2. CHAPTER 11 MENDEL & MUTATIONS

3. Father of Genetics  Monk and teacher.  Experimented with purebred tall and short peas.  Discovered some of the basic laws of heredity.  Studied seven purebred traits in peas.  Called the stronger hereditary factor dominant.  Called the weaker hereditary factor recessive.  Presentation to the Science Society in1866 went unnoticed.  He died in 1884 with his work still unnoticed.  His work rediscovered in 1900.  Known as the “Father of Genetics”.

4. Mendel’s Observations  He noticed that peas are easy to breed for pure traits and he called the pure strains purebreds.  He developed pure strains of peas for seven different traits (i.e. tall or short, round or wrinkled, yellow or green, etc.)  He crossed these pure strains to produce hybrids.  He crossed thousands of plants and kept careful records for eight years.

5. Mendel’s Observations  He noticed that peas are easy to breed for pure traits and he called the pure strains purebreds.  He developed pure strains of peas for seven different traits (i.e. tall or short, round or wrinkled, yellow or green, etc.)  He crossed these pure strains to produce hybrids.  He crossed thousands of plants and kept careful records for eight years.

6. Mendel’s Peas  In peas many traits appear in two forms (i.e. tall or short, round or wrinkled, yellow or green.)  The flower is the reproductive organ and the male and female are both in the same flower.  He crossed pure strains by putting the pollen (male gamete) from one purebred pea plant on the pistil (female sex organ) of another purebred pea plant to form a hybrid or crossbred.

7. Analyzing Mendel’s Results Analyses using Punnett squares demonstrate that Mendel’s results reflect independent segregation of gametes. Analyses using Punnett squares demonstrate that Mendel’s results reflect independent segregation of gametes. The Testcross: The Testcross: Can be used to determine the genotype of an individual when two genes are involved. Can be used to determine the genotype of an individual when two genes are involved.

8. MENDEL’S LAWS OF HEREDITY MENDEL’S LAWS OF HEREDITY WHY MENDEL SUCCEEDED WHY MENDEL SUCCEEDED Gregor Mendol – father of genetics Gregor Mendol – father of genetics 1 st studies of heredity – the passing of characteristics to offspring 1 st studies of heredity – the passing of characteristics to offspring Genetics – study of heredity Genetics – study of heredity The characteristics passed on called traits The characteristics passed on called traits

9. From Genotype to Phenotype Multiple Alleles: Multiple Alleles: Sometimes more than two alleles (multiple alleles) exist for a given trait in a population. Sometimes more than two alleles (multiple alleles) exist for a given trait in a population. EX. ABO blood designation. EX. ABO blood designation. A and B are codominant. A and B are codominant. Rh Blood group: Rh Blood group: Rh is a cell surface marker on red blood cells Rh is a cell surface marker on red blood cells About 85% of the population is Rh+ (have the marker) About 85% of the population is Rh+ (have the marker) Problems: Mother is Rh negative has an Rh+ fetus. Problems: Mother is Rh negative has an Rh+ fetus.

10. MENDEL CHOSE HIS SUBJECT CAREFULLY MENDEL CHOSE HIS SUBJECT CAREFULLY Used garden peas to study Used garden peas to study Have male & female gametes (sex cells) Have male & female gametes (sex cells) Male & female same flower Male & female same flower Know what pollination & fertilization mean Know what pollination & fertilization mean He could control the fertilization process He could control the fertilization process Not many traits to keep track of Not many traits to keep track of

12. MENDEL WAS A CAREFUL RESEARCHER USED CAREFULLY CONTROLLED EXPERIMENTS USED CAREFULLY CONTROLLED EXPERIMENTS STUDIED ONE TRAIT AT A TIME STUDIED ONE TRAIT AT A TIME KEPT DETAILED DATA KEPT DETAILED DATA

