Mendel and the Gene Idea

1. Chapter 14 Mendel and the Gene Idea

2. Overview: Drawing from the Deck of Genes • The “blending” hypothesis • genetic material from the two parents blends together • The “particulate” hypothesis • parents pass on discrete heritable units • Traits reappear after several generations • Mendel used experiments with garden peas

3. Figure 14.1

4. Concept 14.1: Mendel identified two laws of inheritance • Discovered the basic principles of heredity by breeding garden peas

5. Mendel’s Experimental, Quantitative Approach • Advantages of pea plants for genetic study • There are many varieties of heritable features(characters) • Character variants (Traits) • Controlled Mating • Individual sex organs • Cross-pollination

6. 3 2 1 4 5 TECHNIQUE Figure 14.2 Parentalgeneration(P) Stamens Carpel RESULTS First filialgenerationoffspring(F1)

7. What Mendel Used • Mendel chose to track only those characters that occurred in two distinct alternative forms • True-breeding (plants that produce offspring of the same variety when they self-pollinate)

8. Terms of the Experiment • Hybridization: mating two contrasting, true-breeding varieties • P generation: true-breeding parents • F1 generation: hybrid offspring of the P generation • F2 generation: F1 individuals self-pollinate or cross- pollinate with other F1 hybrids

9. The Law of Segregation • Crossing contrasting true-breeding white and purple pea plants: all of the F1 hybrids were purple • Crossing the F1 hybrids: many of the F2 plants had purple flowers, but some had white • Three to one ratio purple to white flowers in the F2 generation

10. EXPERIMENT Figure 14.3-3 P Generation (true-breedingparents) Purpleflowers Whiteflowers F1 Generation(hybrids) All plants had purple flowers Self- or cross-pollination F2 Generation 705 purple-floweredplants 224 whitefloweredplants

11. Ideas that stemmed from observations • Only the purple flower factor was affecting flower color in the F1 hybrids • purple flower color: dominant trait • white flower color: recessive trait • Six other pea plant characters had same patterns • “Heritable factor”: we now call a gene

13. Mendel’s Model • Alternative versions of genes: account for variations in inherited characters • Alternative versions of a gene are now called alleles • Each gene resides at a specific locus

14. Figure 14.4 Allele for purple flowers Pair ofhomologouschromosomes Locus for flower-color gene Allele for white flowers

15. Mendel’s Model • Organisms inherits two alleles, one from each parent • Dominant alleles determine the organism’s appearance • Recessive alleles have no noticeable effect on appearance • Law of segregation: two alleles for a heritable character separate during gamete formation

16. Punnett Square • Diagram for predicting the results of a genetic cross between individuals of known genetic makeup • A capital letter represents a dominant allele • A lowercase letter represents a recessive allele

17. P Generation Figure 14.5-3 Appearance: White flowers Purple flowers Genetic makeup: pp PP p Gametes: P F1 Generation Appearance: Purple flowers Genetic makeup: Pp p 1/2 1/2 P Gametes: Sperm from F1 (Pp) plant F2 Generation p P P Pp PP Eggs from F1 (Pp) plant p pp Pp 3 : 1

18. Useful Genetic Vocabulary • Homozygous: An organism with two identical alleles for a character • Heterozygous: An organism that has two different alleles for a gene • Unlike homozygotes, heterozygotes are not true-breeding • Phenotype: physical appearance • Genotype: genetic makeup

19. Genotype Phenotype Figure 14.6 PP(homozygous) Purple 1 Pp(heterozygous) 3 Purple 2 Pp(heterozygous) Purple pp(homozygous) White 1 1 Ratio 3:1 Ratio 1:2:1

20. The Testcross • How can we tell the genotype of an individual with the dominant phenotype? • Could be either homozygous dominant or heterozygous

21. TECHNIQUE Figure 14.7 Dominant phenotype,unknown genotype:PP or Pp? Recessive phenotype,known genotype:pp Predictions If purple-floweredparent is PP or If purple-floweredparent is Pp Sperm Sperm p p p p P P Pp Pp Pp Pp Eggs Eggs P p pp pp Pp Pp RESULTS or All offspring purple 1/2 offspring purple and1/2 offspring white

