Observing Patterns in Inherited Traits CHAPTER 11

Early Ideas about Heredity People knew that sperm and eggs transmitted information about traits Blending theory Problem: Would expect variation to disappear Variation in traits persists

Gregor Mendel Strong background in plant breeding and mathematics Using pea plants, found indirect but observable evidence of how parents transmit genes to offspring

Crossing garden pea plants Gregor Mendel Crossing garden pea plants

F2 Dominant-to- Recessive Ratio Trait Studied Dominant Form Recessive Form F2 Dominant-to- Recessive Ratio SEED SHAPE 5,474 round 1,850 wrinkled 2.96:1 SEED COLOR 6,022 yellow 2,001 green 3.01:1 POD SHAPE 882 inflated 299 wrinkled 2.95:1 POD COLOR 428 green 152 yellow 2.82:1 FLOWER COLOR 705 purple 224 white 3.15:1 FLOWER POSITION 651 long stem 207 at tip 3.14:1 STEM LENGTH 787 tall 277 dwarf 2.84:1

Genes Units of information about specific traits Passed from parents to offspring Each has a specific location (locus) on a chromosome

Alleles Different molecular forms of a gene Arise by mutation Dominant allele masks a recessive allele that is paired with it

Allele Combinations Homozygous (purebred) having two identical alleles at a locus AA or aa Heterozygous (hybrid) having two different alleles at a locus Aa

A pair of homologous chromosomes each in the unduplicated state most often, one from a male parent and its partner from a female parent gene locus (plural, loci), the location for a specific gene on a chromosome Alleles are at corresponding loci on a pair of homologous chromosomes A pair of alleles may be identical or non-identical. 3 pairs of genes at three loci on this pair of homologous chromosomes same thing as three pairs of alleles

Genotype & Phenotype Genotype refers to particular genes (alleles) an individual carries Phenotype refers to an individual’s observable traits Cannot always determine genotype by observing phenotype

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 14

Monohybrid Cross Testcross

Dominant phenotype, unknown genotype: PP or Pp? Recessive phenotype, 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 16

Tracking Generations Parental generation P mates to produce First-generation offspring F1 mate to produce Second-generation offspring F2

Monohybrid Crosses AA X aa Aa (F1 monohybrids) Aa X Aa ? Experimental intercross between two F1 heterozygotes AA X aa Aa (F1 monohybrids) Aa X Aa ?

(true-breeding parents) 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) 19

(true-breeding parents) 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) 20

All plants had purple flowers 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 21

Punnett Squares a A aa female gametes male gametes a A aa Aa a A aa Aa Stepped Art

Probability The chance that each outcome of a given event will occur is proportional to the number of ways that event can be reached Male Female 9th 210 192 52% 48% 10th 198 184 11th 211 190 53% 47% 12th 213 227 TOTAL 832 793 51% 49%

Mendel’s Theory of Segregation An individual inherits a unit of information (allele) about a trait from each parent During gamete formation, the alleles segregate from each other

Mendel’s Theory of Segregation Homozygous dominant parent Homozygous recessive parent Mendel’s Theory of Segregation (chromosomes duplicated before meiosis) meiosis I meiosis II (gametes) (gametes) fertilization produces heterozygous offspring

Law of Independent Assortment The results of Mendel’s dihybrid experiments are the basis for the law of independent assortment each pair of alleles segregates independently of each other pair of alleles during gamete formation applies to genes on different, non-homologous chromosomes or those far apart on the same chromosome Genes located near each other on the same chromosome tend to be inherited together 26

independent assortment 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 27

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 28

Independent Assortment Nucleus of a diploid (2n) reproductive cell with two pairs of homologous chromosomes Independent Assortment Possible alignments of the two homologous chromosomes during metaphase I of meiosis The resulting alignments at metaphase II Allelic combinations possible in gametes 1/4 AB 1/4 ab 1/4 Ab 1/4 aB

