Other Patterns of
Inheritance
Multiple Alleles
When there are three or more alleles in the gene pool at a specific gene locus
Involves dominance hierarchies and can involve codominance
Codominance is where both alleles present in the genotype are expressed in the
phenotype
Example: Blood Groups
o IAIA/O = group A
o IBIB/O = group B
o IAIB = group AB
o IOIO = group O
o A and B are codominant, and O is recessive
Example: Sickle Cell Anaemia
o HbN = normal allele Hb5 = mutated allele
o Heterozygous individuals have both normal and sickle-shaped RBCs but can
survive normally with half the amount of functioning RBCs
o Heterozygous individuals are immune to malaria so this can be an advantage and
is more common in areas with Anopheles mosquitos
Linkage
Sex Linkage – gene loci present on the sex chromosome
Usually more common in the sex with the XY chromosome pattern (males in humans)
since the Y chromosome is shorter
E.g. colour blindness, haemophilia
Autosomal Linkage – gene loci present on the same autosome (non-sex chromosome)
that are often inherited and expressed together
E.g. sweet pea colour and grain length
Inheritance
Multiple Alleles
When there are three or more alleles in the gene pool at a specific gene locus
Involves dominance hierarchies and can involve codominance
Codominance is where both alleles present in the genotype are expressed in the
phenotype
Example: Blood Groups
o IAIA/O = group A
o IBIB/O = group B
o IAIB = group AB
o IOIO = group O
o A and B are codominant, and O is recessive
Example: Sickle Cell Anaemia
o HbN = normal allele Hb5 = mutated allele
o Heterozygous individuals have both normal and sickle-shaped RBCs but can
survive normally with half the amount of functioning RBCs
o Heterozygous individuals are immune to malaria so this can be an advantage and
is more common in areas with Anopheles mosquitos
Linkage
Sex Linkage – gene loci present on the sex chromosome
Usually more common in the sex with the XY chromosome pattern (males in humans)
since the Y chromosome is shorter
E.g. colour blindness, haemophilia
Autosomal Linkage – gene loci present on the same autosome (non-sex chromosome)
that are often inherited and expressed together
E.g. sweet pea colour and grain length
