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Chromosome Theory of Inheritance• Chromosomes
actually segregate (homologs) and assort (nonhomologous) in meiosis• Compare with
dihybrid cross from chapter 14
• Influenced by Mendel’s laws
Determining Sex• Anatomical and
physiological differences are numerous
• Chromosomal basis for determining sex is presence of the Y chromosome– Significantly smaller
• Only short regions on either end of Y are homologous with X
Gametic Sex• Males produce sperm either X or Y• Females produce eggs all X• Offspring’s sex determined the moment of
conception– X sperm fertilizes egg = female– Y sperm fertilizes egg = male
• Gonads, reproductive organs, are generic until 2 months– Development depends on SRY gene, located on Y
chromosomes– Actually codes for proteins that regulate other genes– Without, gonads become ovaries; default sex is female
Alternate Chromosomal Systems• X-Y system
– Mammals– Determined by sperm, X or Y
• X-0 system– Insects, only one sex chromosome, X– Females XX, males X0
• Z-W system– Birds and fishes– Determined by egg, Z– Female ZW, male ZZ
• Haploid-diploid system– Bees and ants– Fertilization determines– Fertilized = female = diploid = 2
parents– Unfertilized = male = haploid = no
father
Sex (X)-linked GenesGenes located on either sex chromosome,
especially the X (in humans)Fathers pass X to all daughters but no sonsMothers pass X to all offspringRecessive alleles
Female express phenotype only if homozygousMales express only if egg bore the recessive
allele Males are hemizygous because other terms can’t
applyMore males have sex-linked recessive
disorders
Color Blindness
• N represents color-blind gene• Loci carried on sex (X) chromosomes• Muscular dystrophy and hemophilia are similar
examples
F: normal; M: affected F: carrier; M: carrierF: carrier; M: normal
X Inactivation• Females have 2 X chromosomes, but don’t make
double the proteins– Barr body is the inactivated and condensed X
chromosome• Reactivated in ovaries so both X’s are transferable
– Males and females each with same dose (1 copy) of genes
• Decision is random and independent, can be mom or dad’s allele, in each embryonic cell– Inactivity is conserved in all mitotic descendents– Females are mosaics
• Heterozygous for a sex-linked = ½ cells express 1 allele and ½ express other • E.g coat color in cats
Tortiseshell/Calico Cats• X-linked gene (O)
determines orange fur• Males have one X so if
O is orange, o is black• Females have 2 X’s
– Heterozygotes (Oo) have 1 X inactivated
– Some cells express O others o = patches of orange and black
• Rare, but possible to have male’s this color
Phenotype Alterations• Small-scale
– Random mutations create all new alleles = new phenotypic traits
• Large-scale– Physical and chemical disturbances can damage
chromosomes– Errors in meiosis can alter chromosomal number– Often lead to miscarriages or genetic disorders
• Plants often tolerate better than animals
Nondisjunction• Separation during
anaphase doesn’t occur properly
• Fertilization of gamates from nondisjunction results in aneuploidy– Abnormal chromosome
number• extra = trisomy, fewer
= monosomy
• Polyploidy 2+ sets of chromosomes• Common in plants
Human DisordersFetal testing can diagnose
most, often times affected individuals are miscarried
Down SyndromeTrisomy 21; 47
chromosomes; risk increases with age
Sex chromosome relationshipsLess disruptive
Y chromosome carries few genes X chromosome extra copies are
inactivated
Sex Chromosome Disorders
Sex Chromosome Abnormalities
Genotype Gender Syndrome Physical TraitsXXY, XXYY, XXXY Male Klinefelter
syndromeSterile, small testicles, breast enlargement
XYY Male Jacob’s syndrome Normal male traits, previously thought ‘super-males’, karyotype ID
XO Female Turner syndrome sex organs don't mature, sterile, short stature
XXX female Trisomy X tall stature, learning disabilities, limited fertility, karyotype ID
Altering Chromosomal Structure• Deletion: a
chromosomal fragment is lost
• Duplication: segments repeat– Pieces from deletions
can attach to another chromosome
• Inversion: a segment breaks and flips 180°
• Translocation: chromosome segment moves from 1 to another non-homolog
Altered Chromosome DisordersCri du chat (cry of the cat)
Deletion in chromsome 5Small head, unusual facial
features, cry of a catHuntington’s disease
Duplication of CAG sequence in DNA on chromosome 4 Effected has 42-120 copies,
normal is 11-34 copiesCodes for glutamine,
produces a polyglutamine <36 glutamines produces
huntingtin protein, more is mutant
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