3.1 Essential ideas

3.1.4 Inheritance

Mendelian inheritance

  • Gregor Mendel discovered the principles of inheritance while studying the characteristics of pea plants over many generations.
  • He had no knowledge of genes or chromosomes, but he predicted the existence of heritable factors and his work disproved the theory of ‘blended inheritance’.

Genotype and phenotype, dominant and recessive alleles

  • The combination of alleles present at locus on homologous chromosomes is the genotype.
  • The genotype is represented by a pair of letters, as shown in the figure below:

Figure 3.1.4a – The relationship of alleles to genotypeFigure 3.1.4a – The relationship of alleles to genotype

Figure 3.1.4b – The relationship of genotype to phenotypeFigure 3.1.4b – The relationship of genotype to phenotype

ABO blood groups: an example of codominance

  • Most traits are influenced by multiple alleles, and often one allele is not completely dominant over the other.

  • In cases of codominance, both alleles in the heterozygote affect the phenotype. An example is ABO blood groups.

  • ABO blood type is determined by the type of antibodies present on the surface of red blood cells.

Figure 3.1.4c – ABO blood groups are an example of codominanceFigure 3.1.4c – ABO blood groups are an example of codominance
  • Notice that there are three different alleles (IA, IB, and i), but there are four different phenotypes, because IA and IB are codominant.


Phenotype (blood group)









Colour blindness: An example of sex linkage

  • Unlike the 22 pairs of autosomes, the human sex chromosomes, X and Y are not homologous. The X chromosome is much larger and contains more genes.

Figure 3.1.4d – Human sex chromosomes, X and YFigure 3.1.4d – Human sex chromosomes, X and Y
  • Genes that occur on the X chromosome and not the Y chromosome are called sex-linked, or X-linked, genes.

Figure 3.1.4e – Patterns of inheritance in sex-linked genes are different from those in autosomal genesFigure 3.1.4e – Patterns of inheritance in sex-linked genes are different from those in autosomal genes
  • Red-green colour blindness (Xb) is a recessive sex-linked trait.

  • Males (XbY) who inherit the recessive allele will always show the trait, while females (XBXb) who are heterozygous for the trait are called ‘carriers’.

Genetic diseases can be autosomal or sex-linked

  • Thousands of genetic diseases have been identified in humans but most are very rare.

  • Many genetic diseases are due to recessive alleles, but some are due to dominant or codominant alleles. An example of each is listed in the table below.



Inheritance pattern

Cystic fibrosis

1 in 2000 born with CF in Europe

Autosomal recessive allele (chromosome 7)

Huntington’s disease

1 in 20 000 affected worldwide

Autosomal dominant allele (chromosome 11)

Sickle cell anemia

1 in 10 000 worldwide (most common in people with African/South American ancestry)

Autosomal recessive allele (chromosome 11)


1 in 5000 males

1 in 12 000 females

Sex-linked recessive allele

Mutation rates and cancer
  • Mutation is a natural, random process that affects the structure of DNA.
Figure 3.1.4f – Mutations in the germline cause heritable diseasesFigure 3.1.4f – Mutations in the germline cause heritable diseases
  • Radiation and mutagenic chemicals increase the mutation rate and cause cancer and genetic disease.

  • When mutations occur in germline cells (gametes), it can cause heritable genetic diseases.

Watch Hank Green explain the Gregor Mendel controversy.

Essential idea

Genetic inheritance follows patterns.

Figure 3.1.4g – Gregor MendelFigure 3.1.4g – Gregor Mendel
Gregor Mendel (1822–84) made quantitative measurements with many replicates. He was able to perform statistical tests that helped him to discover mathematical relationships between heritable ‘factors’.

Figure 3.1.4h – Red-green colour blindnessFigure 3.1.4h – Red-green colour blindness
8% of men and 0.5% of women can’t see the butterfly in this picture. Why is red-green colour blindness more prevalent in males?

Food for thought

Are dominant traits more prevalent than recessive traits? Discuss your ideas and then do some research into factors that affect allele frequencies.

Course links

  • Review the role of mutagens and oncogenes in the development of cancer in 1.2.6.
  • Recall that new alleles are formed by mutation in 3.1.1.
  • HL students will learn more about ABO blood groups in 11.1.1.
  • Learn more about colour blindness in 12.2.3.