Chapter 13 Observing Patterns in Inherited Traits

1. Chapter 13Observing Patterns in Inherited Traits

2. Mendel’s Experimental Approach • Gregor Mendel was a monk with training in plant breeding and mathematics • He studied the garden pea (Pisum sativum), which breeds true for a number of traits • Mendel discovered that traits of offspring of cross-fertilized pea plants often appear in predictable patterns • Mendel’s work led him to conclude that hereditary information passes from one generation to the next in discrete units

3. Gregor Mendel

4. Garden Pea Plant: Self Fertilization and Cross-Fertilization B C A carpel anther D E

5. Terms Used in Modern Genetics • Genes are heritable units of information about traits • Each gene has a specificlocuson a chromosome • Diploid cells (chromosome number 2n) have pairs of genes on homologous chromosomes • Alleles are different molecular forms of a gene

6. Loci of Some Human Genes

7. LDL receptor (coronary artery disease) insulin receptor brown hair color green/blue eye color (Warfarin resistance) HCG, β chain 19 LH, β chain Figure 13-3c p205

8. dystrophin (muscular dystrophy) (anhidrotic ectodermal dysplasia) IL2RG (SCID-X1) XIST X chromosome inactivation control (hemophilia B) (hemophilia A) (red-deficient color blind) X (green-deficient color blind) Figure 13-3e p205

9. Terms Used in Modern Genetics • The particular set of alleles that an individual carries is the individual’s genotype • An individual with two identical alleles of a gene is homozygous for that gene • An individual with nonidentical alleles of a gene is heterozygous for that gene

10. Terms Used in Modern Genetics • An allele is dominantif its effect masks the effect of a recessive allele paired with it • Capital letters (P) signify dominant alleles; lowercase letters (p) signify recessive alleles • Homozygous dominant(PP) • Homozygous recessive(pp) • Heterozygous(Pp)

11. Genotypes Give Rise to Phenotypes PP (homozygous for dominant allele P) pp (homozygous for recessive allele p) Pp (heterozygous at the P gene locus) genotype: phenotype:

12. Take-Home Message:How do alleles contribute to traits? • Gregor Mendel discovered the role of alleles in inheritance by breeding pea plants and tracking traits of their offspring • Genotype refers to the particular set of alleles carried by an individual’s somatic cell; phenotype refers to the individual’s set of observable traits; genotype is the basis of phenotype • A homozygous individual has two identical alleles at a particular locus; a heterozygous individual has nonidentical alleles at the locus • Dominant alleles mask the effects of recessive ones in heterozygous individuals

13. 13.3 Mendel’s Law of Segregation • Pairs of genes on homologous chromosomes separate during meiosis, so they end up in different gametes • Mendel showed that garden pea plants inherit two “units” of information for a trait, one from each parent

14. Gene Segregation • Homologous chromosomes (and all the alleles they carry) segregate into separate gametes during meiosis • Plants homozygous for the dominant allele (PP) can only make gametes that carry the allele P • Plants homozygous for the recessive allele (pp) can only make gametes that carry the allele p • Heterozygous plants produce both type of gametes

15. Calculating Probabilities • Probability • A measure of the chance that a particular outcome will occur • Punnett square • A grid used to calculate the probability of genotypes and phenotypes in offspring

16. DNA replication meiosis I 2 1 meiosis II 3 gametes (P) gametes (p) zygote (Pp) female gametes 4 male gametes Stepped Art Figure 13-5 p206

17. female gametes male gametes Figure 13-5b p206

18. Testcrosses • A testcross is a method of determining if an individual is heterozygous or homozygous dominant • An individual with unknown genotype is crossed with one that is homozygous recessive (PP x pp) or (Pp x pp)

19. Generations in a Monohybrid Cross • P stands for parents,Ffor filial (offspring) • F1: First generation offspring of parents • F2: Second generation offspring of parents

20. Mendel’s Monohybrid Crosses • Mendel used monohybrid crosses to find dominance relationships among pea plant traits • When he crossed plants that bred true for white flowers with plants that bred true for purple flowers, all F1 plants had purple flowers • When he crossed two F1 plants, ¾ of the F2 plants had purple flowers, ¼ had white flowers

21. Table 13-1 p207

22. Mendel’s Dihybrid Cross PT Pt pT pt 4 PP PPTT PPTt PpTT PpTt parent plant homozygous for purple flowers and long stems parent plant homozygous for white flowers and short stems Pt PPTt PPtt PpTt Pptt pptt PPTT pT PpTT PpTt ppTT ppTt pt PT 1 PpTt 2 dihybrid pt PpTt Pptt ppTt pptt PT Pt pT pt four types of gametes 3 Stepped Art

