Download Chapter 15 Chromosomes

Chapter 15 Chromosomes
The chromosomal basis of sex
In humans, Y chromosome determines sex
XX
X
XY
Gametes?
p(son)
p (daughter)
Humans X/Y system
Chromosome theory of inheritance
• Genes located on chromosomes = gene locus
• Thomas Morgan
X-linked genes in humans
Female genotypes
XCXC XCXc
XcXc
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?
No green photoreceptors
Example: colorblindness is X-linked
• 1/1O males
• 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.
X-chromosome inactivation in females
• One X inactivated during embryonic development 
Barr body (see nuclear envelope)
• # in female cells? male cells?
FYI
• Barr body testing was introduced in the 1966
Olympic games, in an effort to detect male
athletes trying
to "pass" as females, to gain a
.
competitive advantage
• Genes on this X are not expressed
• Lyon hypothesis
– Females mosaics for X-linked traits
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 calico = XorangeXblack
Genotype of orange male? = XorangeY
Genotype of black male?
Linkage
Unlinked genes
-On different chromosomes
-Independent assortment!
FYI: Example in pea plants:
color gene (Chromosome 1)
shape gene (Chromosome 7)
FYI: Example in humans:
Blood type (Chromosome 9)
Lactose intolerance (Chromosome 2)
Linked Genes
-on same chromosome
-do not assort independently
FYI: Example humans:
Freckles (Chromosome 16)
Red hair (Chromosome 16)
Chromosomal abnormalities
• Disjunction anaphase of meiosis
 Fertilization
(human)
Aneuploidy – abnormal number of chromosomes
• Nondisjunction anaphase I
• Nondisjunction anaphaseII
• Fertilization animation at Learn Genetics
1. Monosomy (2n-1)
Ex. Turner syndrome 45, X
1/5,000 births
Only viable monosomy in humans
Possible symptoms
Short stature
Lymphadema of the hands and feet
Broad chest
Low hairline
Low-set ears
Reproductive sterility
Rudimentary ovaries gonadal streak
absence of a menstrual period
Increased weight, obesity
Shield shaped thorax of heart
Shortened hand bones
Small fingernails
Characteristic facial features
Webbed neck
Aorta abnormalities
Poor breast development
Horseshoe kidney
Visual impairments
Ear infections and hearing loss
High waist-to-hip ratio (the hips are not much bigger than the waist)
Attention Deficit/Hyperactivity Disorder
Nonverbal Learning Disability (problems with math, social skills and spatial relations)
2. Trisomy (2n+1)
Ex. Trisomy 21 (Down syndrome)
Possible symptoms
Common physical signs include:
Excess skin at the nape of the neck
Flattened nose
Single crease in the palm of the hand
Small ears
Small mouth
Upward slanting eyes
Wide, short hands, short fingers
White spots on eye
Physical development is often slower than normal.
Delayed mental and social development.
Chromosome structure abnormalities
1. Deletion – fragment lost
2. Duplication – repeated fragment
3. Translocation – fragment attaches to other
chromosome
4. Inversion – fragment inverts
Inheritance of mitochondrial DNA
and chloroplast DNA (plants)
• Extranuclear