Observing Patterns in Inherited Traits Chapter 10 Early

Observing Patterns in Inherited Traits Chapter 10

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

The Garden Pea Plant • Self-pollinating • True breeding (different alleles not normally introduced) • Can be experimentally crosspollinated

Genetic Terms A pair of homologous chromosomes A gene locus A pair of alleles Three pairs of genes

Genes • Units of information about specific traits • Passed from parents to offspring • Each has a specific location (locus) on a chromosome - akin to a postal address!

Alleles • Different molecular forms of a gene (green eye vs. brown eyes) • Arise by mutation • Dominant allele masks a recessive allele that is paired with it (brown eye allele and green eye allele = brown eye are dominant)

Allele Combinations • Homozygous – having two identical alleles at a locus – AA or aa (Brown OR Green eyes) • Heterozygous – having two different alleles at a locus – Aa (Brown allele AND Green allele)

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

Tracking Generations • Parental generation mates to produce P • First-generation offspring mate to produce F 1 • Second-generation offspring F 2

Monohybrid Crosses • Use F 1 offspring of parents that breed true for different forms of a trait: (AA x aa = Aa) • The experiment itself is a cross between two identical F 1 heterozygotes, which are the “monohybrids” (Aa x Aa)

Monohybrid Crosses Homozygous dominant parent Homozygous recessive parent (chromosomes duplicated before meiosis) meiosis II (gametes) fertilization produces heterozygous offspring

Mendel’s Monohybrid Cross Results F 2 plants showed dominant-torecessive ratio that averaged 3: 1

Probability The chance that each outcome of a given event will occur is proportional to the number of ways that event can be reached Roll a die (how many sides = 6) (probability of rolling a 3 = 1/6)

Punnett Square of a Monohybrid Cross Female gametes A Male gametes A AA a Aa aa Dominant phenotype can arise 3 ways, recessive only 1

F 1 Results of One Monohybrid Cross

F 2 Results of Monohybrid Cross

Testcross • Individual that shows dominant phenotype is crossed with individual with recessive phenotype • Examining offspring allows you to determine the genotype of the dominant individual

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

Dihybrid Cross Experimental cross between individuals that are homozygous for different versions of two traits

A Dihybrid Cross - F 1 Results

F 1 Results of Mendel’s Dihybrid Crosses • All plants displayed the dominant form of both traits • We now know: – All plants inherited one allele for each trait from each parent – All plants were heterozygous (Aa. Bb)

Phenotypic Ratios in F 2 Aa. Bb X Aa. Bb Four Phenotypes: – Tall, purple-flowered (9/16) – Tall, white-flowered (3/16) – Dwarf, purple-flowered (3/16) – Dwarf, white-flowered (1/16)

Explanation of Mendel’s Dihybrid Results If the two traits are coded for by genes on separate chromosomes, sixteen gamete combinations are possible

Independent Assortment • Mendel concluded that the two “units” for the first trait were to be assorted into gametes independently of the two “units” for the other trait • Members of each pair of homologous chromosomes are sorted into gametes at random during meiosis

Independent Assortment

Tremendous Variation Number of genotypes possible in offspring as a result of independent assortment and hybrid crossing is 3 n (n is the number of gene loci at which the parents differ)

Impact of Mendel’s Work • Mendel presented his results in 1865 • Paper received little notice • Mendel discontinued his experiments in 1871 • Paper rediscovered in 1900 and finally appreciated

Dominance Relations • Complete dominance • Incomplete dominance – Heterozygote phenotype is somewhere between that of two homozyotes (evening primrose - red, white, and pink) • Codominance – Non-identical alleles specify two phenotypes that are both expressed in heterozygotes (Blood groups of humans)

Genetics of ABO Blood Types: Three Alleles • Gene that controls ABO type codes for enzyme that dictates structure of a glycolipid on blood cells • Two alleles (IA and IB) are codominant when paired • Third allele (i) is recessive to others

ABO Blood Type: Allele Combinations • Type A - IAIA or IAi • Type B - IBIB or IBi • Type AB - IAIB • Type O - ii

ABO Blood Type: Glycolipids on Red Cells • Type A - Glycolipid A on cell surface • Type B - Glycolipid B on cell surface • Type AB - Both glyocolipids A & B • Type O - Neither glyocolipid A nor B

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

Flower Color in Snapdragons: Incomplete Dominance Red-flowered plant X White-flowered plant (homozygote) Pink-flowered F 1 plants (heterozygotes)

Flower Color in Snapdragons: Incomplete Dominance Pink-flowered plant X Pink-flowered plant (heterozygote) White-, pink-, and red-flowered plants in a 1: 2: 1 ratio

Flower Color in Snapdragons: Incomplete Dominance • Red flowers - two alleles allow them to make a red pigment • White flowers - two mutant alleles; can’t make red pigment • Pink flowers have one normal and one mutant allele; make a smaller amount of red pigment

Comb Shape in Poultry Alleles at two loci (R and P) interact • • Walnut comb - RRPP, RRPp, Rr. PP, Rr. Pp Rose comb - RRpp, Rrpp Pea comb - rr. PP, rr. Pp Single comb - rrpp

Pleiotropy • Alleles at a single locus may have effects on two or more traits • Classic example is the effects of the mutant allele at the beta-globin locus that gives rise to sickle-cell anemia

Genetics of Sickle-Cell Anemia • Two alleles 1) Hb. A Encodes normal beta hemoglobin chain 2) Hb. S Mutant allele encodes defective chain • Hb. S homozygotes produce only the defective hemoglobin; suffer from sicklecell anemia

Pleiotropic Effects of Hb. S/Hb. S • At low oxygen levels, cells with only Hb. S hemoglobin “sickle” and stick together • This impedes oxygen delivery and blood flow • Over time, it causes damage throughout the body

Genetics of Coat Color in Labrador Retrievers • Two genes involved - One gene influences melanin production • Two alleles - B (black) is dominant over b (brown) - Other gene influences melanin deposition • Two alleles - E promotes pigment deposition and is dominant over e

Allele Combinations and Coat Color • Black coat - Must have at least one dominant allele at both loci – BBEE, Bb. Ee, BBEe, or Bb. EE • Brown coat - bb. EE, bb. Ee • Yellow coat - Bbee, Bb. EE, bbee

Albinism • Phenotype results when pathway for melanin production is completely blocked • Genotype - Homozygous recessive at the gene locus that codes for tyrosinase, an enzyme in the melaninsynthesizing pathway

Continuous Variation • A more or less continuous range of small differences in a given trait among individuals • The greater the number of genes and environmental factors that affect a trait, the more continuous the variation in versions of that trait

Human Variation • Some human traits occur as a few discrete types – Attached or detached earlobes – Many genetic disorders • Other traits show continuous variation – Height – Weight – Eye color

Number of individuals with some value of the trait (line of bell-shaped curve indicates continuous variation in population) Number of individuals with some value of the trait Describing Continuous Variation Range of values for the trait

Temperature Effects on Phenotype • Himalayan rabbits are Homozygous for an allele that specifies a heatsensitive version of an enzyme in melaninproducing pathway • Melanin is produced in cooler areas of body

Environmental Effects on Yarrow Plants