Monday, December 7, 2009

Genetics - What are Genes?

Kids like asking questions about science. Find out about genetics, what are genes and why parents with brown eyes have a child with blue eyes

Genetics - What are Genes?

Inside every cell of each living thing (plant or animal) are sets of instructions called genes. The genes provide the instructions on what is the plant or animal, what it looks like, how it is to survive, and how it will interact with its surrounding environment. The genes are strung together in long stands of material called deoxyribonucleic acid (DNA) and these long strands are called chromosomes. Most living things have pairs of chromosomes (one from each parent), though they may have a different number of chromosomes from another living thing. For example, humans have 23 pairs of chromosomes and the fruit fly has 4 pairs.

Each gene is made up of long combinations of four different nucleotide bases. It is the various combinations of the nucleotide bases that determine everything about a living creature.

The four nucleotides are called:

* adenine(A),
* cytosine (C),
* guanine (G), and
* thymine (T).

The gene for green eyes might have this nucleotide sequence.


The gene for blue eyes might have this nucleotide sequence.


Notice how the nucleotide sequences below are very similar. The only difference being the last two bases. They both describe an eye with the last two characters stating the color of the eye.

Note: These gene formula are not accurate, the correct sequence could be up to several thousand bases long).

Many of the genes have a quality known as Dominate or Recessive. Dominant traits are more common than Recessive traits due to this quality.

Why do I have blue eyes when both my mother and father have brown eyes?

Each person has two genes for eye color. When a person has two identical genes, he will have eyes of that color. Another person may two different genes, and she will have eyes the color of the dominant gene. With eye color, the gene for brown eyes is dominant (B). The gene for Blue eyes is recessive (b). If a person has one B gene and one b gene or two B genes, then that person will have brown eyes. If a person has bb eye genes, then the person will have blue eyes.

Parents with Brown Eyes and Recessive Genes

Parents with Brown Eyes and Recessive GenesIn this diagram, both the mother and father have brown eyes, but they both have the recessive gene for blue eyes as part of their genetic makeup. There is a 1 in 4 or 25% chance that their child will have blue eyes.

One Brown Eyed Parent with Recessive Gene

One Brown Eyed Parent with Recessive GeneIn this diagram, the mother has brown eyes with the recessive blue eye gene and the father has blue eyes. There is a 1 in 2 or 50% chance that their child will have blue eyes.

Brown Eyed Parent without Recessive Gene

Brown Eyed Parent without Recessive GeneIn this diagram, the father has blue eyes and the mother has brown eyes without the recessive gene. In all of the possible combinations for the children, there is one of the dominant B genes. All of the children will have brown eyes.

The results would be the same in diagram 2 if the Mother had blue eyes and the Father had Brown eyes with a recessive blue gene, or in diagram 3 if the Father had the brown eyes and the Mother had blue eyes.

Word Definitions

* Dominant - a gene in one strand of DNA that is stronger than the corresponding gene in another strand of DNA.
* Recessive - a gene in one strand of DNA that is weaker than the corresponding gene in another strand of DNA.
* Trait - a distinguishing feature in a person.

Trivia Answers

1. Why is DNA described as the “double helix of life”?

Double is from the two strands of DNA. A helix is anything spiral in shape or structure. The DNA is found in two spiral strands. Without the DNA, there would be no life.

2. Where are chromosomes found within a living cell?

The chromosomes are found in the nucleus of each cell.

What Is DNA?

Every cell in every living thing (or organism) has DNA, a molecule that contains all the information about that organism. Building on research by others before them, the structure of DNA was discovered in 1953 by the British scientist Francis Crick and the American scientist James Watson. James Watson was a World Almanac reader as a kid.

Lengths of connected DNA molecules, called genes, are like tiny pieces of a secret code. They determine what each organism is like in great detail. Almost all the DNA and genes come packaged in rod-like structures called chromosomes--humans have 46. There are 22 almost identical pairs, plus the X and Y chromosomes, which determine if a human is male (one X chromosome and one Y chromosome) or female (two X chromosomes).

Genes are passed on from parents to children, and no two organisms (except clones and identical twins) have the same DNA. Many things--the color of our eyes or hair, whether we're tall or short, our chances of getting certain diseases--depend on the genes we get from our parents

If the RNA theory of the origin of life is right, then after a short time, about four billion years ago, the oceans on Earth had a lot of these strands of RNA molecules floating around in them, making proteins, and also making new strands of RNA. But RNA molecules were weak, and they were always breaking and making copies that were not really exactly right - not exactly the same as the original RNA molecule. This was partly helpful, because there got to be a lot of different kinds of RNA molecules, and therefore a lot of different kinds of proteins. But it was also partly bad, because if you had a good healthy RNA molecule it was hard to copy it exactly.

Some of these RNA molecules evolved to make two sides to their spiral staircases instead of just one. These formed a double helix: two spirals twisting inside each other. It was a much more complicated kind of molecule, but it was much stronger, just as a ladder is stronger with two sides than with only one. These new molecules are called deoxyribonucleic acid, or DNA for short. DNA stairs are made of two purines - adenine and guanine - and two pyramidines - thymine and cytosine.

When a DNA molecule needed to make a copy of itself, it had nowhere for the new purines to fasten on to. So it had to unzip itself, just like the zipper on your jacket, and then it could make some new molecules and then zip itself back up again.

Here is a video that shows how DNA zips and unzips: