Reading Exercise: What are Alleles?

Chromosomes are found inside the nucleus of cells and consist of long strands of DNA. Each human cell has 46 chromosomes (23 pairs), apart from gametes (sex cells) which have only 23.

You can think of chromosomes as a set of books. Each book (chromosome) contains a set of instructions (genes). All of the books together contain all the instructions needed to make a certain organism (a living thing), for example a human or a flower.

There are always two copies of the same gene in an organism because chromosomes come in pairs. These two versions of the same gene are called alleles.

The two alleles for one characteristic do not have to be the same. They can be different. If both alleles for one gene are the same, they are homozygous (from homo = the same). If the alleles are different, they are heterozygous (from hetero = different).

For example, everyone has two alleles of the gene that decides eye colour. If someone has brown eyes and is homozygous for that gene, they will have two alleles for brown eyes. We can see this for individual B in the middle of the image..

A person could also have brown eyes and be heterozygous for that gene. They have one allele for brown eyes and another allele for blue eyes. We can see this for individual A to the left in the image. The reason that this person has brown eyes is that alleles can be either dominant or recessive.

The gene for brown eyes is dominant and we will always see this characteristic, no matter what other gene is present. The gene for blue eyes is recessive and we only see it when the dominant gene for brown eyes is not present. We can see this for individual C to the right in the image.

Characteristics that we see on the outside are called the phenotype. The characteristics in our genes are called the genotype. Phenotype and genotype can be slightly different. In our example we have seen that someone can have brown eyes as their phenotype, but both brown and blue eyes in their genotype.


  1. What are chromosomes and where are they found?
  2. What are alleles?
  3. What is meant by homozygous?
  4. What is meant by heterozygous?
  5. What is meant by dominant?
  6. What is meant by recessive?
  7. Why does a person have brown eyes even when they have genes for both brown and blue eyes?
  8. What is meant by phenotype?
  9. What is meant by genotype?
  10. What is the phenotype (= the colour seen on the outside) of a rose that is homozygous and has two dominant alleles for red colour?
  11. What is the phenotype of a rose that is homozygous and has two recessive alleles for white colour?
  12. What is the phenotype of a rose that is heterozygous and has one dominant allele for red colour and one recessive allele for white colour?


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