genotype and phenotype

~Environmental variation can be caused by factors like climate, diet, physical accidents, culture and lifestyle. Many kinds of variation are influenced by both environmental and genetic factors, because although our genes decide what characteristics we inherit, our environment affects how these inherited characteristics develop.
~For example, an individual might inherit a tendency to tallness, but an inadequate diet during childhood will result in poor growth and a shorter individual.
~Identical twins are a good example of the interaction between inheritance and environment, as they are genetically the same. All the differences you see between identical twins, therefore - in personality, tastes, and aptitude for example - are due to differences in their experience or environment.
~Genotype and phenotype
~Our genetic make-up, the sum total of our genes, is called a genotype, but the characteristics which show up in our appearance are referred to as a phenotype.
~Continuous and discontinuous variation
Variation, the small differences that exist between individuals, can be described as either continuous or discontinuous.
~Continuous variation
In continuous variation there is a complete range of measurements from one extreme to the other . Height is an example of continuous variation - individuals can have a complete range of heights, for example, 2.00, 2.01 or 2.005 metres high.
~Other examples of continuous variation include:
Height
Weight
Hand span
Length of feet
Milk yield in cows
~Continuous variation is the combined effect of many genes and is significantly affected by environmental influences. Milk yield in cows, for example, is determined not only by breeding programmes (ie, genetics) but is also significantly affected by environmental factors such as pasture quality and diet, weather, and the comfort of their surroundings.
~Discontinuous variation
This is where individuals fall into a number of distinct classes or categories, and is based on features that cannot be measured across a complete range. You either have the characteristic or you don't. Blood group is a good example: you are either one blood group or another - you can't be in between.
~Discontinuous variation is controlled by alleles of a single gene or a small number of genes. The environment has little effect on this type of variation.
~Environmental factors including life style and diet affect how our genes are translated into what is seen, our phenotype. For example, an identical twin who takes regular exercise will have better muscle tone compared with her twin who does not.
~Genetic variation
Inside the nucleus of all plant and animal cells are thread-like chromosomes which carry the genes controlling characteristics such as the colour of a flower, the shape of a leaf, and a person's height or hair colour.
~Inside a normal human body cell there are 23 pairs of chromosomes. Other animal and plant species have different numbers of chromosomes.
~During meiosis the chromosomes split and pair up again in different combinations to form the gametes or sex-cells (eg sperm, pollen or egg cells) - which contain half the number of chromosomes as the parent cells. (See the Revision bite on Cell division).
~After two gametes have met and fused on fertilisation, the resulting zygote carries a full, unique set of chromosomes, half from the mother and half from the father.
~It is this mixing of genetic material during sexual reproduction that introduces genetic variation. It happens at two distinct points:
~During meiosis, when homologous chromosomes cross over and randomly exchange bits from each other to produce gametes each with a unique make-up, and
~During fertilisation, when any two gametes randomly meet-up and fuse, to produce a unique combination of genetic information in the zygote.
~Mutation
Sometimes genes change by mutation - a random mistake during the copying of genetic material. During meiosis and gamete formation, genes are copied and chromosomes replicated, and if the genetic material is not copied exactly a mutation occurs. Mutations may occur by chance, or can be the result of an environmental factor such as exposure to ionising radiation and certain chemicals.
~In chromosome mutations a major change occurs and may affect part of or a whole chromosome. Down's syndrome is caused by the presence of an extra chromosome in chromosome set 21.
~In gene mutations a chemical change occurs in an individual gene. Although this change may be very small, it can give rise to abnormalities such as albinism, sickle cell disease or cystic fibrosis. Gene mutations are usually recessive and the effects can be hidden by a dominant allele (see the Revision bite on DNA and genes).
~For instance, cystic fibrosis is caused by a recessive allele, so to have the disease a person must have two recessive alleles. Heterozygous people are called 'carriers' because they carry the recessive allele but do not get the disease.) For more on this topic see the Revision bite on Genetic diseases.)
~This fruit fly has suffered a mutation in the genes controlling wing growth
~Most mutations are harmful or even fatal to the organism. However sometimes a random mutation results in a new characteristic which helps the organism survive. For example if a plant which grows in wet, marshy soil undergoes a mutation which makes its roots more resistant to rotting, the mutant individual is more likely to survive and breed than the unmutated plants - so the new characteristic will be passed on to the mutant's offspring and spread through the population.