5.1 Mendel’s Work

Ch.5 – What were the results of Mendel’s experiments?

Heredity: passing of physical characteristics from parents to offspring

Trait: Each different form of a characteristic, such as stem height or seed color

Genetics: the scientific study of heredity

Mendel’s experiments

Mendel’s experiments

Flower’s petals surround the pistil and the stamens. The pistil produces female sex cells (eggs). The stamens produce pollen, which contains male sex cells (sperm).

A new organism begins to form when egg and sperm join in the process called fertilization.

Crossing Pea Plants

Mendel crossed two pea plants (cross-pollination): he removed pollen from a flower on one plant, then brushed the pollen onto a flower on a second plant.

Purebred organism is the offspring of many generations that have the same trait – EX. Purebred short pea plants always come from short parent plants.

The F1 Offspring

Parent generation : P generation Offspring : Filial (child) generation – F1 and

F2 and so on. 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 ¼ of the plants.

F1 and F2 generation

Dominant and Recessive Alleles

Mendel concluded that individual factors, or set of genetic “information” must control the inheritance of traits in the peas.

The factors that control each trait exist in pairs. The female parent contributes one factor, while the male parent contributes the other factor.

Genes and Alleles

Gene: factors that control a trait. Alleles: different forms of a gene. – EX. The

gene that controls stem height in peas has one allele for tall stems and one allele for short stems.

An organism’s traits are determined by the alleles it inherits from its parents. Some alleles are dominant, while others are recessive.

Alleles

Dominant allele’s trait always shows up and recessive allele is hidden whenever dominant allele is present.

Only pea plants that inherit two recessive alleles for short stems will be short.

Alleles in Mendel’s Crosses

Examples of Alleles

P - TT and tt

F1 – Tt and Tt

F2 – TT, Tt, Tt, and tt

Tall allele - Capital T,

Short allele – lowercase t

Significance of Mendel’s Contribution

Mendel’s work changed scientist’s ideas about heredity.

Before Mendel, people thought you can blend genetic information to produce new traits (ex. Red + white = Pink), but now we know that traits are determined by individual, separate alleles inherited from each parent.

If a trait is from a recessive allele, the trait can seem to disappear in the offspring.