1. Mendel's Dihybrid Crosses
1.1. The first generation
1.2. The second generation
1.3. The law of independent assortment
2. Punnett Squares
2.1. Monohybrid Crosses
2.2. Dihybrid Crosses
3. Probability
4. Why Mendel Succeeded
4.1. Mendel chose his subject carefully
4.1.1. Mendel used pea plants because they produced both male and female gametes (sex cells) within the same plant.
4.1.2. When using cross pollination (mixing of pollen from one plant - male gamete - with the pistil of another pea plant - female gamete), Mendel could be sure which parents were involved in his crosses in order to observe the phenotypes of the offspring.
4.2. Mendel was a careful researcher
4.2.1. Mendel was careful in controlling variables in his experiment; for example, he would only observe one trait at a time to control variables.
5. Phenotypes and Genotypes
5.1. Phenotypes:
5.2. Genotypes:
6. Mendel's Monohybrid Crosses
6.1. The first generation
6.1.1. When Mendel crossed a purebred tall pea plant with a purebred short pea plant, he found that all of the offspring in the first generation (F1 generation) all displayed the tall phenotype!
6.2. The second generation
6.2.1. Mendel allowed the tall plants in the F1 generation to self pollinate. He found that out of 1000 pea plants in the next generation (F2 generation), 3/4 of them were tall and 1/4 of them were short. The short trait reappeared out of no where!
6.2.2. Mendel continued these procedures observing 7 different traits, and he noticed the same trend among generations for all the other traits; one trait seemed to disappear in the F1 generation only to reappear in a small fraction of the F2 generation.
6.3. The rule of unit factors
6.3.1. Mendel concluded that each organism has 2 factors that control its traits. While he didn't realize what they were at the time, we now that each organism has 23 sets of 2 chromosomes that house our genes (alleles) that determine our characteristics. During sexual reproduction, one of each set of chromosomes is inherited from the dad and the other from mom.
6.4. The rule of dominance
6.4.1. Mendel concluded that one of these unit factors (alleles) must trump the other in order for a trait to be suppressed in an entire generation. He called the observed trait dominant and the trait that disappeared recessive.
6.5. The law of segregation
6.5.1. The first of Mendel's 2 laws of heredity, Mendel formed this law when he concluded that the possible allele combinations (genotypes) a generation of offspring receives is completely dependent on the alleles possessed by the parents. For example, the plants in the F2 population had one "tall" allele and one "short" allele, which meant they could produce two types of gametes; one carrying the tall allele and one carrying the short. During fertilization, the male/female gametes combine to form one of 4 possible combinations of alleles.