13. MENDEL’S MONOHYBRID CROSSES MENDEL STUDIED 7 TRAITS CAREFULLY MENDEL STUDIED 7 TRAITS CAREFULLY 11.1 11.1 Mendel crossed plants w/ diff. traits to see what traits the offspring would have Mendel crossed plants w/ diff. traits to see what traits the offspring would have These offspring are called hybrids – offspring of parents w/ different traits These offspring are called hybrids – offspring of parents w/ different traits A monohybrid cross is one that looks at only one trait (let’s look at plant height – tall or short) A monohybrid cross is one that looks at only one trait (let’s look at plant height – tall or short)

14. THE 1 ST GENERATION Mendel crossed two plants – 1 tall & 1 short (they came from tall & short populations) Mendel crossed two plants – 1 tall & 1 short (they came from tall & short populations) These plants are called the parental generation (P generation) These plants are called the parental generation (P generation) The offspring were all called the 1 st filial generation (F 1 generation) The offspring were all called the 1 st filial generation (F 1 generation) All the offspring were tall (the short plants were totally excluded) All the offspring were tall (the short plants were totally excluded)

15. THE 2 ND GENERATION THE 2 ND GENERATION Next, Mendel crossed two plants from the F 1 generation Next, Mendel crossed two plants from the F 1 generation The offspring from this cross are called the 2 nd filial generation (F 2 GENERATION) The offspring from this cross are called the 2 nd filial generation (F 2 GENERATION) Mendel found that ¾ of the offspring were tall & ¼ were short (the short plants reappeared!!!!!!) Mendel found that ¾ of the offspring were tall & ¼ were short (the short plants reappeared!!!!!!)

17. 11.3 Mendel Proposes a Theory By convention, genetic traits are assigned a letter symbol referring to their more common form By convention, genetic traits are assigned a letter symbol referring to their more common form dominant traits are represented by uppercase letters, and lower-case letters are used for recessive traits dominant traits are represented by uppercase letters, and lower-case letters are used for recessive traits for example, flower color in peas is represented as follows for example, flower color in peas is represented as follows P signifies purple P signifies purple p signifies white p signifies white

18. Mendel Proposes a Theory Mendel Proposes a Theory The results from a cross between a true-breeding, white- flowered plant (pp) and a true breeding, purple-flowered plant (PP) can be visualized with a Punnett square The results from a cross between a true-breeding, white- flowered plant (pp) and a true breeding, purple-flowered plant (PP) can be visualized with a Punnett square A Punnett square lists the possible gametes from one individual on one side of the square and the possible gametes from the other individual on the opposite side A Punnett square lists the possible gametes from one individual on one side of the square and the possible gametes from the other individual on the opposite side The genotypes of potential offspring are represented within the square The genotypes of potential offspring are represented within the square

19. Figure 11.7 A Punnett square analysis

20. Figure 11.8 How Mendel analyzed flower color

21. TO GO ANY FURTHER, WE MUST UNDERSTAND ALLELES, DOMINANCE, & SEGREGATION Genes – a section of DNA that codes for one protein Genes – a section of DNA that codes for one protein These genes are what control & produce traits These genes are what control & produce traits The genes Mendel studied came in two forms (tall/short - round/wrinkled - yellow/green…….etc.) The genes Mendel studied came in two forms (tall/short - round/wrinkled - yellow/green…….etc.) Alternate forms of a gene are called alleles Alternate forms of a gene are called alleles Alleles are represented by a one or two letter symbol (e.g. T for tall, t for short) Alleles are represented by a one or two letter symbol (e.g. T for tall, t for short)

23. ALLELES CONT’D THESE 2 ALLELS ARE NOW KNOWN TO BE FOUND ON COPIES OF CHROMOSOMES – ONE FROM EACH PARENT THESE 2 ALLELS ARE NOW KNOWN TO BE FOUND ON COPIES OF CHROMOSOMES – ONE FROM EACH PARENT

