Gene Location and Pedigrees
• We refer to genes as either sex-linked (on
the sex-determining chromosomes) or
autosomal (not on the sex-linked
chromosomes)
Sex Linkage
• Sex-linked traits: traits that are controlled by
genes located on the sex chromosomes
• ex. X-linked recessive:
– red-green colourblindness in humans: a recessive
allele located on the X gene
• More prevalent in males since females would have to inherit
2 recessive alleles to be red-green colourblind
• Since males inherit only one X chromosome they require only
one recessive allele to be colourblind
• Ex. X-linked dominant
Why use Pedigrees?
• Punnett squares work well for organisms that have
large numbers of offspring and controlled
matings, but humans are quite different:
1. Small families - Even large human
families have 20 or fewer children.
2. Uncontrolled matings, often with
heterozygotes.
Goals of Pedigree Analysis
1. Determine the mode of inheritance:
dominant, recessive, sex-linked,
autosomal.
2. Determine the probability of an
affected offspring for a given cross.
Basic Symbols
More Symbols
Fraternal Twins
Identical Twins
Try and figure out the method of inheritance…
X-linked recessive
Try and figure out the method of inheritance…
Autosomal recessive
Try and figure out the method of inheritance…
X-linked recessive
Case Study: Sickle Cell Anemia
• One of the most common genetic diseases that
afflicts persons of African ancestry.
– about 10% of such persons carry the allele for this trait
– in some areas of Africa, upwards of 40% carry the allele
• Homozygous individuals with sickle-cell disease
suffer from:
– “Crises” in joints and bones
– Strokes, blindness
– Damage to lungs, kidneys or heart
• Untreated, many sufferers die before the age of 20
– Modern medical treatments can prolong life to age 40-50
(for individuals who are homozygous for Sickle cell
anemia)
• Normal allele (A) and Sickle allele (S) are
CODOMINANT
– In heterozygous individuals (AS), both normal and
abnormal hemoglobin are produced
– Individuals are usually healthy, but may notice problem in
conditions of low oxygen (high altitude)
– Known as ‘carriers’
Sickle Cell Disease
• What does it actually do?
– Affects the structure of hemoglobin
• Help bind oxygen in blood, carry to body cells
• Hemoglobin becomes ‘sticky’, and red blood cells
collapse when there is no oxygen
Normal red blood cells
Sickled red blood cells
Hemoglobin
Red Blood Cell
Oxygen molecule
Hemoglobin
subunit
• Sickle cell traits
- malaria
Effects of Sickle Cell Mutation
CTGACTCCTGAGGAGAAGTCT
DNA
GACTGAGGACTCCTCTTCAGA
L
T
P
E
E
K
S
CTGACTCCTGTGGAGAAGTCT
GACTGAGGACACCTCTTCAGA
order of
amino acids
L
T
protein
shape
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
cell
function
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
P
V
E
K
S
Case Study: Sickle Cell Anemia
• Why do we see this more often in Africa, especially
when it is deadly?
– There must be a benefit to having this abnormal allele!
• Malaria!
– Caused by a blood parasite
– Infect red blood cells
– When a blood cell with defective hemoglobin are infected,
the cells sickle and die
• The parasite is trapped and infection is reduced