Chapter 15

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