Download - Chapter 15 Chromosomal Basis of Inheritance
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