Mendel’s Laws Of Heredity

Gregor Mendel, an Austrian monk, carried out important studies of heredity. Characteristics that are inherited are called traits.Mendel was the first person to succeed in predicting how traits are transferred from one generation to the next.A complete explanation requires the careful study of genetics.

• Mendel worked with pea plants.• Pea plants reproduce sexually, which means

that they produce male and female sex cells, called gametes.

• When male gamete unites with female gamete it is called fertilization.

• Then the fertilized cell is called a zygote. Zygote then develops in to a seed in plants and an embryo in animals.

• The transfer of pollen grains from a male reproductive organ to a female reproductive organ in a plant is called pollination.

• Since pea plants have both organs they self-pollinate. (male and female gametes come from the same plant)

Mendel cross pollinated tall pea plants with short pea plants.

Mendel’s Monohybrid

• Mendel cross pollinated tall pea plants with short pea plants.

• Mendel referred to the offspring of this cross as hybrids.

• A hybrid is the offspring of parents that have different forms of traits, such as tall and short height.

Mendel’s monohybrid crosses

The first generation (F1)(P1) selected 6 ft pea plant and crossed

with less than 2 ft pea plant. All F1 offspring grew to be as tall as the taller parent. In this first generation, it was as if the shorter parent had never existed.

Mendel’s monohybrid crosses

The second generation (F2)Mendel allowed the tall plants in this first

generation to self pollinate. He planted the more than 1000 seeds.

Mendel found that 3/4 of the plants were as tall as the tall plans in the parent and first generations.

He also found that ¼ of the offspring were as short as the short plants in the parent generation.

The second generation (F2)In the second generation, tall and short

plants occurred in a ratio 3 tall plants to 1 short plant.

The rule of unit factors

Mendel concluded that each organism has two factors that control each of its traits. We know that these factors are genes and that they are located on chromosomes.

Genes exist in alternative forms. We call these different gene forms alleles.

Mendel’s pea plants had two alleles of the gene that determined its height.

The rule of unit factors

A plant could have 2 alleles for tallness, 2 alleles for shortness, or 1 allele for tallness and 1 allele for shortness.

An organism’s two alleles are located on different copies of a chromosome- one inherited from the female parent and one from the male parent.

The rule of dominance

In F1 generation the trait for tallness was shown.Mendel called the observed trait dominant and the trait that disappeared recessive. So in the case of the F1 generation, the allele for tall plants is dominant to the allele for the short plants.

The law of segregation

The law of segregation states that every individual has two alleles of each gene and when gametes are produced, each gamete receives one of these alleles.

Phenotypes and Genotypes

Phenotype- the way an organism looks and behaves.Genotype- the allele combination an organism contains.

phenotype

Genotype

Homozygous and heterozygous

Homozygous- two alleles for the trait are the same. (TT) homozygous dominant, (tt) homozygous recessive.Heterozygous- two alleles for the trait differ from each other. (Tt) heterozygous

Mendel’s dihybrid cross

Di means two. Dihybrid cross- a cross involving two different traits.

Law of independent assortment

Mendel’s second for different law states that genes for different traits-for example, seed shape and seed color- are inherited independently of each other. This conclusion is known as law of independent assortment.Genotype RrYy- the alleles R and r will separate from each other as well as from the alleles Y and y.

Punnett squares

Monohybrid

Punnett squares

Dihybrid