Chapter 11Chapter 11

Mendel & The Mendel & The Gene Gene

IdeaIdea

Mendelian geneticsMendelian genetics CharacterCharacter

heritable feature, i.e., fur colorheritable feature, i.e., fur color TraitTrait

variant for a character, i.e., variant for a character, i.e., brownbrown

True-bred True-bred all offspring of same varietyall offspring of same variety

HybridizationHybridization crossing of 2 different true-bredscrossing of 2 different true-breds

P generationP generation parentsparents

FF11 generation generation first filial generationfirst filial generation

FF22 generation generation second filial generationsecond filial generation

The The Law of DominanceLaw of Dominance In a cross of parents that are pure for In a cross of parents that are pure for

contrasting traits, only one form of the trait contrasting traits, only one form of the trait will appear in the next generation.will appear in the next generation.

Offspring that are hybrid for a trait will have Offspring that are hybrid for a trait will have only the dominant trait in the phenotype.only the dominant trait in the phenotype.

When an organism has two different When an organism has two different alleles for a trait, the allele that is alleles for a trait, the allele that is expressed, overshadowing the expressed, overshadowing the expression of the other allele, is said to expression of the other allele, is said to be be dominantdominant. The gene whose . The gene whose expression is overshadowed is said to expression is overshadowed is said to be recessive. be recessive.

Mendel’s Mendel’s Law of DominanceLaw of Dominance

Leading to the Leading to the Law of Law of SegregationSegregation

Alternative versions of genes Alternative versions of genes (alleles) account for variations in (alleles) account for variations in inherited characteristicsinherited characteristics

For each character, an organism For each character, an organism inherits 2 alleles, one from each inherits 2 alleles, one from each parentparent

If the two alleles differ, then one, the If the two alleles differ, then one, the dominant allele, is fully expressed in dominant allele, is fully expressed in the organism’s appearance; the the organism’s appearance; the other, the recessive allele, has no other, the recessive allele, has no noticeable effect on the organism’s noticeable effect on the organism’s appearanceappearance

The alleles for each character The alleles for each character segregate (separate) during gamete segregate (separate) during gamete production (meiosis). production (meiosis).

Mendel’s Mendel’s Law of SegregationLaw of Segregation

The The Law of Independent Law of Independent AssortmentAssortment

Law of Segregation Law of Segregation involves 1 character. involves 1 character. What about 2 (or more) What about 2 (or more) characters?characters?

Monohybrid cross vs. Monohybrid cross vs. dihybrid crossdihybrid cross

The two pairs of alleles The two pairs of alleles segregate independently segregate independently of each other.of each other.

Mendel’s Mendel’s Law of Law of Independent Independent AssortmentAssortment

Genetic vocabulary…….Genetic vocabulary…….

Punnett squarePunnett square: predicts the : predicts the results of a genetic cross results of a genetic cross between individuals of known between individuals of known genotypegenotype

HomozygousHomozygous: pair of identical : pair of identical alleles for a characteralleles for a character

HeterozygousHeterozygous: two different : two different alleles for a genealleles for a gene

PhenotypePhenotype: an organism’s : an organism’s traitstraits

GenotypeGenotype: an organism’s : an organism’s genetic makeupgenetic makeup

TestcrossTestcross: breeding of a : breeding of a recessive homozygote X recessive homozygote X dominate phenotype (but dominate phenotype (but unknown genotype)unknown genotype)

Alternative Inheritance Alternative Inheritance PatternsPatterns

Incomplete dominanceIncomplete dominance: : appearance between the appearance between the phenotypes of the 2 parents. Ex: phenotypes of the 2 parents. Ex: snapdragonssnapdragons

CodominanceCodominance: : two alleles affect two alleles affect the phenotype in separate, the phenotype in separate, distinguishable ways. Ex: Tay-distinguishable ways. Ex: Tay-Sachs diseaseSachs disease

Multiple allelesMultiple alleles: : more than 2 more than 2 possible alleles for a gene. Ex: possible alleles for a gene. Ex: human blood typeshuman blood types

Alternative Inheritance Alternative Inheritance PatternsPatterns

PleiotropyPleiotropy: : genes with multiple genes with multiple phenotypic effect. Ex: sickle-cell phenotypic effect. Ex: sickle-cell anemiaanemia

EpistasisEpistasis: : a gene at one locus a gene at one locus (chromosomal location) affects (chromosomal location) affects the phenotypic expression of a the phenotypic expression of a gene at a second locus. Ex: gene at a second locus. Ex: mice coat colormice coat color

