LAWS of PROBABILITY

Product rule:

The probability of the simultaneous occurrence of two independent events is equal to the product of their individual probabilities.

Summation rule:

The probability of the occurrence of any of a set of mutually exclusive events is equal to the sum of the probabilities of the events.

Geneticists often make controlled matings to infer the underlying mode of inheritance for traits. The individuals chosen for the original mating are referred to as the parental (P) generation. Their offspring are referred to as the F1 generation (first filial generation). If individuals from the F1 generation are crossed, their offspring represent the F2 generation (second filial generation).

The physical or physiologically measurable expression of the genes is referred to as the phenotype. The underlying message is called the genotype.

If an experimental cross is made to study a single phenotypic trait controlled by a single gene it is referred to as a monohybrid cross. Mendel was the first to correctly interpret the results of a monohybrid cross. Among the important things that Mendel's research indicated was that inheritance is particulate rather than blending. He also proposed the Law of Segregation which suggests that organisms have two copies of messages controlling each phenotypic trait and these copies separate from each other during gamete production such that half of the gametes produced have one message, half the other.

A diploid individual may have two identical messages for a gene (homozygous) or have two different messages (heterozygous).

Mendel's F2 results for a monohybrid cross that is initiated with two true breeding (homozygous) parents that differ in their phenotype are:

Phenotypic ratio- 3:1

Genotypic ratio- 1:2:1

These results apply only for genes that are on autosomal chromosomes and not sex chromosomes. Many species utilize a pair of chromosomes (the sex chromosomes) as gender determiners. In mammals, females contain two relatively large chromosomes referred to as the X chromosomes. Mammalian males have one X chromosome and one smaller Y chromosome. The X chromosome has many genes that are not contained on the Y chromosome. Inheritance of traits controlled by these genes is referred to as X-linked inheritance. The X and Y chromosomes behave as homologous chromosomes during meiosis.

Because mammalian females can only produce gametes that contain X chromosomes they are called homogametic; males can produce gametes having either X or Y chromosomes and are called heterogametic. In other taxa, females may be heterogametic and males homogametic (e.g., birds and butterflies).

X-linked inheritance requires that the gender identity of individuals be considered. When males are heterogametic, a phenotype often is expressed in a male and his grandsons but not in his sons. Another characteristic of X-linked inheritance is that there are different patterns shown from reciprocal crosses.

Modes of inheritance can not be studied in humans through controlled matings. Commonly, geneticists use pedigree analysis to infer the mode of inheritance in humans. Typically genetics students must use pedigree charts to infer the mode of inheritance of a condition and the risk that future offspring may be phenotypically affected.

DNA that is found in organelles (mitochondria) is not inherited in a Mendelian fashion, they are inherited maternally