Recessive – the characteristic is only expressed if there is no dominant allele present
* Homozygous – two identical alleles
* Heterozygous – two different alleles
* Codominance – both alleles contribute to the phenotype
Linkage is the phenomenon where genes for different characteristics are located at
different loci on the same chromosome and so are inherited together.
Monogenic inheritance – when a phenotype or trait is controlled by a single gene. For
instance, cystic fibrosis where the individuals with doubly recessive genootype are affected.
Dihybrid cross – inheritance of two genes
Sex linkage – expression of an allele dependent on the gender of the individual as the gene
is located on a sex chromosome, for instance, males are more likely to inherit an Xchromosome linked condition because they only have a single copy of the X chromosome. An
example of sex linkage is haemophilia which is a recessive condition (hh).
Autosomal linkage – genes which are located on the same chromosome (which is not a sex
chromosome) and tend to be expressed together in the offspring
Codominance – when both alleles are expressed in a heterozygote, that is, both alleles
contribute towards the phenotype. Examples include blood type.
Epistasis – the interaction of different loci on the gene, one gene locus affects the other gene
locus. One gene loci can either mask or suppress the expression of another gene locus.
Recessive epistasis occurs when the presence of a recessive allele prevents the expression
of another allele at a second locus. Recessive epistasis gives the ratio of 9:3:4.
Dominant epistasis is when a dominant allele at one locus completely masks the alleles at
a second locus. Dominant epistasis gives a ratio of 12:3:1.
