3 Allele for purple flowers Homologouspair ofchromosomesLocus for flower-color geneAllele for white flowers
4 The TestcrossHow can we tell the genotype of an individual with the dominant phenotype?Could be DD or DdThe answer is to carry out a testcross: breeding the mystery individual with a homozygous recessive individualIf any offspring display the recessive phenotype, the mystery parent must be heterozygous. Same thing for a 2 trait but testcross wouldinvolve the double recessive.
5 LE 14-8 P Generation YYRR yyrr Gametes YR yr YyRr F1 Generation Hypothesis ofdependentassortmentHypothesis ofindependentassortmentSperm1YR1YryRyrSperm441414Eggs1YRyr2121YREggs4YYRRYYRrYyRRYyRr1YRF2 Generation(predictedoffspring)2YYRRYyRr1Yr4YYRrYYrrYyRrYyrr1yr2YyRryyrr1yR4YyRRYyRryyRRyyRr34141yr4Phenotypic ratio 3:1YyRrYyrryyRryyrr916316316316Phenotypic ratio 9:3:3:1
6 Using a dihybrid cross, Mendel developed the law of independent assortment This principle states that the alleles for a trait separate when gametes are formed. These allele pairs are then randomly united at fertilization.Genes located near each other on the same chromosome tend to be inherited together – these are called linked genes and we will do work them!
7 The Spectrum of Dominance Complete dominance occurs when phenotypes of the heterozygote and dominant homozygote are identicalIn codominance, two dominant alleles affect the phenotype – both expressed!In incomplete dominance, the phenotype of F1 hybrids is somewhere between the phenotypes of the two parental varieties
9 LE 14-12 20/64 15/64 6/64 1/64 AaBbCc AaBbCc aabbcc Aabbcc AaBbcc Fraction of progeny6/641/64
10 Pedigree AnalysisA pedigree is a family tree that describes the interrelationships of parents and children across generationsInheritance patterns of particular traits can be traced and described using pedigreesMales = squares, females = circlesThe trait is shaded in
11 Dominant trait (widow’s peak) LE 14-14aFirst generation(grandparents)WwwwwwWwSecond generation(parents plus auntsand uncles)WwwwwwWwWwwwThirdgeneration(two sisters)WWwworWwWidow’s peakNo widow’s peakDominant trait (widow’s peak)
12 Recessive trait (attached earlobe) LE 14-14bFirst generation(grandparents)FfFfffFfSecond generation(parents plus auntsand uncles)FF or FfffffFfFfffThirdgeneration(two sisters)ffFForFfAttached earlobeFree earlobeRecessive trait (attached earlobe)
13 Recessively Inherited Disorders Many genetic disorders are inherited in a recessive mannerRecessively inherited disorders show up only in individuals homozygous for the alleleCarriers are heterozygous individuals who carry the recessive allele but are phenotypically (look) normal, so they don’t know they have bad genes!
14 Cystic Fibrosis http://www. muschealth. com/video/Default. aspx Cystic fibrosis is the most common lethal genetic disease in the United States,striking one out of every 2,500 people of European descent - why???The cystic fibrosis allele results in defective or absent chloride transport channels in plasma membranes –so lungs full of mucusSymptoms include mucus buildup in some internal organs and abnormal absorption of nutrients in the small intestine (so stunted growth)
15 Sickle-Cell Disease – Watch HHMI video Sickle-cell disease affects one out of 400 African-AmericansThe disease is caused by the substitution of a single amino acid in the hemoglobin protein in red blood cellsSymptoms include physical weakness, pain, organ damage, and even paralysis
16 Dominantly Inherited Disorders Some human disorders are due to dominant allelesOne example is achondroplasia, a form of dwarfism that is lethal whenhomozygous for thedominant allele