Lecture 23: It's in the genes

Alleles and Genotypes

• Not all alleles are normal; some may be recessive or have a reduced function
• Heterozygous genotypes have two different alleles, e.g., IAi
• Recessive alleles can be expressed if an individual is homozygous recessive, e.g., ii

Dominant and Recessive Alleles

• Dominant alleles mask the effect of recessive alleles
• Recessive alleles can retain some normal function, but at a reduced capacity or be completely dysfunctional
• Recessive alleles are not necessarily rare in a population
• Recessive alleles don't necessarily cause genetic diseases

Co-dominance

• Co-dominance occurs when two different alleles and phenotypes are expressed equally
• Examples: AB blood type, Sickle cell anemia

Inheritance Patterns

• Autosomal recessive: dry earwax, Hemochromatosis, Sickle cell disease
• Autosomal dominant: bitter taste perception, wet earwax, Huntington's disease
• Co-dominant: AB blood type, Sickle cell anemia
• Incomplete dominance: Sickle cell anemia
• X-linked recessive: g6pd deficiency, Red-green color blindness, Haemophilia A
• X-linked dominant: Rett's syndrome
• Y-linked: "Webbed toes"

Earwax Type

• Controlled by the gene ABCC11 on chromosome 16
• Encodes for a protein transporter
• Two alleles: dominant allele codes for a functioning protein, recessive allele codes for a non-functioning protein
• Recessive phenotype: dry earwax, dominant phenotype: wet earwax

ABO Blood Group

• Three alleles: IA, IB, and i
• IA and IB are co-dominant
• i allele encodes for an inactive enzyme
• Heterozygous genotype: IAi or IBi
• Homozygous recessive genotype: ii

Genotype vs Phenotype

• Genotype: combination of alleles for a particular gene
• Phenotype: observable or measurable characteristic arising from the genotype and its interaction with environmental factors

Dihybrid Cross

• Determining the phenotype and genotype for two traits
• Example: earwax type and bitter taste perception

Sickle Cell Disease

• Mutation in HBB gene resulting in abnormal haemoglobin beta subunit
• Heterozygotes exhibit the sickle cell trait and are carriers of the abnormal allele
• Co-dominant phenotype: mixture of normal and sickle red blood cells
• Incomplete phenotype: lower blood oxygen levels

Why Sickle Cell Allele Persists

• Heterozygous individuals have greater resistance to malaria
• Malaria infected red blood cells tend to sickle and are removed by macrophages

Pedigrees

• A family tree indicating the presence or absence of a trait for each member
• Conventions: unaffected male, unaffected female, affected male, affected female

Huntington's Disease

• Autosomal dominant disorder
• Mutant alleles characterized by additional CAG repeats
• Normal alleles encode for a normal HTT protein
• Abnormal HTT allele prone to misfolding