chapter 6 genetics and...

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Chapter 6Chapter 6

Genetics and InheritanceGenetics and Inheritance

Lecture 2: Genetics and Patterns of Lecture 2: Genetics and Patterns of Inheritance (continued)Inheritance (continued)

Incomplete DominanceIncomplete Dominance

•• Sometimes there is not one clear dominant alleleSometimes there is not one clear dominant allele

•• In a heterozygous individual, both alleles are In a heterozygous individual, both alleles are expressedexpressed

•• Phenotype is a blend of both traitsPhenotype is a blend of both traits


•• Example: snapdragon colorExample: snapdragon color

•• Both red (RR) and white (Both red (RR) and white (rrrr) are dominant) are dominant

•• Heterozygous (Heterozygous (RrRr) = pink) = pink

•• Use a Use a PunnettPunnett square to predict the ratio of square to predict the ratio of red:pink:whitered:pink:white offspring if 2 pink snapdragons offspring if 2 pink snapdragons

are crossedare crossed


•• Genotype?Genotype?


•• Example in humans: hairExample in humans: hair

•• Both curly (CC) and straight (SS) are dominantBoth curly (CC) and straight (SS) are dominant

•• Heterozygous (CS) = wavyHeterozygous (CS) = wavy

•• Use a Use a PunnettPunnett square to predict the probability square to predict the probability of a child with wavy hair from a father with wavy of a child with wavy hair from a father with wavy

hair and a mother with straight hair hair and a mother with straight hair


•• Genotype?Genotype?

Maternal allelesMaternal alleles

S S SS

Paternal alleles

Paternal alleles

C S

C S

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CodominanceCodominance

•• Commonly seen when more than 2 alleles exist Commonly seen when more than 2 alleles exist for the same genefor the same gene

•• Both dominant alleles are expressed at onceBoth dominant alleles are expressed at once

•• Not a blend of the 2 traits Not a blend of the 2 traits –– both distinct both distinct traits can be seen at the same timetraits can be seen at the same time

Incomplete vs. Incomplete vs. CodominanceCodominance

Dominant Dominant•• Incomplete dominanceIncomplete dominance

and and codominancecodominance areare

NOT the same thing!!NOT the same thing!!

•• Incomplete dominance:Incomplete dominance:

phenotype is a blendphenotype is a blend

of the two traitsof the two traits

•• CodominanceCodominance: both: both

traits are seen attraits are seen at

the same timethe same time

CodominanceCodominance

•• Human example: A, B, O blood typesHuman example: A, B, O blood types

•• Both type A and type B are dominant (IBoth type A and type B are dominant (IAA and Iand IBB))

•• Make different Make different glycoproteinsglycoproteins on the on the

membrane of red blood cellsmembrane of red blood cells

•• Type O is recessiveType O is recessive

•• Makes no such glycoprotein due to a Makes no such glycoprotein due to a frameshiftframeshift mutation that produces a STOP mutation that produces a STOP

codoncodon

•• If IIf IAA and Iand IBB are both present, both will be are both present, both will be expressedexpressed

Chaplin Paternity CaseChaplin Paternity Case

•• Before the days of DNA testing, blood type was Before the days of DNA testing, blood type was used to settle paternity suitsused to settle paternity suits

•• Doesn’t always work thoughDoesn’t always work though

•• Charlie Chaplin was involved in such a case in Charlie Chaplin was involved in such a case in 1942 with actress Joan Barry1942 with actress Joan Barry


•• Charlie Chaplin’s blood type: ABCharlie Chaplin’s blood type: AB

•• Joan Barry’s blood type: OJoan Barry’s blood type: O

•• Child’s blood type: OChild’s blood type: O

•• Use a Use a PunnettPunnett square to determine whether square to determine whether Charlie Chaplin could have been the child’s Charlie Chaplin could have been the child’s

fatherfather



•• Only possible genotype: Only possible genotype:

•• Joan Berry’s blood type: OJoan Berry’s blood type: O




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•• Only possible genotype: IOnly possible genotype: IAAIIBB

•• Joan Berry’s blood type: OJoan Berry’s blood type: O

•• Only possible genotype: iiOnly possible genotype: ii


•• Only possible genotype: iiOnly possible genotype: ii

Independent AssortmentIndependent Assortment

•• All of our examples of inheritance patterns have All of our examples of inheritance patterns have focused on single genesfocused on single genes

•• Humans have 25,000 genes!Humans have 25,000 genes!

