Mendel and the Gene Idea 11 Mendel and the Gene Idea

Overview: Drawing from the Deck of Genes What genetic principles account for the passing of traits from parents to offspring? The “blending” hypothesis is the idea that genetic material from the two parents blends together (the way blue and yellow paint blend to make green) 2

The “particulate” hypothesis is the idea that parents pass on discrete heritable units (genes) Mendel documented a particulate mechanism through his experiments with garden peas 3

Figure 11.1 Figure 11.1 What principles of inheritance did Gregor Mendel discover by breeding garden pea plants? 4

Concept 11.1: Mendel used the scientific approach to identify two laws of inheritance Mendel discovered the basic principles of heredity by breeding garden peas in carefully planned experiments 5

Mendel’s Experimental, Quantitative Approach Mendel probably chose to work with peas because There are many varieties with distinct heritable features, or characters (such as flower color); character variants (such as purple or white flowers) are called traits He could control mating between plants 6

Technique Parental generation (P) Stamens Carpel Results First filial Figure 11.2 Technique 1 2 Parental generation (P) 3 Stamens Carpel 4 Figure 11.2 Research method: crossing pea plants Results 5 First filial generation offspring (F1) 7

Mendel chose to track only characters that occurred in two distinct alternative forms He also used varieties that were true-breeding (plants that produce offspring of the same variety when they self-pollinate) 8

The true-breeding parents are the P generation In a typical experiment, Mendel mated two contrasting, true-breeding varieties, a process called hybridization The true-breeding parents are the P generation The hybrid offspring of the P generation are called the F1 generation When F1 individuals self-pollinate or cross- pollinate with other F1 hybrids, the F2 generation is produced 9

The Law of Segregation When Mendel crossed contrasting, true-breeding white- and purple-flowered pea plants, all of the F1 hybrids were purple When Mendel crossed the F1 hybrids, many of the F2 plants had purple flowers, but some had white Mendel discovered a ratio of about three to one, purple to white flowers, in the F2 generation 10

(true-breeding parents) Figure 11.3-1 Experiment P Generation (true-breeding parents) Purple flowers White flowers Figure 11.3-1 Inquiry: When F1 hybrid pea plants self- or cross-pollinate, which traits appear in the F2 generation? (step 1) 11

(true-breeding parents) Figure 11.3-2 Experiment P Generation (true-breeding parents) Purple flowers White flowers F1 Generation (hybrids) All plants had purple flowers Self- or cross-pollination Figure 11.3-2 Inquiry: When F1 hybrid pea plants self- or cross-pollinate, which traits appear in the F2 generation? (step 2) 12

All plants had purple flowers Figure 11.3-3 Experiment P Generation (true-breeding parents) Purple flowers White flowers F1 Generation (hybrids) All plants had purple flowers Self- or cross-pollination Figure 11.3-3 Inquiry: When F1 hybrid pea plants self- or cross-pollinate, which traits appear in the F2 generation? (step 3) F2 Generation 705 purple-flowered plants 224 white-flowered plants 13

Mendel reasoned that in the F1 plants, the heritable factor for white flowers was hidden or masked in the presence of the purple-flower factor He called the purple flower color a dominant trait and the white flower color a recessive trait The factor for white flowers was not diluted or destroyed because it reappeared in the F2 generation 14

What Mendel called a “heritable factor” is what we now call a gene Mendel observed the same pattern of inheritance in six other pea plant characters, each represented by two traits What Mendel called a “heritable factor” is what we now call a gene 15

Table 11.1 Table 11.1 The results of Mendel’s F1 crosses for seven characters in pea plants 16

Table 11.1a Table 11.1a The results of Mendel’s F1 crosses for seven characters in pea plants (part 1) 17

Table 11.1b Table 11.1b The results of Mendel’s F1 crosses for seven characters in pea plants (part 2) 18

Mendel’s Model Mendel developed a model to explain the 3:1 inheritance pattern he observed in F2 offspring Four related concepts make up this model 19

These alternative versions of a gene are now called alleles First, alternative versions of genes account for variations in inherited characters For example, the gene for flower color in pea plants exists in two versions, one for purple flowers and the other for white flowers These alternative versions of a gene are now called alleles Each gene resides at a specific locus on a specific chromosome 20

Allele for purple flowers Figure 11.4 Allele for purple flowers Pair of homologous chromosomes Locus for flower-color gene Figure 11.4 Alleles, alternative versions of a gene Allele for white flowers 21

