Patterns of inheritance

How a particular trait is inherited depends on the mode of in which it is inherited.

Types of dominance

Complete dominance occurs when there is enough protein for a particular phenotype due to the action of a single alleles coding for  a dominant trait. In complete dominance the heterozygote displays the dominant phenotype.

Co dominance occurs when both alleles in the genotype are expressed fully and equally in the phenotype.

Example: The AB blood type in ABO blood types.

When representing codominance use a capital letter for the trait and superscripts representing the alleles. Eg the genotype IAIB is the genotype for ABO blood group.

Multiple alleles

Multiple allele systems have more than two different alleles that can contribute two a phenotype, but two alleles are still inherited at one time. The ABO blood group is an example of this where there are three alleles.

Types of inheritance

Autosomal Dominant Inheritance :only once copy of an allele is necessary to express the phenotype.

Autosomal recessive inheritance: two copies of an allele are required to show the phenotype.

X linked dominant: Only one copy of allele on the X chromosome is required to express the phenotype.

X linked recessive: Both copies of the X chromosome in females or the one copy of X chromosome in males must have the allele to express the phenotype.


In cases whereby a particular genotype results in an individual it will result in death at some time of the indiviual’s lifespan due to the lethal allele.

Lethality often occurs before birth, so the individual dies before birth. In this case when you do a punnet square  (with AA being the lethal genotype) and cross for example Aa x Aa  you get a 2: 1 ratio instead of 3:1.



(Click to enlarge)

Phenotypic ratio:

2 Normal: 1 Diseased

Genotypic ratio

2 Aa       : 1 aa

See also