14. Variation and inheritance GapFill

Variation

There are a lot of differences between individuals of the same  groupsizeclonehabitatplacespecies. No two people in your class look alike, even though we are all human.

Differences between individual animals or plants come about through either genetic variation or environmental variation.

Genetic variation

We inherit our characteristics from both our parents through their  cellsgenesspermbloodcytoplasmmitochondria. Even with the same parents, brothers and sisters can be very different. Each one of us receives a unique combination.

An exception to this is where you get identical twins, who have developed from the same  unfertilised eggovuleovaryfertilised eggbody cellsperm. Both will be the same sex and share identical genes.

Identical twins are genetically the same but as they grow up they can still have differences. One might end up being much stronger or fitter than the other; this is due to differences in their  hormonesenzymeslifestylechromosomesgenesDNA.

Characteristics that are caused only by genes usually have a small number of values. For example, there are only four   tissuespermbloodcellmuscleorgan groups; A, B, AB and O. This is called discontinuous variation. You would show this information as a bar chart.

Environmental variation

The environment that you grow up in or live in has an effect on how your characteristics develop. For example, if you had the genes to be very tall, but you didn’t get enough to eat when you were growing up, you might not grow as tall as someone with the same genes who had a healthy balanced diet all through their life.

If you look at plants you can easily see the effects of environmental variation. For example, imagine two identical plants grown in the same soil, but one is in the shade of a wall or a large tree. After six months' growth the plant that is in the shade will   be a different species frombe healthier thanbe smaller thanbe the same size ashave bigger leaves thanbe bigger than the one that gets full sunlight all day.

A characteristic such as height will have a wide range of values, from very short to very tall, with most individuals in the middle. This is called   categoricalcontinuousdiscreterangingcellularmeasurement variation, and it's usually shown as a line graph.

Inheritance

Most of our characteristics are the result of several genes acting together, and some are also influenced by environmental factors.

Some characteristics, such as eye colour or the shape of the earlobe, are controlled by a single pair of genes. These genes may have different forms, which are called  chromosomesDNAalternativesallelesbasesequivalents. The gene for eye colour has one for blue eye colour and one for brown eye colour.

Alleles are either dominant or recessive. For example, the allele for brown eyes in humans is dominant, while the allele for blue eyes is recessive.

A dominant allele will be expressed (show its characteristic) if it is present on one or both chromosomes in a pair.

A recessive allele will only be expressed if  the allele is present on both chromosomes in a pairthe environment is rightthe allele comes from the fatherthe allele is present on only one chromosomethe allele is not presentthe allele comes from the mother.

This means that a person will only have blue eyes if they inherit two copies of the allele for blue eyes. 

A person will have brown eyes if they inherit   either one or two allelesmore than one alleleexactly two allelesidentical allelesexactly one alleleno alleles for brown eyes.

Scientists use letters to show which alleles an individual has. The dominant allele is shown as a capital letter, while the recessive allele is shown as a lower-case of the same letter.

For eye colour:

A person with brown eyes could have the alleles BB or Bb.

A person with blue eyes can only have bb.

Therefore, two brown-eyed parents could have a blue-eyed child if  they are both Bbthe mother has Bb and the father has BBthey are both BBthey are both bbthe mother has BB and the father has bbthey have very light brown eyes. The child would inherit a b allele from both parents, and so the child would have the alleles bb.