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Chapter 15

BIO152H5 Chapter Notes - Chapter 15: Homologous Chromosome, X-Inactivation, Y Chromosome

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- Mendelian genes have specific loci (position) along chromosomes
-it is the chromosome that that undergo segregation and independent assortment
-Homologous chromosomes - segregation of alleles
at each genetic locus to different gametes
-Non-homologous chromosomes: accounts for independent assortment of the alleles for 2 or more genes
located on different chromosomes
Moga’s Epeietal Eidee: Sietifi Iui
Morgan selected a specie of fruit flies-drosophila melanogaster- common insect that feeds on
fungi growing on fruits
o A single mating will produce a hundreds offspring and a new generation bred every two
o Another advantage: they have only 4 pairs of chromosomes (easily distinguishable
under light microscope
o 3 pairs of autosomes, 1 sex chromosome
o Females have X chromosomes and males have X and Y
o After 2 years of work he discovered a single male fly with white eyes instead of red
o Phenotype for a character most commonly observed in natural population is called wild
o Mutant phenotype- alternatives to white wild traits
o Alleles for white eyes = w; a subscript + identities (w+) for the red eyes
Coelatig ehaio of a Gee’s allele ith ehaio of a hoosoe pai
o Moran mated his white-eyed ales with red-eyed females- all F1 offsprings had red eyes
Wild-type allele was dominant
o When he bred F1 with each otherhe observed a classical 3:1 phenotypic ratio among
o White eyes only showed in males; all F2 females had red eyes
Half males had red and half males had right
o Morgan concluded that fly eye color was linked to sex chromosomes
o If eve color was unrelated to sex, then half red and half white not specified to gender
o Females have two X chromosomes (XX) while a male fly has an X and Y (XY)
o White eyed mutant was located exclusively on X chromosome
Sex-Linked genes exhibit unique patterns of inheritance
Chromosomal basis of Sex:
o The Y chromosome is much smaller than the X
o Short segments at either end of the Y chromosome that are only regions that are
homologous with corresponding regions of the X
These regions allow the X and Y chromosomes in males to pair and behave like
homologs during meiosis in testes
Each egg contains one X
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Sperm falls into 2 categories: half the sperm cells the male produces contain an
X chromosome and half contain a Y chromosome
Sex determination is a matter of chance (50-50)
Gene located on either sex chromosome is called sex-linked
Those located on Y chromosomes are called y-linked
o Disorders that are y linked are passed from fathers to sons
X chromosomes contains approx. 1100 genes called X- chromosomes
Inheritance of X chromosomes genes:
A colorblind father transfers mutant allele to all daughter and no sonsif mom is normal each
daughter will be a carrier for that disease
If a carrier mates with a male who has normal color vision there is a 50 percent chance that each
daughter will be a carrier like mother and 50% that each son will have the disorder
If a carrier mates with color bind male there is 50% chance that each child born to them will
have the disorder regardless of sex
o Daughters with normal color vision will be carriers
o While males with normal color vision will be free of recessive allele
Hemophilia is a X linked recessive disorder defined by one or more of the proteins required for
blood clotting
X inactivation in Female mammals
Most of one X chromosome in each cell in female becomes inactivated during early embryonic
o Result: cell of male/female have the same effective dose (one copy) of most x-linked
Inactive x in females condenses into a compact object called barr body
o Lies among the inside of the nuclear envelope
o In the overies, bar bodies chromosomes are reactivated in the cells that give rise to eggs
so evey female gamete has an active X
o Barr body occurs randomly and independently in each embryonic cell present at the
time of X inactivation
o Result: females consist of a mosaic of 2 types: with the active X derived from the father
and those with the active X derived from the mother
o After an X chromosome is inactivated in a particular cell all mitotic descendants of that
cell have the same inactive X
Thus if a female is heterozygous for a sex-linked trait about ½ cells will express
one allele and the other ½ will express the alternative allele
Ex: inactivation of the tortoiseshell cat half black and half orange: orange is
active and the black patches have cells in which X chromosomes with the black
allele is active
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