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 cross-pollinated

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 Heterozygous 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 P mates to produce First-generation offspring F1 mate to produce Second-generation offspring F2

F1 Results of One Monohybrid Cross

F2 Results of Monohybrid Cross

Mendel’s Monohybrid Cross Results 5,474 round 1,850 wrinkled 6,022 yellow 2,001 green 882 inflated 299 wrinkled 428 green 152 yellow F2 plants showed dominant-to-recessive ratio that averaged 3:1 705 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 Male gametes A a 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 a aa Aa Homozygous recessive a a A Aa Two phenotypes All dominant phenotype

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

A Dihybrid Cross - F1 Results purple flowers, tall white flowers, dwarf TRUE- BREEDING PARENTS: AABB x aabb GAMETES: AB AB ab ab AaBb F1 HYBRID OFFSPRING: All purple-flowered, tall

F1 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 F2 Four Phenotypes: Tall, purple-flowered (9/16) AaBb X AaBb 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 1/4 1/4 1/4 AB Ab aB ab 1/4 1/16 1/16 1/16 1/16 AB AABB AABb AaBB AaBb 1/4 1/16 1/16 1/16 1/16 Ab AABb AAbb AaBb Aabb 1/4 1/16 1/16 1/16 1/16 aB AaBB AaBb aaBB aaBb 1/4 1/16 1/16 1/16 1/16 ab AaBb Aabb aaBb aabb

16 Allele Combinations in F2 1/4 1/4 1/4 1/4 AB Ab aB ab 1/4 1/16 1/16 1/16 1/16 AB AABB AABb AaBB AaBb 1/4 1/16 1/16 1/16 1/16 Ab AABb AAbb AaBb Aabb 1/4 1/16 1/16 1/16 1/16 aB AaBB AaBb aaBB aaBb 1/4 1/16 1/16 1/16 1/16 ab AaBb Aabb aaBb 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 OR A A a a A A a a B B b b b b B B Metaphase II: A A a a A A a a B B b b b b B B Gametes: B B b b b b B B A A a a A A a a 1/4 AB 1/4 ab 1/4 Ab 1/4 aB

(n is the number of gene loci at which the parents differ) Tremendous Variation Number of genotypes possible in offspring as a result of independent assortment and hybrid crossing is 3n (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 Pink-flowered F1 plants (homozygote) (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) (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

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) HbA Encodes normal beta hemoglobin chain 2) HbS Mutant allele encodes defective chain HbS homozygotes produce only the defective hemoglobin; suffer from sickle-cell anemia

Pleiotrophic Effects of HbS/HbS At low oxygen levels, cells with only HbS 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, 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

Alleles at two loci (R and P) interact 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

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

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 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