24. THE RULE OF DOMINANCE A dominant trait is the trait that will always be expressed if at least one dominant allele is present A dominant trait is the trait that will always be expressed if at least one dominant allele is present The dominant allele is always represented by a capital letter The dominant allele is always represented by a capital letter A recessive trait will only be expressed if both alleles are recessive A recessive trait will only be expressed if both alleles are recessive Recessive traits are represented by a lower case letter Recessive traits are represented by a lower case letter

25. DOMINANCE CONT’D LET’S USE TALL & SHORT PEA PLANTS FOR AN EXAMPLE LET’S USE TALL & SHORT PEA PLANTS FOR AN EXAMPLE WHICH OF THESE WILL SHOW THE DOMINANT & RECESSIVE TRAIT? WHICH OF THESE WILL SHOW THE DOMINANT & RECESSIVE TRAIT? TT Tt tt DOMINANT TRAIT RECESSIVE TRAIT DOMINANT TRAIT RECESSIVE TRAIT

26. THE LAW OF SEGREGATION MENDEL ASKED HIMSELF……..”HOW DID THE RECESSIVE SHORT PLANTS REAPPEAR IN THE F2 GENERATION?” MENDEL ASKED HIMSELF……..”HOW DID THE RECESSIVE SHORT PLANTS REAPPEAR IN THE F2 GENERATION?” HE CONCLUDED THAT EACH TALL PLANT FROM THE F1 GENERATION CARRIED TWO ALLELES, 1 DOMINANT TALL ALLELE & ONE RECESSIVE SHORT ALLELE HE CONCLUDED THAT EACH TALL PLANT FROM THE F1 GENERATION CARRIED TWO ALLELES, 1 DOMINANT TALL ALLELE & ONE RECESSIVE SHORT ALLELE SO ALL WERE Tt SO ALL WERE Tt

27. SEGREGATION CONT’D HE ALSO CONCLUDED THAT ONLY ONE ALLELE FROM EACH PARENT WENT TO EACH OFFSPRING HE ALSO CONCLUDED THAT ONLY ONE ALLELE FROM EACH PARENT WENT TO EACH OFFSPRING HIS CORRECT HYPOTHESIS WAS THAT SOMEHOW DURING FERTILIZATION, THE ALLELES SEPARATED (SEGREGATED) & COMBINED WITH ANOTHER ALLELE FROM THE OTHER PARENT HIS CORRECT HYPOTHESIS WAS THAT SOMEHOW DURING FERTILIZATION, THE ALLELES SEPARATED (SEGREGATED) & COMBINED WITH ANOTHER ALLELE FROM THE OTHER PARENT The law of segregation states that during gamete formation, the alleles separate to different gametes The law of segregation states that during gamete formation, the alleles separate to different gametes

28. F1 GENERATION FATHERMOTHER T t T t T t T TT TT TT T T tT tT tT t t t t t F2 GENERATION - the law of dominance explained the heredity of the offspring of the f1 generation - the law of dominance explained the heredity of the offspring of the f1 generation - the law of segregation explained the heredity of the f2 generation - the law of segregation explained the heredity of the f2 generation

31. PHENOTYPES & GENOTYPES PHENOTYPE – THE WAY AN ORGANISM LOOKS AND BEHAVES – ITS PHYSICAL CHARACTERISTICS (i.e. – TALL, GREEN, BROWN HAIR, BLUE EYES, ETC.) PHENOTYPE – THE WAY AN ORGANISM LOOKS AND BEHAVES – ITS PHYSICAL CHARACTERISTICS (i.e. – TALL, GREEN, BROWN HAIR, BLUE EYES, ETC.) GENOTYPE – THE GENE COMBONATION (ALLELIC COMBINATION) OF AN ORGANISM – (i.e. – TT, Tt, tt, ETC.) GENOTYPE – THE GENE COMBONATION (ALLELIC COMBINATION) OF AN ORGANISM – (i.e. – TT, Tt, tt, ETC.) HOMOZYGOUS – 2 ALLELES ARE THE SAME HOMOZYGOUS – 2 ALLELES ARE THE SAME HETEROZYGOUS – 2 ALLELES DIFFERENT HETEROZYGOUS – 2 ALLELES DIFFERENT

