GENES AND HEREDITY

1. GENES AND HEREDITY Mendelian Genetics Part 1

2. Introduction • The passing of traits from parents to offspring is called heredity. • Your traits are a result of the interactions of the genes from both your parents (50% from mom, 50% from dad) • There are over 8 x106 combinations from the 23 chromosomes inherited from each parent. • Your makeup is one of over 6.4 x 1013 (64 trillion) combinations of your parents gametes.

3. Early Beliefs and Mendel • Early beliefs on heredity were flawed in many ways. • Gregor Mendel is known as the father of genetics because his experiments laid the foundation for the science of genetics. • Mendel was an Austrian monk (1822-1884) who worked with garden peas.

4. Mendel cont’d • Pea plants were ideal because they: 1) had many contrasting traits, 2) reproduced quickly and in large numbers, 3) self-fertilized as well as cross-fertilized. • Mendel crossed pure-breeding plants that always produced the same type of offspring with other pure-breeding plants. • He crossed traits like: tall plants x short plant, round seeds x wrinkled seeds, green seeds x yellow seeds and looked for a pattern in the offspring.

6. Mendel’s Experiments • Prior to Mendel it was believed that the crossing of traits would create a blend. • Mendel showed that this was not the case for when he crossed pure tall plants with pure short plants, all the offspring were tall. • Mendel showed that one trait always dominated over another. • He reasoned that things called “factors” control the traits of a plant.

7. Mendel’s Experiment cont’d • These factors were later called genes. • He also realized that there were alternate forms of genes called alleles. • Smooth (S) and wrinkled (s) are alleles for seed shape and the smooth is dominant while wrinkled is recessive. • Notice the upper case letter denotes the dominant allele.

8. Monohybrid Crosses • When pure smooth plants (SS) are crossed with pure wrinkled (ss) plants, all the first generation plants (F1) were smooth (Ss). • The next year Mendel crossed the F1 plants together to see if the wrinkled trait would reappear in the second generation (F2)

9. The 3:1 Ratio • Mendel recovered 7324 seeds, 5474 were smooth while 1850 were wrinkled. • The ratio 5474:1850 can be simplified to 3:1. • The chart to the right is called a punnett square and explains how these alleles could form offspring in a 3:1 ratio.

10. Summary of Mendel’s Work • Traits are controlled by genes which occur in pairs. Different forms of a gene are called alleles. • One allele can mask the expression of another which is called the principle of dominance (eg. S-smooth is dominant over s-wrinkled) • Therefore SS is smooth, Ss is also smooth, while only ss is wrinkled. • Pairs of alleles separate during sex cell formation so that each sex cell (sperm or egg) only has one member of each pair -- law of segregation.

11. Meiosis Supports Mendel • 25 years after Mendel’s work, the study of meiosis in cells explains how genes segregate into sex cells or gametes. • A hybrid parent Ss will produce 50% S gametes and 50% s gametes. • Now mathematics can be applied in biology to solve heredity problems ---- Mendelian genetics!

12. Review Why did Mendel choose pea plants? • Pea plants have many contrasting traits, reproduce quickly with lots of offspring, and can be cross-fertilized. What are alleles? Give some examples. • Alleles are different versions of genes. E.g. Tall & short plants, brown & blue eyes

13. Review At what stage of meiosis do pairs of alleles separate? (Mendel’s law of segregation) • When homologous chromosomes separate during anaphase 1. If you are hybrid for freckles (Ff), what % of your gametes will have the dominant freckles allele (F) ? • 50%

14. Review What are the chances of a couple who are hybrid brown eyed (Bb x Bb) of producing a child with blue eyes (bb)? Bb x Bb B b B BB Bb b Bb bb there is a ¼ chance of producing blue-eyed child, or 25%