Second, for each character, an organism inherits two alleles, one from each parent Mendel made this deduction without knowing about the existence of chromosomes Two alleles at a particular locus may be identical, as in the true-breeding plants of Mendel’s P generation Alternatively, the two alleles at a locus may differ, as in the F1 hybrids 22

Third, if the two alleles at a locus differ, then one (the dominant allele) determines the organism’s appearance, and the other (the recessive allele) has no noticeable effect on appearance In the flower-color example, the F1 plants had purple flowers because the allele for that trait is dominant 23

Fourth (now known as the law of segregation), the two alleles for a heritable character separate (segregate) during gamete formation and end up in different gametes Thus, an egg or a sperm gets only one of the two alleles that are present in the organism This segregation of alleles corresponds to the distribution of homologous chromosomes to different gametes in meiosis 24

P Generation Appearance: Genetic makeup: Purple flowers PP Figure 11.5-1 P Generation Appearance: Genetic makeup: Purple flowers PP White flowers pp Gametes: P p Figure 11.5-1 Mendel’s law of segregation (step 1) 25

P Generation F1 Generation Appearance: Genetic makeup: Purple flowers Figure 11.5-2 P Generation Appearance: Genetic makeup: Purple flowers PP White flowers pp Gametes: P p F1 Generation Appearance: Genetic makeup: Purple flowers Pp Gametes: ½ ½ P p Figure 11.5-2 Mendel’s law of segregation (step 2) 26

P Generation F1 Generation F2 Generation Appearance: Genetic makeup: Figure 11.5-3 P Generation Appearance: Genetic makeup: Purple flowers PP White flowers pp Gametes: P p F1 Generation Appearance: Genetic makeup: Purple flowers Pp Gametes: ½ ½ P p Sperm from F1 (Pp) plant Figure 11.5-3 Mendel’s law of segregation (step 3) F2 Generation P p P Eggs from F1 (Pp) plant PP Pp p Pp pp 3 : 1 27

Mendel’s segregation model accounts for the 3:1 ratio he observed in the F2 generation of his numerous crosses The possible combinations of sperm and egg can be shown using a Punnett square, a diagram for predicting the results of a genetic cross between individuals of known genetic makeup A capital letter represents a dominant allele, and a lowercase letter represents a recessive allele For example, P is the purple-flower allele and p is the white-flower allele 28

Useful Genetic Vocabulary An organism with two identical alleles for a character is said to be homozygous for the gene controlling that character An organism that has two different alleles for a gene is said to be heterozygous for the gene controlling that character Unlike homozygotes, heterozygotes are not true-breeding 29

Because of the effects of dominant and recessive alleles, an organism’s traits do not always reveal its genetic composition Therefore, we distinguish between an organism’s phenotype, or physical appearance, and its genotype, or genetic makeup In the example of flower color in pea plants, PP and Pp plants have the same phenotype (purple) but different genotypes 30

PP (homozygous) Pp (heterozygous) Pp (heterozygous) pp (homozygous) Figure 11.6 Phenotype Genotype PP (homozygous) Purple 1 3 Pp (heterozygous) Purple 2 Pp (heterozygous) Purple Figure 11.6 Phenotype versus genotype pp (homozygous) 1 White 1 Ratio 3:1 Ratio 1:2:1 31

The Testcross How can we tell the genotype of an individual with the dominant phenotype? Such an individual could be either homozygous dominant or heterozygous The answer is to carry out a testcross: breeding the mystery individual with a homozygous recessive individual If any offspring display the recessive phenotype, the mystery parent must be heterozygous 32

Dominant phenotype, unknown genotype: PP or Pp? Recessive phenotype, Figure 11.7 Technique Dominant phenotype, unknown genotype: PP or Pp? Recessive phenotype, known genotype: pp Predictions If purple-flowered parent is PP or If purple-flowered parent is Pp Sperm Sperm p p p p P P Pp Pp Pp Pp Eggs Eggs Figure 11.7 Research method: the testcross P p Pp Pp pp pp Results or All offspring purple ½ offspring purple and ½ offspring white 33

The Law of Independent Assortment Mendel derived the law of segregation by following a single character The F1 offspring produced in this cross were monohybrids, individuals that are heterozygous for one character A cross between such heterozygotes is called a monohybrid cross 34