•• Genes on the same chromosome are inherited Genes on the same chromosome are inherited togethertogether

•• Genes on different chromosomes are inherited Genes on different chromosomes are inherited separatelyseparately

•• If we consider just 2 chromosomes, how many If we consider just 2 chromosomes, how many different gametes could be produced by different gametes could be produced by

meiosis?meiosis?

Independent AssortmentIndependent Assortment Independent AssortmentIndependent Assortment

•• 4 possible gametes produced4 possible gametes produced

•• Consider 2 pea plantsConsider 2 pea plants

•• Heterozygous for 2 traits on 2 different Heterozygous for 2 traits on 2 different

chromosomeschromosomes

•• Can create a Can create a dihybriddihybrid PunnettPunnett square to square to

examine all offspring possibilitiesexamine all offspring possibilities


•• Trait 1: seed shapeTrait 1: seed shape

•• Dominant = spherical (S)Dominant = spherical (S)

•• Recessive = dented (s)Recessive = dented (s)

•• Trait 2: seed colorTrait 2: seed color

•• Dominant = yellow (Y)Dominant = yellow (Y)

•• Recessive = green (y)Recessive = green (y)


•• Both plants to be crossed: Both plants to be crossed: SsYySsYy

•• Phenotype? Phenotype?

•• What are the possible gamete combinations?What are the possible gamete combinations?

•• (Each will get one allele of each gene)(Each will get one allele of each gene)

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RecombinationRecombination

•• Sometimes it appears that 2 traits on the same Sometimes it appears that 2 traits on the same chromosome sort independentlychromosome sort independently

•• Recall the process of crossing over during Recall the process of crossing over during meiosis…meiosis…


•• DNA is DNA is replicatedreplicated

•• 4 copies of 4 copies of each each

chromatidchromatidform a tetradform a tetrad

•• Portions of Portions of homologous homologous

chromosomes chromosomes are swappedare swapped


•• This process is called This process is called recombinationrecombination

•• Remember: 25,000 genes in human genomeRemember: 25,000 genes in human genome

•• Between recombination and independent Between recombination and independent

assortment there are essentially assortment there are essentially infinite infinite genetic combinations for the gametes of any genetic combinations for the gametes of any individualindividual

Genetics of GenderGenetics of Gender

•• Chromosomal basis of gender discovered in 1905 Chromosomal basis of gender discovered in 1905 by Nettie Stevensby Nettie Stevens

•• Homogametic:Homogametic: sex chromosomes are the samesex chromosomes are the same

•• Heterogametic:Heterogametic: sex chromosomes are not the sex chromosomes are not the samesame

•• Homogametic sex is usually (but not always) Homogametic sex is usually (but not always) default default


•• Humans (and most animals)Humans (and most animals)

•• Males = XYMales = XY

•• Females = Females = XXXX

•• Birds, some insectsBirds, some insects

•• Females = ZWFemales = ZW

•• MMales = ZZales = ZZ


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•• Chromosomal basis for gender discovered by Chromosomal basis for gender discovered by studying 2 disorders:studying 2 disorders:

•• Turner’s Syndrome (XO)Turner’s Syndrome (XO)

•• Kleinfelter’sKleinfelter’s Syndrome (XXYSyndrome (XXY))

•• Both caused by nonBoth caused by non--disjunction of sex disjunction of sex

chromosomeschromosomes


(Sperm

could also

be XY)

Turner’s Syndrome (XO)Turner’s Syndrome (XO)

