Basic genetics

Human Genetics-How you became you!

If you are reading this you are probably interested in how you genetically became you, and chances are you are human in which case you probably have 43 chromosomes (23 pairs). If you were a fruit fly you would 8 chromosomes and if you were a cat you would have 38. Chromosomes are the genetic material that is used to pass along the stuff that plays a part in making up a person. Each of your parents contributed half of your genetic material. Your father contributed 23 chromosomes in his sperm cell and your mother contributed 23 chromosomes in her egg cell. The sperm cell and the egg cell are referred to as gametes, they are the reproductive cells, each has a single set of the 23 chromosomes one of which is a single sex chromosome, either X or Y. Your mother being female has only an X to contribute, but your father could contribute either an X or a Y. If you are female your mother’s X and your father’s X made you female. If you are male your mother’s X and your father’s Y made you male. In either case, the sex chromosomes and the 22 other autosomes from each parent joined to form a cell called a zygote, which was the beginning of you. This zygote has both contributions from your mother and father and therefore contains 46 chromosomes.

As the human develops from a zygote to a sexually mature adult, the genes are passed on with precision to all somatic cells of the body by the process of mitosis. This means that almost all the cells in your body have the same amount, 46 chromosomes, as did the original zygote produced by your parents. Mitosis is cell division whereby the number of chromosomes stays the same by equally allocating replicated chromosomes to each of the daughter cells. The only cells in the body not produced by mitosis are the gametes, which develop in the gonads (ovaries in females and testes in males). These cells undergo a special form of cell division called Meiosis. Meiosis reduces the number of chromosomes by half, to 23 chromosomes. This is done so that when a zygote is formed it will have 46 chromosomes and not 92!

You may now wonder, how can you account for genetic variation apparent in families and in everyone you meet. Some of this can be accounted for by simply remembering you receive half of your chromosomes from you mother and half from you father. That means that although you may have received your fathers gene for black hair, what about your mother’s genes; why not the gene for blonde hair from your mother? The way that works is you do have your mother’s gene for blonde hair it is just not expressed. Black hair is dominant over blonde hair. If we were to call black hair “B” and blonde hair “b” then your genotype would be “Bb”. You received the “B” from your father and the “b” from your mother. In order for your mother to be blonde she must have both recessive alleles therefore “bb”, but your father could be “Bb” like you or “BB”. This means if you were to have a child with someone who also has black hair and is also Bb then it is possible for you to have a child with blonde hair. You may contribute your “b” and she may contribute her “b” giving rise to a blonde haired child even though neither of you have blonde hair. The term allele is given to describe the characteristics possible. In the above example there is two alleles one for black and one for blonde. If the alleles are different, as in “Bb” the genes are called heterozygous, if they are the same “bb” or “BB” they are called homozygous. The above example does not show all the possible outcomes. You may have another child whose hair colour is black. A simple way to show all the possible outcomes is to use a Punnett Square. You place the father’s genotype at the top and the mother’s genotype along the side and pull down and across what each parent can contribute. As shown in the Punnett Square below you can see all the possibilities of hair colour.

.......“B” “b”“B” BB Bb

“b” Bb bb

^ mother

In other words there is a 75%(3/4) chance your child will have black hair(remember this is phenotype, “BB” and “Bb” both mean black hair). There is a 25%(1/4) chance your child will have blonde hair. These are the odds every time you have a child, even though the odds are lower you can have 4 blonde children. Now that you know how to use a Punnett Square a partial listing of some dominant and recessive genes is given below:

Dominant................Recessive

Black Skin..............White Skin

Freckles................No Freckles

Pigmented Skin..........Albinism

Mid-Digital Hair........No Mid-Digital Hair

Curly Hair..............Straight Hair

Non-Red Hair............Red Hair

White forelock of Hair..Natural Colour

V-shaped Hairline.......Straight Hairline

Brown Eyes..............Blue Eyes

Nearsightedness.........Normal Vision

Farsightedness..........Normal Vision

Long Eyelashes..........Short Eyelashes

Free Ear lobes..........Attached Ear lobes

Full lips...............Thin lips

Normal hearing..........Deafness

Normal vision...........Blindness

Webbed fingers..........Non-webbed fingers

Short fingers...........Normal Length

Extra Finger............Normal Number

Dwarfism................Normal Height

There is of course many other considerations and aspects to genetics that have not been mentioned. This is intended only to give a basic outline of how genetics works, and how you came you. There is a lot of debate going on about how genetics is only a small part of how you became you. The environment also plays a big part in shaping individuals.