Artificial Insemination
Gene Therapy
Protein Synthesis
Variation and Meiosis

Other Sections

Cell Biology
Human Biology
Plant Biology


What is a Genotype?

A genotype is the genetic constitution of an organism; it refers to the combination of alleles and genes.

A phenotype is the expression of the genotype and its expression with the environment. For example, there are genes for height, however, if growing up, someone doesn't have good nutrition, they will never grow very tall.

At this level, you should already know about alleles and dominance and recessiveness (see here if you don't). We use the term heterozygous when there is a pair of different alleles (Aa) and homozygous when they are the same (BB).

Sex Determination

The sex of someone is determined not by individual genes but by an entire chromosome pair. In humans, there are 23 pairs of chromosomes, the last pair is normal in females (is said to be XX), but in males, one of the chromosomes is shorter than the other (males are said to be XY). See the diagram below

males and female chromosomes 45 and 46

It is also possible to have other chromosome combinations for example XXY and XO (only one X chromosome), and these lead to genetic disorders; for more see here. However, it is interesting to note that having the combination XYY has no observable affect, and those with this configuaration are normal males.

Monohybrid Inheritance

There are different forms of gene called alleles. These different forms of gene are what cause variation amoungst living things. There are two types of allele: dominant and recessive. Let's look at an example to understand this.

One type of gene is for eye colour. The allele for brown is dominant and thus represented as 'B' (capital letter). The allele for blue eyes is recessive and represented as 'b' (lower case).

the MOTHER has brown eyes and the genotype Bb
the FATHER has blue eyes and the genotype bb.

If they have a baby, it gets an allele from each parent. You can work out the possible outcomes by representing it in a 'punnet square'.

example of a punnet square for monohybrid inheritance of eye colour

Two of the outcomes are Bb. This means the baby has a 50% chance of having brown eyes. As, even though it has a b (blue eye) allele, it has one B allele, and as this is dominant the baby will have this characteristic. In order to have blue eyes, the baby must have two recessive blue alleles (bb).

Inheriting Blood Group

Monohybrid inheritance is fairly simple. But now we willl look at codominance and dihybrid inheritance by studying blood groups and the associated genetics.

You will not always have one recessive and one dominant allelle; sometimes, there might be two or more that are codominat. Take the example of blood group, where A and B are dominant to O, and A and B are codominant. This means that if you have the genotype AO or BO then your blood type will be A or B, but having AB means you have both A and B blood group, and the only way to have blood group O is to have the genotype OO.

summary of blood group genetics

However, there are other markers that affect blood group. One of these is known as the Rhesus factor and is either positive or negative. Positive is dominant and negative is recessive. This means we have two types of gene for the same thing. It is possible to work out the possible outcomes of offspring when these are crossed; this is called: dihybrid inheritance, and it works as follows:

example of comdominance in blood groups

Sex Linkage

As shown in the chromosome diagram above, the male Y chromosome has much less genetic information; this means that it doesn't have some genes that the homologous X does. So males only have one copy of a gene and therefore are more likely to be affected by recessive disorders.

inheritance of haemophillia

If they had a daughter, there would be a 50% chance of HH, and 50% chance of Hh (heterozygous) and a 0 chance of being affected because two affected allelles are needed.


Epistasis is an interaction between two genes where one allelle affects the expression of another, this can lead to a graduation of phenotype. It is best explained by example, so here is a (fictitious) example of rat colouration.

example of epistasis in rat colouration

The default colour in this example. Is to have white fur. If the rat has the dominant gene A, then a pigment is produced to make a brown rat. Then if the rat has the dominant allelle B on top of this the pigment is modified again to make a black rat. If the rat has the B allelle but not A, then it will remain white because it is a continuum . If you wanted to work out the genetic possibilities, you would use the dihybrid inheritance explained above.