Chapter 6

1. Chapter 6 Mendelian and Quantitative Genetics Are You Only As Smart As Your Genes?

2. Chapter 6 Section 1 The Inheritance of Traits

3. 6.1 The Inheritance of Traits The Inheritance of Traits • Offspring resemble their parents, but not exactly. • Siblings resemble each other, but not exactly. • How much is because of environment? • How much is inherited? • Nature Versus Nurture

4. 6.1 The Inheritance of Traits Mother’s egg and father’ssperm each contain halfofthe information to “build a human.” This single cell containsall the information on“how to build a human.” Egg Meiosis Body axisestablishment,tissuedifferentiation,organ systemformation Mitosisanddifferentiation Fertilization Zygote Sperm Adult Gametes Single-celled embryo Multicellular embryo The human life cycle: • Adults produce gametes in their gonads by meiosis. • Sperm cells fertilize egg cells to form single-celled zygotes. • Repeated cell divisions form the embryo. Figure 6.1

5. 6.1 The Inheritance of Traits Birth Mitosisanddifferentiation Mitosisanddifferentiation Mitosisanddifferentiation Fetus Baby Child Adult The human life cycle, cont.: • The embryo grow to become a fetus. • After birth, the individual continues to grow until reaching adulthood. Figure 6.1

6. 6.1 The Inheritance of Traits • Genes are segments of DNA that code for proteins. • Analogous to words in an instruction manual for building a human Genesexpressed inmuscle cell build strong heart muscle build grow long dark blood brown strong hair for eyes heart small red muscle Genes expressedin eye cell build dark brown eyes Figure 6.3

7. 6.1 The Inheritance of Traits • Chromosomes are analogous to pages in the instruction manual. • Each “page” contains thousands of “words” • Different types of cells use different words, in different orders

8. 6.1 The Inheritance of Traits - Producing Diversity in Offspring • Mistakes in copying DNA (mutations) produce different versions of genes (alleles), with different results. Mutation Mutation Mutation Original Copy Copy Original Copy Original nerve grey Normal allele: strong Mutant allele: gray nzrve string (a) The mutant allele has the samemeaning (mutant allele functionthe same as the original allele). (b) The mutant allele has a differentmeaning (mutant allele functionsdifferently than the original allele). (c) The mutant allele has nomeaning (mutant allele isno longer functional). Figure 6.4

9. 6.1 The Inheritance of Traits - Producing Diversity in Offspring Egg Sperm Zygote + = The 23 pages of each instruction manualare roughly equivalent to the 23chromosomes in each egg and sperm. The zygote has 46pages, equivalent to 46chromosomes. • Parent cell has two complete copies of the manual: 23-page copy from mom and 23-page copy from dad • 23 pairs of homologous chromosomes Figure 6.5

10. 6.1 The Inheritance of Traits - Producing Diversity in Offspring Meiosis creates variation in offspring • Segregation: in meiosis, one member of each homologous pair goes into a gamete • Gamete gets just one copy of each page of the manual • Independent assortment randomly determines which member of a pair of chromosomes goes into a gamete • Due to random alignment during metaphase I • About 8 million different combinations of chromosomes.

11. 6.1 The Inheritance of Traits - Producing Diversity in Offspring Parent cells have 2 copies of each chromosome—that is, 2 full sets ofinstruction manual pages, 1 from each parent. Sperm and egg cells each have only 1 full set—a random combinationof maternal and paternal instruction manual pages. Possible sperm cell 1 Possible sperm cell 2 Page 9Eye-colorgenes fromdad Page 3Blood-groupgene from dad Page 9Eye-color genesfrom mom Page 3Blood-groupgene from mom • Siblings share 50% of alleles with each other, on average Figure 6.6

12. 6.1 The Inheritance of Traits - Producing Diversity in Offspring (a) Dizygotic (fraternal) twins (b) Monozygotic (identical) twins Egg Sperm Egg Sperm Egg Sperm Zygote Zygote Zygote Embryo Embryo Embryo Embryosplits Twoembryos 50% identical (no moresimilar than siblings born atdifferent times) 100% genetically identical • Random fertilization produces more diversity: 64 trillion possibilities! • No two humans are genetically identical, except for monozygotic twins. Figure 6.7

13. 6.1 The Inheritance of Traits Processes that create variation in offspring • Crossing-Over: in prophase I homologous chromosomes exchange segments • Segregation: one member of each homologous pair goes into a gamete • Independent assortment: randomly determines which member of a pair of chromosomes goes into a gamete • Due to random alignment during metaphase I • Random Fertilization: Which sperm will fertilize an egg. • Leads to about 64 trillion genetic possibilities from two parents!