32. ANSWER ON YOUR SHEET TRAITS = BLUE SKIN & YELLOW SKIN BB – IS THIS HOMOZYGOUS OR HETEROZYGOUS? IS BLUE SKIN OR YELLOW SKIN DOMINANT? HOMOZYGOUS BLUE

33. MENDEL’S DIHYBRID CROSSES MONOHYBRID – MENDEL LOOKED AT ONE TRAIT MONOHYBRID – MENDEL LOOKED AT ONE TRAIT IN HIS DIHYBRID CROSSES – HE LOOKED AT 2 TRAITS IN HIS DIHYBRID CROSSES – HE LOOKED AT 2 TRAITS WANTED TO SEE IF TRAITS ARE INHERITED TOGETHER OR INDEPENDENTLY WANTED TO SEE IF TRAITS ARE INHERITED TOGETHER OR INDEPENDENTLY

34. DIHYBRID CROSS TOOK TWO TRUE BREEDING PLANTS FOR 2 DIFFERENT TRAITS (ROUND/WRINKLED SEEDS ------- YELLOW/GREEN SEEDS) TOOK TWO TRUE BREEDING PLANTS FOR 2 DIFFERENT TRAITS (ROUND/WRINKLED SEEDS ------- YELLOW/GREEN SEEDS) 1 ST GENERATION 1 ST GENERATION WHAT WOULD HAPPEN IF HE CROSSED JUST TRUE BREEDING ROUND W/ TRUE BREEDING WRINKLED (ROUND IS DOMINANT) WHAT WOULD HAPPEN IF HE CROSSED JUST TRUE BREEDING ROUND W/ TRUE BREEDING WRINKLED (ROUND IS DOMINANT) ALL THE OFFSPRING ARE ROUND

35. DIHYBRID CROSS – 1 ST GENERATION CONT’D SO WHAT DO YOU THINK HAPPENED WHEN HE CROSSED TRUE BREEDING ROUND/YELLOW SEEDS WITH TRUE BREEDING WRINKLED/GREEN SEEDS SO WHAT DO YOU THINK HAPPENED WHEN HE CROSSED TRUE BREEDING ROUND/YELLOW SEEDS WITH TRUE BREEDING WRINKLED/GREEN SEEDS ALL THE F1 WERE ROUND AND YELLOW

36. DIHYBRID CROSS – 2 ND GENERATION TOOK THE F1 PLANTS AND BRED THEM TOGETHER (PHENOTYPE WAS ROUND/YELLOW X ROUND/YELLOW) TOOK THE F1 PLANTS AND BRED THEM TOGETHER (PHENOTYPE WAS ROUND/YELLOW X ROUND/YELLOW) 2 ND GENERATION 2 ND GENERATION FOUND ROUND/YELLOW - 9 FOUND ROUND/YELLOW - 9 FOUND ROUND/GREEN - 3 FOUND ROUND/GREEN - 3 FOUND WRINKLED/YELLOW - 3 FOUND WRINKLED/YELLOW - 3 FOUND WRINKLED/GREEN - 1 FOUND WRINKLED/GREEN - 1 ( 9 : 3 : 3 : 1 RATIO) ( 9 : 3 : 3 : 1 RATIO)

37. EXPLANATION OF 2 ND GENERATION MENDEL CAME UP W/ 2 ND LAW – THE LAW OF INDEPENDENT ASSORTMENT MENDEL CAME UP W/ 2 ND LAW – THE LAW OF INDEPENDENT ASSORTMENT GENES FOR DIFFERENT TRAITS ARE INHERITED INDEPENDENTLY FROM EACH OTHER GENES FOR DIFFERENT TRAITS ARE INHERITED INDEPENDENTLY FROM EACH OTHER THIS IS WHY MENDEL FOUND ALL THE DIFFERNENT COMBONATIONS OF TRAITS THIS IS WHY MENDEL FOUND ALL THE DIFFERNENT COMBONATIONS OF TRAITS

