Pleiotropy, Polygenes and Environmental

Influences

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.

Sickle cell Anaemis

Sickle Cell Anaemia

• Refer to pg 176 text book

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.

Polygenetic Inheritance

• If you plot the phenotypes of these from a large sample, they give a normal distribution curve.

• Remember continuous variation.

No

of In

divi

dual

s

Feature being recorded e.g. height

Increasingly abnormal

Increasingly abnormal

Normal Range

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.

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

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.

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

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.

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

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.

Thalidamide Babies

Drugs

• Smoke– Babies of smokers have low birth weights

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.

FAS

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.

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.

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.

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.

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.

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.

Temperature

• The temperature that certain reptile eggs are incubated in can determine the sex of the offspring.

• Lower temperatures – females• Higher temperatures - males

Ionising Radiation

• This can have a huge effect of genes• (refer to mutagenic agents)

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.