Chapter 7: Genes and Inheritance

1. Chapter 7: Genes and Inheritance Family resemblance: how traits are inherited Lectures by Mark Manteuffel, St. Louis Community College

2. How can a single bad gene make you smell like a rotten fish? 7.1 Family resemblance: your mother and father contribute equally to your genetic makeup.

5. Take-home message 7.1 • Offspring resemble their parents because they inherit genes from their parents. • Genes are instruction sets for biochemical, physical, and behavioral traits.

6. 7.2 Some traits are controlled by a single gene. • Heredity • the passing of characteristics from parent to offspring through their genes

7. Selective Breeding: Observing Heredity

8. Single-Gene Traits Some traits are determined by the instructions an organism carries on one gene.

9. Are there multiple-gene traits too? • Yes • Let’s first examine the mechanism by which single-gene traits pass from parent to child • We’ll then expand this model of heritability

10. Take-home message 7.2 • Some traits are determined by instructions that an individual carries on a single gene, and these traits exhibit straightforward patterns of inheritance.

11. 7.3 Mendel learned about heredity by conducting experiments.

14. Take-home message 7.3 • In the mid-1800s, Gregor Mendel conducted studies that help us understand heredity. • He focused on easily observed and categorized traits in garden peas and applied methodical experimentation and rigorous hypothesis testing to determine how traits are inherited.

15. 7.4 Segregation: you’ve got two copies of each gene but put only one copy in each sperm or egg.

16. A dominant trait masks the effect of a recessive trait.

18. Three Ideas Mendel Used for Explaining This Pattern of Inheritance • Each parent puts into every sperm or egg it makes a single set of instructions for building the trait.

19. Three Ideas Mendel Used for Explaining This Pattern of Inheritance • Offspring receive two copies of the instructions for any trait (called alleles).

20. Three Ideas Mendel Used for Explaining This Pattern of Inheritance • The trait observed in an individual depends on the two copies of the gene it inherits from its parents. • homozygous and heterozygous

21. Take-home message 7.4 • Each parent puts a single set of instructions for building a particular trait into every sperm or egg he or she makes. • This instruction set is called a gene. • The trait observed in an individual depends on the two copies (alleles) of the gene it inherits from its parents.

22. 7.5 Observing an individual’s phenotype is not sufficient for determining its genotype.

24. Phenotypes and Genotypes • The outward appearance of an individual is called their phenotype. • Underlying the phenotype is the genotype. • This is an organism’s genetic composition.

25. Genotypes • Homozygous dominant • Heterozygous

26. How do we analyze and predict the outcome of crosses? • Assign symbols to represent the different variants of a gene. • Generally we use an uppercase letter for the dominant allele and lowercase for the recessive allele.

28. Take-home message 7.5 • It is not always possible to determine an individual’s genetic makeup, known as its genotype, by observation of the organism’s outward appearance, known as its phenotype.

29. Take-home message 7.5 • For a particular trait, an individual may carry a recessive allele whose phenotypic effect is masked by the presence of a dominant allele. • Much genetic analysis makes use of clever experiments and careful recordkeeping, often using Punnett squares, to determine organisms’ genotypes.

30. Probability and chance play central roles in genetics.

31. Probability has a central role in genetics for two reasons: 7.6 Chance is important in genetics. • The first is a consequence of segregation. • The second reason is that fertilization, too, is a chance event.

32. Probabilities • Any gamete produced by an individual heterozygous for a trait has a 50% probability of carrying the dominant allele and a 50% probability of carrying the recessive allele.

33. Probabilities • If a male is heterozygous for albinism (Mm) and a female is homozygous for albinism (mm), what is the probability that their child will be homozygous for albinism (mm)?

36. Take-home message 7.6 • Probability plays a central role in genetics. • In segregation, each gamete that an individual produces receives only one of the two copies of each gene the individual carries • It is impossible to know which allele goes into the gamete.

37. Take-home message 7.6 • Chance plays a role in fertilization, too: all of the sperm or eggs produced by an individual are different from one another, and any one of those gametes may be the gamete involved in fertilization.

38. 7.7 A test-cross enables us to figure out which alleles an individual carries.

39. You would like to produce white alligators via a mating program. • The problem is that you cannot be certain of the genotype of your alligators.

40. They might be homozygous dominant, MM, or they might be heterozygous, Mm. • In either case their phenotype is normal coloration. • How can you figure out which of these two possibilities is the actual genotype?

42. Take-home message 7.7 • In a test-cross, an individual that exhibits a dominant trait but has an unknown genotype is mated with an individual that is homozygous recessive.

43. Take-home message 7.7 • The phenotypes of the offspring reveal whether the unknown-genotype individual is homozygous dominant (all of the offspring exhibit the dominant trait) or heterozygous (half of the offspring show the dominant trait and half show the recessive trait).

44. 7.8 Using pedigrees to decipher and predict the inheritance patterns of genes. Pedigree: a type of family tree

45. Analyzing Which Individuals Manifest the Trait and Which Do Not

46. Sex-Linked Traits

47. A Trait’s Mode of Inheritance Is Not Always Completely Obvious • Some traits may not show complete dominance. • Many traits are influenced by the environment

48. Take-home message 7.8 • Pedigrees help scientists, doctors, animal and plant breeders, and prospective parents determine: • the genes that individuals carry • the likelihood that the offspring of two individuals will exhibit a given trait.

49. How are genotypes translated into phenotypes?

50. 7.9 Incomplete dominance and codominance: the effects of both alleles in a genotype can show up in the phenotype.