Chapter 7: Mendelian Inheritance

1. Chapter 7: Mendelian Inheritance Family resemblance: how traits are inherited Lectures by Tom Chen Santa Monica College

2. Learning Objectives • Explain the concept of a single-gene trait • Describe Mendel’s contributions to the field of genetics • Define the terms gene, allele, dominant, recessive, homozygous, and heterozygous • Describe the difference between an organism’s genotype and its phenotype • Demonstrate the ability to perform Punnett squares to predict the offspring of parents with particular genotypes • Understand how to use the rules of probability to predict the inheritance of traits • Explain how a test-cross can be used to determine the genotype of an organism • Predict patterns of inheritance based on pedigree analysis • Explain how each of the following genetic “rules” works: incomplete dominance, codominance, multiple alleles, polygenic inheritance, and pleiotropy • Understand how the ABO and Rh markers contribute to human blood type • Explain how blood type compatibility is determined • Describe how sex linked traits are inherited • Understand that phenotypes are a combination of genotypes and the environment • Explain why linked genes do not assort independently

3. Human’s Life Cycle 23 One set of chromosomes 23 46 23 23 Two sets of chromosomes How do we know this? Thanks, Mendel. See slides 16 & 17. Segregation. 46 46

4. 7.1 Family resemblance: your mother and father contribute equally to your genetic makeup. Haploid 1n Diploid 2n

5. Definitions: Must know these!!! • Trait—A variable characteristic of organism • Genome: All the DNA in a haploid number for a given species (gametes) Don’t forget mit/chl • Genotype: what our genes say (ATGC ) • Transcription  mRNA  translation & where? • Alleles: Different molecular forms of a gene (represented by alphabets) • Arise through mutation • Diploid cell has a pair of alleles at each locus (e.g., AA, Aa, aa) • Alleles on homologous chromosomes may be same or different • Phenotype: how we look/behave • Doesn’t always represent genotype • e.g., attached earlobe (ee), but free earlobe could have a genotype of either EE or Ee. Because all you need is one dominant allele E from either one of your parents.

6. Locus and Allele A gene’s specific location on a chromosome One of several alternative forms of a particular gene. Ambitions of DNA?

7. Homozygous (same pair) Heterozygous (different pair) Same allele at a given gene locus Different allele at a given gene locus E/E or e/e Attached E/e Homozygous dominant, homozygous recessive, heterozygote

8. 7.1 Family resemblance: your mother and father contribute “equally” to your genetic makeup. How can a single bad gene make you smell like a rotten fish?

9. 7.1 Family resemblance: your mother and father contribute equally to your genetic makeup. Haploid 1n Diploid 2n

10. 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.

11. 7.2 Some traits are controlled by a single gene. Selective Breeding: Observing Heredity • Heredity • the passing of characteristics from parent to offspring through their genes

12. Are any human traits determined by a single gene? Traits that are determined by the instructions a person carries at one gene are called single-gene traits. 12 examples in the Human Genetics lab. Take-home message 7.2 • More than 9,000 human traits are determined by a single gene.

13. Simple Inheritance, pp. 95-96 & 99 Your Phenotype Your genotype • Bent little finger (B/) can • Attached earlobe (ee) attached • Tongue rolling (R) can • Widow’s peak (W) yes • Thumb placement (Tp) left up • Arm placement (A) left up • Hitchhiker’s thumb (hh) yes • Middigital hairs (M) yes • PTC tasting (T) Te, Tl, tt • Pigmented iris (Pi) yes • Index finger shorter than ring finger (S) S in males • Chin shape (rr) square chin

14. 7.3 Mendel learned about heredity by conducting experiments.

15. True-Breeding Take-home message 7.3 • In the mid-1800s, Gregor Mendel conducted studies that helped us understand how traits are inherited. • Gregor Mendel used the scientific method carefully. • He focused on easily observed and categorized traits in garden peas.

16. A dominant trait “masks” the effect of a recessive trait. The dominant gene codes for the production of a functional protein that can do the job it is meant to do. 7.4 Segregation: you’ve got two copies of each gene but put only one copy in each sperm or egg.

18. An example: Using Punnett SquareATTACHED ERLOBEIf a person is homozygous recessive (ee) for this trait, the earlobes attach directly to the head and do not hang free. George has attached ear lobes.  ee Barbara has free earlobes, but she is heterozygous for this trait.  Ee What are the chances of their child having free earlobes?  look for E/

19. George has attached ear lobes.  eeBarbara has free earlobes, but she is heterozygous for this trait.  EeWhat are the chances of their child having free earlobes?  look for E/ E e e e

20. George has attached ear lobes.  eeBarbara has free earlobes, but she is heterozygous for this trait.  EeWhat are the chances of their child having free earlobes?  look for E/ E e e e

21. 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. • Offspring thus find themselves with two copies of the instructions for any trait (called alleles). • The actual trait produced by an individual depends on the two copies of the gene that they inherit from their parents. • Homozygous (AA or aa)and heterozygous (Aa) Take-home message 7.4 • Each parent puts a single set of instructions for a particular trait into every sperm or egg. • The 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. Phenotypes vs. Genotypes • The outward appearance of an individual is called their phenotype. • Underlying the phenotype is the genotype. • This is an organism’s genetic composition. 7.5 Observing an individual’s phenotype is not sufficient for determining its genotype. Genotypes • Homozygous dominant or homozygous recessive • Heterozygous

23. 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.

