Genetics

What is Genetics? Genetics is the study of heredity. Heredity is the passing of characteristics from parents to offspring (children). Traits are inherited characteristics. Traits are controlled by genes found on chromosomes.

Who is Mendel? Gregor Mendel is called the father of genetics. He was the first person to successfully predict how traits are passed on from one generation to the next. Mendel did the majority of his experiments with pea plants.

Why Pea Plants? Pea plants reproduce sexually, producing male and female gametes (sex cells). Mendel could control breeding (called crossing) of plants. Mendel removed the male organs. He then dusted the female organ with pollen (male gamete) from a chosen plant.

Using this process, called cross pollination, Mendel could be control the parents in the cross.

What did Mendel study? He studied only one trait at a time to control variables. For example flower color, seed shape, or plant height. He selected plants that were considered true breeding for the particular traits. The pea plants were from populations of plants that the particular trait many generations and had always produced that trait.

What were Mendel’s experiments? He performed monohybrid crosses, crossing two plants that differed from each other by a single trait, such as height. He cross-pollinated a tall plant and a short plant and the results were all tall plants for the first generation. Then he allowed the offspring to self-pollinate and the results were ¾ of the plants were tall and ¼ were short in the second generation.

The original true breeding parents are called the P1-generation. The offspring of the parent plants are called the F1 generation. The offspring of the F1 plants are called the F2 generation. P1 Short pea plant Tall pea plant F1 All tall pea plants F2 3 tall : 1 short

What did Mendel find? He did many monohybrid crosses with different traits following the same procedure. In every case, he found that one trait of a pair seemed to disappear in the F1 generation, only to reappear unchanged in one-fourth of the F2 plants. He called the observed trait dominant and the trait that disappeared recessive.

Traits in pea plants Recessive trait Dominant Seed shape color Flower position Pod Plant height round yellow purple axial (side) green inflated wrinkled white terminal (tips) constricted short

What did Mendel conclude? Mendel concluded that each organism has two factors that control each of its traits. These factors are now know to be genes and that they are located on chromosomes. Genes exist in alternative forms. Each alternate form is called an allele. An organism’s two alleles are located on different chromosomes, one inherited from the female parent and one from the male parent.

What are Mendel’s Laws? The Law of Dominance Dominant traits mask recessive traits and are represented with a capital letter For example T= tall Recessive traits are hidden by dominant traits and are represented by the same letter as the dominant, only lower case. For example t=short.

The Law of Independent Assortment The Law of Segregation Every individual has two alleles of each gene and when gametes are produced, each gamete receives one of these alleles. During fertilization, these gametes randomly pair to produce four combinations of alleles. The Law of Independent Assortment Genes for different traits are inherited independently; they are not linked. For example a pea plant can be tall with purple or white flowers.

Other important terms Genotype Genetic makeup of an organism or alleles. For example Tt in pea plants. Phenotype The physical characteristics or traits of an organism determined by the genotype. For example tall in pea plants.

Heterozygous An organism with two different alleles for a trait, also called hybrid. Tt (a capital T and a lower case t) Homozygous An organism with two of the same alleles for a trait, also called pure. Homozygous Dominant T T (two capital T’s) Homozygous Recessive tt (two lower case t’s)