Observable Patterns of Inheritance Chapter 14 Earlobe Variation
Observable Patterns of Inheritance Chapter 14
Earlobe Variation • Whether a person is born with attached or detached earlobes depends on a single gene • Gene has two molecular forms (alleles)
Earlobe Variation • You inherited one allele for this gene from each parent • Dominant allele specifies detached earlobes • Recessive allele specifies attached lobes
Dominant & Recessive Alleles • If you have attached earlobes, you inherited two copies of the recessive allele • If you have detached earlobes, you may have either one or two copies of the dominant allele
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
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 Chromosomes – having two identical alleles at a locus – AA or aa • Heterozygous Chromosomes – having two different alleles at a locus – Aa
Genetic Terms A pair of homologous chromosomes A gene locus A pair of alleles Three pairs of genes
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
F 1 Results of One Monohybrid Cross
F 2 Results of Monohybrid Cross
Mendel’s Monohybrid Cross Results F 2 plants showed dominant-torecessive ratio that averaged 3: 1 5, 474 round 1, 850 wrinkled 6, 022 yellow 2, 001 green 882 inflated 299 wrinkled 428 green 152 yellow 705 purple 224 white 651 long stem 207 at tip 787 tall 277 dwarf
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
Probability The chance that each outcome of a given event will occur is proportional to the number of ways that event can be reached
Punnett Square of a Monohybrid Cross Female gametes A Male gametes a A AA Aa aa Dominant phenotype can arise 3 ways, recessive only one
Test Cross • Individual that shows dominant (heterozygous or Homozygous dominant) phenotype is crossed with individual with recessive phenotype • Examining offspring allows you to determine the genotype of the dominant individual
Punnett Squares of Test Crosses Homozygous recessive a a A Aa Aa a aa aa A Aa Aa Two phenotypes All dominant phenotype
Dihybrid Cross Experimental cross between individuals that are homozygous for different versions of two traits
A Dihybrid Cross - F 1 Results purple flowers, tall TRUEBREEDING PARENTS: AABB GAMETES: AB x AB white flowers, dwarf aabb ab ab Aa. Bb F 1 HYBRID OFFSPRING: All purple-flowered, tall
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 1/4 AB 1/4 Ab 1/4 a. B 1/4 ab 1/16 AABB AABb Aa. BB Aa. Bb 1/16 AABb AAbb Aa. Bb Aabb 1/16 Aa. BB Aa. Bb aa. BB aa. Bb 1/16 Aa. Bb Aabb aa. Bb aabb
16 Allele Combinations in F 2 1/4 AB 1/4 Ab 1/4 a. B 1/4 ab 1/16 AABB AABb Aa. BB Aa. Bb 1/16 AABb AAbb Aa. Bb Aabb 1/16 Aa. BB Aa. Bb aa. BB aa. Bb 1/16 Aa. Bb Aabb aa. Bb aabb
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 Metaphase I A A a a B B b b OR A A a a b b B B Metaphase II: Gametes: A A a a B B b b B B B A 1/4 AB b a 1/4 ab b A 1/4 Ab B a 1/4 a. B
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 • Codominance – Non-identical alleles specify two phenotypes that are both expressed in heterozygotes
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
Genetics of ABO Blood Types: Three Alleles: Codominance • 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
Pleitropy • 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
Pleiotrophic 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
Epistasis • Interaction between the products of gene pairs • Common among genes for hair color in mammals
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
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
Campodactyly: Unexpected Phenotypes • Effect of allele varies: – Bent fingers on both hands – Bent fingers on one hand – No effect • Many factors affect gene expression
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
(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 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 Plant Phenotype • Hydrangea macrophylla • Action of gene responsible for floral color is influenced by soil acidity • Flower color ranges from pink to blue
Web Sites • BSCI 124 Lecture Notes -- Mendelian Genetics • Biology Project - site map • Genome Project
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