Inheritance is the transmission of genetic information from generation to generation.  Genetic information is stored in the form of DNA within the cell nucleus. Here are some key definitions that you must be aware of: 

  • Chromosome is a thread like structure of DNA and almost every cell has 46 or 23 pairs – the two chromosomes of a pair are called homologous chromosomes
  • A gene is a length of DNA which codes for a specific protein and are found on chromosomes
  • An allele is a version of a gene 

For example, imagine a gene that codes for eye colour. There are variations in this gene called alleles. One allele for this gene may code for brown eyes, whereas another allele may code for blue eyes. 

Sex inheritance – Normal human cells have 23 pairs of chromosomes. One of the pairs of chromosomes codes for sex inheritance. Sex inheritance depends on the presence of X and Y chromosomes.

  • Males have one X chromosome and one Y chromosome (XY)
  • Females have two X chromosomes (XX)

DNA structure and function – The function of DNA is to control cell function by controlling the production of proteins i.e. enzymes, antibodies, cell receptors, etc. 

The DNA is made of a double helix backbone, which are connected by pairs of bases. 

  • Adenine always pairs with thymine 
  • Cytosine always pairs with guanine 

The sequence of bases along a particular length of a DNA strand codes for the manufacture of a specific protein. 

Consider the particular length of DNA (gene) in the diagram above. The code within that length is ATTCGAA. This base sequence within the gene codes for the specific types and order of amino acids that become joined to form a protein. 

All body cells contain the same genes, but not all genes are expressed because the cell only makes certain proteins that it really needs. 

Protein manufacture – As we’ve discussed above, a gene is a particular length of a DNA molecule containing a specific sequence of bases. This base sequence codes for which amino acids should be joined in what order, to build a particular protein. 

DNA held within the nucleus of the cell. Let us consider a particular gene (gene A) that makes a certain protein (protein A). 

  • A molecule called mRNA ‘copy’ the DNA base sequences found within gene A – This is called transcription
  • mRNA molecule leaves the nucleus 
  • mRNA passes through the ribosomes in the cytoplasm
  • Ribosomes ‘read’ the base sequences and assemble various amino acids in a specific order based on the base sequences – This is called translation 

Diploid v/s Haploid

As mentioned before, a normal human cell contains 23 pairs of chromosomes. This particular number is extremely important for normal bodily function. Cells like this are called diploid cells, and are majority of the cells in the human body. Gametes i.e. sperm cells/egg cells, have only a single set of 23 chromosomes. This is half the normal amount in order to maintain 46 chromosomes after fertilization. 

  • Diploid cells have 23 pairs of chromosomes 
  • Haploid cells have a single set of 23 chromosomes 

Cell Division

Mitosis – The nuclear division giving rise to genetically identical cells. This is very important for various bodily functions such as growth, repair and cell replacement

Imagine diploid cell A (46 chromosomes) undergoing mitosis. If this cell were to simply divide in half, then it the resulting cells will only have 23 chromosomes. Therefore, the cell duplicates its chromatids before mitosis occurs (i.e. from 46 to 92) in order to maintain the chromosome number after the cell division occurs. Then it splits in half and gives genetically identical cells – 1 cell becomes 2, which become 4, etc. and all contain identical 46 chromosomes.

Meiosis – Type of nuclear division giving rise to cells that are genetically different. This is a reduction division to form haploid cells which produce gametes. 

Essentially, diploid cell A (46 chromosomes) will form haploid cells with 23 chromosomes. 

Meiosis 1 –Condensation of chromatids indicates beginning of meiosis. Chromosomes are then defined and nuclear membrane diffuses into cytoplasm. Chromosomes are aligned in a line with their homologous pairs along a transverse plane of alignment. Recombinant chromosomes are formed through hybridization. Two centrioles form spindle-like fibres that attach to one of the chromosomes from the homologous pairs each. They pull the chromosomes to the opposite ends of the cell. Cytokinesis occurs – splitting of cytoplasm to form two distinct cells – and a distinct nuclear membrane forms between the two.

Meiosis 2 – Centrioles form in the daughter cells of meiosis 1. They move towards opposite ends and the spindle-like fibres attach to one of the chromatids each. They pull the chromatids to the opposite ends of the cell. Cytokinesis occurs and a distinct membrane forms between the two cells again. Each daughter cells form 2 new cells with 23 chromosomes each and have different characteristics amongst each other.