Mendel identified his second law of inheritance by following two characters at the same time Crossing two true-breeding parents differing in two characters produces dihybrids in the F1 generation, heterozygous for both characters A dihybrid cross, a cross between F1 dihybrids, can determine whether two characters are transmitted to offspring as a package or independently 35

independent assortment Figure 11.8 Experiment P Generation YYRR yyrr Gametes YR yr F1 Generation YyRr Predictions Hypothesis of dependent assortment Hypothesis of independent assortment Sperm or Predicted offspring in F2 generation ¼ YR ¼ Yr ¼ yR ¼ yr Sperm ½ YR ½ yr ¼ YR YYRR YYRr YyRR YyRr ½ YR YYRR YyRr ¼ Yr Eggs YYRr YYrr YyRr Yyrr Figure 11.8 Inquiry: Do the alleles for one character segregate into gametes dependently or independently of the alleles for a different character? Eggs ½ yr YyRr yyrr ¼ yR YyRR YyRr yyRR yyRr ¾ ¼ ¼ yr Phenotypic ratio 3:1 YyRr Yyrr yyRr yyrr 9 16 3 16 3 16 1 16 Phenotypic ratio 9:3:3:1 Results 315 108 101 32 Phenotypic ratio approximately 9:3:3:1 36

Experiment YYRR yyrr P Generation Gametes F1 Generation YyRr YR yr Figure 11.8a Experiment YYRR yyrr P Generation Gametes YR yr F1 Generation YyRr Figure 11.8a Inquiry: Do the alleles for one character segregate into gametes dependently or independently of the alleles for a different character? (part 1: experiment) 37

independent assortment Figure 11.8b Hypothesis of dependent assortment Hypothesis of independent assortment Sperm Predicted offspring in F2 generation ¼ YR ¼ Yr ¼ yR ¼ yr Sperm ½ YR ½ yr ¼ YR YYRR YYRr YyRR YyRr ½ YR YYRR YyRr ¼ Yr Eggs YYRr YYrr YyRr Yyrr Eggs ½ yr YyRr yyrr ¼ yR YyRR YyRr yyRR yyRr ¾ ¼ ¼ yr Phenotypic ratio 3:1 Figure 11.8b Inquiry: Do the alleles for one character segregate into gametes dependently or independently of the alleles for a different character? (part 2: results) YyRr Yyrr yyRr yyrr 9 16 3 16 3 16 1 16 Phenotypic ratio 9:3:3:1 Results 315 108 101 32 Phenotypic ratio approximately 9:3:3:1 38

The results of Mendel’s dihybrid experiments are the basis for the law of independent assortment It states that each pair of alleles segregates independently of each other pair of alleles during gamete formation This law applies to genes on different, nonhomologous chromosomes or those far apart on the same chromosome Genes located near each other on the same chromosome tend to be inherited together 39

Concept 11.2: The laws of probability govern Mendelian inheritance Mendel’s laws of segregation and independent assortment reflect the rules of probability When tossing a coin, the outcome of one toss has no impact on the outcome of the next toss In the same way, the alleles of one gene segregate into gametes independently of another gene’s alleles 40

The Multiplication and Addition Rules Applied to Monohybrid Crosses The multiplication rule states that the probability that two or more independent events will occur together is the product of their individual probabilities This can be applied to an F1 monohybrid cross Segregation in a heterozygous plant is like flipping a coin: Each gamete has a chance of carrying the dominant allele and a chance of carrying the recessive allele 41

Segregation of alleles into eggs Segregation of alleles into sperm Figure 11.9 Rr  Rr Segregation of alleles into eggs Segregation of alleles into sperm Sperm ½ R ½ r R R r ½ R R Figure 11.9 Segregation of alleles and fertilization as chance events ¼ ¼ Eggs r r R r ½ r ¼ ¼ 42

The addition rule states that the probability that any one of two or more mutually exclusive events will occur is calculated by adding together their individual probabilities It can be used to figure out the probability that an F2 plant from a monohybrid cross will be heterozygous rather than homozygous 43

Solving Complex Genetics Problems with the Rules of Probability We can apply the rules of probability to predict the outcome of crosses involving multiple characters A dihybrid or other multicharacter cross is equivalent to two or more independent monohybrid crosses occurring simultaneously In calculating the chances for various genotypes, each character is considered separately, and then the individual probabilities are multiplied 44

For example, if we cross F1 heterozygotes of genotype YyRr, we can calculate the probability of different genotypes among the F2 generation 45