•• Only nonOnly non--fatal complete fatal complete monosomymonosomy

•• Incidence: 1:2000Incidence: 1:2000

•• Short statureShort stature

•• NonNon--functioning ovariesfunctioning ovaries

•• Webbed neckWebbed neck

•• Swelling of hands and feetSwelling of hands and feet

•• Skeletal abnormalitiesSkeletal abnormalities

•• High blood pressureHigh blood pressure

•• Heart defectsHeart defects

•• Kidney problemsKidney problems

Kleinfelter’sKleinfelter’s Syndrome (XXY)Syndrome (XXY)

•• Not a true Not a true trisomytrisomy, but 47 , but 47 chomosomeschomosomes presentpresent

•• Incidence 1:500 Incidence 1:500 –– 1:10001:1000

•• Highly variedHighly varied

•• Sometimes obvious at pubertySometimes obvious at puberty

•• Sometimes only discovered Sometimes only discovered

when seeking help for infertilitywhen seeking help for infertility

•• Less testosterone = Less testosterone =

(sometimes) less muscle,(sometimes) less muscle,

poor beard growth, poor beard growth,

tall stature, breast developmenttall stature, breast development

MosaicismMosaicism

•• CChromosome nonhromosome non--disjunction takes place in disjunction takes place in mitosis during embryonic developmentmitosis during embryonic development

•• Some cells will have normal chromosome countsSome cells will have normal chromosome counts

•• Some cells will have 45 or 47 chromosomesSome cells will have 45 or 47 chromosomes

•• Extreme case: Extreme case: hermaphroditismhermaphroditism

MosaicismMosaicism

•• Also occurs normally with X chromosomesAlso occurs normally with X chromosomes

•• Called X silencingCalled X silencing

•• One X chromosome is silenced in every cell of One X chromosome is silenced in every cell of

the female bodythe female body

•• Which X is silenced depends on the cellWhich X is silenced depends on the cell

•• Usually not noticeableUsually not noticeable

•• Example: calico catExample: calico cat

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SexSex--Linked InheritanceLinked Inheritance

•• When one gender is affected by a genetic When one gender is affected by a genetic disorder more than the otherdisorder more than the other

•• In humans = most often malesIn humans = most often males

•• X chromosome = largeX chromosome = large

•• Y chromosome = smallY chromosome = small

SexSex--Linked InheritanceLinked Inheritance

•• Many genes on X chromosomeMany genes on X chromosome

•• Males are essentially Males are essentially monosomaticmonosomatic for these for these genesgenes

•• Whatever allele is present will be expressedWhatever allele is present will be expressed

•• Examples: hemophilia, muscular dystrophy, color Examples: hemophilia, muscular dystrophy, color

blindness, many blindness, many lysosomallysosomal storage diseases, storage diseases, Fragile X syndromeFragile X syndrome

Fragile X SyndromeFragile X Syndrome

•• XX--linked dominant disorderlinked dominant disorder

•• Variably expressedVariably expressed

•• Results from >200 CGG Results from >200 CGG

repeats that lead to DNA repeats that lead to DNA

methylationmethylation and silencing of and silencing of FMR1FMR1

•• Causes physical and Causes physical and

intellectual changesintellectual changes

•• Nearly all children with Nearly all children with

Fragile X meet criteria for Fragile X meet criteria for

autism diagnosisautism diagnosis

ColorblindnessColorblindness

•• Genes for color vision are on X chromosomeGenes for color vision are on X chromosome

•• No corresponding gene on Y chromosomeNo corresponding gene on Y chromosome

•• Normal color vision is dominant, so males are Normal color vision is dominant, so males are

most often affectedmost often affected

•• Consider…Consider…

•• Male with normal color vision: XMale with normal color vision: XCCYY--

•• Female carrier with normal color vision: XFemale carrier with normal color vision: XCCxxCC

•• What is the probability that their child will be What is the probability that their child will be

colorblind?colorblind?

XX--Linked Recessive PedigreeLinked Recessive Pedigree

•• Females are affected by XFemales are affected by X--linked disorderslinked disorders

•• Far less commonFar less common

•• Consider the following pedigree for a family with Consider the following pedigree for a family with

colorblindnesscolorblindness

•• Assign genotypes to each individualAssign genotypes to each individual

•• How many carriers are there?How many carriers are there?

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