Genetics

1. Genetics Feb. 23, 2010

2. 10.1 Section Objectives – page 253 Section Objectives: • Relate Mendel’s two laws to the results he obtained in his experiments with garden peas. • Predict the possible offspring of a genetic cross by using a Punnett square.

3. Section 10.1 Summary – pages 253-262 Why Mendel Succeeded • It was not until the mid-nineteenth century that Gregor Mendel, an Austrian monk, carried out important studies of heredity—the passing on of characteristics from parents to offspring. • Characteristics that are inherited are called traits.

4. Section 10.1 Summary – pages 253-262 Why Mendel Succeeded • 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—the branch of biology that studies heredity.

5. Section 10.1 Summary – pages 253-262 Mendel chose his subject carefully • Mendel chose to study traits in pea plants. • When he wanted to breed, or cross, one plant with another, Mendel opened the petals of a flower and removed the male organs.

6. Section 10.1 Summary – pages 253-262 Mendel chose his subject carefully • He then dusted the female organ with pollen from the plant he wished to cross it with. Pollen grains Transfer pollen Female part Male parts Cross-pollination

7. Section 10.1 Summary – pages 253-262 Mendel chose his subject carefully • This process is called cross-pollination. • By using this technique, Mendel could be sure of the parents in his cross.

8. Section 10.1 Summary – pages 253-262 The first generation • Mendel selected a six-foot-tall pea plant that came from a population of pea plants, all of which were over six feet tall. • He cross-pollinated this tall pea plant with pollen from a short pea plant. These two plants are called the P generation for “parent”. • All of the offspring from this cross were tall and are referred to as the F1 generation.

9. Section 10.1 Summary – pages 253-262 The second generation • Mendel allowed the tall plants in this first generation to self-pollinate. • After the seeds formed, he planted them and counted more than 1000 plants in this second generation. • 75% of the plants were as tall as the tall plants in the parent and first generations. • 25% of the plants were short. • This is the F2 generation.

10. Section 10.1 Summary – pages 253-262 Mendel studied one trait at a time. • Mendel studied short vs. tall in pea plants. • Plants receive one allele for height from the mother plant and one from the father plant. An allele is a piece of a gene. • T = tall and t = short • So. . . TT = tall, tt = short • What about Tt?

11. Mendel studied one trait at a time. • Plants that inherit tallness from both parents are homozygous for that trait (TT). • Plants that inherit shortness from both parents are homozygous for that trait (tt). • Parents that inherit a short and tall form of the trait are hybrids and are called heterozygous(Tt).

12. Section 10.1 Summary – pages 253-262 The rule of dominance • Mendel called the observed trait dominant and the trait that disappeared recessive. • Mendel concluded that the allele for tall plants is dominant to the allele for short plants.

13. Section 10.1 Summary – pages 253-262 The rule of dominance • An uppercase letter is used for the dominant allele and a lowercase letter for the recessive allele. Short plant Tall plant t t T T t T F1 • The dominant allele is always written first. All tall plants t T

14. Section 10.1 Summary – pages 253-262 The rule of unit factors • Mendel concluded that each organism has two alleles that control each of its traits. • We now know that these alleles are parts of genes and that they are located on chromosomes.

15. Section 10.1 Summary – pages 253-262 The rule of unit factors • 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.

16. Section 10.1 Summary – pages 253-262 Traits in Pea Plants Seed shape Flower color Pod color Seed color Flower position Pod shape Plant height Dominant trait axial (side) purple yellow round green tall inflated Recessive trait terminal (tips) green short white yellow wrinkled constricted

17. Section 10.1 Summary – pages 253-262 Phenotypes and Genotypes • The way an organism looks and behaves is called its phenotype. • The allele combination an organism contains is known as its genotype. • An organism’s genotype can’t always be known by its phenotype.

18. Section 10.1 Summary – pages 253-262 Punnett Squares • If you know the genotypes of the parents, you can use a Punnett square to predict the possible genotypes of their offspring.

19. Section 10.1 Summary – pages 253-262 Monohybrid crosses • A Punnett square for this cross is two boxes tall and two boxes wide because each parent can produce two kinds of gametes for this trait. Heterozygous tall parent T t T t T t T T TT Tt t t Tt tt T t Heterozygous tall parent

20. Section 10.1 Summary – pages 253-262 Monohybrid crosses • The two kinds of gametes from one parent are listed on top of the square, and the two kinds of gametes from the other parent are listed on the left side. Heterozygous tall parent T t T t T t T T TT Tt t t Tt tt T t Heterozygous tall parent