Mendel's Model

In the 1860s, Gregor Mendel developed a particulate theory of inheritance based on his experiments with garden peas. Today, we call the hereditary units genes.


1. By breeding garden peas, Mendel demonstrated that parents pass on to their offspring discrete genes that retain their identity generation after generation.

2. Mendel's success is attributed to his quantitative approach and his choice of organism. He worked with seven pea characters, each of which occurred in two distinct, alternative forms, His peas were true-breeding, and he was able to control mating.

3. By producing hybrid offspring and allowing them to self-pollinate, Mendel arrived at the principle of segregation. The hybrids (F1) exhibited the dominant trait. In the next generation (F2), 75% of offspring had the dominant trait and 25% had the recessive trait, for a 3:1 ratio.

4. To account for these results, Mendel postulated that genes have alternative forms (now called alleles) and that each organism inherits one allele for each gene from each parent. These separate (segregate) from each other during gamete formation (meiosis), so that a sperm or an egg carries only one allele per character. After fertilization, if the two alleles of the pair are different, one (the dominant allele) is fully expressed in the offspring and the other (the recessive allele) is completely masked (Mendel's principle of dominance) Today we know that the genes are in chromosomes. We use the term locus to refer to the site a gene occupies in the chromosome. Alleles occupy corresponding loci on homologous chromosomes.

5. Homozygous individuals have two identical alleles for a given character and are true-breeding. Heterozygous individuals have two different alleles for a given character.

6. The genotype of an organism showing a dominant trait can be determined by breeding it to a recessive homozygote in a testcross. For example, researchers usually test cross the progeny from an F1 generation (Aa or AA) with a homozygous recessive individual (aa).

7. The principle of segregation operates according to the rules of probability. According to the rule of multiplication, the probability of a compound event is equal to the product of the separate probabilities of the independent single events. The rule of addition states that the probability of an event that can occur in two or more independent ways is the sum of the separate probabilities.

8. Mendel proposed the principle of independent assortment based on dihybrid crosses between plants contrasting in two or more characters, such as flower color and seed shape. Alleles for each character segregate into gametes independently of alleles for other characters. F1 progeny are all heterozygous (AaBa), and F2 progeny will exhibit a 9,3,3,1 genotype pattern)