23. Offspring of Mendel’s Monohybrid Cross

24. Mendel’s Law of Segregation • Mendel observed a phenotype ratio of 3:1 in the F2 offspring of his monohybrid crosses • Consistent with the probability of the pp genotype in the offspring of a heterozygous cross (Pp x Pp) • This is the basis of Mendel’s law ofsegregation • Diploid cells have pairs of genes on pairs of homologous chromosomes • The two genes of each pair separate during meiosis, and end up in different gametes

25. Take-Home Message:What is Mendel’s law of segregation? • Diploid cells carry pairs of genes, on pairs of homologous chromosomes • The two genes of each pair are separated from each other during meiosis, so they end up in different gametes • Mendel discovered patterns of inheritance in pea plants by tracking the results of many monohybrid crosses

26. 13.4 Mendel’s Law of Independent Assortment • Mendel’s law of independent assortment • During meiosis, members of a pair of genes on homologous chromosomes get distributed into gametes independently of other gene pairs

27. Dihybrid Crosses • Dihybrid crossestest for dominance relationships between alleles at two loci • Individuals that breed true for two different traits are crossed (PPTT x pptt) • F2 phenotype ratio is 9:3:3:1 (four phenotypes) • Individually, each dominant trait has an F2 ratio of 3:1 – inheritance of one trait does not affect inheritance of the other

28. The Contribution of Crossovers • Independent assortment also occurs when the genes are on the same chromosome, but far enough apart that crossing over occurs between them very frequently • Genes that have loci very close to one another on a chromosome tend to stay together during meiosis and not assort independently

29. Linkage Groups • All genes on one chromosome are called a linkage group • The farther apart two genes are on a chromosome, the more often crossing over occurs between them • Linked genes are very close together; crossing over rarely occurs between them • The probability that a crossover will separate alleles of two genes is proportional to the distance between those genes

30. Codominance • Codominance • Two nonidentical alleles of a gene are both fully expressed in heterozygotes, so neither is dominant or recessive • May occur in multiple allele systems • Multiple allele systems • Genes with three or more alleles in a population • Example: ABO blood types

31. Codominance in ABO Blood Types AA or AO BB or BO Genotypes: AB OO Phenotypes (blood type): A AB B O

32. Incomplete Dominance • Incomplete dominance • One allele is not fully dominant over its partner • The heterozygote’s phenotype is somewhere between the two homozygotes, resulting in a 1:2:1 phenotype ratio in F2 offspring • Example: Snapdragon color • RR is red • Rr is pink • rr is white

33. heterozygous (Rr) homozygous (RR) homozygous (rr) Figure 13-10 p210

34. Figure 13-10b p210

35. Epistasis • Epistasis • Two or more gene products influence a trait • Typically, one gene product suppresses the effect of another • Example: Coat color in dogs • Alleles B and b designate colors (black or brown) • Two recessive alleles ee suppress color

36. Coat Colors in Labrador Retrievers

37. Figure 13-11b p211

38. Pleiotropy • A pleiotropicgene influences multiple traits • Example: Some tall, thin athletes have Marfan syndrome, a potentially fatal genetic disorder

39. Environment and Epigenetics • Environmentally driven changes in gene expression patterns can be permanent and heritable • Such changes are implemented by gene controls such as chromatin modifications and RNA interference that act on DNA itself • Example: Many environmental factors affect DNA methylation patterns, enhancing or suppressing gene expression

40. Effects of Temperature on Gene Expression

41. a Mature cutting at high elevation (3,060 meters above sea level) b Mature cutting at mid-elevation (1,400 meters above sea level) c Mature cutting at low elevation (30 meters above sea level) Figure 13-14a p212

42. 13.7 Complex Variations in Traits • Individuals of most species vary in some of their shared traits • Many traits (such as eye color) show a continuous range of variation

43. Continuous Variation • Continuous variation • Traits with a range of small differences • The more factors that influence a trait, the more continuous the distribution of phenotype • Bell curve • When continuous phenotypes are divided into measurable categories and plotted as a bar chart, they form a bell-shaped curve

44. Regarding the Unexpected Phenotype • Phenotype results from complex interactions among gene products and the environment • Enzymes and other gene products control steps of most metabolic pathways • Mutations, interactions among genes, and environmental conditions may result in unpredictable traits • Example: Camptodactyly can affect any fingers on either or both hands