The genetics of inheritance patterns

  • Dominant and recessive alleles
  • Sex linked vs autosomal inheritance
  • Codominance vs incomplete dominance
  • Polygenic [Extension]
  • How to read and interpret pedigree & punnett squares


Genes are the instructions which code for traits. They determine certain proteins which combine in highly organized structures to form tissue, which then combine together to form organs. Just as differences in a recipe can lead to a slightly different product, genes also contain variances. These variances between genes which code for the same trait are known as alleles. For example different alleles code for different color eyes.

In humans, a version of each gene is inherited from both the mother and father. This means you can have different alleles. The type of allele, if two differing types are inherited, is determined by predictable patterns. You may have noticed if one parent has blue eyes and the other has brown eyes, you will probably have brown eyes.

The reason for this is dominant vs recessive alleles. Some alleles are represented in your phenotype, which is the physical expression of your genotype, over other allels. Different people have different blood types, however, if your mother is O and your father is A, you will be blood type A (assuming your father has genome AA). This is because A is dominant and O is recessive. However, your genome will contain both A and O.

However, as you know males have an XY chromosome and females have XX. This means that males only have one X chromosome, therefore, traits can be sex linked in their inheritance patterns. A common example of this is color blindness which affects 1 in 3 males since it is carried on the x chromosome. Therefore, if they inherint an x chromosome with a color blind allele – they will be colour blind. Whereas, females would need to inherent two colour blind allels since clolour blindness is recessive. If an allele is not affected by gender it is not sex linked and known as autosomal.

Punnett Squares: Watch the video below to learn how punnett squares work.

Incomplete dominance occurs when the dominant alleles is not fully dominant hence the recessive can also be seen. An example of this is the pink snap dragon where both the white and red snap dragon flower alleles overlap with one another.

Codominance occurs when two dominant allels are expressed at the same time. This can be seen in blood groups when an A and a B are both present. This results in the blood type AB. Additionally, when red cows are breed with white cows, both hair colors are present in the offspring.

Polygenic traits are determined by multiple alleles for multiple genes which code for one trait. An example of this is skin color – this gives rise to an array of different color’s and variations since it is more complex in inheritance.

Pedigree charts: Watch the video below to know how pedigree’s work.


  • An allele is a variance of a gene
  • Alleles can be inherited as dominate or resecive, incomplete dominance, codominant or as a polygenic trait


Question 1. [4 marks]

Using examples compare and contrast incomplete dominance and codominance

Question 2. [1 mark each]

Question 3. [2 marks]

Determine what is the likelihood that the child is colorblind if both grandfathers are color blind and neither grandmothers are color blind or carriers of color blind alleles. Color blind alleles are sex linked recessive.

Question 4.

Determine the inheritance patterns of the following pedigrees

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