• Outline the relationship between DNA, genes, chromosomes and alleles
  • Define DNA, genes, chromosomes and alleles
  • Describe the structure of DNA
  • Explain how genes are made of varying lengths of DNA, are the unit of heredity and determine the characteristics of organisms
  • Explain the structure and arrangement of chromosomes in the nucleus of the cell
  • Understand that alleles are different forms of the same gene, and can be dominant or recessive
  • Define species
  • Define heredity
  • Explain the process of recombination in heredity


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Every creature is made up of cells. These cells have a nucleus, which contains the chromosomes, which hold the DNA, and the DNA is made up of multiple genes.

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DNA (deoxyribonucleic acid) is a complex molecule that carries genetic information. DNA is contained in chromosomes, which are found in the nucleus of most cells. 

DNA consists of two strands that are twisted around each other to form a double helix.

Each strand has a ‘chain' of base molecules. There are four different types of bases (nucleotides) each with their own name and shape: Adenine (A), Thymine (T), Guanine (G) and Cytosine (C).

The strands are held together by weak bonds between base pairs, where only adenine pairs with Thymine, and Guanine pairs with Cytosine.



DNA consists of different genes (short sections of DNA), and it is these genes that are life’s information carriers.

A gene is defined as the basic unit of heredity and is made up of short sections of DNA (and therefore these nucleotides).

Genes contain instructions for the cells, and determine the characteristics (traits) that living creatures have. It is this DNA that is unique to every creature – it is slightly different from individual to individual, which is why each individual has slightly different traits. For example, some of us have blue eyes or brown eyes, or blonde hair or brown hair.

Genes can vary in size, from a few hundred nucleotides to more than 2 million nucleotides, and they act as instructions to make molecules called proteins.

Some characteristics, such as eye colour and the shape of the earlobe, are controlled by a single gene. These genes may have different forms. These different forms of the same gene are called alleles.

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Chromosomes are structures made of DNA that codes for all the characteristics of an organism. They are thread-like structures located inside the nucleus of animal and plant cells. Each chromosome single molecule of DNA tightly wrapped around a protein. Without this packaging, DNA molecules would be too long to fit inside cells.

Humans, along with other animals and plants, have chromosomes arranged in pairs within the nucleus of the cell. The only human cells that do not contain pairs of chromosomes are the reproductive cells (gametes) which carry just one copy of each chromosome. When two reproductive cells unite, they become a single cell that contains two copies of each chromosome.

All human body cells (except gametes) have 46 chromosomes in 23 pairs in humans. They are known as diploid cells. Human gametes contain 23 chromosomes and are known as haploid cells.


Alleles (pronounced al-eels) are different forms of the same gene. The gene for eye colour has an allele for blue eye colour and an allele for brown eye colour.

Alleles are dominant or recessive:

  • the characteristic controlled by a dominant allele develops if the allele is present on one or both chromosomes in a pair

  • the characteristic controlled by a recessive allele develops only if the allele is present on both chromosomes in a pair

For example, the allele for brown eyes is dominant, while the allele for blue eyes is recessive. An individual who inherits one or two alleles for brown eyes will have brown eyes. An individual will only have blue eyes if they inherit two copies of the allele for blue eyes.


Image by  Freepik

Image by Freepik


A species is a group of closely related organisms that are capable of interbreeding to produce fertile offspring (babies that are capable of reproducing when they reach sexual maturity (puberty).

The members of a species may strongly resemble each other in appearance but they all have slightly different traits (characteristics). This is diversity. For example, they may be a bit bigger, faster or stronger than other organisms within their species.

However, sometimes a species may have different kinds (breeds) that make them look very different to one another, but these individuals still belong to the same species. One example of this is dogs, where you see breeds that look very different, from Chihuahuas to Huskies, but they are still capable of producing offspring with each other.



Heredity means the passing on of DNA (and therefore traits) from parent organisms to their offspring, and leads to diversity within a species.

During heredity, recombination and mutation occurs. We will talk about recombination below and mutation later (in “Mechanisms of evolution”).

Recombination is the random mixing of the DNA of two creatures.

When two creatures mate, they recombine their genes twice:

  1. The first time the genes recombine is in each creature separately through the generation of gametes (meiosis). In females, these gametes are their egg cells, and in males, these are their sperm cells). The gametes are take half of the genes and shuffle them. The gametes are formed from cells in the reproductive organs (ovaries in females and testes in males) by a type of cell division called meiosis. During meiosis, the chromosomes of the parent cell are copied, and the cell divides twice, producing four gametes. During this process, the genes are recombined, and each gamete receives half of the genes from the parent cell.

  2. The second time recombination occurs is during mating, when a male inseminates a female. These individuals, the parents, each provide 50% of their DNA (that is, 50% of their unique traits and characteristics). The DNA that each parent provides is then recombined, resulting in a new offspring. These offspring therefore have a random mix of the DNA and a random mix of the traits and characteristics from their parents. This increases the diversity and differences within a species even further.

Image by  BBC Bitesize

Image by BBC Bitesize