Monohybrid Crosses

Monohybrid Crosses and Mendels Theory of Segregation

Mendel did a number of experiments crossing pea plants with different characteristics. The standard terminology in crosses is to use the letter P to designate the parent generation, F₁ (F stands for filial) to designate the First-generation offspring, and F₂ to designate the second generation offspring.

Mendel's first experiments were monohybrid crosses. Monohybrid crosses have two parents that are true-breeding for contrasting forms of a trait.

One form of the trait disappears in the first generation (F₁), only to show up in the second generation.

We now know that all members of the first generation are heterozygous because one parent could produce only an A gamete and the other could produce only an a gamete.

Results of the F₂ generation required mathematical analysis. The numerical ratios of crosses suggested that genes do not blend. For example, the F₂ offspring showed a 3:1 phenotypic ratio. for each trait that was individually tested.

Mendel assumed that each sperm has an equal probability of fertilizing an egg. This can be seen most easily by using a Punnett square. Thus, each new plant has three chances in four of having at least one dominant allele.

The Mendelian theory of segregation states that diploid organisms inherit two genes per trait, and each gene segregates from the other during meiosis such that each gamete will receive only one gene per trait.

Testcrosses

Mendel found that pea plants expressing a recessive trait were pure-breeding. In a testcross, an individual that shows a dominant phenotype is crossed with an individual with a recessive phenotype. Examining the offspring allows you to determine the genotype of the dominant individual.

To support his concept of segregation, Mendel crossed heterozygous F₁ plants with homozygous recessive individuals. A 1:1 ratio of recessive and dominant phenotypes supported his hypothesis.

This animation (Audio - Important) describes monohybrid crosses.