3F – research and describe the history of biology and contributions of scientists

How Genetics Began

Gregor Mendel, an Austrian monk,lived in the 1800s. He experimented with pea plants in the monastery gardens.Pea plants usually reproduce by self-fertilization. This means that the female gamete is fertilized by a male gamete in the same flower.

  • What is a gamete? ______Give examples of gametes. ______
  • What does fertilization mean? ______
  • Describe self-fertilization in your own words. ______

Mendel discovered a way to cross-pollinate peas by hand. He removed the male gametes from a flower. He then fertilized the flower with the male gamete from a different flower.Through these experiments, Mendel made several hypotheses about how traits are inherited. Examples of a trait could be eye color and leaf shape. In 1866, he published his findings. That year marks the beginning of the science of genetics, the science of heredity. Mendel is called the father of genetics.

  • What does genetics study? _____

The Inheritance of Traits

Mendel used true-breeding pea plants—plants whose traits stayed the same from generation to generation. Mendel studied seven traits—flower color, seed color, seed pod color, seed shape, seed pod shape, stem length, and flower position.

What did Mendel find when he crossed pea plants with different traits?

Mendel called the original plants the parent, or P, generation. The offspring were called the F1 generation. The offspring of the F1 plants were called the F2 generation. In one experiment, Mendel crossed yellow-seeded and green seeded plants. All the F1 offspring had yellow seeds. The green seed trait seemed to disappear. Mendel allowed the F1 plants to self-fertilize. He planted thousands of seeds from these plants. He saw that in these offspring, the F2 generation, three-fourths of the plants had yellow seeds and one-fourth had green seeds, a 3:1 ratio. The green seed plants reappeared in the F2.

  • What experiment did Mendel perform? ______
  • What did the F1 generation look like? ______
  • Could you provide a possible explanation for why there were no green-seeded plants in the F1? ______
  • What did the F2 generation look like after the F1 generation was allowed to self-fertilize? _____

Mendel performed similar experiments for other six

traits. Each time, he observed the same 3:1 ratio.

It was obvious that some regular mechanism was

Operating here.

How did Mendel explain his results?

Law of Dominance

Mendel concluded that there must be two forms of the seed trait in the pea plants – yellow-seed and green-seed – and that each was controlled by a factor, which now called an allele. An allele is alternative form or a version of a single gene passed from generation to generation. Therefore, the gene for yellow seeds and the gene for green seeds are each different forms of a single gene. Mendel concluded that the 3:1 ratio observed during his experiments could be explained if the alleles were paired in each of the plants. He called the form of the trait that appeared in the F1 generation DOMINANT and the form of the trait that was masked in the F1 generation recessive.The yellow seed was the dominant form (Y) of the trait and the green seed was the recessive form (y) of the trait.

The Law of Dominance states that when a dominant and recessive allele pair up in one trait, only the dominant trait shows in phenotype. When Mendel allowed the F1 generation to self-fertilize, he showed that recessive allele for green seeds (y) had not disappeared but was masked by a DOMINANT allele (Y).

  • Restate the Law of Dominance in your own words. ______

Law of Segregation

Mendel used homozygous yellow-seed (YY) and green-seed (yy) plants in his P cross. Recall that the chromosome number is divided in half during meiosis. The resulting gametes contain only one of the pair of seed-color alleles. He law of segregation states that the two alleles for each trait separate during meiosis. During fertilization, two alleles for that trait unite.

  • What does the Law of Segregation state?______

During gamete formation in the YY or yy plant, the two alleles separate, resulting in Y or y in the gametes. Gametes from each parent unite during fertilization.

Law of Independent Assortment

Mendel allowed F1 pea plants with the genotype YyRr to self fertilize (R – round seed; r – wrinkled seed). Mendel calculated the genotypic and phenotypic ratios of the offspring in the F1 and F2generations. From these results, he developed the law of independent assortment, which states that a random distribution of alleles occurs during gamete formation (meiosis). There is an equal chance that each pair of alleles (Yy and Rr) can randomly combine with each other.

  • Restate the Law of Ind. Assortment in your own words. ______