Tuesday, Sept. 24 - Genetics - Basic Principles

Key Terms

Allele – One of two or more forms of a gene.

Gene – A segment of DNA that contributes to the characteristics, or traits, of an organism.

Genetics – The field of biology devoted to studying heredity.

Heredity – The passing of traits from one generation to the next.

Trait – A heritable characteristic.

You look like your family because you share traits – the shape of your nose, color of your eyes and even your blood type. Some traits you can see, others you can’t. This passing on of traits is heredity. The study of heredity is genetics. Although we study genetics on a molecular level now – think of DNA – for thousands of years people have been breeding animals and plants to have specific traits. For example, a mule is created by mating a horse and a donkey.

Gregor Mendel

Mendel was an Austrian Monk who observed pea plants in his garden. He noticed traits inherited from one generation to the next, so he began experimenting with those plants. In one of his earliest experiment, he crossed a tall pea plant with a short pea plant (He called this the P generation). He noticed their children, (the F1 generation) were all tall pea plants. It was as if the short pea plant gene had disappeared. However, in the grandchildren (the F2) generation, some plants were tall and some plants were short.

So you had:

P = tall and short

F1 = tall, tall, tall, tall

F2 = tall, tall, tall, short.

Mendel discovered this happened with other traits as well – there is a dominant trait and a recessive trait. What he called traits were genes – a segment of DNA that contributes to the traits of an organism. Genes are found in chromosomes, which can carry thousands of genes.

So in humans, for example, brown eyes and blue eyes. Each parent contributes one chromosome, which makes up a pair of chromosomes. So, you have a blue-eyed mother and a brown-eyed father.  They each contribute a chromosome with the gene on it that determines eye color.

Let’s assume B=Brown and b=blue. Blue is a recessive trait. A capital letter usually means a dominant trait or gene (B) and a small letter means a recessive trait or gene (b).

The mother’s has bb (if she had a B, she would have brown eyes, since brown is dominant). The father could have Bb ( a brown and a blue gene) or BB (two brown genes). If he has BB, he can only contribute a B and the child has brown eyes. If however, he has Bb, then he could contribute a B (child has brown eyes) or a b (child would then have a b from the father and a b from the mother = bb) and the child would have blue eyes.

Different forms of a gene are called alleles. So pea plants have two different alleles for height – tall and short. One is dominant and one is recessive. Dominant means that if you have that gene, that is the one that will appear – it doesn’t mean it’s a better gene.