24. Take-home message 7.5 • It is not always possible to determine an individual’s genotype from its phenotype. • A recessive allele’s effects may be masked by a dominant allele. • Genetic analysis makes use of clever experiments and Punnett squares.

25. Probability has a central role in genetics for two reasons: 7.6 Chance is important in genetics. 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. • If a male is heterozygous for albinism (Aa) and a female is homozygous for albinism (aa), what is the probability that their child will be homozygous for albinism (aa)? • The first is a consequence of segregation. • The second reason is that fertilization, too, is a chance event.

27. Take-home message 7.6 • Probability plays a central role in genetics. • In segregation, each gamete receives only one of the two copies of each gene. • It is impossible to know which allele goes into which gamete. • Chance plays a role in fertilization too. • All of an individual’s sperm or eggs are different. • Any of these gametes may be the gamete involved in fertilization.

28. 7.7 A test-cross enables us to figure out which alleles an individual carries. • 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. • 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?

29. Take-home message 7.7 • In a test-cross, an individual with a dominant phenotype and an unknown genotype is mated with a homozygous recessive individual. • The phenotypes of the offspring reveal the unknown genotype.

30. 7.8 Using pedigrees to decipher and predict the inheritance patterns of genes. Pedigree: a type of family tree : Analyzing Which Individuals Manifest the Trait and Which Do Not

31. Sex-Linked Traits

34. A Trait’s Mode of Inheritance Is Not Always Completely Obvious Why do breeders value “pedigreed” horses and dogs so much? Take-home message 7.8 • Pedigrees help scientists, doctors, animal and plant breeders, and prospective parents determine: • the genes that individuals carry • the genes that their offspring will likely carry • complete dominance or… • the influence of the environment

35. 7.9 Incomplete dominance and codominance: the effects of both alleles in a genotype can show up in the phenotype.

36. Incomplete dominance, in which the heterozygote appears to be intermediate between the two homozygotes.

37. Codominance, in which the heterozygote displays characteristics of both homozygotes. Take-home message 7.9 • Sometimes the effects of both alleles in a heterozygous genotype are visible. • Incomplete dominance—a heterozygote displays a characteristic somewhere between the characteristics of the two homozygotes. • Codominance—a heterozygote displays characteristics of both homozygotes.

38. 7.10 What’s your blood type? Some genes may have more than two alleles. Multiple Allelism • in which a single gene has more than two alleles • each individual still carries only two alleles Inheritance of the ABO Blood Groups • A, B, and O alleles • The A and B alleles are both completely dominant to O. • The A and B alleles are codominant to each other. • Individuals can be one of four different blood types: A, B, AB, and O.

40. Blood type

41. Type A with A antigens on the red cells and anti B antibodies in the plasma • Type B with B antigens on the red cells and anti A antibodies in the plasma • Type AB with both A and B antigens on the red cells and no type antibodies in the plasma (universal recipient) • Type O with no type antigens on the red cells and both anti A and anti B antibodies in the plasma (universal donor)

42. Human ABO Blood GroupA Case of Co-dominance Type Genotype RBCs Anti-bodies Re-ceives Donates Freq A AA or AO B A or O A or AB 40% 10% B BB or BO A B or O B or AB AB AB Neither AB, A,B, O(universal) AB 4% OO Both O O O,AB,A,B(universal) 46%

43. Rh system • The Rh system represents another antigen on the surface of RBCs. Rh positive (Rh+) indicates presence of the antigen; Rh negative (Rh-) indicates its absence. • Rh factor is especially important in Rh- mothers who carry Rh+ fetuses because of the possibility of hemolytic disease in the newborn

45. 7.11 Multi-gene Traits How are continuously varying traits such as height influenced by genes? Additive Effects Polygenic Trait • A trait that is influenced by many different genes What happens when the effects of alleles from multiple genes all contribute to the ultimate phenotype

46. Why might computer nerds be more likely to have autistic children? Take-home message 7.11 • Many traits, including continuously varying traits such as height, eye color, and skin color, are influenced by multiple genes. 7.12 Pleiotropy: How can one gene influence multiple traits? What is the benefit of “almost” having sickle cell disease?

47. The SRY Gene Take-home message 7.12 • In pleiotropy, one gene influences multiple unrelated traits. • Most, if not all, genes may be pleiotropic. • “Sex-determining Region on the Y-chromosome” • Causes fetal gonads to develop as testes shortly after fertilization. • Following the gonads’ secretion of testosterone, other developmental changes also occur.

48. 7.13 Why are more men than women color-blind? Sex-linked traits differ in their patterns of expression in males and females. If a man is color-blind, did he inherit this condition from his mother, his father, or both parents?

49. If female is colorblind and male is also colorblindFemale: CCMale: CYGirl: %; boy: % If female has the disorder and male has the disorder, Chances of girl vs. boy: girl 100%; boy 100% If female has the disorder and male does not have the disorder, Girl: %; boy: % If female is a carrier, male does not have the disorder, Girl: %; boy: % If female is a carrier, male has the disorder, Girl: %; boy: % If female is homozygous normal, male has the disorder, Girl: %; boy: %

50. Take-home message 7.13 • The patterns of inheritance of most traits do not differ between males and females. • When a trait is coded for by a gene on a sex chromosome, such as color vision on the X chromosome, the effects differ in males and females. 7.14 Environmental effects: identical twins are not identical. Drinking diet soda can be deadly if you carry a single bad gene. What gene is it and why is it so deadly?