Unit 5: Genetics—The Science of Heredity

Unit 5: Genetics—The Science of Heredity

Heredity is the passing of physical characteristics from parents to offspring. Gregor Mendel was curious about the different forms of characteristics, or traits, of pea plants. Mendel's work was the foundation of genetics, the scientific study of heredity.

A new organism forms when the egg and sperm join in fertilization. Before fertilization can happen in pea plants, pollen must reach the pistil of a pea flower through pollination. Pea plants are usually self-pollinating, meaning pollen from a flower lands on the pistil of the same flower. Mendel developed a method by which he cross-pollinated, or 'crossed,' pea plants.

Mendel crossed two pea plants that differed in height. He crossed purebred tall plants with purebred short plants. These parent plants, the P generation, were purebred because they always produced offspring with the same traits as the parent. In all of Mendel's crosses, only one form of the trait appeared in the F1 generation. However, in the F2 generation, the 'lost' form of the trait always reappeared in about one-fourth of the plants. From his results, Mendel reasoned that individual factors, one from each parent, control the inheritance of traits. Today, scientists call the factors that control traits genes. The different forms of a gene are called alleles.

The organism inherits from its parents, who control an organism's traits. Some alleles are dominant, while other alleles are recessive. A dominant allele is one whose trait always appears in the organism when the allele is present. A recessive allele is hidden whenever the dominant allele is present. A trait controlled by a recessive allele will only show up if the organism does not have the dominant allele.

The purebred tall plant has two alleles of Mendel's stems in Mendel's cross. The purebred short plant has two alleles for short stems. The F1 plants are all hybrids: they have two different alleles for the traits allele for tall stems and one for short stems. Geneticists use a capital letter to represent a dominant allele and a lowercase version of the same letter for the recessive allele.

Mendel's discovery was not recognized during his lifetime. Mendel's three different scientists rediscovered Mendel's work. Because of his work, Mendel is often called Mendel'sher of Genetics.