genetics page 2

Where do you get your genes from?

You get your genes from your biological parents.

Your mother contributes one set of 23 chromosomes from her egg.
Your father contributes another set of 23 chromosomes from his sperm.
In this way, you have 23 pairs of chromosomes (46 altogether). They are said to be homologous pairs.

The two chromosomes of each pair carry genes that control the same characteristics in the same positions. You therefore have two copies of each gene. One from your mother and one from your father.
There is an exception to this rule. While females do have 23 matched pairs of chromosomes, one of the male's pairs are not matched. These are called the sex chromosomes.
Females have two large chromosomes called X chromosomes, while males have one X chromosome and a smaller one called the Y chromosome.
The Y chromosome does not carry many genes - only ones that make the individual grow into a male. The X chromosome has hundreds of genes which control all sorts of characteristics, most of them have no effect on gender.

Karyotypes

The pictures below show karyotypes of a male and female. A karyotype is a picture made by squashing a cell just before it divides (to spread out the coiled up chromosomes), photographing the results, then cutting around the images of the chromosomes and arranging them in homologous pairs, in order of size.

Here is a karyotype of a human which has been made by treating the chromosomes with a stain that sticks to only certain parts of them. This makes it easier to tell the pairs apart from other pairs of a similar size.
This is another karyotype that uses stylised drawings to show the banding pattern on chromosomes.
At the Human Genome Project Information site you can click on a picture of any one of our chromosomes and see a list of selected traits and disorders associated with that chromosome. This is their picture of chromosome number 1.

Does the number of chromosomes matter?

Humans should have 46 chromosomes. Individuals born with any other number do usually show harmful effects.
Each species has a characteristic number of chromosomes, but many share the same number. For example, New Zealand native Karaka trees and poroporo shrubs happen to have 46, as does Kirk's dik dik (an African species of antelope).
What is most important is having genes that control the same characteristics at the same places on homologous chromosomes.
Below is a list of species with their chromosome numbers.

Plants				Animals
kauri rimu totara karaka beech kowhai tawa pine	26 20 34 46 26 18 24 24	onion rice corn tomato garden pea potato clover yeast	16 24 20 24 14 48 32 8	human chimp gorilla cow dog cat horse mouse	46 48 48 60 78 38 64 40	sheep pig chicken goldfish house fly mosquito honeybee red ant	54 38 78 94 12 6 32 48

The full number of chromosomes of a species is called its diploid number, while the half-number found in its gametes (= sperm or eggs) is called its haploid or monoploid number.
It is also common to call the monoploid number of a species (or its number of pairs of chromosomes), n. So for humans, n = 23. There is a pretty good chance that you are a 2n individual - meaning you have 46 chromosomes.
If you are a gorilla however, for you n = ____? and 2n = ____?

How do genes work?

Genes store information that is used in the making of proteins.
Proteins have a number of jobs to do in cells.
Some are structural, they form solid structures such as hair, skin and muscle fibres.
Others help transport materials within an organism. For example, haemoglobin in red blood cells helps carry oxygen.
The majority of proteins are enzymes. In order to stay alive, cells carry out many chemical reactions. Enzymes are biological catalysts that help these reactions take place.
You may have heard that enzymes help you digest food, this is just one job. Other enzymes are used to build-up or break-down hundreds of chemicals inside cells. Each reaction requires a different enzyme to help it take place.
When an enzyme is needed, a copy of the information on the appropriate gene is made and moves out into the cell where it is "decoded" and helps in the making of the right protein to do the job.
The enzyme must be exactly the right structure to do its job. If the gene has a mistake in it, it may code for the wrong protein and the job may not be done, or done differently than usual.
Any change to a chromosome or gene is called a mutation.
You may have heard of albinos - these are people or animals with no pigmentation (examples: people moose, squirrel, koala, hedgehog. Melanin is a pigment which gives colour to animals. It is made when enzymes act on a substance called tyrosine to turn it into melanin. People or animals whose genes do not code for the right enzyme, can not make melanin and therefore lack pigmentation.
The good news is that we usually have 2 copies of each gene so even if the one you got from one parent doesn't work, hopefully the gene from the other parent does work so that you don't suffer any ill effects.
Geneticists use the word alleles to describe different forms of a gene. For example, eye colour is controlled by genes. One person may have alleles (forms of the gene for eye colour) that give them blue eyes. Another person may have alleles that give them brown coloured eyes. Having a mixture of alleles may result in other colours. (More on that later)
In many cases the different alleles for a gene all work in some way without harming the individual. They produce variation within a population of a species. E.g. a range of colours, sizes and abilities. This is considered to be a good thing. Having variety in a species means that if conditions change, some individuals may be able to still survive. If they were all the same, whole populations might be wiped out by a change such as cooling, warming or loss of a certain food supply.