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 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 cross- pollinated

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 P mates to produce First-generation offspring F 1 mate to produce 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 fertilization produces heterozygous offspring meiosis II meiosis I (chromosomes duplicated before meiosis) Homozygous dominant parent Homozygous recessive parent (gametes)

Mendel’s Monohybrid Cross Results F 2 plants showed dominant-to- recessive 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 Male gametes Dominant phenotype can arise 3 ways, recessive only 1 aA aaAa AA A a

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 (AaBb)

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

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 (I A and I B ) are codominant when paired Third allele (i) is recessive to others

ABO Blood Type: Allele Combinations Type A - I A I A or I A i Type B - I B I B or I B i Type AB - I A I B 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 Pink-flowered F 1 plants (homozygote) (heterozygotes)

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

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, RrPP, RrPp Rose comb - RRpp, Rrpp Pea comb - rrPP, rrPp 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 sickle-cell 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, BbEe, BBEe, or BbEE Brown coat - bbEE, bbEe Yellow coat - Bbee, BbEE, 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 melanin- synthesizing 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

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

Temperature Effects on Phenotype Himalayan rabbits are Homozygous for an allele that specifies a heat- sensitive version of an enzyme in melanin- producing pathway Melanin is produced in cooler areas of body

Environmental Effects on Yarrow Plants