Chapter 15 Chapter 15 Chromosomal Basis of Chromosomal Basis of

InheritanceInheritance

Mendel & ChromosomesMendel & Chromosomes• Mendel was ahead of his time. 19Mendel was ahead of his time. 19thth C C

cytology suggested a mechanism for cytology suggested a mechanism for his earlier findings. What did they his earlier findings. What did they find?find?

Chromosomes and genes are both Chromosomes and genes are both present in pairs in diploid cells.present in pairs in diploid cells.

Homologous chromosomes separate and Homologous chromosomes separate and alleles segregate during meiosis.alleles segregate during meiosis.

Fertilization restores the paired condition Fertilization restores the paired condition for both chromosomes and genes.for both chromosomes and genes.

Chromosome Theory of InheritanceChromosome Theory of Inheritance• Mendelian genes Mendelian genes

have specific loci have specific loci on chromosomeson chromosomes

• Chromosomes are Chromosomes are what physically what physically undergo undergo segregation and segregation and independent independent assortment.assortment.

Morgan’s Fruit FliesMorgan’s Fruit Flies• Morgan first associated a specific gene Morgan first associated a specific gene

with a specific chromosome.with a specific chromosome.

• Why fruit flies?Why fruit flies?• Breed quickly (two week generations)Breed quickly (two week generations)• 4 pairs of chromosomes (3 pair of 4 pairs of chromosomes (3 pair of

autosomes, 1 pair of sex chromosomes)autosomes, 1 pair of sex chromosomes)• Females = XXFemales = XX• Males = XYMales = XY

Morgan’s Fruit FliesMorgan’s Fruit Flies• Wild Type flies are the most common Wild Type flies are the most common

natural phenotype. (Red Eyes)natural phenotype. (Red Eyes)• After a series of crosses, Morgan After a series of crosses, Morgan

produced mutants with white eyes.produced mutants with white eyes.• After a few generations, Morgan noted that After a few generations, Morgan noted that

only males displayed the white eyes.only males displayed the white eyes.• He concluded that certain genes are He concluded that certain genes are

located on the sex chromosome and thus located on the sex chromosome and thus linked to sex.linked to sex.• Sex-linked genes (ie: hemophilia)Sex-linked genes (ie: hemophilia)

Sex-linked TraitsSex-linked Traits

Sex-linked TraitsSex-linked Traits• Morgan concluded Morgan concluded

the gene with the the gene with the white-eyed white-eyed mutation is on the mutation is on the X chromosome. X chromosome. Y chromosome = Y chromosome = no info no info

• Males (XY) only Males (XY) only need one copy of need one copy of recessive allele to recessive allele to show trait.show trait.

Linked GenesLinked Genes• All genes located on the same All genes located on the same

chromosome tend to be inherited together.chromosome tend to be inherited together.• Chromosome passed on as a unit.Chromosome passed on as a unit.• Testcross results varied from those Testcross results varied from those

predicted by the law of independent predicted by the law of independent assortment.assortment.• This showed that certain genes will This showed that certain genes will

assort together. (on same chromosome)assort together. (on same chromosome)

Linked GenesLinked Genes

Linked GenesLinked Genes• Body color and wing shape are usually inherited Body color and wing shape are usually inherited

together (same chromosome)together (same chromosome)

RecombinantsRecombinants• Where did the other phenotypes come Where did the other phenotypes come

from? (grey-vestigial and black normal)from? (grey-vestigial and black normal)• Genetic recombination= offspring with Genetic recombination= offspring with

new combinations of traits inherited from new combinations of traits inherited from two parentstwo parents

• How?? How?? • independent assortment of genes (non-independent assortment of genes (non-

homologous)homologous)• crossing over of genes (homologous)crossing over of genes (homologous)

RecombinantsRecombinants• Mendel’s dihybrid crosses produced Mendel’s dihybrid crosses produced

recombinant genotypes. recombinant genotypes. • 50% parental : 50% recombinant 50% parental : 50% recombinant

genotypes typical for nonhomologuesgenotypes typical for nonhomologues• Metaphase IMetaphase I

• YR, Yr, yR, and yrYR, Yr, yR, and yr• Seed shape and color tetrads are Seed shape and color tetrads are

independent from one anotherindependent from one another

RecombinantsRecombinants• Linked genes tend to move together Linked genes tend to move together

during meiosis/fertilizationduring meiosis/fertilization• If Independent assortment of genesIf Independent assortment of genes

• Expect a 1:1:1:1 phenotype ratioExpect a 1:1:1:1 phenotype ratio• If Complete linkage of genesIf Complete linkage of genes

• 1:1:0:0 ratio (all parental)1:1:0:0 ratio (all parental)• Observed 17% recombinant fliesObserved 17% recombinant flies

• Suggested Incomplete linkage of genesSuggested Incomplete linkage of genes

Crossing OverCrossing OverProphase I: homologous chromosomes can “swap” allelesProphase I: homologous chromosomes can “swap” allelesMore variable gametes than simple mendelian rules More variable gametes than simple mendelian rules

would predictwould predict

Therefore, Crossing Over Therefore, Crossing Over Explains:Explains:

Linkage MapsLinkage Maps• Ordered list of genetic loci along Ordered list of genetic loci along

chromosomechromosome• Based on recombination frequencies Based on recombination frequencies

between two genesbetween two genes• Higher % of recombination = farther apartHigher % of recombination = farther apart

• More places in between genes for More places in between genes for crossing over to occur and separate the crossing over to occur and separate the genesgenes

Linkage MapsLinkage Maps•The recombination frequency between cn and b is 9%.•The recombination frequency between cn and vg is 9.5%.•The recombination frequency between b and vg is 17%.