38. PUNNETT SQUARES A QUICK WAY TO FIND THE GENOTYPES IN UPCOMING GENERATIONS A QUICK WAY TO FIND THE GENOTYPES IN UPCOMING GENERATIONS 1 ST DRAW A BIG SQUARE AND DIVIDE IT IN 4’S 1 ST DRAW A BIG SQUARE AND DIVIDE IT IN 4’S

39. PUNNETT SQUARE CROSS T T X Tt

40. CONT’D T T X T t T T T t T TTT TtTt

41. DIHYBRID CROSSES A LITTLE DIFFERENT A LITTLE DIFFERENT H h G g X H h G g H h G g X H h G g MUST FIND OUT ALL THE POSSIBLE ALLELIC COMBONATIONS MUST FIND OUT ALL THE POSSIBLE ALLELIC COMBONATIONS USE THE FOIL METHOD LIKE IN MATH USE THE FOIL METHOD LIKE IN MATH

42. H h G g X H h G g 1. HG 2. Hg 3. hG 4. hg FOIL – FIRST, OUTSIDE, INSIDE, LAST BOTH PARENTS ARE THE SAME

43. NOW LET’S DO A DIHYBRID CROSS H h G g X H h G g HGHghG hg hg HG Hg hG hg HHGGHHGgHhGGHhGg HHGgHHggHhGgHhgg HhGGHhGghhGGhhGg HhGgHhgghhGghhgg

44. WHAT ARE THE PHENOTYPIC RATIO’S? H h G g X H h G g HGHghG hg hg HG Hg hG hg HHGGHHGgHhGGHhGg HHGgHHggHhGgHhgg HhGGHhGghhGGhhGg HhGgHhgghhGghhgg

45. Figure 11.10 Analysis of a dihybrid cross

46. PROBABILITY WILL REAL LIFE FOLLOW THE RESULTS FROM A PUNNETT SQUARE? WILL REAL LIFE FOLLOW THE RESULTS FROM A PUNNETT SQUARE? NO!!!!!! – A PUNNETT SQUARE ONLY SHOWS WHAT WILL PROBABLY OCCUR NO!!!!!! – A PUNNETT SQUARE ONLY SHOWS WHAT WILL PROBABLY OCCUR IT’S A LOT LIKE FLIPPING A COIN – YOU CAN ESTIMATE YOUR CHANCES OF GETTING HEADS, BUT REALITY DOESN’T ALWAYS FOLLOW PROBABILITY IT’S A LOT LIKE FLIPPING A COIN – YOU CAN ESTIMATE YOUR CHANCES OF GETTING HEADS, BUT REALITY DOESN’T ALWAYS FOLLOW PROBABILITY

47. MEIOSIS MEIOSIS GENES, CHROMOSOMES, AND NUMBERS GENES, CHROMOSOMES, AND NUMBERS CHROMOSOMES HAVE 100’S OR 1000’S OF GENES CHROMOSOMES HAVE 100’S OR 1000’S OF GENES GENES FOUND ON CHROMOSOMES GENES FOUND ON CHROMOSOMES

48. DIPLOID & HAPLOID CELLS ALL BODY CELLS (SOMATIC CELLS) HAVE CHROMOSOMES IN PAIRS ALL BODY CELLS (SOMATIC CELLS) HAVE CHROMOSOMES IN PAIRS BODY CELLS ARE CALLED DIPLOID CELLS (2n) BODY CELLS ARE CALLED DIPLOID CELLS (2n) HUMANS HAVE THE 2n # OF CHROMOSOMES HUMANS HAVE THE 2n # OF CHROMOSOMES