14. 6.1 The Inheritance of Traits End Chapter 6 Section 1 The Inheritance of Traits

15. 6.2 Mendelian Genetics Chapter 6 Section 2 Mendelian Genetics

16. 6.2 Mendelian Genetics: When the Role of Genes Is Clear Gregor Mendel: first to accurately describe rules of inheritance for simple traits • Controlled mating between pea plants

17. 6.2 Mendelian Genetics: When the Role of Genes Is Clear Gregor Mendel • Studied traits due to a single gene with a few alleles • Discovered that both parents contribute equally to offspring (genetically) • Mendel’s principles also apply to many genetic diseases in humans

18. 6.2 Mendelian Genetics: When the Role of Genes Is Clear • Phenotype: physical traits of an individual • Genotype: description of the alleles for a particular gene in an individual • Homozygous (-ote): both alleles for a gene are identical • Heterozygous (-ote): the gene has two different alleles • Recessive: the phenotype of an allele is seen only when homozygous • Dominant: the phenotype is seen when homozygous or heterozygous

19. 6.2 Mendelian Genetics - Genetic Diseases in Humans Genetic Diseases in Humans • Cystic fibrosis: a recessive human genetic disease • Defect in chloride ion transport • Causes recurrent lung infections, dramatically shortened lifespans • Heterozygotes (carriers) do not show the symptoms • Most common recessive disease among Europeans

20. 6.2 Mendelian Genetics - Genetic Diseases in Humans Huntington’s disease • a dominant human genetic disease • Progressive, incurable, always fatal • Symptoms occur in middle age • Mutant protein forms clumps inside nerve cell nuclei, killing the cells • Having a normal allele cannot compensate for this

21. 6.2 Mendelian Genetics Using Punnett Squares to Predict Offspring Genotypes & Phenotypes • Punnett square: graphic way to predict possible outcomes of a cross • Consider a cross between two cystic fibrosis carriers • “F” = normal dominant allele; • “f” = recessive disease allele • The cross would be: F f x F f • What offspring could result?

22. 6.2 Mendelian Genetics - Using Punnett Squares to Predict Offspring Genotypes Possible types of eggs SpermsampleFf Ff FF Ff F Possible types of sperm Femalecarrier Ff ff Ff f 25% chance that a child willnot have cystic fibrosis 50% chance that a child willbe an unaffected carrier ofthe cystic fibrosis allele 25% chance that a child willhave cystic fibrosis Figure 6.13

23. 6.2 Mendelian Genetics END Chapter 6 Section 2 Mendelian Genetics

24. 6.3 Chapter 6 Section 3 Quantitative & Qualitative Genetics

25. 6.3 Quantitative Genetics: When Genes and Environment Interact Qualitative traits are on or off traits - such as yellow or green peas Quantitative traits show continuous variation • Large range of phenotypes • E.g., height, weight, intelligence • Variation due to both genetic and environmental differences • Heritability: proportion of the variation within a population due to genetic differences among individuals

26. 6.3 Quantitative Genetics: When Genes and Environment Interact (a) Normal distribution of student height in onecollege class 5 ft, 10 in (1.78 m ) Mean Bell-shapedcurve Number of men Variability Height (ft, in) Distribution of Phenotypes in Population • Mean: sum up all the phenotypic values and divide by the number of individuals; same as the average. Figure 6.16a

27. 6.3 Quantitative Genetics: When Genes and Environment Interact (b) Variance describes the variability around themean. Number ofjockeys Low variance Mean = 114 lbs(51.7 kg) Number of14-year-old boys High variance Weight (lbs) • Variance: a measure of how much variability there is in the population • The amount an individual varies from the mean, on average Figure 6.16b

