Observing Patterns in Inherited Traits Chapter 11 HsuehFen

Observing Patterns in Inherited Traits Chapter 11 Hsueh-Fen Juan Oct. 16, 2012

Impacts, Issues: The Color of Skin § Like most human traits, skin color has a genetic basis; more than 100 gene products affect the synthesis and deposition of melanins

Video: Genetics of skin color

11. 1 Mendel, Pea Plants, and Inheritance Patterns § Recurring inheritance patterns are observable outcomes of sexual reproduction § Before the discovery of genes, it was thought that inherited traits resulted from a blend of parental characters

Mendel’s Experimental Approach § 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

Garden Pea Plant: Self Fertilization and Cross. Fertilization carpel anther A Garden pea flower, cut in half. Sperm form in pollen grains, which originate in male floral parts (anthers). Eggs develop, fertilization takes place, and seeds mature in female floral parts (carpels). B Pollen from a plant that breeds true for purple flowers is brushed onto a floral bud of a plant that breeds true for white flowers. The white flower had its anthers snipped off. Artificial pollination is one way to ensure that a plant will not self-fertilize. C Later, seeds develop inside pods of the cross-fertilized plant. An embryo in each seed develops into a mature pea plant. D Each new plant’s flower color is indirect but observable evidence that hereditary material has been transmitted from the parent plants. Fig. 11 -3, p. 170

Animation: Crossing garden pea plants

Terms Used in Modern Genetics § Genes • Heritable units of information about traits • Parents transmit genes to offspring • Each gene has a specific locus on a chromosome § Diploid cells (chromosome number 2 n) have pairs of genes on homologous chromosomes

Terms Used in Modern Genetics § A mutation is a permanent change in a gene • May cause a trait to change • Alleles (等位基因) are different molecular forms of a gene § A hybrid has nonidentical alleles for a trait • Offspring of a cross between two individuals that breed true for different forms of a trait are hybrids

Terms Used in Modern Genetics § An individual with nonidentical alleles of a gene is heterozygous for that gene (異型合子) § An individual with identical alleles of a gene is homozygous for that gene (同型合子)

Terms Used in Modern Genetics § An allele is dominant (顯性) if its effect masks the effect of a recessive (隱性) allele paired with it • Capital letters (A) signify dominant alleles; lowercase letters (a) signify recessive alleles • Homozygous dominant (AA) • Homozygous recessive (aa) • Heterozygous (Aa)

Terms Used in Modern Genetics § Gene expression (基因表現) • The process by which information in a gene is converted to a structural or functional part of a cell or body • Expressed genes determine traits

Terms Used in Modern Genetics § Genotype (基因型) • The particular alleles an individual carries § Phenotype (表型) • An individual’s observable traits

Terms Used in Modern Genetics § P stands for parents, F for filial (offspring) § F 1: First generation offspring of parents § F 2: Second generation offspring of parents

11. 1 Key Concepts Where Modern Genetics Started § Gregor Mendel gathered the first experimental evidence of the genetic basis of inheritance § His meticulous work gave him clues that heritable traits are specified in units § The units, which are distributed into gametes in predictable patterns, were later identified as genes

11. 2 Mendel’s Law of Segregation § Garden pea plants inherit two “units” of information for a trait, one from each parent

Testcrosses § Testcross (試交) • 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 (AA x aa) or (Aa x aa)

Monohybrid Experiments § Monohybrid experiments (單性雜交) • Testcrosses that check for a dominance relationship between two alleles at a single locus (某基因位上，等位基因僅兩種才能用) • May be crosses between true breeding (homozygous) individuals (AA x aa), or between identical heterozygotes (Aa x Aa)

Mendel’s Monohybrid Experiments § Mendel used monohybrid experiments 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 F 1 plants had purple flowers • When he crossed two F 1 plants, ¾ of the F 2 plants had purple flowers, ¼ had white flowers

Segregation of Alleles at a Gene Locus

Mendel’s Monohybrid Experiments

Calculating Probabilities § Probability • A measure of the chance that a particular outcome will occur § Punnett square (旁氏表) • A grid used to calculate the probability of genotypes and phenotypes in offspring

Construction of a Punnett Square

Phenotype Ratios in a Monohybrid Experiment

Phenotype Ratios in a Monohybrid Experiment

Mendel’s Law of Segregation (分離律) § 重點：單性雜交實驗驗證了分離律 § Mendel observed a phenotype ratio of 3: 1 in the F 2 offspring of his monohybrid crosses • Consistent with the probability of the aa genotype in the offspring of a heterozygous cross (Aa x Aa) § This is the basis of Mendel’s law of segregation in modern terms (當時尚未知染色體的存在) 1. Diploid cells have pairs of genes on pairs of homologous chromosomes 2. The two genes of each pair separate during meiosis, and end up in different gametes

11. 2 Key Concepts Insights from Monohybrid Experiments § Some experiments yielded evidence of gene segregation: When one chromosome separates from its homologous partner during meiosis, the alleles on those chromosomes also separate and end up in different gametes

11. 3 Mendel’s Law of Independent Assortment § Mendel’s law of independent assortment • Many genes are sorted into gametes independently of other genes

Dihybrid Experiments § Dihybrid experiments (兩性雜交) • Tests for dominance relationships between alleles at two loci • Individuals that breed true for two different traits are crossed (AABB x aabb) • F 2 phenotype ratio is 9: 3: 3: 1 (four phenotypes) • Individually, each dominant trait has an F 2 ratio of 3: 1 – inheritance of one trait does not affect inheritance of the other

