3.4.2. Genetic diseases

Caused by a gene in an autosomal chromosome

Recessive, dominant and co-dominant

• Caused by recessive alleles

• Most common genetic disease

Individual must have two copies of the recessive allele in order to develop the disease

• Heterozygous

Individuals who have one allele for the genetic disease and one dominant allele (i.e. heterozygous individuals)

Cystic Fibrosis

Mutation to CFTR gene on chromosome 7

Codes for the production of chloride ion channels required for secretion of sweat, mucus and digestive juices

Recessive alleles of affected gene produce malfunctioning chloride channels

• Sweat contains excessive sodium chloride, but digestive juices and mucus have insufficient sodium chloride

• Hence, not enough water moves into the secretions by osmosis, making them more viscous

• Sticky mucus builds up in the lungs, causing infections and respiratory failure

• Pancreatic duct is blocked (pancreatic cyst) so digestive enzymes secreted by the pancreas do not reach the small intestine

1/400

• Caused by a dominant allele

• Small proportion of genetic diseases

Only one dominant allele of the gene is required to develop the disease

• Homozygous dominant (100% chance)

• Heterozygous (50%)

Huntington’s disease

Caused by the dominant allele of the HTT gene on chromosome 4

Produced a protein called Huntington, whose function is unknown

• Dominant allele causes degenerative changes in the brain

• Symptoms usually start between 30 to 50 years old

  • Changes to behavior, thinking and emotions become increasingly severe

  • Life expectancy after the start of symptoms is 20 years

  • Individual eventually dies of heart failure, pneumonia or other infections

• Caused by co-dominant alleles

• Very rare

In individuals carrying one normal allele and one affected allele, both are expressed

Sickle cell anemia

• Normal allele for hemoglobin is Hb^A and the sickle cell allele is Hb^S

• Phenotype of individuals with one Hb^A and one Hb^S (heterozygous) differ from those who have two copies of either allele, so the alleles are co-dominant

Caused by a gene in a sex chromosome

When the ratios of inheritance for a disease are different in males and females

Sex-linked genes are only present on one sex chromosome and not the other, so the sex of an individual affects what alleles are passed onto offspring

X-chromosome

• Y chromosome is shorter and contains only a few genes, while the X is longer and contains genes not present in the Y

Males (XY) → one copy

Females (XX) → two copies

More common in females because if ither chromosome (maternal or paternal) carries a dominant allele for the disease, the female will have it

More common in males because they only have one copy of the gene, so if they have the affected allele, they will express the disease

Males → cannot be carriers as Y chromosome cannot mask the disease

Females → since they do not inherit affected gene from an unaffected father, they can be carriers

Red-green colorblindness and haemophlia

Disorder in which the individual fails to discriminate between red and green

A recessive allele of a gene that synthesizes photoreceptor proteins

Males since the presence of an abnormal allele will necessarily given them the condition

Affected father + inherit the X-chromosome carrying the recessive gene from their mother

Mother is affected → 100% chance since mother must be homozygous recessive

Mother is carrier → 50% chance since mother is heterozygous

Disorder in which the body’s ability to control blood clotting is impaired

A recessive allele of a gene that codes for a protein called factor VIII, which is needed to make blood clots

Dominant F → codes for normal factor VIII

Recessive f → lack of factor VIII

Same as red-green colorblindness

Used to investigate patterns of inheritance within a family

Autosomal dominant

• Cannot be recessive → two affected parents would not have unaffected offspring

• Parents must be heterozygous

Autosomal recessive

• Cannot be dominant → two unaffected parents would not have affected offspring

• Parents must be heterozygous

X-linked dominant

• Sex linkage cannot be confirmed

• 100% incidence of affected daughters from an affected father suggests X-linked dominance

X-linked recessive

• Sex linkage cannot be confirmed

• 100% incidence of affected sons from an affected mother suggests X-linked recessive