Mendel’s Laws

By the 1800s, people generally understood that offspring inherited traits from their parents. Initially, however, it was thought that the traits of both parents blended together in their offspring. Called the “blending hypothesis”, this theory explained inheritance like two different liquids mixing together. Gregor Mendel’s careful work with thousands of pea plants in the 1860 proved the blending hypothesis wrong and explained how inheritance really happens.

Mendel’s Experiments

Mendel studied several different traits of a pea plant. For example, some pea plants have purple flowers and others have white flowers. Pea plants can either self-fertilize or cross-fertilize. Crossing two plants is called hybridization. To start, however, Mendel needed plants that were true-breeding. This means that after generations of self-breeding, the pea plant expressed only one version of the trait. After many generations a purple flowered plant only produced plant never produced a white flowered plant.

Mendel then crossed a true-breeding purple flower plant and a true-breeding white flower plant. This is called a monohybrid experiment. A test cross is performed between two plants that breed true for one trait, and the resulting trait for each offspring plant is determined

The first, true-breeding generation, is called the parent, P generation. The first generation of offspring, the first filial generation, is the F₁ generation. Mendel found that these plants all had purple flowers. The F₁ generation was crossed with itself. The next generation, F₂ generation, had a 3:1 ration of purple to white flowers.

If the blending hypothesis was correct, the F₁ generation should have all had light purple flowers. Instead, all the F₁ plants had dark purple flowers. How did Mendel explain the complete loss of the white flower characteristic in the first generation and its reappearance in the second generation?

The Law of Segregation

Mendel explained what he saw using the law of segregation. Each gene can have different alleles. For example, one gene determines flower color in pea plants. Different versions of that same gene are alleles. The two different alleles here are purple flowers and white flowers. Each plant has two copies of each gene: one copy from each parent plant. The F₁ generation inherits a purple allele and a white allele. Even though the plant has one copy of each, we only see the trait from the purple flower gene. When a plant has one copy of each gene, the gene we see is the dominant allele. We indicate this with a capital letter for the allele, “P”. The gene that is masked by the dominant allele is recessive. Here the white allele is recessive, and we represent this using a lower case letter for the gene “p”.

The F₁ plants all have one purple allele (P) and one white allele (p). The genotype is the alleles for each gene in the plant. Here the genotype is Pp. A plant with two of the same alleles is homozygous. A plant with two different alleles is heterozygous. The characteristic that we see with our eyes is the phenotype. For Pp, the phenotype is purple flowers.

This inheritance model is explained using a Punnett square.

The law of segregation states that during gamete formation, the two genes each end up in different gametes. The gametes from each parent form a zygote, and the pairing of genes is random. The distribution of genes in the offspring is therefore dictated by probability. In a Punnett square, the alleles from one parent are written across the top and the genes from the other parent are written on the left side. Each box is filled with one allele from the top and one from the left. The ratio of genotypes for the offspring, is the ratio of genotypes from each of these boxes. The F1 generation is 100% Pp. The F2 generation is 25% PP (homozygous dominant, purple flowers), 50% Pp (heterozygous, purple flowers), and 25% pp (homozygous recessive, white flowers). This is how the white characteristic reappears in the second generation. The phenotype is 3:1, purple to white.

The Law of Independent Assortment

The law of independent assortment states that the alleles for two different genes sort independently into gametes. Crosses that examine two different traits are called dihybrid crosses. For example, round seeds (R) are dominant to wrinkled seeds (r), and yellow seeds (Y) are dominant to green seeds (y). For our test cross we have two true-breeding P generation plants: RRYY and rryy.

The F₁ generation has a genotype of RrYy and a round, yellow phenotype. The F₁ generation is then crossed with itself: RrYy x RrYy. The law of independent assortment means that the gametes have an equal chance of having RY or Ry. RY and ry do not have to sort together into gametes. Again, probability determines the genotype and phenotype for the F₂ generation.

The Punnett square for a dihybrid cross has the possibilities for each trait from both parents across the top or on the left side. Now we are looking at two traits instead of one. The F₂ generation now has a 9:3:3:1 ratio of round, yellow: round, green: wrinkled, yellow: wrinkled, green.

Mendel’s Laws Practice Quiz