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Unit 2 - Genetics

This GCSE Biology quiz is all about genetics, the study of variation and inheritance. Genetics can also mean the genetic properties, characteristics or features of an organism. The rules of genetics were first worked out by Gregor Mendel in 1866 from studies of pea plants. He discovered that genetic particles (heredity units), which we now call genes, determine the features of the offspring. Later, with the discovery of DNA, we began to understand genetics much better.

Different versions of the same gene found on a pair of chromosomes in the nucleus of a cell are called alleles. These can determine features such as eye colour. Mendel recognised that there were dominant and recessive forms of these alleles. He also realised that genes are passed on from both parents and that recombination of genes during fertilisation is a random event. This means that every person on the Earth (other than identical twins who came from the same egg cell) has a unique set of genes. This can be analysed to provide a genetic fingerprint which looks a bit like a bar code!

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For the exams, it is important to know the relationship of chromosomes and genes. Chromosomes are made from long strands of DNA (deoxyribonucleic acid). Genes are small sections of the chromosomes. But what exactly do genes do? In short, they are 'instructions' for making proteins. Some of them make structural proteins whilst others make enzymes, which are very complex protein molecules that act as catalysts for many chemical reactions in the body.

Certain diseases are caused by the inheritance of faulty genes that are passed on to offspring from their parents. Two of the ones that are usually used in exam questions are cyctic fibrosis and Huntington's disease. When one or both of the parents are carriers of the faulty gene, genetics can show how likely it will be that their children will have the disease.

Do you know the difference between genes, chromosomes and DNA? Were you aware that your gender is an inheritance from your father - even if you are female? Try this quiz and refresh your memory on the subject of genetics.

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  1. What offspring ratio would you expect from two parents who are carriers - Tt crossed with Tt?
    Only the offspring that receive the recessive genes from both parents will be unaffected by Huntington's disease
  2. Sperm cells carry which of the following chromosomes?
    The egg cell contains only the X chromosome so it is the sperm that determines the gender of the offspring
  3. What are alleles?
    Alleles are given letters. A different letter of the alphabet is used for each gene. The dominant allele is given an upper case letter e.g. T and the recessive allele is given a lower case letter e.g. t
  4. Humans have two sex chromosomes. What are the ones in women known as?
    Chromosomes occur in pairs. Men have an XY chromosome combination
  5. What is the name of the technique used to test a foetus for genetic disorders?
    Genetic tests are not available for all inherited diseases and sometimes they can give a false positive result
  6. In humans, genes are found in...
    Genes are always found in pairs. One of the pair was inherited from Mum and the other one of the pair of genes comes from Dad
  7. If the genes were Tt, this means that the alleles are...
    You can tell that by the fact that there is a capital letter and also a small letter - genetics are case sensitive!
  8. In a recessive condition, such as cystic fibrosis, how many copies of the abnormal allele are needed for the person to be a sufferer?
    Both parents need to be carriers for the child to inherit cystic fibrosis. Parents would be Cc and the child with cystic fibrosis would be cc
  9. What is a carrier?
    This most commonly occurs with recessive conditions
  10. For a condition such as Huntington's disease, how many parents need to be affected for the children to inherit this condition?
    Huntington's disease is a dominant condition so only one copy of the faulty gene is needed to cause it. Normally, it is only diagnosed in middle-aged people, which means that they have already passed it on to their children. There is a form that affects children but it is very rare

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