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Inheritance Patterns Many inherited disorders in humans are
controlled by a single gene. All of our somatic cells possess two copies of each gene, one inherited from your mother and one inherited
from your father.
46,XY male karyotype
– Autosomes are the 22 pairs of chromosomes that are not the sex chromosomes
– Alleles are all of the alternate forms of a gene (ie. B and b)
– Recessive means it takes two abnormal copies to exhibit the full blown phenotype
– Examples of autosomal recessive disorders are : cystic fibrosis, sickle-cell disease, PKU
– A Punnet Square demonstrating inheritance of an autosomal recessive deafness is found at the right
Most inherited disorders are caused
by autosomal recessive alleles
Figure 9.9A
D D
d d
NormalDd
NormalDd
DDNormal
DdNormal(carrier)
DdNormal(carrier)
ddDeaf
Eggs Sperm
PARENTS
OFFSPRING
Found on Chromosome 12
• A few inherited disorders are caused by dominant alleles. Dominant alleles hide recessive alleles, phenotypically.
Figure 9.9B
– Examples: achondroplasia, Huntington’s disease
Autosomal Dominant Inheritance
Table 9.9
• Most sex-linked human disorders are due to recessive alleles
• These sex linked alleles are forms of genes found on the X chromosome. A male has only one X chromosome
– Examples: hemophilia, red-green color blindness
– These are mostly seen in males, but can be seen in females.
– A male receives a single X-linked allele from his mother, and will have the disorder, while a female has to receive the allele from both parents to be affected
Sex-linked disorders affect mostly males
Figure 9.23A
– Their inheritance pattern reflects the fact that males have one X chromosome and females have two
Figure 9.22B-D
– These figures illustrate inheritance patterns for white eye color (r) in the fruit fly, an X-linked recessive trait
Female Male Female Male Female Male
XrYXRXR
XRXr
XRY
XR Xr
Y
XRXr
XR
Xr XRXR
XR
Y
XRY
XrXR XRY
XrY
XRXr
XR
Xr
Xr
YXRXr
XrXr XRY
XrY
XrY
R = red-eye alleler = white-eye allele
X-Linked Recessive Inheritance
X-Linked Dominant Inheritance
• When an offspring’s phenotype—such as flower color— is in between the phenotypes of its parents, it exhibits incomplete dominance
Incomplete dominance results in intermediate phenotypes
P GENERATION
F1 GENERATION
F2 GENERATION
RedRR
Gametes R r
Whiterr
PinkRr
R r
R R
r r
1/21/2
1/2
1/21/2
1/2 SpermEggs
PinkRr
PinkrR
Whiterr
RedRR
Figure 9.12A
• Incomplete dominance in human hypercholesterolemia (high levels of cholesterol in the blood)
Figure 9.12B
GENOTYPES:
HHHomozygous
for ability to makeLDL receptors
HhHeterozygous
hhHomozygous
for inability to makeLDL receptors
PHENOTYPES:
LDL
LDLreceptor
Cell
Normal Mild disease Severe disease
Codominance-The individual expresses both phenotypes and neither is
dominant.
Type AB express both antigens
Mitochondrial Gene Inheritance
Mitochondrial Disorders
Mitochondrial Disorders• mtDNA Point mutations
CardiomyopathyLeber's optic neuropathyLeigh's syndromeMELASMERRFNARP/MILS
Single deletion or duplicationAtaxia, LeukodystrophyDiabetes: Maternal inheritance Kearns-SayrePearson'sPEO: SporadicMultiple deletionsAgingMyositis Inclusion body COX- muscle fibersMNGIEPEOWolfram
•Depletion of mtDNA Infantile myopathy Fatal "Later-onset"AZT treatmentSeveral types of mtDNA defectDeafnessDiabetesExternal ophthalmoplegia (PEO) Sporadic Maternal Dominant RecessiveLeigh'sMyopathyRhabdomyolysisSensory neuropathySystemic disorders
Mitochondrial Inheritance
Mitochondrial disease begins to become apparent once the number of affected mitochondria reaches a certain level; this phenomenon is called 'threshold expression'.
In class……………………….
• What is the mode of inheritance of the disorder you are researching?
• If you are studying a particular cell type, are there diseases associated with this cell type when it is mutated and how are those diseases inherited?