Linkage MapsLinkage Maps• Map units are the distances between Map units are the distances between

genes on a chromosome.genes on a chromosome.• 1 map unit = 1% recombination1 map unit = 1% recombination• 50% recombination = so far apart that 50% recombination = so far apart that

crossing over is all but certaincrossing over is all but certain• Remember, 50% recomb. = ind. Remember, 50% recomb. = ind.

assortment (non-homologous)assortment (non-homologous)• Linkage maps show relative Linkage maps show relative

order/distanceorder/distance• More recent studies show exact More recent studies show exact

distances and orderdistances and order

Sex ChromosomesSex Chromosomes

X-Y Sex DeterminationX-Y Sex Determination• X and Y behave as homologuesX and Y behave as homologues• Each egg receives an X from XX motherEach egg receives an X from XX mother• One sperm receives X and one YOne sperm receives X and one Y• Results in 50/50 chance of male or femaleResults in 50/50 chance of male or female• SRY GeneSRY Gene

• Present (on Y) : gonads develop into testes Present (on Y) : gonads develop into testes (male)(male)

• Not present (no Y): gonads become Not present (no Y): gonads become ovaries (female)ovaries (female)

• SRY also regulates other genes SRY also regulates other genes

Sex-Linked GenesSex-Linked Genes• Sex chromosomes also contain other Sex chromosomes also contain other

genes. (ie: drosophila eye color)genes. (ie: drosophila eye color)• Females must be homozygous recessive Females must be homozygous recessive

to display trait (XX – second X can mask to display trait (XX – second X can mask recessive)recessive)• Females can be carriersFemales can be carriers

• Males only need to inherit a single copy to Males only need to inherit a single copy to show traitshow trait• Can a male be a carrier?Can a male be a carrier?

Sex-Linked DisordersSex-Linked Disorders

• Duchenne Muscular Dystrophy Duchenne Muscular Dystrophy • 1/3500 males1/3500 males• Progressive muscular weakeningProgressive muscular weakening

• Die by mid-20’sDie by mid-20’s• Missing X-linked geneMissing X-linked gene

• No production of dystrophin (muscle No production of dystrophin (muscle protein)protein)

Sex-Linked DisordersSex-Linked Disorders

• HemophiliaHemophilia • Absence of one or more clotting Absence of one or more clotting

factorsfactors• affected individuals cannot stop affected individuals cannot stop bleeding normallybleeding normally

• treated with protein injectionstreated with protein injections

Barr BodiesBarr Bodies• Only one of the females X chromosomes Only one of the females X chromosomes

is activeis active• The other becomes a Barr bodyThe other becomes a Barr body• When assorted into an ovum, the Barr When assorted into an ovum, the Barr

body becomes activated againbody becomes activated again• Which X becomes Barr body is random in Which X becomes Barr body is random in

each celleach cell• Approx. 50% express each allele (if Approx. 50% express each allele (if

hetero)hetero)

X-Inactivation in FemalesX-Inactivation in Females

NondisjunctionNondisjunction

• Errors with meiotic spindleErrors with meiotic spindle• Meiosis I: Homologous tetrad doesn’t Meiosis I: Homologous tetrad doesn’t

separate ORseparate OR• Meiosis II: Sister chromatids don’t Meiosis II: Sister chromatids don’t

separateseparate• Some gametes receive two of the same Some gametes receive two of the same

type of chromosome and another gamete type of chromosome and another gamete receives no copyreceives no copy

AneuploidyAneuploidy• Results from fertilization involving Results from fertilization involving

nondisjoined gamete(s)nondisjoined gamete(s)

• TrisomyTrisomy three copies of a particular three copies of a particular chromosome (2n + 1)chromosome (2n + 1)

• MonosomyMonosomy only one copy of a particular only one copy of a particular chromosome (2n – 1)chromosome (2n – 1)

Down SyndromeDown Syndrome

• Three copies of Three copies of chromosome chromosome 2121

• 1/700 children 1/700 children born each yearborn each year

• Definite link Definite link with maternal with maternal ageage

Aneuploidy in Sex ChromosomesAneuploidy in Sex Chromosomes

• XXY Male (Klinefelter’s Syndrome)XXY Male (Klinefelter’s Syndrome)• Male sex organs, sterile w/ Male sex organs, sterile w/

femininityfemininity

• XYY MalesXYY Males• Tend to be taller than normalTend to be taller than normal

Aneuploidy in Sex ChromosomesAneuploidy in Sex Chromosomes

• XXX FemalesXXX Females• Will develop as normal femalesWill develop as normal females

• XO Females (monosomy – Turner XO Females (monosomy – Turner syndrome)syndrome)• Immature femalesImmature females• 1/2500 live female births1/2500 live female births

Changes in ChromosomesChanges in Chromosomes