Polygenic InheritancePolygenic Inheritance: : an an additive effect of two or more additive effect of two or more genes on a single phenotypic genes on a single phenotypic character Ex: human skin character Ex: human skin pigmentation and heightpigmentation and height

Human disordersHuman disorders The family pedigreeThe family pedigree

Recessive disorders:Recessive disorders:

Cystic fibrosisCystic fibrosis Tay-SachsTay-Sachs Sickle-cell Sickle-cell

Dominant disorders:Dominant disorders: Huntington’sHuntington’s

Testing:Testing: AmniocentesisAmniocentesis chorionic villus sampling chorionic villus sampling

(CVS)(CVS)

Chapter 12Chapter 12

The Chromosomal The Chromosomal Basis of Basis of InheritanceInheritance

Chromosomal Theory Chromosomal Theory of of InheritanceInheritance

Genes have Genes have specific loci on specific loci on chromosomes and chromosomes and chromosomes chromosomes undergo undergo segregation and segregation and independent independent assortmentassortment

Chromosomal LinkageChromosomal Linkage MorganMorgan Drosophilia melanogasterDrosophilia melanogaster

XX (female) vs. XY (male)XX (female) vs. XY (male) Sex-linkageSex-linkage: genes located on : genes located on

a sex chromosomea sex chromosome Linked genesLinked genes: genes located : genes located

on the same chromosome that on the same chromosome that tend to be inherited togethertend to be inherited together

Genetic RecombinationGenetic Recombination Crossing overCrossing over

Genes that DO NOT assort Genes that DO NOT assort independently of each otherindependently of each other

Genetic mapsGenetic maps The further apart 2 genes are, the The further apart 2 genes are, the

higher the probability that a higher the probability that a crossover will occur between crossover will occur between them and therefore the higher the them and therefore the higher the recombination frequencyrecombination frequency

Linkage mapsLinkage maps Genetic map based on Genetic map based on

recombination frequenciesrecombination frequencies

Human Sex-LinkageHuman Sex-Linkage SRY gene: gene on Y chromosome that triggers the development of SRY gene: gene on Y chromosome that triggers the development of

testestestes Fathers= pass X-linked alleles to all daughters only (but not to sons)Fathers= pass X-linked alleles to all daughters only (but not to sons) Mothers= pass X-linked alleles to both sons & daughtersMothers= pass X-linked alleles to both sons & daughters Sex-Linked Disorders: Color-blindness; Duchenne muscular dystropy Sex-Linked Disorders: Color-blindness; Duchenne muscular dystropy

(MD); hemophilia(MD); hemophilia

X-inactivation: 2nd X chromosome in females condenses into a Barr X-inactivation: 2nd X chromosome in females condenses into a Barr body (e.g., tortoiseshell gene gene in cats)body (e.g., tortoiseshell gene gene in cats)

Chromosomal ErrorsChromosomal Errors

NondisjunctionNondisjunction:: members of a pair of homologous members of a pair of homologous

chromosomes do not separate chromosomes do not separate properly during meiosis I or sister properly during meiosis I or sister chromatids fail to separate during chromatids fail to separate during meiosis IImeiosis II

AneuploidyAneuploidy: : chromosome number is abnormalchromosome number is abnormal Monosomy – missing chromosomeMonosomy – missing chromosome Trisomy - extra chromosome Trisomy - extra chromosome

(ex. Trisomy 21 Down (ex. Trisomy 21 Down Syndrome)Syndrome)

Polyploidy - extra sets of Polyploidy - extra sets of chromosomeschromosomes

Chromosomal ErrorsChromosomal Errors Alterations of chromosomal structure:Alterations of chromosomal structure: Deletion: removal of a chromosomal segmentDeletion: removal of a chromosomal segment Duplication: repeats a chromosomal segmentDuplication: repeats a chromosomal segment Inversion: segment reversal in a chromosomeInversion: segment reversal in a chromosome Translocation: movement of a chromosomal segment to anotherTranslocation: movement of a chromosomal segment to another

Genomic ImprintingGenomic Imprinting A parental effect on gene A parental effect on gene

expressionexpression

Identical alleles may have Identical alleles may have different effects on offspring, different effects on offspring, depending on whether they depending on whether they arrive in the zygote via the arrive in the zygote via the ovum or via the sperm.ovum or via the sperm.

Fragile X syndrome: higher Fragile X syndrome: higher prevalence of disorder and prevalence of disorder and mental disability in malesmental disability in males