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| Chapter
11 |
Genetics
and sex |
| 1. |
Genetics
and sex
- The nuclei of all cells (except sex cells)
in our bodies have 23 pairs of chromosomes. Sex cells (egg
cells and sperms) have 23 single chromosomes. Chromosomes
are divided into genes.
- The study of genes is called genetics.
Chromosomes (and therefore genes) are passed on from parents
to their offspring. They control how we grow and what we
look like.
- One pair of chromosomes determines our
sex – whether we are male or female. These are called
the sex chromosomes. All
males have one X and one Y sex chromosome (XY). All females
have two X chromosomes (XX).
This means that all egg cells have one X chromosome, but
sperms may have one X or one Y chromosome.
- At fertilisation:
– If the father’s X chromosome joins an X chromosome
from the mother to form an XX pair, the baby will be a girl.
– If the father’s Y chromosome joins an X chromosome
from the mother to form an XY pair, the baby will be a boy.
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| 2. |
Genes
and inheritance
- Many of our characteristics (such
as eye colour) are controlled by a homologous pair of genes.
One gene in the pair comes from the mother, the other comes
from the father. This is called monohybrid
inheritance.
- Genes may be dominant
(brown eyes, B) or recessive
(blue eyes, b). Dominant genes always result in the characteristic
being shown. Recessive genes only result in the characteristic
if both of the genes
in the pair are recessive. The dominant and recessive genes
which can make up a pair, e.g. B and b, are called alleles.
- The possible pairings of genes
(e.g. BB, Bb
and bb) are called
genotypes (genetic
types). These genotypes determine the characteristic (phenotype)
of an organism.
- Genotypes
with two identical genes (e.g. BB and bb) are described
as homozygous.
Genotypes with two different genes (e.g. Bb) are described
as heterozygous.
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| 3. |
Inherited
characteristics
- The phenotypes
of offspring can be predicted (as in the diagram below)
by drawing a Punnet square if the genotypes of the parents
are known.
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| 4. |
Faulty
genes and inherited diseases
- Faulty genes may be passed from parents
to their children. This may cause a disease, like cystic
fibrosis, if a child inherits the faulty gene from both
parents. In most cases, the parents are healthy because
they have the faulty, recessive gene on only one chromosome
in their homologous pair.
- Some other diseases (e.g. muscular dystrophy,
colour blindness and haemophilia) are much more common in
males than females. This is because they are linked to recessive
genes on the X chromosome.
Males with an XY chromosome
pair are more likely to have the disease than
females with an XX pair.
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| 5. |
Variation
- We are all humans, but we live in a world
of very different people. These differences in humans and
other organisms are called variations.
Variations can occur because of our genes (genetically)
or because of where we live and how we live (environmentally).
- Genetically caused variations result from:
- fertilisation (chromosomes from both parents);
- mutation (chromosomes may be changed by chemicals or radiation);
- meiosis (chromosome pairs
separate randomly).
- Environmental causes of variation include
differences in diet, climate and fashion.
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| 6. |
Adaptation, selection
and evolution
- Variations within a species may result
in some individuals being better adapted to survive than
others. The animals and plants that are better adapted are
usually fitter and stronger (they can avoid predators; they
are
free of disease; they can survive the winter; and they are
strong competitors for food).
- This survival of the fittest due to variations
in a species is known as natural selection.
- As a result of variation, changes can
occur in a species over millions of years resulting in a
new species (e.g. humans from apes). This is called evolution.
- The best evidence for evolution comes
from fossils.
- In some cases, the organisms within a
species are unable to adapt to changing conditions and they
die out. This is called extinction.
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