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Chromosome Location Condition and Inheritance Pattern

Protein Affected

Symptoms and Comments

Gaucher Disease

1, recessive

glycohydrolase glucocerebrosidase,

Common among European Jews. Lipid accumulation

a lipid metabolism enzyme

in liver, spleen, and bone marrow. Treat with

enzyme replacement

Achondroplasia

4, dominant

fibroblast growth factor receptor 3

Causes dwarfism. Most cases are new mutations,

not inherited

Huntington's Disease

4, dominant

huntingtin, function unknown

Expansion of a three-nucleotide portion of the gene

causes late-onset neurodegeneration and death

Juvenile Onset Diabetes

6, 11, 7, others

IDDM1, IDDM2, GCK, other genes

Multiple susceptibility alleles are known for this

form of diabetes, a disorder of blood sugar

regulation. Treated with dietary control and insulin

injection

Hemochromatosis

6, recessive

HFE protein, involved in iron absorption

Defect leads to excess iron accumulation, liver

from the gut

damage. Menstruation reduces iron in women.

Bloodletting used as a treatment

Cystic Fibrosis

7, recessive

cystic fibrosis transmembrane regulator,

Sticky secretions in the lungs impairs breathing, and

an ion channel

in the pancreas impairs digestion. Enzyme

supplements help digestive problems

Friedreich's Ataxia

9, recessive

frataxin, mitochondrial protein of

Loss of function of this protein in mitochondria

unknown function

causes progressive loss of coordination and

heart disease

Albinism

11, recessive

tyorsinase

Lack of pigment in skin, hair, eyes; loss of visual

acuity

Best Disease

11, dominat

VMD2 gene, protein function unknown

Gradual loss of visual acuity

Sickle Cell Disease

11, recessive

hemoglobin beta subunit, oxygen

Change in hemoglobin shape alters cell shape,

transport protein in blood cells

decreases oxygen-carrying ability, leads to joint

pain, anemia, and infections. Carriers are

resistant to malaria. About 8% of US black

population are carriers

Phenylketonuria

12, recessive

phenylalanine hydroxylase, an amino

Inability to breakdown the amino acid phenylalanine

acid metabolism enzyme

causes mental retardation. Dietary avoidance

can minimize effects. Postnatal screening is

widely done

Marfan Syndrome

15, dominant

fibrillin, a structural protein of

Scoliosis, nearsightedness, heart defects, and

connective tissue

other symptoms

Tay-Sachs Disease

15, recessive

beta-hexosaminidase A, a lipid

Accumulation of the lipid GM2 ganglioside in

metabolism enzyme

neurons leads to death in childhood

Breast Cancer

17, 13

BRCA1, BRCA2 genes

Susceptibility alleles for breast cancer are thought

to involve reduced ability to repair damaged DNA

Myotonic Dystrophy

19, dominant

dystrophia myotonica protein kinase, a

Muscle weakness, wasting, impaired intelligence,

regulatory protein in muscle

cataracts

familial hypercholesterolemia

19, incomplete dominance

low-density lipoprotein (LDL) receptor

Accumulation of cholesterol-carrying LDL in the

adenosine deaminase, nucleotide

bloodstream leads to heart disease and heart

metabolism enzyme

attack

Severe Combined Immune

20, recessive

respiratory complex proteins

Immature white blood cells die from accumulation

Deficiency ("Bubble Boy"

of metabolic products, leading to complete loss

Disease)

of the immune response. Gene therapy has been

a limited success

Leber's Hereditary Optic

mitochondria,

transfer RNA

degeneration of the central portion of the optic

Neuropathy

maternal inheritance

nerve, loss of central vision

Mitochondrial Encephalopathy,

mitochondria,

lignoceroyl-CoA ligase, in peroxisomes

recurring, stroke-like episodes in which sudden

Lactic Acidosis, and Stroke

maternal inheritance

headaches are followed by vomiting and seizures;

(MELAS)

musle weakness

Adrenoleukodystrophy

X

dystrophin, muscle structural protein

Defect causes build-up of long-chain fatty acids.

Degeneration of the adrenal gland, loss of myelin

insulation in nerves. Featured in the film

"Lorenzo's Oil"

Duchenne Muscular Dystrophy

X

Factor VIII, part of the blood clotting

Lack of dystrophin leads to muscle breakdown,

cascade

weakness, and impaired breathing

Hemophilia A

X

Uncontrolled bleeding, can be treated with injections

or replacement protein

Rett Syndrome

X

methyl CpG-binding protein 2, regulates

Most boys die before birth. Girls develop mental

DNA transcription

retardation, mutism and movment disorder

Table 1. Autosomal Recessive Inheritance

Table 1. Autosomal Recessive Inheritance

Autosomal recessive inheritance is due to recessive alleles carried on autosomes. An individual possessing only one recessive allele is known as a carrier. An individual must inherit two recessive alleles, one from each parent, in order to express the recessive trait. When two carrier parents have offspring, each offspring has a 25 percent chance of inheriting two alleles and ooooc expressing the trait. The two recessive alleles need not be precisely identical, as long as each is nonfunctional. An individual possessing two different alleles with the same effect is known as a compound heterozygote. Compound heterozygotes account for some cases of the neurologic disorder known as Friedreich's ataxia.

Medical conditions due to autosomal recessive traits also number in the many hundreds. These include cystic fibrosis (affecting ion transport in the lungs and pancreas), Tay-Sachs disease (affecting lipid metabolism and storage, especially in the brain), and hemochromatosis (affecting iron metabolism and storage in a variety of organs).

The number of people with such conditions is actually much higher than that for autosomal dominant conditions. This is because inheritance of one harmful recessive allele does not produce symptoms, and so the individual can reproduce and pass the allele on to children easily. Thus, most harmful recessive alleles are not deleted from a population's gene pool as rapidly as most dominant ones, and the likelihood of inheriting two copies is consequently higher. Most humans harbor a small handful of known harmful alleles; it is only when they mate with another who has the same set that there is a chance of bearing children that express the disorder. Customs that warn against marrying close relations have the effect of minimizing the likelihood of offspring with homozygous recessive conditions.

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