• Question: What is the difference between homozygosity and heterozygosity?

    Asked by thechosenone to Cassie, Greg, Karen, Sofia, Tommy on 18 Nov 2013.
    • Photo: Karen Bacon

      Karen Bacon answered on 18 Nov 2013:

      This is to do with genes and genetics. Most genes have at least two different alleles that help to determine what form of a particular trait, like hair or eye colour, is expressed in a particular individual. We all have two copies of each gene, one from our father and one from our mother. The different forms a a gene for a particular trait are called alleles. If both of the copies of the gene for a particular trait are the same, then the individual is homozygous for that trait. If on the other hand, an individual has two different alleles for a particular trait, then they are heterozygous for that trait. Having different mixes of alleles can change how people (or other animals or plants) express different traits.
      An example is eye colour (although this is a very simplified description and it is actually more complex than what I’m saying here). There are different alleles for eye colour in humans. The allele for brown eyes is dominant to the allele for blue eyes. That means that one brown eye colour allele will have more of an influence on eye colour than one blue eye colour allele. So if someone has blue eyes then they inherited the blue eye colour allele from both parents and are homozygous for eye colour. If they have brown eyes then they could be either homozygous for brown eyes and have inherited the brown eye colour allele from both parents or they could have one brown eye colour allele and one blue eye colour allele, one from each parent, making them heterozygous for eye colour.
      This type of interaction can be really important for some diseases too. One example is haemophilia. This is an inherited disease whereby the blood does not clot properly when the individual cuts themselves, making even small cuts potentially very dangerous as they will just keep bleeding. This is a recessive disorder – which means that only people who are homozygous for the disease express it. This particular disease is linked to the sex chromosomes – it is linked to the X chromosome and as a result it expresses in males – that is because males only have one X chromosome so if they get the haemophilia allele they do not have another X chromosome to resist the disorder. Females can have one disease-causing allele on one X chromosome and one non-disease-causing allele on the other X chromosome, which means that they could pass the disease to any sons that they have but will not have the disease themselves. Females with two haemophilia X chromosomes are highly unusual but it can happen. This disease is historically interesting because it effected many of the royal families in Europe during the 18th and 19th centuries.
      The same type of genetic interactions occur in both plants and animals and the first serious investigation into genetics were undetaken by a monk called Gregor Mendal – he had to work on pea plants because he wasn’t allowed to study genetics on animals (in this case guinnea pigs) because it upset his bosses in the church. Some traits have much more complex interactions between genes (and genes and the environment) but there is often an element of homozygous v heterozygous gene interactions determining traits and determining the expression of traits.

    • Photo: Sofia Franco

      Sofia Franco answered on 19 Nov 2013:

      As Karen explained, homozygous is when you have the same to alleles on one gene and heterozygous is when you have two different alleles of that gene. Think of it as playing cards with more than one deck and each card is an allele: you can have 2kings of spades, so it’s homozigous, or one king of spades and one of hearts, so it’s heterozigous, though you are always looking at the king, which in this example would be any gene! When you have two different alleles you will code for a trait and two of the same will code for another, and this is how you get all the diversity in our genetic code!

    • Photo: Thomas Doherty-Bone

      Thomas Doherty-Bone answered on 19 Nov 2013:

      All good from my esteemed colleagues. Just a quick schematic from moi.

      Two alleles, “A” and “a” (lets say they code for the trait when you go “aaaaaaaaaaaaaa”, A being the dominant trait, and is loud)

      Mum: Aa (heterzygous….and loud)
      Dad: aa (homozygous….and quiet)

      Aa (heterozygous…and loud)
      aa (homozygous….and quiet)

      hetero means different
      homo means the same

      Hope this helps. Problems, read answers of the guys above.

    • Photo: Cassandra Raby

      Cassandra Raby answered on 20 Nov 2013:

      These are ace explainations of the differences from everyone!

      So just to sum up…
      …it is whether the alleles your genes match up (like the card example that Sofia used)… and if they do match they are homozygous, and if they don’t then they are heterozygous (like Tom and Karen explained!).
      If the do match then you get whichever trait they show, and if they don’t match then it’s whichever is the dominant trait that wins!