Independent Assortment at Meiosis One of two possible alignments a Chromosome alignments at metaphase I: The only other possible alignment A Aa a B Bb b b b. B B b The resulting alignments at metaphase II: A A a a B B b b B B c Possible B combinations of alleles in gametes: A A a a AB B b ab b b Ab b B a. B B

Mendel’s Dihybrid Experiments P generation A Meiosis in homozygous individuals results in one kind of gamete. parent plant homozygous for purple for white flowers and long and short stems aabb AABB B A cross between plants AB homozygous for two different traits yields one possible combination of gametes: x ab Fig. 11 -9 a, p. 175

F 1 generation All F 1 offspring are Aa. Bb, with purple flowers and tall stems. Aa. Bb C Meiosis in Aa. Bb dihybrid plants results in four kinds of gametes: AB Ab a. B F 2 generation ab These gametes can meet up in one of 16 possible wayswhen the dihybrids are crossed (Aa. Bb X Aa. Bb): Fig. 11 -9 b, p. 175

AB Ab a. B ab AB AABb Aa. BB Aa. Bb Ab AABb AAbb Aa. Bb Aabb a. B Aa. Bb aa. BB aa. Bb ab Aa. Bb Aabb aa. Bb aabb D Out of 16 possible genetic outcomes of this dihybrid cross, 9 will result in plants that are purple-flowered and tall; 3, purple-flowered and short; 3, white-flowered and tall; and 1, white-flowered and short. The ratio of phenotypes of this dihybrid cross is 9: 3: 3: 1. Fig. 11 -9 c, p. 175

Mendel’s Law of Independent Assortment § Mendel’s dihybrid experiments showed that “units” specifying one trait segregated into gametes separately from “units” for other traits ( 亦即一對等位基因之分離，與另一對等位基因之分 離毫不相干) § 重點：兩性雜交實驗驗證了自由配合律 § Exception: Genes that have loci very close to one another on a chromosome tend to stay together during meiosis (基因連鎖則不成立)

11. 3 Key Concepts Insights from Dihybrid Experiments § Some experiments yielded evidence of independent assortment: Genes are typically distributed into gametes independently of other genes

11. 4 Beyond Simple Dominance § Mendel focused on traits based on clearly dominant and recessive alleles; however, the expression patterns of genes for some traits are not as straightforward

Codominance in ABO Blood Types § Codominance (等顯性) • 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 (複對偶基因) • Genes with three or more alleles in a population • Example: ABO blood types

Codominance in ABO Blood Types

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 F 2 offspring § Example: Snapdragon color • RR is red • Rr is pink • rr is white

Incomplete Dominance in Snapdragons

Fig. 11 -11 a, p. 176

Fig. 11 -11 b, p. 176

Epistasis § Epistasis (上位基因) • Two or more gene products influence a trait • Typically, one gene product suppresses the effect of another, so the resulting phenotype is somewhat unexpected. • 通常會有一個基因在眾基因的交互作用中佔優勢， 稱此基因為上位基因(例：藍眼瑪莉花色基因) § Example: Coat color in dogs • Alleles B and b designate colors (black or brown) • Two recessive alleles ee suppress color • Allele E promotes the deposition of melanin in fur, but two recessive alleles (ee) reduce it.

Epistasis in Chicken Combs Dramatic variations in their combs

Animation: Comb shape in chickens

Epistasis in Coat Colors

EE/Ee/ee為上位基因，決定是否有黑色素，因此只 要出現基因型ee，不論是BB, Bb或bb，皆因無黑色 素而呈表型yellow EB Eb e. B eb EB EEBB black EEBb black Ee. BB black Ee. Bb black Eb EEBb black EEbb chocolate Ee. Bb black Eebb chocolate e. B Ee. BB black Ee. Bb black ee. BB yellow ee. Bb yellow eb Ee. Bb black Eebb chocolate ee. Bb yellow eebb yellow Allele B (black) is dominant to b (brown). Allele E promotes the deposition of melanin in fur, but two recessive alleles (ee) reduce it. Fig. 11 -13 a, p. 177

Pleiotropy § Pleiotropy (基因多效性) • One gene product influences two or more traits • Example: Some tall, thin athletes have Marfan syndrome, a potentially fatal genetic disorder • 單基因(�物)多表型 FBN 1, which encodes a connective protein called fibrillin-1 Long fibers of fibrillin impart elasticity to the tissues of the heart, skin, blood vessels, tendons and other body parts.

11. 5 Linkage Groups § The farther apart two genes are on a chromosome, the more often crossing over (互 換) occurs between them § Linkage group (基因連鎖群) • All genes on one chromosome are called a linkage group • Linked genes are very close together; crossing over rarely occurs between them

Linkage and Crossing Over

Animation: Crossover review

The Distance Between Genes § The probability that a crossover event will separate alleles of two genes is proportional to the distance between those genes

11. 6 Genes and the Environment § Expression of some genes is affected by environmental factors such as temperature, altitude, or chemical exposure § The result may be variation in traits

Effects of Temperature on Gene Expression § Enzyme tyrosinase (酪胺酸酶), works at low temperatures

Animation: Coat color in the Himalayan rabbit

Effects of Altitude on Gene Expression

Effects of Predation on Gene Expression § Predators of daphnias emit chemicals that trigger a different phenotype

11. 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

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

Continuous Variation and the Bell Curve

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 affect one or more steps

11. 4 -11. 7 Key Concepts Variations on Mendel’s Theme § Not all traits appear in Mendelian inheritance patterns • An allele may be partly dominant over a nonidentical partner, or codominant with it • Multiple genes may influence a trait; some genes influence many traits • The environments also influences gene expression