49. DIPLOID AND HAPLOID CELLS CONT’D HAPLOID CELLS HAPLOID CELLS ONLY HAVE 1 OF EACH TYPE OF CHROMOSOME (DIPLOID CELLS HAVE 2 OF EACH TYPE) ONLY HAVE 1 OF EACH TYPE OF CHROMOSOME (DIPLOID CELLS HAVE 2 OF EACH TYPE) SYMBOL IS (n) SYMBOL IS (n) SEX CELLS HAVE THE n # OF CHROMOSOMES SEX CELLS HAVE THE n # OF CHROMOSOMES

50. HOMOLOGOUS CHROMOSOMES HOMOLOGOUS CHROMOSOMES ARE THE PAIRED CHROMOSOMES THAT CONTAIN THE SAME TYPE OF GENTIC INFORMATION, SAME BANDING PATTERNS, SAME CENTROMERE LOCATION, ETC. HOMOLOGOUS CHROMOSOMES ARE THE PAIRED CHROMOSOMES THAT CONTAIN THE SAME TYPE OF GENTIC INFORMATION, SAME BANDING PATTERNS, SAME CENTROMERE LOCATION, ETC. THEY MAY HAVE DIFFERENT ALLELES, SO NOT PERFECTLY IDENTICAL THEY MAY HAVE DIFFERENT ALLELES, SO NOT PERFECTLY IDENTICAL WHY DO THEY HAVE DIFFERENT ALLELES? WHY DO THEY HAVE DIFFERENT ALLELES? CAME FROM DIFFERENT PARENTS

51. IMPORTANT THINGS TO KNOW IMPORTANT THINGS TO KNOW CROSSING OVER – OCCURS DURING PROPHASE I CROSSING OVER – OCCURS DURING PROPHASE I CROSSING OVER CROSSING OVER CREATES GENETIC VARIABILITY (RECOMBINATION OF GENES) CREATES GENETIC VARIABILITY (RECOMBINATION OF GENES)GENETIC VARIABILITY GENETIC VARIABILITY IN MEIOSIS I, HOMOLOGOUS CHROMOSOMES SEPARATE (ANAPHASE I) IN MEIOSIS I, HOMOLOGOUS CHROMOSOMES SEPARATE (ANAPHASE I) IN MEIOSIS II, SISTER CHROMATIDS SEPARATE IN MEIOSIS II, SISTER CHROMATIDS SEPARATE TETRAD – WHAT THE HOMOLOGOUS CHROMOSOMES ARE CALLED WHEN THEY PAIR UP DURING PROPHASE I TETRAD – WHAT THE HOMOLOGOUS CHROMOSOMES ARE CALLED WHEN THEY PAIR UP DURING PROPHASE I

52. Figure 11.11 The journey from DNA to phenotype

53. 11.6 Why Some Traits Don’t Show Mendelian Inheritance Often the expression of phenotype is not straightforward Often the expression of phenotype is not straightforward Continuous variation Continuous variation characters can show a range of small differences when multiple genes act jointly to influence a character characters can show a range of small differences when multiple genes act jointly to influence a character this type of inheritance is called polygenic this type of inheritance is called polygenic

54. Figure 11.12 Height is a continuously varying character

55. 11.6 Why Some Traits Don’t Show Mendelian Inheritance Pleiotropic effects Pleiotropic effects an allele that has more than one effect on the phenotype is considered pleiotropic: one gene affects many characters an allele that has more than one effect on the phenotype is considered pleiotropic: one gene affects many characters these effects are characteristic of many inherited disorders, such as cystic fibrosis and sickle-cell anemia these effects are characteristic of many inherited disorders, such as cystic fibrosis and sickle-cell anemia

