Mendel’s GeneticsBiology: The Living Environment

Ms. Romano

Why Do Offspring Resemble Their Parents?

• Family resemblances may be inherited from one generation to another.

• Heredity is the study of how characteristics are passed from parents to offspring.

• In the late 1800’s, an Austrian monk named Gregor Mendel performed important studies explaining how physical characteristics are inherited.

The Origins of Genetics

• Genetics is the branch of biology that studies heredity.

• Characteristics that are inherited are called traits.

• Gregor Mendel was the first person to successfully predict how traits are transferred from one generation to the next.

Experiments with Garden Pea Plants

• Mendel used garden pea plants in his experiments, because they reproduce sexually. Pea plants have distinct male and female sex cells called gametes. Male and female gametes are in the same flower.

Fertilization

• The male gamete is in the pollen grain and the female gamete is in the ovule.

• Fertilization is the uniting of male and female gametes, when the pollen grain fuses with the female gamete in the ovule.

• The transfer of male pollen grains to the female organ of a flower is pollination.

• The fertilized ovule matures into a seed.

Mendel’s Experiments

• Mendel transferred pollen from one pea plant to another plant with different traits. This is called making a cross.

• Mendel studied only one trait at at time to control variables. This is called a monohybrid cross. He chose plants that had a stable characteristic for generations. Such plants are said to be true bred for the trait.

How Do You Cross Plants?

• He removed the male parts (containing pollen). He dusted the female part (pistil) with pollen from the plant he wished to cross it with, so he could be sure of the parents in his cross.

Mendel’s Results

• Mendel crossed-pollinated plants (parents) that either produced only yellow or only green peas. The first offspring generation (F1) always had yellow peas. The following generation (F2) always had a 3:1 ratio of yellow to green peas.

Monohybrid Cross

• Mendel did similar monohybrid crosses for other pairs of traits. In every case, one trait seemed to “disappear” in the F1 generation.

The Rule of Unit Factors

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

• Genes exist in different forms called alleles. Alleles are located on different copies of a chromosome, one inherited from each parent.

The Rule of Dominance

• When only one trait is observed during a cross, the observed trait is called the dominant trait and the one that disappeared is called recessive.

• In Mendel’s experiment, the allele for yellow color is dominant to the allele for green color peas.

• Pea plants with two alleles for yellowness were yellow. Peas with one allele for yellow color and one for green color were also yellow because yellow is the dominant allele.

• Peas with two alleles for green color are green. Green is a recessive trait.

The Rule of Dominance

• When recording the results of crosses, it is customary to use the same letter for different alleles of the same gene.

• The uppercase letter is used for the dominant allele, and a lowercase letter for the recessive allele.

• The dominant allele is always written first.• Example: T= allele for tallness and t=allele for

shortness.

The Law of Segregation• Mendel concluded that

the two alleles for each trait must separate when gametes are formed. A parent passes on, at random, only one allele for each trait to each offspring.

• This is called the law of segregation and is the first of Mendel’s two laws of heredity.