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?