199 physical accidents
3 identical twins
3.1 Identical twins are genetically the same
3.2 An identical twin who takes regular exercise will have better muscle tone than one who does not
exercise. All of the differences that you see between identical twins, for example, in personality,
tastes and aptitude, are due to differences in their experiences or environment.
4 Sex cells
4.1 When an egg and sperm cell come together, the now fertilised egg
contains 23 pairs of chromosomes. Sex chromosomes are
responsible for certain genetic traits.
4.2 When an egg and sperm cell come
together, the now fertilised egg contains
23 pairs of chromosomes. Sex
chromosomes are responsible for certain
4.2.1 Female sex cells are called egg cells, or ova
4.2.2 Male sex cells are called sperm
4.3 A pair of chromosomes carry the same genes in the same place, on each
chromosome within the pair. However, there are different versions of a
gene called alleles. These alleles may be the same (homozygous) on each
pair of chromosomes, or different (heterozygous), for example, to give blue
eyes or brown eyes.
4.3.1 Which chromosome we get from
each pair is completely random. This
means different children in the same
family will each get a different
combination. This is why children in
the same family look a little like each
other and a little like each parent, but
are not identical to them.
4.4 The highlighted pair of chromosomes are called the sex
chromosomes; they are a pair. The longer sex
chromosome is called the X chromosome, the shorter
one the Y chromosome. Females are XX, Males are XY.
5 Determination of gender
5.1 When sex cells form, the pairs of
sex chromosomes (XX and XY) are
5.1.1 All normal egg cells
produced by a human ovary
have an X chromosome half
the sperm carry an X
chromosome, and half a Y.
6 Higher Tier
6.1 The Y chromosome carries a gene called the
‘sex-determining region Y’, or SRY for short. The
SRY gene causes testes to develop in an XY
embryo. These produce androgens: male sex
hormones. Androgens cause the embryo to
become a male: without them, the embryo
develops into a female.
6.2 Some people think parents should be able to choose the sex
of their future children, especially if they had a child that died,
or already have three or four children of the same sex. Other
people think we should not be able to choose, because this
could affect the balance of males and females in society, or
because they believe it is against God or nature.