biology 102 lecture 10: chromosomes and sex...

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Biology 102Biology 102

Lecture 10: Chromosomes Lecture 10: Chromosomes and Sex Inheritanceand Sex Inheritance

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 examine all offspring possibilitiespossibilities

•• This is what you will do in Lab 5This is what you will do in Lab 5


•• 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 What are the possible gamete combinations?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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Gametes for Parent 1Gametes for Parent 1

SY SY SySy sYsY sysy

Gam

ete

s fo

r Pa

rent

2Gam

ete

s fo

r Pa

rent

2

SY

SY Sy

Sy

sYsYsysy

SsYySsYy(spherical, (spherical,

yellow)yellow)

SsyySsyy(spherical, (spherical,

green)green)

ssYyssYy(dented,(dented,

yellow)yellow)

ssyyssyy(dented,(dented,

green)green)

SsYYSsYY(spherical,(spherical,

yellow)yellow)

SsYySsYy(spherical,(spherical,

yellow)yellow)

ssYYssYY(dented,(dented,

Yellow)Yellow)

ssYyssYy(dented,(dented,

Yellow)Yellow)

SSYySSYy(spherical,(spherical,

yellow)yellow)

SSyySSyy(spherical,(spherical,

green)green)


yellow)yellow)

SsyySsyy(spherical,(spherical,

green)green)

SSYYSSYY(spherical,(spherical,

yellow)yellow)

SSYySSYy(spherical,(spherical,

yellow)yellow)

SsYYSsYY(spherical,(spherical,

yellow)yellow)


yellow)yellow)


•• Phenotypic ratio?Phenotypic ratio?

•• Spherical yellow = 9/16Spherical yellow = 9/16

•• SSYY, SSYY, SsYYSsYY, , SSYySSYy, , SsYySsYy

•• Spherical green = 3/16Spherical green = 3/16

•• SSyySSyy, , SsyySsyy

•• Dented yellow = 3/16Dented yellow = 3/16

•• ssYYssYY, , ssYyssYy

•• Dented green = 1/16Dented green = 1/16

•• ssyyssyy

9:3:3:1 9:3:3:1 inheritance inheritance

pattern typical pattern typical for straightfor straight--

forward forward complete complete dominance dominance situationssituations

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 recombinationThis process is called recombination

•• 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 infinite assortment there are essentially 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

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•• 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 = XXFemales = XX

•• Birds, some insectsBirds, some insects

•• Females = ZWFemales = ZW

•• Males = ZZMales = ZZ


•• 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 (XXY)Syndrome (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: 1:2Incidence: 1:2--3,0003,000

•• 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 chromosomes chromosomes 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

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TrisomyTrisomy X Syndrome (XXX)X Syndrome (XXX)

•• Another sex chromosome Another sex chromosome trisomytrisomy

•• Occurrence: 1 in 1,000 womenOccurrence: 1 in 1,000 women

•• No noticeable defectsNo noticeable defects

•• Tendency to be tall, Tendency to be tall,

learning disabledlearning disabled

•• Fertile, offspring have no Fertile, offspring have no

•• higher chance of higher chance of trisomytrisomy

•• Additional mechanism Additional mechanism

must prevent inheritance of must prevent inheritance of

additional Xadditional X

Jacob Syndrome (XYY)Jacob Syndrome (XYY)

•• Another sex chromosome Another sex chromosome trisomytrisomy

•• Occurrence: 1 in 1,000 menOccurrence: 1 in 1,000 men

•• High levels of testosteroneHigh levels of testosterone

•• TallTall

•• Severe acneSevere acne

•• Normal intelligenceNormal intelligence

•• Delayed emotional maturityDelayed emotional maturity

(insert joke here)(insert joke here)

MosaicismMosaicism

•• Chromosome nonChromosome 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

•• Males only have one X chromosome, yet function Males only have one X chromosome, yet function normallynormally

•• Remember: most Remember: most monosomiesmonosomies are fatalare fatal

•• Only one copy of the genes on this chromosome Only one copy of the genes on this chromosome must be requiredmust be required

•• But females have two X chromosomesBut females have two X chromosomes

•• How does the body deal with competing How does the body deal with competing

expression?expression?

MosaicismMosaicism

•• All females are technically mosaics!All females are technically mosaics!

•• One One X chromosome is silenced in every cell of X chromosome is silenced in every cell of the female the female bodybody

•• Called X silencing or X Called X silencing or X inactivationinactivation

•• Forms collapsed structure called a Barr bodyForms collapsed structure called a Barr 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

XX--InactivationInactivation

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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

•• 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 chromosome (YNo corresponding gene on Y chromosome (Y--))

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

most often affectedmost often affected

•• Proper notation:Proper notation:

•• Normal allele: XNormal allele: XCC

•• Colorblind allele: Colorblind allele: XXcc

•• Really difficult to tell the difference on Really difficult to tell the difference on

PowerPointPowerPoint

•• We will use XWe will use XCC for normal, Xfor normal, Xcbcb for colorblindfor colorblind


•• Consider…Consider…

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

•• Female carrier with normal color vision: Female carrier with normal color vision: XXCCXXcbcb

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


Maternal allelesMaternal alleles

XXCC XXcbcb

Pate

rnal allel

esPa

tern

al allel

es

XXCC

YY--

•• Genotypes?Genotypes?

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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?

XX--Linked Recessive PedigreeLinked Recessive Pedigree

1. Assign 1. Assign genotypes to affected genotypes to affected individualsindividuals

•• Circles Circles are are femalesfemales

•• Squares Squares are are malesmales

2. Assign 2. Assign genotype to unaffected genotype to unaffected malesmales

3. Assign 3. Assign one normal allele to unaffected one normal allele to unaffected femalesfemales

4. 4. Work backwards to determine missing alleles Work backwards to determine missing alleles

(if possible(if possible))

5. How 5. How many carriers are there in the pedigree?many carriers are there in the pedigree?

Can you tell?Can you tell?

biology 102 lecture 10: chromosomes and sex...

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