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Interactives -- DNA

Genetics : More on Mendel's Discovery

In one of his many experiments, Mendel decided to cross a tall pea plant with a short one. He expected that the height of the offspring would be an average of the height of the parent plants. Instead, all of the offspring were tall.

Mendel got a bigger surprise when he crossed two of those offspring. He expected that breeding two tall plants would lead to only tall plants, but he instead got tall and short plants in a 3 to 1 ratio--three tall plants for every short one.

This result and the outcome of many other crosses led Mendel to a stunning realization: there were two copies of a gene for each of the seven traits. The genes come in two forms: dominant and recessive. The dominant gene (for example, the gene for tallness in garden peas) determines the trait that you will see in a plant, and the recessive gene (for shortness) will only produce an observable trait if the plant has two copies of it, with no dominant gene present.

Mendel also came up with two concepts explaining the patterns he saw. These are now known as Mendel's Laws of Heredity.

The Law of Segregation states that two genes in a pair separate from each other in sex cells, or gametes--the eggs and sperm--so that each sex cell will have only one gene for each trait. Therefore an egg will have either a gene for tallness or a gene for shortness, but not both, and a sperm will also carry only one or the other. One half of the sex cells will have one type of gene; the other half, the other version.

The Law of Independent Assortment states that the separation of the genes for one trait (for example, height) into two sex cells will occur independently of the separation of the gene pairs for other traits (for example, flower color). Therefore, the inheritance of one trait does not affect the inheritance of any other trait.


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