Mendelian Genetics and Monohybrid Crosses

The progression of a Mendelian experiment follows a predictable series of generations:

1. P Generation (Parental): The initial cross between two different true-breeding varieties.
2. F1 Generation (First Filial): The immediate offspring of the P generation. In a monohybrid cross between two different true-breeding parents, these offspring are hybrids.
3. F2 Generation (Second Filial): The offspring produced when the F1 generation is allowed to self-fertilize for several generations.

Exam Tip: When analyzing a problem, always identify if you are looking at the F1 or F2 generation. In a standard monohybrid cross of true-breeding parents, the F1 generation will show 100% of the dominant phenotype, while the F2 generation typically shows a 3:1 phenotypic ratio.

Mendel's First Law: The Principle of Segregation

By observing the F2 generation, Mendel noticed something profound: traits that seemed to "disappear" in the F1 generation (because they were masked by a dominant trait) would reappear in the F2 generation. This proved that traits are controlled by discrete units—now known as genes—rather than blending fluids.

This led to Mendel's First Law, also known as the Principle of Segregation. This law states that the two alleles (versions of a gene) for a heritable character segregate (separate) from each other during the formation of gametes (sperm and egg). Consequently, an offspring receives one allele from each parent, resulting in two alleles for every gene.

To predict the outcome of these crosses, scientists use a Punnett Square, a diagram that maps out the possible combinations of alleles from the parents to the offspring.

Worked Example: The 3:1 Ratio

Imagine crossing two heterozygous pea plants ($Yy \times Yy$), where $Y$ is the allele for yellow seeds and $y$ is the allele for green seeds.

The Punnett Square would look like this:

Results:

• Genotypes: 25% $YY$, 50% $Yy$, 25% $yy$ (a $1:2:1$ ratio).
• Phenotypes: 75% Yellow, 25% Green (a $3:1$ ratio).

Determining Genotypes: The Testcross

Sometimes, an organism displays a dominant phenotype, but we do not know if it is homozygous (two identical alleles, e.g., $BB$) or heterozygous (two different alleles, e.g., $Bb$). To solve this, we use a testcross.

A testcross involves breeding the individual of unknown genotype with an organism that is homozygous recessive ($bb$).

• If the unknown parent is $BB$, all offspring will be $Bb$ (showing the dominant phenotype).
• If the unknown parent is $Bb$, approximately 50% of the offspring will be $bb$ (showing the recessive phenotype).

By observing the phenotypes of the offspring, we can work backward to determine the genotype of the parent.