Stem Cells – As an embryo develops, the cells take on different roles – called differentiation. This happens as although every cell has the same genes, only one set of genes is expressed.

Embryonic stem cells are undifferentiated types of stem cells and can produce any type of specialized cell. In adults, very few cells remain stem cells but their range becomes limited. E.g.: stem cells in bone marrow can produce RBCs, WBCs, and platelets but cannot produce liver cells or nerve cells.

Stem cells may be able to cure diseases where cell functions fail – e.g.: type 1 diabetes where the islets of Langerhans cannot produce insulin – and are therefore interesting to medical researchers.

Monohybrid Inheritance

Important Definitions 

  • Genotype = The genetic make-up of an organism in terms of the alleles present 
  • Phenotype = Observable features of an organism 
  • Alleles = Variations of a given gene
    • Homozygous = Two identical alleles of a particular gene 
    • Heterozygous = Two different alleles of a particular gene 
    • Dominant allele = Allele that is always expressed if present 
    • Recessive allele = Allele that is only expressed if the dominant allele is not present

Monohybrid Crosses

A monohybrid cross is a genetic mix between two individuals who have homozygous genotypes, or genotypes that have completely dominant or completely recessive alleles, which result in opposite phenotypes for a certain genetic trait.

Eye colour – There are different alleles that code for eye colour:

  • Blue eye allele (b) is recessive 
  • Brown eye allele is dominant (B) 

A brown-eyed individual can therefore have two possible genotypes:  BB or Bb. A blue-eyed individual can only have one possible genotype: bb.

Example 1 – Homozygous recessive (bb) X Heterozygous (Bb)

Consider a brown eyed person with genotype BB having a child with another person with blue eyes genotype bb.

You can use the Punnett square to look at all the possible genotypes of the child. 

In this scenario, the chance of a blue-eyed child is 2/4 and the chance of a brown eyed child is 2/4, therefore the ratio is 1:1 

Example 2 – Homozygous recessive (bb) vs Homozygous dominant (BB)

Consider a brown eyed person with genotype BB having a child with another person with blue eyes genotype bb. 

In this scenario, the chance of a blue-eyed child is 0/4, and the chance of a brown eyed child is 4/4. 

Example 3 – Heterozygous (Bb) X heterozygous (Bb)

Consider a brown eyed person with genotype Bb having a child with another person with brown eyes genotype Bb

In this case, the chance of a blue-eyed child is 1/4, and the chance of a brown eyed child is 3/4. The ratio of blue to brown is 1 to 3 


Co-dominance is when a pair of alleles are neither dominant or recessive to one another. Both alleles can therefore impact the phenotype. 

Blood type is a good example of co-dominance. The alleles of blood types are: IA, IB and IO

  • IA and IB are co-dominant
  • IA and IB are dominant over IO

The combination of these alleles can therefore result in blood groups A, B, AB, and O. 

Sex-linked characteristics

Sex linked characteristics are characteristics in which the gene responsible is located on the sex chromosome, and therefore making it more common in one sex than the other. 

Red-green colour deficiency – The colour deficient gene is an abnormal gene found in the X chromosome (Xc).

A male (with XY chromosomes) with these gene will inevitably have colour deficiency because males only have one X chromosome, so this abnormal gene will always be expressed. 

Females on the other hand, may have the colour deficient gene but have normal colour vision. This is because females have two X chromosomes (XX).

The normal colour vision allele dominates the abnormal allele, so as long as the female has one normal X chromosome then they will have good colour vision. 

To summarize: 

  • Male (XY) = Normal colour vision 
  • Male (XcY) = Colour deficient 
  • Female (XX) = Normal colour vision
  • Female (XcX) = Normal colour vision i.e. carrier 
  • Female (XcXc) = Colour deficient

True Breeding – All children produced are identical to that of parent and have homozygous alleles.

Protein Synthesis – Transcription is a process where a messenger RNA strand is made by using one of the strands of DNA as a template. This messenger RNA contains the codons needed to make the protein. Codons are made up of three bases of DNA (e.g.: CCG, AAT, ATT, etc.) and one codon is an amino acid. Once the messenger RNA is made, it is detached from the DNA and exits out of the nucleus, through the nuclear pore into the cytoplasm where a ribosome attaches to it. These amino acids are then aligned to form a protein by the mRNA.


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