Mendel and Heredity

1. Mendel and Heredity Biology Ch. 8 Ms. Haut

2. Pre-Mendelian Theory of Heredity • Blending Theory—hereditary material from each parent mixes in the offspring • Individuals of a population should reach a uniform appearance after many generations • Once traits are blended, they can no longer be separated out to appear in later generations

3. Pre-Mendelian Theory of Heredity • Problems—inconsistent with observations: • Individuals of a population don’t reach uniform appearance • Traits can skip generations

4. Modern Theory of Heredity • Based on Gregor Mendel’s fundamental principles of heredity • Parents pass on discrete inheritable factors (genes) to their offspring • These factors remain as separate factors from one generation to the next

5. Useful Genetic Vocabulary • Homozygous—having 2 identical alleles for a given trait (PP or pp) • Heterozygous—having 2 different alleles for a trait (Pp); ½ gametes carry one allele (P) and ½ gametes carry the other allele (p) • Phenotype—an organism’s expressed traits (purple or white flowers) • Genotype—an organism’s genetic makeup (PP, Pp, or pp)

6. Developed true-breeding lines—populations that always produce offspring with the same traits as the parents when parents are self-fertilized Counted his results and kept statistical notes on experimental crosses Mendel’s Discoveries

7. Crosses Tracking One Characteristic: Flower Color

8. Ratio 3.15:1 3.14:1 3.01:1 2.96:1 2.95:1 2.82:1 2.84:1 x x x 3:1 x x x x

9. Purple Purple Purple PP (homozygous) 1 3 Pp (heterozygous) Pp (heterozygous) 2 White 1 pp (homozygous) 1 Genotype versus Phenotype Genotypic Ratio 1:2:1 Phenotypic Ratio 3:1

10. The Testcross • The cross of any individual to a homozygous recessive parent • Used to determine if the individual is homozygous dominant or heterozygous CAUTION: Must perform many, many crosses to be statistically significant

11. Mendel’s Principles of Heredity • First Law of Genetics: Law of Segregation • alternate forms of genes are responsible for variations in inherited traits • for each trait, an organism inherits 2 alleles, one from each parent • If 2 alleles differ, one is fully expressed (dominant allele); the other is completely masked (recessive allele) • 2 alleles for each trait segregate during gamete production

12. Mendel’s Principles of Heredity • Second Law of Genetics: Law of Independent Assortment • During gamete formation, the segregation of the alleles of one allelic pair is independent of the segregation of another allelic pair • Law discovered by following segregation of 2 genes

14. Dihybrid Cross

15. Mendelian Inheritance Reflects Rules of Probability • Rules of Multiplication: The probability that independent events will occur simultaneously is the product of their individual probabilities.

16. Mendelian Inheritance Reflects Rules of Probability • Question: In a Mendelian cross between pea plants that are heterozygous for flower color (Pp), what is the probability that the offspring will be homozygous recessive? Answer: • Probability that an egg from the F1 (Pp) will receive a p allele = ½ • Probability that a sperm from the F1 will receive a p allele = ½ • Overall probability that 2 recessive alleles will unite at fertilization: ½ x ½ = ¼

17. Mendelian Inheritance Reflects Rules of Probability Works for Dihybrid Crosses: • Question: For a dihybrid cross, YyRr x YyRr, what is the probability of an F2 plant having the genotype YYRR? Answer: • Probability that an egg from a YyRr parent will receive the Y and R alleles = ½ x ½ = ¼ • Probability that a sperm from a YyRr parent will receive the Y and R alleles = ½ x ½ = ¼ • Overall probability of an F2 plant having the genotype YYRR: ¼ x ¼ = 1/16

18. Mendelian Inheritance Reflects Rules of Probability • Rules of Addition: The probability of an event that can occur in two or more independent ways is the sum of the separate probabilities of the different ways.

19. Mendelian Inheritance Reflects Rules of Probability • Question: In a Mendelian cross between pea plants that are heterozygous for flower color (Pp), what is the probability that the offspring will being a heterozygote? Answer: • There are 2 ways in which a heterozygote may be produced: the dominant allele may be in the egg and the recessive allele in the sperm, or the dominant allele may be in the sperm and the recessive allele in the egg.