56. Figure 11.13 Pleiotropic effects of the cystic fibrosis gene, cf

57. 11.6 Why Some Traits Don’t Show Mendelian Inheritance Incomplete dominance Incomplete dominance not all alternative alleles are either fully dominant or fully recessive in heterozygotes not all alternative alleles are either fully dominant or fully recessive in heterozygotes in such cases, the alleles exhibit incomplete dominance and produce a heterozygous phenotype that is intermediate between those of the parents in such cases, the alleles exhibit incomplete dominance and produce a heterozygous phenotype that is intermediate between those of the parents

58. Figure 11.14 Incomplete dominance

59. 11.6 Why Some Traits Don’t Show Mendelian Inheritance Environmental effects Environmental effects the degree to which many alleles are expressed depends on the environment the degree to which many alleles are expressed depends on the environment for example, some alleles are heat-sensitive for example, some alleles are heat-sensitive arctic foxes only produce fur pigment when temperatures are warm arctic foxes only produce fur pigment when temperatures are warm the ch allele in Himalayan rabbits and Siamese cats encodes a heat-sensitive enzyme, called tyrosinase, that controls pigment production the ch allele in Himalayan rabbits and Siamese cats encodes a heat-sensitive enzyme, called tyrosinase, that controls pigment production tyrosinase is inactive at high temperatures tyrosinase is inactive at high temperatures

60. Figure 11.15 Environmental effects on an allele

61. 11.6 Why Some Traits Don’t Show Mendelian Inheritance Epistasis Epistasis in some situations, two or more genes interact with each other, such that one gene contributes to or masks the expression of the other gene in some situations, two or more genes interact with each other, such that one gene contributes to or masks the expression of the other gene in epistasis, one gene modifies the phenotypic expression produced by the other in epistasis, one gene modifies the phenotypic expression produced by the other for example, in corn, to produce and deposit pigment, a plant must possess at least one functional copy of each of two genes for example, in corn, to produce and deposit pigment, a plant must possess at least one functional copy of each of two genes one gene controls pigment deposition one gene controls pigment deposition the other gene controls pigment production the other gene controls pigment production

62. Figure 11.16 How epistasis affects kernel color

63. Why is coat color in Labrador retrievers an example of epistasis? E gene determines if dark pigment will be deposited in fur or not E gene determines if dark pigment will be deposited in fur or not genotype ee, no pigment will be deposited in the fur, and it will be yellow genotype ee, no pigment will be deposited in the fur, and it will be yellow genotype E_, pigment will be deposited in the fur genotype E_, pigment will be deposited in the fur A second gene, the B gene, determines how dark the pigment will be A second gene, the B gene, determines how dark the pigment will be Yellow dogs with the genotype eebb will have brown pigment on their nose, lips, and eye rims, while yellow dogs with the genotype eeB_ will have black pigment in these areas. Yellow dogs with the genotype eebb will have brown pigment on their nose, lips, and eye rims, while yellow dogs with the genotype eeB_ will have black pigment in these areas.

64. Figure 11.17 The effect of epistatic interactions on coat color in dogs

65. 11.6 Why Some Traits Don’t Show Mendelian Inheritance Codominance Codominance a gene may have more than two alleles in a population a gene may have more than two alleles in a population often, in heterozygotes, there is not a dominant allele but, instead, both alleles are expressed often, in heterozygotes, there is not a dominant allele but, instead, both alleles are expressed these alleles are said to be codominant these alleles are said to be codominant

66. 11.6 Why Some Traits Don’t Show Mendelian Inheritance The gene that determines ABO blood type in humans exhibits more than one dominant allele The gene that determines ABO blood type in humans exhibits more than one dominant allele the gene encodes an enzyme that adds sugars to lipids on the membranes of red blood cells the gene encodes an enzyme that adds sugars to lipids on the membranes of red blood cells these sugars act as recognition markers for cells in the immune system these sugars act as recognition markers for cells in the immune system the gene that encodes the enzyme, designated I, has three alleles: I A,I B, and i the gene that encodes the enzyme, designated I, has three alleles: I A,I B, and i different combinations of the three alleles produce four different phenotypes, or bloodtypes (A, B, AB, and O) different combinations of the three alleles produce four different phenotypes, or bloodtypes (A, B, AB, and O) both I A and I B are dominant over i and also codominant both I A and I B are dominant over i and also codominant

