Chapter 14

Chapter 14Mendel and the Gene IdeaGregor MendelMonkPea Plants many varieties, easy to reproduce and control, tracked traits that were either-or, started with true breeding plantsExperimentsP generation = true breeding parentsPurple x WhiteF1 generation = hybrids from P generation crossAll purple plantsF2 generation = offspring from F1 self-pollination3:1 ratio of purple to white plants

P Generation

(true-breeding parents) PurpleflowersWhiteflowersF1 Generation (hybrids)All plants hadpurple flowersF2 Generation Mendels ConclusionsAlternate versions of genesALLELESFound on Homologous ChromosomesRepresented with lettersFigure 14.4

Allele for purple flowersLocus for flower-color geneHomologouspair ofchromosomesAllele for white flowersEach trait is controlled by 2 alleles one contributed from each parent

The Law of SegregationThe 2 alleles for a trait segregate during gamete formation (meiosis) and end up in different gametes

Mendels Other Law: The Law of Independent AssortmentGenes located on different chromosomes are inherited independentlyExceptions: genes far away on same chromosome (many map units apart) allowing crossing over to occur can be inherited independently* Genes located close together and are usually inherited together are called linked ex: red hair, frecklesSome alleles are dominant while others are recessiveDominant alleles mask recessiveDominant alleles = capital lettersRecessive alleles = lowercase letters

VocabHomozygous = 2 of the same allelesHomozygous Dominant = DDShows dominant traitHomozygous Recessive = ddShows recessive traitHeterozygous = 1 dominant, 1 recessive alleleHeterozygous = DdShows dominant trait and masks recessiveGenotype = allele make up (ex: DD)Phenotype = physical traitDetermine ProbabilitiesPunnett SquaresAka Monohybrid Cross 1 traitExplains Law of Segregation

TestcrossAllows us to determine the genotype of an organism with the dominant phenotype, but unknown genotypeIs the organism DD or Dd?!

Crosses an individual with the dominant phenotype with an individual that is homozygous recessive for a trait

The testcrossFigure 14.7

Dominant phenotype,unknown genotype:PP or Pp? Recessive phenotype,known genotype:ppIf PP,then all offspringpurple:If Pp,then 12 offspring purpleand 12 offspring white:ppPPPpPpPpPpppppPpPpPpppAPPLICATION An organism that exhibits a dominant trait,such as purple flowers in pea plants, can be either homozygous forthe dominant allele or heterozygous. To determine the organismsgenotype, geneticists can perform a testcross.

TECHNIQUE In a testcross, the individual with theunknown genotype is crossed with a homozygous individualexpressing the recessive trait (white flowers in this example). By observing the phenotypes of the offspring resulting from this cross, we can deduce the genotype of the purple-flowered parent.

RESULTSDihybrid CrossShows inheritance of two traitsEx: seed color AND seed shape

Typical Outcome for 2 heterozygous individuals: 9:3:3:1Types of DominanceComplete DominancePhenotype of Dd is the same as DDCodominanceShows 2 dominant traitsEx: BB = black, WW = white, BW = black AND whiteIncomplete DominanceShows blended patternEx: CRCR = Red, CwCw = White, CRCW = PinkCodominance vs. Incomplete Dominance

Multiple AllelesMost traits are controlled by 2 alleles (ex: DD)Some traits have more than 2 alleles ABO Blood TypeIA, IB, i are the 3 alellesTable 14.2

Pleiotropy1 gene has multiple phenotypic effectsEx: Sickle Cell Anemia

Multiple Genes (Polygenic)Ex: Hair, Skin, Eye ColorAdditive Effects

AaBbCcAaBbCcaabbccAabbccAaBbccAaBbCcAABbCcAABBCcAABBCC20641564664164Fraction of progenyEpistasisA gene at one locus alters the phenotypic expression of a gene at a second locus (masks the other gene)

Ex: Laborador Retrievers2 genes: (E,e) & (B,b)pigment (E) or no pigment (e)pigment concentration: black (B) to brown (b)

Outcome differs from 9:3:3:1Multifactorial TraitsControlled by genetics and environmentEx: Skin color in humans, hydrangea colorNature vs. Nurture

PedigreesCircles = FemalesSquares = MalesShaded = Expresses phenotypeHalf-Shaded = CarriersParents joined by horizontal linesOffspring listed below parents in birth order

Human Genetic DisordersMost are recessive (require homozygous recessive genotype)Carriers = heterozygous individuals that do not show trait, but carry the gene for it (Ex: Aa)

Cystic Fibrosis: defective chloride channel in cell membranes lead to build up of thickened mucusTay-Sachs disease: brain cells cannot metabolize lipidsSickle-Cell Anemia: misshaped blood cellsHuman Genetic DisordersHuntingtons Disease: Dominant, degenerative disease of nervous system

Achondroplasia: Dominant form of dwarfism (AA is lethal)

Many disorders are multifactorialHeart diseaseCancerDiabetes

Carrier RecognitionFetal TestingAmniocentesis: withdrawl of fluid from amniotic sac to collect cells for viewing and karyotypingChorionic villus sampling (CVS): small amount of tissue suctioned from placentaUltrasound: non-invasive, reveals structuresNewborn ScreeningBlood: test for PKU (Phenylketonuria)Recessive disorder that leads to mental retardationRequires a change in diet