20. Mendelian Inheritance Reflects Rules of Probability • Probability that the dominant allele will be in the egg with the recessive in the sperm is ½ x ½ = ¼ • Probability that the dominant allele will be in the sperm with the recessive in the egg is ½ x ½ = ¼ • Therefore, the overall probability that a heterozygote offspring will be produced is ¼ + ¼ = ½

21. Pedigree Analysis • Analysis of existing populations • Studies inheritance of genes in humans • Useful when progeny data from several generations is limited • Useful when studying species with a long generation time

22. I II Symbols: = female = male = affected individual = mating = offspring in birth order I and II are generations = Identical twins = Fraternal twins

23. I II III Dominant Pedigree: • For dominant traits: • Affected individuals have at least one affected parent • The phenotype generally appears every generation • 2 unaffected parents only have unaffected offspring

24. I II III Recessive Pedigree: • For recessive traits: • Unaffected parents can have affected offspring • Affected progeny are both male and female

25. Recessive Human Disorders • Sickle-cell anemia; autosomal recessive • Caused by single amino acid substitution in hemoglobin • Abnormal hemoglobin packs together to form rods creating crescent- shaped cells • Reduces amount of oxygen hemoglobin can carry

26. Genetic Testing & Counseling • Genetic counselors can help determine probability of prospective parents passing on deleterious genes • Genetic screening for various known diseases alleles (gene markers)

27. GeneticTesting & Counseling • Fetal testing Amniocentesis • needle inserted into uterus and amniotic fluid extracted • Test for certain chemicals or proteins in the fluid that are diagnostic of certain diseases • Karyotype-can see chromosome abnormalities

28. Genetic Testing & Counseling • Fetal testing • Chorion Villus Sampling • Suctions off a small amount of fetal tissue from the chorionic villus of placenta • Karyotype-can see chromosome abnormalities

29. Ultrasound at 12 weeks --can see any physical abnormalities

30. Variations to Mendel’s First Law of Genetics • Incomplete dominance—pattern of inheritance in which one allele is not completely dominant over the other • Heterozygote has a phenotype that is intermediate between the phenotypes of the 2 homozygous dominant parent and homozygous recessive parent

31. Incomplete Dominance in Snapdragon Color F2 Genotypic ratio: 1 CRCR: 2 CRCW: 1 CWCW Phenotypic ratio: 1 red: 2 pink: 1 white

32. Variations to Mendel’s First Law of Genetics • Codominance—pattern of inheritance in which both alleles contribute to the phenotype of the heterozygote

33. Multiple Alleles • Some genes may have more than just 2 alternate forms of a gene. • Example: ABO blood groups • A and B refer to 2 genetically determined polysaccharides (A and B antigens) which are found on the surface of red blood cells (different from MN blood groups) • A and B are codominant; O is recessive to A and B

34. Multiple Alleles for the ABO Blood Groups 3 alleles: IA, IB, i

35. Polygenic Traits • Skin pigmentation in humans --3 genes with the dark-skin allele (A, B, C) contribute one “unit” of darkness to the phenotype. • These alleles are incompletely dominant over the other alleles (a, b, c) --An AABBCC person would be very dark; an aabbcc person would be very light --An AaBbCc person would have skin of an intermediate shade

36. Discovery of Sex-Linkage • Thomas Hunt Morgan provided convincing evidence for Chromosomal Theory of Inheritance • Experiments with Drosophila revealed sex-linkage traits. Why Drosophila? • Easily cultured • Prolific breeders • Short generation times • Only 4 pairs of chromosomes, visible under microscope

37. Chromosomal Basis of Sex Varies with Organism • Fruit flies (Drosophila), like Mammals use an X-Y system • XX = female, XY = male • Y chromosome has many fewer genes than X • chromosome • Y chromosome has SRY gene, determines • testes development

38. Sex-Linked Genes have Unique Patterns of Inheritance • For genes on X chromosomes, females have 2 copies of gene—can have 2 different alleles • For genes on X chromosomes, males have only one allele; the allele they express • Males’ X comes from mom (dad contributes Y) • Males are said to be hemizygous • If allele is recessive, it will be expressed • Genes on X chromosome are said to be sex-linked • Males are more likely to have disorders that are inherited as sex-linked recessives

39. Sex-Linked Recessive Trait

40. F1 Generation: All red eyes • F2 Generation: 3 red eyes: 1 white eyes • But, the recessive trait—white eyes—was linked to sex. • All females had red eyes • ½ males had red eyes and ½ had white eyes The gene responsible is located on the X chromosome