22. The Law of Independent Assortment • Followed a single character • Monohybrids: individuals that are heterozygous for one character • A cross between such heterozygotes is called a monohybrid cross • Dihybrids: Two true-breeding parents differing in two characters produces • A dihybrid cross, a cross between F1 dihybrids

23. EXPERIMENT YYRR yyrr P Generation Figure 14.8 Gametes yr YR F1 Generation YyRr Hypothesis ofdependent assortment Predictions Hypothesis ofindependent assortment Sperm or Predictedoffspring ofF2 generation 1/4 1/4 1/4 1/4 yR yr Yr YR Sperm 1/2 YR 1/2 yr 1/4 YR YYRR YYRr YyRR YyRr 1/2 YR YyRr YYRR 1/4 Yr Eggs YYRr YYrr Yyrr YyRr Eggs 1/2 yr YyRr yyrr 1/4 yR YyRr yyRr YyRR yyRR 3/4 1/4 yr 1/4 Phenotypic ratio 3:1 Yyrr yyRr YyRr yyrr 3/16 3/16 1/16 9/16 Phenotypic ratio 9:3:3:1 RESULTS 108 101 315 Phenotypic ratio approximately 9:3:3:1 32

24. Law of independent assortment • Pairs of alleles segregate independently of each other pair of alleles during gamete formation

25. Rr Rr  Segregation ofalleles into sperm Segregation ofalleles into eggs Figure 14.9 Sperm r 1/2 1/2 R R R r R R 1/2 1/4 1/4 Eggs r r r R 1/2 r 1/4 1/4

26. The Multiplication and Addition Rules Applied to Monohybrid Crosses • probability that two or more independent events will occur together is the product of their individual probabilities • Each gamete has a ½ chance of carrying the dominant allele and a ½ chance of carrying the recessive allele

27. The addition rule • Probability that any one of two or more exclusive events will occur is calculated by adding together their individual probabilities • The rule of addition can be used to figure out the probability that an F2 plant from a monohybrid cross will be heterozygous rather than homozygous

28. Figure 14.UN01  1/4 (probability of YY)   1/4 (RR) 1/16 Probability of YYRR   Probability of YyRR  1/4 (RR) 1/2 (Yy) 1/8 1/4 (probability of pp)  1/2 (yy)  1/2 (Rr)  1/16 ppyyRr  1/16 ppYyrr 1/41/21/2  2/16 Ppyyrr 1/21/21/2  1/16 1/41/21/2 PPyyrr ppyyrr  1/16 1/41/21/2  6/16 or 3/8 Chance of at least two recessive traits

29. Extending Mendelian Genetics for a Single Gene • Inheritance of characters by a single gene may deviate from simple Mendelian patterns in the following situations: • When alleles are not completely dominant or recessive • When a gene has more than two alleles • When a gene produces multiple phenotypes

30. Degrees of Dominance • Complete dominance heterozygote and dominant homozygote phenotypes are identical • Incomplete dominance F1 hybrids are somewhere between the phenotypes of the two parental varieties • Codominance two dominant alleles affect the phenotype in separate, distinguishable ways

31. P Generation Figure 14.10-3 White Red CWCW CRCR Gametes CW CR F1 Generation Pink CRCW 1/2 1/2 CR CW Gametes Sperm F2 Generation 1/2 1/2 CW CR 1/2 CR CRCR CRCW Eggs 1/2 CW CRCW CWCW

32. Dominance and Phenotype • A dominant allele does not subdue a recessive allele; alleles don’t interact that way • Alleles are simply variations in a gene’s nucleotide sequence Frequency of Dominant Alleles • Dominant alleles are not necessarily more common • 1/400 babies in the US is born with extra fingers or toes

33. Multiple Alleles • Four phenotypes of the ABO blood group in humans are determined by three alleles for the enzyme (I) that attaches A or B carbohydrates to red blood cells: IA, IB, and i.