Figure 11.UN01 Figure 11.UN01 In-text figure, dihybrid calculations, p. 214 46

For example, for the cross PpYyRr  Ppyyrr, we can calculate the probability of offspring showing at least two recessive traits 47

Figure 11.UN02 Figure 11.UN02 In-text figure, trihybrid probabilities, p. 214 48

Concept 11.3: Inheritance patterns are often more complex than predicted by simple Mendelian genetics Not all heritable characters are determined as simply as the traits Mendel studied However, the basic principles of segregation and independent assortment apply even to more complex patterns of inheritance 49

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 single gene influences multiple phenotypes 50

Degrees of Dominance Complete dominance occurs when phenotypes of the heterozygote and dominant homozygote are identical In incomplete dominance, the phenotype of F1 hybrids is somewhere between the phenotypes of the two parental varieties In codominance, two dominant alleles affect the phenotype in separate, distinguishable ways 51

P Generation Red CRCR White CWCW Gametes CR CW Figure 11.10-1 Figure 11.10-1 Incomplete dominance in snapdragon color (step 1) 52

P Generation Red CRCR White CWCW Gametes Pink CRCW F1 Generation Figure 11.10-2 P Generation Red CRCR White CWCW Gametes CR CW Pink CRCW F1 Generation Gametes ½ CR ½ CW Figure 11.10-2 Incomplete dominance in snapdragon color (step 2) 53

P Generation Red CRCR White CWCW Gametes Pink CRCW F1 Generation Figure 11.10-3 P Generation Red CRCR White CWCW Gametes CR CW Pink CRCW F1 Generation Gametes ½ CR ½ CW Figure 11.10-3 Incomplete dominance in snapdragon color (step 3) Sperm ½ CR ½ CW F2 Generation ½ CR CRCR CRCW Eggs ½ CW CRCW CWCW 54

The Relationship Between Dominance and Phenotype Alleles are simply variations in a gene’s nucleotide sequence When a dominant allele coexists with a recessive allele in a heterozygote, they do not actually interact at all For any character, dominant/recessive relationships of alleles depend on the level at which we examine the phenotype 55

Tay-Sachs disease is fatal; a dysfunctional enzyme causes an accumulation of lipids in the brain At the organismal level, the allele is recessive At the biochemical level, the phenotype (i.e., the enzyme activity level) is incompletely dominant At the molecular level, the alleles are codominant 56

Frequency of Dominant Alleles Dominant alleles are not necessarily more common in populations than recessive alleles For example, one baby out of 400 in the United States is born with extra fingers or toes, a dominant trait called polydactyly 57

Multiple Alleles Most genes exist in populations in more than two allelic forms For example, the four phenotypes of the ABO blood group in humans are determined by three alleles of the gene: IA, IB, and i. The enzyme (I) adds specific carbohydrates to the surface of blood cells The enzyme encoded by IA adds the A carbohydrate, and the enzyme encoded by IB adds the B carbohydrate; the enzyme encoded by the i allele adds neither 58

(a) The three alleles for the ABO blood groups and their carbohydrates Figure 11.11 (a) The three alleles for the ABO blood groups and their carbohydrates Allele IA IB i Carbohydrate A B none (b) Blood group genotypes and phenotypes Genotype IAIA or IAi IBIB or IBi IAIB ii Figure 11.11 Multiple alleles for the ABO blood groups Red blood cell appearance Phenotype (blood group) A B AB O 59

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 60

Extending Mendelian Genetics for Two or More Genes Some traits may be determined by two or more genes 61

Epistasis In epistasis, a gene at one locus alters the phenotypic expression of a gene at a second locus For example, in Labrador retrievers and many other mammals, coat color depends 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 62

¼ ¼ ¼ ¼ ¼ ¼ ¼ ¼ BbEe BbEe Sperm Eggs BBEE BbEE BBEe BbEe BbEE bbEE Figure 11.12 BbEe BbEe Sperm ¼ BE ¼ bE ¼ Be ¼ be Eggs ¼ BE BBEE BbEE BBEe BbEe ¼ bE BbEE bbEE BbEe bbEe ¼ Be BBEe BbEe BBee Bbee Figure 11.12 An example of epistasis ¼ be BbEe bbEe Bbee bbee 9 : 3 : 4 63

Polygenic Inheritance Quantitative characters are those that 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 64

Figure 11.13 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 Figure 11.13 A simplified model for polygenic inheritance of skin color 1 8 1 8 1 64 6 64 15 64 20 64 15 64 6 64 1 64 1 64 Phenotypes: Number of dark-skin alleles: 1 2 3 4 5 6 65