67. Figure 11.19 Multiple alleles controlling the ABO blood groups

68. 67 Inheritance of Blood Type

69. 11.8 Human Chromosomes Nondisjunction may also affect the sex chromosomes Nondisjunction may also affect the sex chromosomes nondisjunction of the X chromosome creates three possible viable conditions nondisjunction of the X chromosome creates three possible viable conditions XXX female XXX female usually taller than average but other symptoms vary usually taller than average but other symptoms vary XXY male (Klinefelter syndrome) XXY male (Klinefelter syndrome) sterile male with many female characteristics and diminished mental capacity sterile male with many female characteristics and diminished mental capacity XO female (Turner syndrome) XO female (Turner syndrome) sterile female with webbed neck and diminished stature sterile female with webbed neck and diminished stature

70. Figure 11.26 Nondisjunction of the X chromosome

71. 11.9 The Role of Mutations in Human Heredity Accidental changes in genes are called mutations Accidental changes in genes are called mutations mutations occur only rarely and almost always result in recessive alleles mutations occur only rarely and almost always result in recessive alleles not eliminated from the population because they are not usually expressed in most individuals (heterozygotes) not eliminated from the population because they are not usually expressed in most individuals (heterozygotes) in some cases, particular mutant alleles have become more common in human populations and produce harmful effects called genetic disorders in some cases, particular mutant alleles have become more common in human populations and produce harmful effects called genetic disorders

72. Table 11.3 Some Important Genetic Disorders

73. 11.9 The Role of Mutations in Human Heredity To study human heredity, scientists examine crosses that have already been made To study human heredity, scientists examine crosses that have already been made they identify which relatives exhibit a trait by looking at family trees or pedigrees they identify which relatives exhibit a trait by looking at family trees or pedigrees often one can determine whether a trait is sex-linked or autosomal and whether the trait’s phenotype is dominant or recessive often one can determine whether a trait is sex-linked or autosomal and whether the trait’s phenotype is dominant or recessive

74. Figure 11.27 A general pedigree

75. 11.9 The Role of Mutations in Human Heredity Sickle-cell anemia is a recessive hereditary disorder Sickle-cell anemia is a recessive hereditary disorder affected individuals are homozygous recessive and carry a mutated gene that produces a defective version of hemoglobin affected individuals are homozygous recessive and carry a mutated gene that produces a defective version of hemoglobin the hemoglobin sticks together inappropriately and produces a stiff red blood cell with a sickle-shape the hemoglobin sticks together inappropriately and produces a stiff red blood cell with a sickle-shape the cells cannot move through the blood vessels easily and tend to form clots the cells cannot move through the blood vessels easily and tend to form clots this causes sufferers to have intermittent illness and shortened life spans this causes sufferers to have intermittent illness and shortened life spans

76. Figure 11.29 Inheritance of sickle-cell anemia

77. 11.9 The Role of Mutations in Human Heredity The sickle-cell mutation to hemoglobin affects the stickiness of the hemoglobin protein surface but not its oxygen-binding ability The sickle-cell mutation to hemoglobin affects the stickiness of the hemoglobin protein surface but not its oxygen-binding ability In heterozygous individuals, only some of their red blood cells become sickled when oxygen levels become low In heterozygous individuals, only some of their red blood cells become sickled when oxygen levels become low this may explain why the sickle-cell allele is so frequent among people of African descent this may explain why the sickle-cell allele is so frequent among people of African descent the presence of the allele increases resistance to malaria infection the presence of the allele increases resistance to malaria infection

78. Figure 11.30 The sickle-cell allele confers resistance to malaria