Mendelian Patterns of Inheritance

1. Mendelian Patterns of Inheritance Mader: Biology 8th Ed.

2. Outline • Blending Inheritance • Monohybrid Cross • Law of Segregation • Modern Genetics • Genotype vs. Phenotype • Punnett Square • Dihybrid Cross • Law of Independent Assortment Mader: Biology 8th Ed.

3. Gregor Mendel • Austrian monk who formulated fundamental laws of heredity in early 1860s. • Studied science and mathematics at University of Vienna. • Conducted breeding experiments with the garden pea Pisum sativum. • Carefully gathered and documented mathematical data from his experiments. Mader: Biology 8th Ed.

4. Blending Inheritance • At the time, most breeders believed parents of contrasting appearance always produce offspring of intermediate appearance. • Mendel’s experiments helped him formulate the particulate theory of inheritance. • Inheritance involves reshuffling of genes from generation to generation. Mader: Biology 8th Ed.

5. Garden Peas Mader: Biology 8th Ed.

6. One-Trait Inheritance • Mendel performed cross-breeding experiments between true-breeding plants. • Chose varieties that differed in only one trait (monohybrid cross) • Performed reciprocal crosses • Parental generation = P • First generation offspring = F1 • Second generation offspring = F2 • Formulated law of segregation Mader: Biology 8th Ed.

7. Mendel’s Monohybrid Cross Mader: Biology 8th Ed.

8. Mendel’s Monohybrid Cross Mader: Biology 8th Ed.

9. Law of Segregation • Each individual has two factors for each trait. • The factors segregate during gamete formation. • Each gamete contains only one factor from each pair of factors. • Fertilization gives each new individual two factors for each trait. Mader: Biology 8th Ed.

10. Modern Genetics View • Each trait in a pea plant is controlled by two alleles (alternate forms of a gene). • Dominant allele (capital letter) masks the expression of the recessive allele (lower-case). • Alleles occur on a homologous pair of chromosomes at a particular gene locus. • Homozygous = identical alleles • Heterozygous = different alleles Mader: Biology 8th Ed.

11. Homologous Chromosomes Mader: Biology 8th Ed.

12. Genotype Versus Phenotype • Genotype • Refers to the alleles an individual receives at fertilization. • Phenotype • Refers to the physical appearance of the individual. Mader: Biology 8th Ed.

13. Punnett Square • All possible kinds of sperm are lined up on one axis, and all possible kinds of eggs are lined up on the other axis. • Every possible combination of alleles is placed within the squares. Mader: Biology 8th Ed.

14. PUNNETT SQUARE Mader: Biology 8th Ed.

15. Testcross • A monohybrid testcross is used to determine if an individual with the dominant phenotype is homozygous dominant or heterozygous for a particular trait. • The individual with the dominant trait in question is crossed with a homozygous recessive Mader: Biology 8th Ed.

16. Testcross Mader: Biology 8th Ed.

17. Two-Trait Inheritance • Mendel performed cross using true-breeding plants differing in two traits. • Dihybrid Cross • Observed phenotypes among F2 plants. • Formulated law of independent assortment Mader: Biology 8th Ed.

18. Law of Independent Assortment • Each pair of factors segregates independently of the other pairs. • All possible combinations of factors can occur in the gametes. Mader: Biology 8th Ed.

19. Dihybrid Cross Mader: Biology 8th Ed.

20. Dihybrid Cross Mader: Biology 8th Ed.

21. Two-Trait Testcross • A two-trait testcross is used to determine if an individual is homozygous dominant or heterozygous for either of the two traits. Mader: Biology 8th Ed.

22. Two-Trait Testcross Mader: Biology 8th Ed.

23. Incomplete Dominance • Exhibited when the heterozygote has an intermediate phenotype between that of either homozygote. Mader: Biology 8th Ed.

24. Incomplete Dominance Mader: Biology 8th Ed.

25. Polygenic Inheritance • Occurs when a trait is governed by two or more sets of alleles. • Each dominant allele has a quantitative effect on the phenotype, and these effects are additive. • Result in continuous variation of phenotypes. Mader: Biology 8th Ed.

26. Height in Human Beings Mader: Biology 8th Ed.

27. Terminology • Pleiotropy - A gene that affects more than one characteristic of an individual. • Sickle-cell anemia (incomplete dominance) • Codominance - More than one allele is fully expressed. • ABO blood type (multiple allelic traits) • Epistasis - A gene at one locus interferes with the expression of a gene at a different locus. • Human skin color (polygenic inheritance) Mader: Biology 8th Ed.

28. Environment and Phenotype • Himalayan Rabbits - Enzyme coding for black fur is active only at low temperatures. • Black fur only occurs on extremities. Mader: Biology 8th Ed.

29. Review • Blending Inheritance • Monohybrid Cross • Law of Segregation • Modern Genetics • Genotype vs. Phenotype • Punnett Square • Dihybrid Cross • Law of Independent Assortment Mader: Biology 8th Ed.

30. Mader: Biology 8th Ed.