Transcript Chromosomal Basis of Inheritance

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 Genes
 Genes located on either sex chromosome, especially
the X (in humans)
 Fathers pass X to all daughters but no sons
 Mothers pass X to all offspring
 Recessive alleles
 Female express phenotype only if homozygous
 Males express only if egg bore the recessive allele

Males are hemizygous because other terms can’t apply
 More males have sex-linked recessive disorders
Color Blindness
F: normal; M: affected
F: carrier; M: normal
F: carrier; M: carrier
• N represents color-blind gene
• Loci carried on sex (X) chromosomes
• Muscular dystrophy and hemophilia are similar examples
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 Disorders
 Fetal testing can diagnose
most, often times affected
individuals are miscarried
 Down Syndrome
 Trisomy 21; 47 chromosomes;
risk increases with age
 Sex chromosome relationships
 Less disruptive


Y chromosome carries few genes
X chromosome extra copies are
inactivated
Sex Chromosome Disorders
Sex Chromosome Abnormalities
Genotype
Gender
Syndrome
Physical Traits
XXY, XXYY, XXXY
Male
Klinefelter syndrome
Sterile, small testicles,
breast enlargement
XYY
Male
Jacob’s syndrome
Normal male traits,
previously thought ‘supermales’, 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 Disorders
 Cri du chat (cry of the cat)
 Deletion in chromsome 5
 Small head, unusual facial
features, cry of a cat
 Huntington’s disease
 Duplication of CAG sequence
in DNA on chromosome 4

Effected has 42-120 copies,
normal is 11-34 copies
 Codes for glutamine, produces
a polyglutamine

<36 glutamines produces
huntingtin protein, more is
mutant