28. 6.3 Quantitative Genetics - Why Traits Are Quantitative • Quantitative traits, with continuous variation, are polygenic traits. • Result of several genes • Each with more than one allele • Interaction of multiple genes with multiple alleles results in many phenotypes. • Example: human eye color

29. 6.3 Quantitative Genetics Why Traits Are Quantitative • Usually influenced by both genes and environment • Monozygotic twins, genetically identical, but different environments Figure 6.17

30. 6.3 Quantitative Genetics Effects of the Environment: Sun Exposure • 72 Year old monk with no sun exposure • 58 year old Native American with lots of sun exposure

31. 6.3 Quantitative Genetics – Measuring Heritability in Animals • Artificial selection: • Only the cow giving the most milk was allowed to breed • The next generation has a higher mean milk production • Milk production has a high heritability

32. 6.3 Quantitative Genetics – Measuring Heritability in Animals Points represent parent-offspringpairs with matching immunity levels. Weak Strong Average Blue tit chick immune response On average,parents andoffspring hadsame level ofimmunity. Blue tit parent immune response • When artificial selection is impossible, correlations between relatives estimates heritability. Figure 6.20

33. 6.3 Quantitative Genetics –Calculating Heritability in Human Populations • Have to use correlation to measure heritability in humans • Scientists seek “natural experiments”, situations in which either the overlap in genes or environment is removed • Twins are often used • Dizygotic twins share environment, but only half their genes • Heritability of IQ from such twin studies estimated to be about 0.52 • Similar treatment of twins might explain why their IQs are so similar

34. 6.3 Quantitative Genetics – Calculating Heritability in Human Populations Another approach: • Monozygotic twins raised apart share all genes • Estimates of IQ heritability for such twins is 0.72 • Drawback: limited number of such twins to study

35. 6.3 END Chapter 6 Section 3 Quantitative & Qualitative Genetics

36. 6.4 Chapter 6 Section 4 Genes, Environment, and the Individual

37. 6.4 Genes, Environment, and the Individual – The Use and Misuse of Heritability • Differences between groups may be environmental, despite a high heritability • A heritability value pertains just to the population in which it was measured, and to the environment of that population • Imagine a laboratory population of mice of varying weights • Divide this population into 2 genetically identical groups • Give one group a rich diet, the other a poor diet • The “rich diet” mice will be bigger than the “poor diet” mice.

38. 6.4 Genes, Environment, and the Individual – The Use and Misuse of Heritability Start with a population ofmice that are variable in size. 3 2 1 Randomly divide miceinto two groups. Feedhalf a poor diet and theother half a rich diet. Average weight of the mice in therich- diet environment is twice theaverage weight of the population inthe poor- diet environment.However, there is no geneticdifference between the two groups. Allow the mice in bothgroups to breed.Measure the weight ofadult offspring. • Allow the mice in each group to breed, maintaining their diets. • Measure the weight of adult offspring; correlation with parents shows high heritability Figure 6.22

39. 6.4 Genes, Environment, and the Individual – The Use and Misuse of Heritability • Instead of body weight in mice, consider IQ in humans. • Affluent group: higher IQs • Impoverished group: lower IQs • Conclude that the difference is probably due to genetics?

40. 6.4 Genes, Environment, and the Individual – The Use and Misuse of Heritability • A highly heritable trait can still respond to environmental change. • Example: Maze-learning ability is highly heritable in rats. • Bright rats have bright offspring • Dull rats have dull offspring • Still, no rats learned well in a restricted environment. • All rats learned better in an enriched environment

41. 6.4 Genes, Environment, and the Individual – The Use and Misuse of Heritability • Heritability does not tell us about individual differences • Heritability is based on variances in populations, not individuals • High heritability value for a trait does not automatically mean that most of the difference between two individuals is genetic.

42. 6.4 Genes, Environment, and the Individual – How Do Genes Matter? • Genes have a strong influence on even complex traits. • But, independent assortment of multiple genes with multiple alleles produces a large number of phenotypes. • Environment can also have a big effect. • For quantitative traits, it is difficult to predict the phenotype of children from the phenotypes of the parents

43. 6.4 END Chapter 6 Section 4 Genes, Environment, and the Individual

44. END Chapter 6 Mendelian and Quantitative Genetics Are You Only As Smart As Your Genes?