Genetic Disorders and Conditions Referenced in Mendelian Genetics

Condition	Gene/Allele	Inheritance Type	Key Feature
Sickle Cell Anemia	HbS allele of beta-globin gene	Pleiotropy; Autosomal recessive (for disease expression)	One allele causes: abnormal haemoglobin → sickle RBCs → anaemia → multi-organ damage; heterozygotes (HbA HbS) show co-dominance with malaria resistance
ABO Blood Groups	$I^A$ , $I^B$ , i alleles of ABO gene (chromosome 9)	Multiple allelism + co-dominance ( $I^A$ $I^B$ ) + dominance ( $I^A$ or $I^B$ over i)	3 alleles, 6 genotypes, 4 phenotypes; controls RBC surface antigens; critical for blood transfusion compatibility
Thalassemia (context)	Alpha or beta globin genes	Incomplete dominance in heterozygotes (thalassemia minor)	Single gene defect in haemoglobin chain; heterozygotes show intermediate disease severity
Snapdragon Flower Colour	R locus	Incomplete dominance	RR = red, Rr = pink (intermediate), rr = white; neither allele fully dominant
Sweet Pea Flower Colour	C and P gene loci	Complementary gene interaction (9:7)	Both C and P dominant alleles needed for colour; absence of either gives white
Labrador Coat Colour	B and E gene loci	Recessive epistasis (9:3:4)	ee genotype epistatic to B/b gene; ee = yellow regardless of B allele
Squash Fruit Colour	Multiple gene loci	Dominant epistasis (12:3:1)	Single dominant allele at one locus masks expression of other locus
Human Skin Colour	Multiple gene loci (MC1R, OCA2, SLC24A5, and others)	Polygenic inheritance	Many genes with additive effects; continuous variation; bell-curve distribution
Albinism	TYR gene (tyrosinase)	Autosomal recessive	Absence of melanin pigment; homozygous recessive for tyrosinase-null allele
Huntington's Disease (context)	HTT gene	Autosomal dominant (exception showing dominant disorder)	Single dominant allele sufficient for disease; not Mendelian recessive pattern