Segregation of alleles into eggs Segregation of alleles into sperm 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 30

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 multi-character 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 31

For example, if we cross F1 heterozygotes of genotype YyRr, we can calculate the probability of different genotypes among the F2 generation Probability of YYRR = Probability of YyRR = For example, for the cross PpYyRr  Ppyyrr, we can calculate the probability of offspring showing at least two recessive traits ppyyRr = ppYyrr = Ppyyrr = PPyyrr = 32

Mendels 4 principles: Individual units, called genes, determine biological characteristics. For each gene, an organism receives one allele from each parent. The alleles separate from each other a process called segregation when reproductive cells are formed

Mendels 4 principles: If an organism inherits different alleles for the same trait, one allele may be dominant over the other. Some genes segregate independently. Independent assortment genes segregate independently and do not influence each others inheritance

Human Inheritance 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 35

Human Inheritance 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 36

Degrees of Dominance Complete dominance Incomplete dominance occurs when phenotypes of the heterozygote and dominant homozygote are identical Incomplete dominance the phenotype of F1 hybrids is somewhere between the phenotypes of the two parental varieties Codominance two dominant alleles affect the phenotype in separate, distinguishable ways 37

Incomplete Dominance Incomplete dominance

P Generation Red CRCR White CWCW Gametes CR CW Figure 11.10-1 Incomplete dominance in snapdragon color (step 1) 39

P Generation Red CRCR White CWCW Gametes Pink CRCW F1 Generation ½ CR ½ CW Figure 11.10-2 Incomplete dominance in snapdragon color (step 2) 40

P Generation Red CRCR White CWCW Gametes Pink CRCW F1 Generation 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 41

Multiple Alleles / co-dominance 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 42

ABO Blood Type Range of genotypes: IAIA IBIB or or IAi IAIB IBi ii

(a) The three alleles for the ABO blood groups and their carbohydrates 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 44

ABO and Transfusions Recipient’s immune system will attack blood cells that have an unfamiliar glycolipid on surface Type O is universal donor because it has neither type A nor type B glycolipid

Pleiotropy pleiotropy most genes have multiple phenotypic effects alleles at a single locus may have effects on two or more traits pleiotropic alleles are responsible for the multiple symptoms of certain hereditary diseases cystic fibrosis mutation in a transporter gene affects lungs, liver, pancreas, and intestines

Pleiotropy Marfan syndrome mutation in gene for fibrillin affects skeleton, cardiovascular system, lungs, eyes, and skin

Pleiotropy sickle-cell disease abnormal red blood cells affects brain, eyes, lungs, liver, heart, kidneys, penis, joints, bones, or skin.

Epistasis Epistasis a gene at one locus alters the phenotypic expression of a gene at a second locus 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 54

Coat Color in Retrievers Coat color in Labrador retrievers

¼ ¼ ¼ ¼ ¼ ¼ ¼ ¼ BbEe BbEe Sperm Eggs BBEE BbEE BBEe BbEe BbEE bbEE Figure 11.12 An example of epistasis ¼ be BbEe bbEe Bbee bbee 9 : 3 : 4 56

Comb Shape in Poultry Comb shape in chickens

Comb Shape in Poultry rose comb pea comb walnut comb single comb X RRpp pea rrPP F1 all walnut RrPp RrPp RrPp F2 9/16 walnut RRPP, RRPp,RrPP, or RrPp 3/16 rose RRpp or Rrpp 3/16 pea rrPP or rrPp 1/16 single rrpp

Coat color in the Himalayan rabbit

Polygenic Inheritance Quantitative characters 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 the greater the number of genes and environmental factors that affect a trait, the more continuous the variation in versions of that trait Human Traits that show continuous variation Height Weight Eye color Skin color 60

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 61

Continuous Variation Variation in human eye color

Describing Continuous Variation

Continuous variation in height

Pleiotropic effects of Marfan syndrome Pleiotropy Pleiotropic effects of Marfan syndrome