Nature and Nurture: The Environmental Impact on Phenotype Another departure from Mendelian genetics arises when the phenotype for a character depends on environment as well as genotype The norm of reaction is the phenotypic range of a genotype influenced by the environment 66

The phenotypic range is generally broadest for polygenic characters Such characters are called multifactorial because genetic and environmental factors collectively influence phenotype 67

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 68

Concept 11.4: Many human traits follow Mendelian patterns of inheritance Humans are not good subjects for genetic research Generation time is too long Parents produce relatively few offspring Breeding experiments are unacceptable However, basic Mendelian genetics endures as the foundation of human genetics 69

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 70

Pedigrees can also be used to make predictions about future offspring We can use the multiplication and addition rules to predict the probability of specific phenotypes 71

FF or Ff FF or Ff WW or Ww Attached earlobe Free earlobe Figure 11.14 Key Male Female Affected male Affected female Mating Offspring, in birth order (first-born on left) Ff Ff ff Ff 1st generation (grandparents) Ww ww ww Ww 2nd generation (parents, aunts, and uncles) FF or Ff ff ff Ff Ff ff Ww ww ww Ww Ww ww 3rd generation (two sisters) ff FF or Ff Figure 11.14 Pedigree analysis WW or Ww ww Widow’s peak No widow’s peak Attached earlobe Free earlobe (a) Is a widow’s peak a dominant or recessive trait? (b) Is an attached earlobe a dominant or recessive trait? 72

(parents, aunts, and uncles) Figure 11.14a Key Male Affected male Mating Offspring, in birth order (first-born on left) Female Affected female 1st generation (grandparents) Ww ww ww Ww 2nd generation (parents, aunts, and uncles) Ww ww ww Ww Ww ww 3rd generation (two sisters) Figure 11.14a Pedigree analysis (part 1: widow’s peak) WW or Ww ww Widow’s peak No widow’s peak (a) Is a widow’s peak a dominant or recessive trait? 73

Figure 11.14aa Figure 11.14aa Pedigree analysis (part 1a: widow’s peak photo) Widow’s peak 74

No widow’s peak Figure 11.14ab Figure 11.14ab Pedigree analysis (part 1b: absence of widow’s peak photo) No widow’s peak 75

(parents, aunts, and uncles) FF or Ff ff ff Ff Ff ff Figure 11.14b Key Male Affected male Mating Offspring, in birth order (first-born on left) Female Affected female 1st generation (grandparents) Ff Ff ff Ff 2nd generation (parents, aunts, and uncles) FF or Ff ff ff Ff Ff ff 3rd generation (two sisters) Figure 11.14b Pedigree analysis (part 2: attached earlobe) ff FF or Ff Attached earlobe Free earlobe (b) Is an attached earlobe a dominant or recessive trait? 76

Attached earlobe Figure 11.14ba Figure 11.14ba Pedigree analysis (part 2a: attached earlobe photo) Attached earlobe 77

Figure 11.14bb Figure 11.14bb Pedigree analysis (part 2b: free earlobe photo) Free earlobe 78

Recessively Inherited Disorders Many genetic disorders are inherited in a recessive manner These range from relatively mild to life-threatening 79

The Behavior of Recessive Alleles Recessively inherited disorders show up only in individuals homozygous for the allele Carriers are heterozygous individuals who carry the recessive allele but are phenotypically normal Most people who have recessive disorders are born to parents who are carriers of the disorder 80

Parents Normal Aa Normal Aa Sperm A a Eggs Aa Normal (carrier) AA Figure 11.15 Parents Normal Aa Normal Aa Sperm A a Eggs Aa Normal (carrier) AA Normal A Aa Normal (carrier) Figure 11.15 Albinism: a recessive trait aa Albino a 81

Figure 11.15a Figure 11.15a Albinism: a recessive trait (photo) 82

If a recessive allele that causes a disease is rare, then the chance of two carriers meeting and mating is low Consanguineous (between close relatives) matings increase the chance of mating between two carriers of the same rare allele Most societies and cultures have laws or taboos against marriages between close relatives 83

Cystic Fibrosis Cystic fibrosis is the most common lethal genetic disease in the United States,striking one out of every 2,500 people of European descent The cystic fibrosis allele results in defective or absent chloride transport channels in plasma membranes leading to a buildup of chloride ions outside the cell Symptoms include mucus buildup in some internal organs and abnormal absorption of nutrients in the small intestine 84

