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Pleiotropy, Polygenes and Environmental
Influences
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Pleiotropy• Most genes have many different effects, a
quality known as Pleiotropy.• We see this is in many genetic diseases
where several symptoms can be traced to a single pair of alleles.
• E.g. sickle cell anaemia.
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Sickle cell Anaemis
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Sickle Cell Anaemia
• Refer to pg 176 text book
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Polygenes
• This is the opposite of Pleiotropy.• When 2 or more independent pairs of
genes have similar and additive effects on the same characteristic.
• We call this polygenetic inheritance.• E.g. height, weight, skin colour and
intelligence.
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Polygenetic Inheritance
• If you plot the phenotypes of these from a large sample, they give a normal distribution curve.
• Remember continuous variation.
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No
of In
divi
dual
s
Feature being recorded e.g. height
Increasingly abnormal
Increasingly abnormal
Normal Range
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Skin Colour in Humans
• It is thought that 3 or 4 pairs of genes code for skin colour and they show incomplete dominance, but for clarity we shall only use only 2.
• The alleles A and B code for dark skin; the more capital letters you have, the darker your skin.
• AABB is very dark and aabb is very light.
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Height in Humans
• It is thought there are about 10 genes for height and they are additive.
• AABBCCDDEEFFGGHHIIJJ – as tall as possible
• AaBbCcDdEeFfGgHhIiJj – medium height• aabbccddeeffgghhiijj – as short as
possible
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The Effect of Environment of the Expression of Genes
• One of the great debates in life is how much effect the environment has on your phenotype.
• Its very difficult to determine this in reality, we would need about 1000 sets of identical twins, separated at birth, exposed to different environments and then compared later in life.
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Internal Environment
• Modifier Genessometimes the expression of a gene at one locus is affected by alleles present at another locus. E.g. eye colour in humans
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Internal Environment
• Sex-limited Genesthese genes are on the autosomal chromosomes but can only be expressed in a particular sex.e.g. a rare type of cancer that affects the uterus is clearly limited to females.Also the secondary sexual characteristics.
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Internal Environment
• Sex-linked Genesthese can show up in both males and females but to a different extent, influenced by sex hormonese.g. pattern baldness
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External Environment
• DrugsWhen taken by pregnant woman these can affect the unborn child.
• Thalidomide– this was a mild sedative, given to mothers for
morning sickness. – This drug caused severe deformities,
particularly unformed limbs, in unborn babies.
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Thalidamide Babies
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Drugs
• Smoke– Babies of smokers have low birth weights
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Drugs
• Alcohol– This can have a very bad effect on a child,
leading to foetal alcohol syndrome.– This can be identified by certain facial traits,
and an unusual behaviour pattern that seems to fail to connect actions with consequences.
– The babies of chronic alcoholics are often severely retarded in their development, leading to deformed joints, imperfectly formed heart and lowered IQ.
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Starvation or Malnutrition
• This can effect the proper development of both plants and animals.
• The effect of severe malnutrition in humans is stunted growth and mental retardation, especially if it is due to a protein deficiency.
• Malnutrition has an especially bad effect on the brain if it occurs under the age of about 8.
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Starvation or Malnutrition
• In many third world countries whole populations are affected.
• This will lead to a future population of mentally slow adults.
• In these circumstances genes do not get a chance to show through.
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Light
• Lack of light can prevent the formation of chlorophyll or development of seedlings.
• Light also affects skin colour. Some people can tan, as the sun affects the deposition of melanin.
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Temperature
• This can affect enzymes and fur colour.• In himalayan rabbits and siamese cats, the
fur colour can be affected by temperature.• The rabbit is normally light-coloured with
black feet, ears, nose and tail – the extremities are colder than the rest of the body.
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Temperature
• If an area of the rabbit is shaved and a cool pack is placed on that area, the low temperature causes the growth of a patch of black hair.
• Coldness affects the expression of the gene.
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Temperature
• In the fruit fly Drosophila, there is a condition that causes the wings to curl.
• If flies with this gene are raised at a temperature of 25ºC, the wings curl.
• If the flies are raised at a temperature of around 16ºC, the trait appears only rarely; most of the flies have straight wings.
• If their offspring are raised at 25ºC, the curly wings reappear.
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Temperature
• The temperature that certain reptile eggs are incubated in can determine the sex of the offspring.
• Lower temperatures – females• Higher temperatures - males
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Ionising Radiation
• This can have a huge effect of genes• (refer to mutagenic agents)
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Polluting Chemicals in the Environment
• Chemicals such as DDT, and many pesticides and herbicides e.g. 2.4.5.T.
• The chemical 2.4.D was part of agent orange, used in the Vietnam war to defoliate the forests.
• It was full of a toxic substance called dioxin.• Heavy metal ions can destroy enzyme
systems so the genes do not show through.
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