Genetics - A beginning
Genetics can be divided into two broad areas of study
1. Mendelian (modern) - the study of patterns of inheritance; predictions; pedigree charts
2. Molecular - the study of the activity of DNA; how genes work; cell cycle; protein synthesis
When looking at differences, remember that between species, we speak of diversity. Within a species, we speak of variation.
Who was Gregor Mendel?
Mendel is considered to be the father of modern genetics; although his studies were largely ignored for more than thirty years after they were published - long after his death. He was an Austrian monk and teacher in the mid 19th century. His most famous work was based on his studies of the inheritance of characteristics in the garden pea plant, Pisum sativum.
Seven characters were traced in the pea plant for seven years with thousands of plants being identified and scored. Mendel discovered a numerical ratio among the inheritance of different varieties of characters which he had named "traits".
His methods were remarkable in their adherence to scientific principles and a model of patience. He took several seasons to develop true-breeding plants. A true-breeding plant will pass its traits to all its offspring without failure. When Mendel had developed true-breeding plants, he crossed those with opposing traits - in one test, he crossed a true-breeding plant with purple flowers with a true-breeding plant carrying white flowers. For this cross, he carefully isolated the flowers and transferred the pollen of one to the pistil of another. This meant that Mendel knew exactly which "factors" had participated in the fertilization. He named the plants involved the P, or parental, generation.
When the seeds from this cross were cultivated, Mendel scored this generation - named the filial or F1 generation - for traits. He noticed two things.
1. The traits had not blended. The purple and white had not run together to make mauve.
2. All the F1 plants were as purple as the parent. No white flowers were present in this generation.
Mendel then allowed the F1 generation to freely interbreed (cross-pollinate). When he scored their offspring - the F2 generation - he noticed that the white flower trait had reappeared and the ratios of purple to white were invariable: 3:1. This ratio was seen when Mendel looked at other characters as well. Seven characters were examined in the pea plant.
Because the purple had somehow masked the expression of the white in the F1 generation, Mendel coined the term "dominant" for the trait, and others like it, and the term "recessive" for the white flower trait and others like it.
Mendel's Laws
From his observations, Mendel developed two laws of inheritance.
1. The Law of Segregation. Factors carrying different traits segregate during gamete formation. The paired condition is restored by the random fusion of gametes at fertilization.
2. The Law of Independent Assortment. Allele pairs of any character will assort independently of allele pairs of any other character.
Magic Six and Probabilities
Magic Six (in class): the six cases of inheritance of one character with a dominant and recessive trait.
The probability that independent events will occur is the product of their individual probabilites. The key is the use of the word "AND".
The probability that an event can occur in two or more independent ways is the sum of the separate probabilities of the different ways. The key is the use of the word "OR".
Take home: AND means multiply. OR means add.
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