Sickle-Cell Disease: A Genetic Disorder with Evolutionary Implications Sickle-cell disease affects one out of 400 African-Americans The disease is caused by the substitution of a single amino acid in the hemoglobin protein in red blood cells In homozygous individuals, all hemoglobin is abnormal (sickle-cell) Symptoms include physical weakness, pain, organ damage, and even paralysis 85

Heterozygotes (said to have sickle-cell trait) are usually healthy but may suffer some symptoms About one out of ten African-Americans has sickle-cell trait, an unusually high frequency of an allele with detrimental effects in homozygotes Heterozygotes are less susceptible to the malaria parasite, so there is an advantage to being heterozygous 86

Dominantly Inherited Disorders Some human disorders are caused by dominant alleles 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 87

Dwarf Dd Normal dd Dd Dwarf dd Normal Dd Dwarf dd Normal Figure 11.16 Parents Dwarf Dd Normal dd Sperm D d Eggs Dd Dwarf dd Normal d Figure 11.16 Achondroplasia: a dominant trait Dd Dwarf dd Normal d 88

Figure 11.16a Figure 11.16a Achondroplasia: a dominant trait (photo) 89

The timing of onset of a disease significantly affects its inheritance Huntington’s disease is a degenerative disease of the nervous system The disease has no obvious phenotypic effects until the individual is about 35 to 45 years of age Once the deterioration of the nervous system begins the condition is irreversible and fatal 90

Multifactorial Disorders Many diseases, such as heart disease, diabetes, alcoholism, mental illnesses, and cancer, have both genetic and environmental components Lifestyle has a tremendous effect on phenotype for cardiovascular health and other multifactorial characters 91

Genetic Counseling Based on Mendelian Genetics Genetic counselors can provide information to prospective parents concerned about a family history for a specific disease Each child represents an independent event in the sense that its genotype is unaffected by the genotypes of older siblings 92

Figure 11.UN03 1 64 6 64 15 64 20 64 15 64 6 64 1 64 Phenotypes: Number of dark-skin alleles: 1 2 3 4 5 6 Figure 11.UN03 Skills exercise: making a histogram and analyzing a distribution pattern 93

PP (homozygous) Pp (heterozygous) Pp (heterozygous) pp (homozygous) Figure 11.UN04 PP (homozygous) Pp (heterozygous) Pp (heterozygous) Figure 11.UN04 Summary of key concepts: monohybrid genotypes pp (homozygous) 94

alleles of a single gene Figure 11.UN05 Relationship among alleles of a single gene Description Example Complete dominance of one allele Heterozygous phenotype same as that of homo- zygous dominant PP Pp Incomplete dominance of either allele Heterozygous phenotype intermediate between the two homozygous phenotypes CRCR CRCW CWCW Codominance Both phenotypes expressed in heterozygotes IAIB Figure 11.UN05 Summary of key concepts: single-gene Mendelian extensions Multiple alleles In the whole population, some genes have more than two alleles ABO blood group alleles IA, IB, i Pleiotropy One gene is able to affect multiple phenotypic characters Sickle-cell disease 95

Relationship among two or more genes Figure 11.UN06 Relationship among two or more genes Description Example Epistasis The phenotypic expression of one gene affects the expression of another gene BbEe BbEe BE bE Be be BE bE Be be 9 : 3 : 4 Polygenic inheritance A single phenotypic character is affected by two or more genes AaBbCc AaBbCc Figure 11.UN06 Summary of key concepts: multi-gene Mendelian extensions 96

Ww ww ww Ww Ww ww ww Ww Ww ww WW or Ww ww Widow’s peak No widow’s peak Figure 11.UN07 Ww ww ww Ww Ww ww ww Ww Ww ww WW or Ww ww Figure 11.UN07 Summary of key concepts: pedigrees Widow’s peak No widow’s peak 97

Figure 11.UN08 Figure 11.UN08 Test your understanding, question 6 (pea plant characters) 98

Figure 11.UN09 Figure 11.UN09 Test your understanding, question 15 (curl cat) 99

George Arlene Sandra Tom Sam Wilma Ann Michael Carla Daniel Alan Tina Figure 11.UN10 George Arlene Sandra Tom Sam Wilma Ann Michael Carla Figure 11.UN10 Test your understanding, question 18 (alkaptonuria) Daniel Alan Tina Christopher 100