34. (a) The three alleles for the ABO blood groups and their carbohydrates Figure 14.11 Allele IA IB i none Carbohydrate B A (b) Blood group genotypes and phenotypes Genotype ii IAIA or IAi IBIB or IBi IAIB Red blood cellappearance Phenotype(blood group) A AB O B

35. Pleiotropy • Most genes have multiple phenotypic effects, a property called pleiotropy • For example, pleiotropic alleles are responsible for the multiple symptoms of certain hereditary diseases, such as cystic fibrosis and sickle-cell disease

36. Epistasis • A gene at one locus alters the phenotypic expression of a gene at a second locus • Coat color of certain animals depend on two genes • One gene determines the pigment color (with alleles B for black and b for brown) • The other gene (with alleles C for color and c for no color) determines whether the pigment will be deposited in the hair

37. Figure 14.12 BbEe BbEe Sperm 1/4 1/4 1/4 1/4 Be BE be bE Eggs 1/4 BE BbEE BBEe BbEe BBEE 1/4 bE BbEE bbEe bbEE BbEe 1/4 Be BBEe BBee Bbee BbEe 1/4 be BbEe bbEe bbee Bbee : 3 9 : 4

38. Polygenic Inheritance • Quantitative characters: vary in the population along a continuum • Quantitative variation usually indicates polygenic inheritance • an additive effect of two or more genes on a single phenotype • Skin color in humans is an example of polygenic inheritance

39. Figure 14.13 AaBbCc AaBbCc Sperm 1/8 1/8 1/8 1/8 1/8 1/8 1/8 1/8 1/8 1/8 1/8 1/8 Eggs 1/8 1/8 1/8 1/8 Phenotypes: 1/64 6/64 15/64 20/64 6/64 1/64 15/64 Number ofdark-skin alleles: 1 2 3 4 5 0 6

40. The effect of environment on phenotype

41. Integrating a Mendelian View of Heredity and Variation • An organism’s phenotype includes its physical appearance, internal anatomy, physiology, and behavior • An organism’s phenotype reflects its overall genotype and unique environmental history

42. Pedigree Analysis • A pedigree is a family tree that describes the interrelationships of parents and children across generations • Inheritance patterns of particular traits can be traced and described using pedigrees

43. Key Figure 14.15 Affected female Mating Male Affectedmale Female Offspring 1stgeneration Ff ff Ff Ff 1stgeneration Ww ww ww Ww 2ndgeneration 2ndgeneration ff ff ff Ff FF or Ff Ff Ww ww ww Ww Ww ww 3rdgeneration 3rdgeneration ff FForFf WWorWw ww No widow’speak Freeearlobe Widow’speak Attachedearlobe b) (a) Is an attached earlobe a dominantor recessive trait? Is a widow’s peak a dominant orrecessive trait?

44. Pedigrees • Can also be used to make predictions about future offspring • Can use the multiplication and addition rules to predict the probability of specific phenotypes

45. Recessively Inherited Disorders • Many genetic disorders are inherited in a recessive manner • These range from relatively mild to life-threatening • Recessively inherited disorders show up only in individuals homozygous for the allele • Carriers are heterozygous individuals who carry the recessive allele

46. Figure 14.16 Parents NormalAa NormalAa Sperm a A Eggs Aa Normal(carrier) AA Normal A Aa Normal(carrier) aa Albino a

47. Dominantly Inherited Disorders • Dominant alleles that cause a lethal disease are rare and arise by mutation • Achondroplasia is a form of dwarfism caused by a rare dominant allele

48. Figure 14.17 Parents DwarfDd Normaldd Sperm d D Eggs Dd Dwarf dd Normal d dd Normal Dd Dwarf d

49. Multifactorial Disorders • Heart disease, diabetes, alcoholism, mental illnesses, and cancer have both genetic and environmental components

50. Fetal Testing • In amniocentesis, the liquid that bathes the fetus is removed and tested • In chorionic villus sampling (CVS), a sample of the placenta is removed and tested • Other techniques, such as ultrasound and fetoscopy, allow fetal health to be assessed visually in utero