Chapter 15 Chromosomes
Chromosome theory of inheritance
• Genes located on chromosomes = gene locus
• Thomas Hunt Morgan, Columbia Univ. “Fly room”
• Drosophila
– 100s of offspring
– 2n = 8
• 3 prs autosomes
• X and Y sex chromosomes
Bithorax
Wildtype
White eye
Wildtype = normal
Mutant = abnormal
Wingless
eyeless
Drosophila genetics
White eye allele = w
Wildtype allele = w+
gene locus on
X chromosome
Sex-linked!
Genotype? ww,
w+w
w+w+
Sex-linked genes
Cross a red female with a white male (pg. 288)
Xw+ Xw+
X
Xw Y
Punnett square results
Cross f1 females with f1 males
f1
Xw+ Xw
X
Xw+ Y
f2
 Some genes are located on the X chromosome
The chromosomal basis of sex
In humans, Y chromosome determines sex
XX
X
XY
Gametes?
p(son)
p (daughter)
Humans have an X/Y system
Development
<2 months gestation – embryo has rudimentary
gonads
2 mos. SRY gene on Y chromosome active
-> testes develop
-> testosterone
-> male
If no SRY
female default pathway  ovaries
X-linked genes in humans
Female genotypes
XCXC XCXc
Xc X c
Male genotypes
XC Y XcY
Terms: homozygous, heterozygous, hemizygous
Concept check:
• From whom do males obtain the Y chromosome?
• From whom do females obtain the X chromosomes?
• Why are X-linked disorders more prevalent in males?
Example: colorblindness
• 1/1O males
• Colorblindness is an X-linked trait. A man is
colorblind. What % of his sons and daughters
are expected to be colorblind. His wife does
not carry the colorblind allele on either X
chromosome.
No green photoreceptors
X-chromosome inactivation in females
• One X inactivated during embryonic development 
Barr body (see nuclear envelope)
• # in female cells? male cells?
• Genes on this X are not expressed
• Lyon hypothesis
– Females mosaics for X-linked traits
– Allele key
C
c
X X
• Patches of
colorblind cells in
retina
• Normal
phenotype
Blue colorblindness is rare
• Genotype of orange female?
• Genotype of black female?
• Genotype of orange male?
• Genotype of black male?
Linkage
Unlinked genes
-On different chromosomes
-Independent assortment!
Example in humans:
Blood type (Chromosome 9)
Lactose intolerance (Chromosome 2)
Linked Genes
-on same chromosome
-do not assort independently
Example humans:
Freckles (Chromosome 16)
Red hair (Chromosome 16)
Genetic recombination of linked genes
Crossing over
Meiosis
Non-sister chromatids of homologous chromosomes
25% of each
Crossing over results in recombinants
Chromosomal abnormalities
• Disjunction anaphase of meiosis
If chromosome do not disjoin
• Non-disjunction
– Meiotic spindle error
– Sister chromatids do not separate in Anaphase II
– Gametes  ?
 Fertilization
(human)
Aneuploidy – abnormal number of chromosomes
1. Monosomy (2n-1)
Ex. Turner syndrome 45, X
Only viable monosomy in humans
2. Trisomy (2n+1)
Ex. Trisomy 21 (Down syndrome)
Higher risk in women > 35
Abnormal meiosis
• Nondisjunction anaphase I
• Nondisjunction anaphaseII
• Fertilization animation at Learn Genetics
Chromosome structure abnormalities
1.
2.
3.
4.
Deletion
Duplication
Translocation
Inversion
Univ. Wisc. Genetics center tumors
Inheritance of mitochondrial DNA
and chloroplast DNA (plants)
• Extranuclear genes
• Maternal inheritance (cytoplasm)