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B1.1 What are genes?

·         Instructions to control how an organism develops and functions are found in the nucleus of its cells and are called genes.

·         Genes consist of molecular instructions for a cell that describe how to make proteins.

·         Proteins may be structural (e.g. collagen) or functional (e.g. enzymes such as amylase)

·         Genes are sections of very long DNA molecules that make up chromosomes in the nuclei of cells.

·         Some characteristics are determined by genes (e.g. dimples), some are determined by environmental factors (e.g. scars), and some are determined by a combination of  genes and the environment (e.g. weight)

·         Many characteristics are determined by several genes working together (e.g. eye colour)

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B1.2 Why can people look like each other?

  • Body cells contain pairs of chromosomes and that sex cells contain only one chromosome from each pair.
  • Chromosomes in a pair carry the same genes in the same place, but that their may be different versions of genes called alleles.
  • An individual person usually has two alleles for each gene.
  • In an individual the two alleles of a gene can be the same (HT, homozygous) or different (Ht heterozygous)
  • During sexual reproduction genes from both parents come together and produce variation in the offspring.
  • the offspring have some similarities to their parents because of the combination of maternal and paternal alleles in the fertilized egg.
  • Different offspring from the same parents can differ from each other because they inherit a different combination of maternal and paternal alleles.
  • An allele can be dominant or recessive.
  • males have XY sex chromosomes and females have XX sex chromosomes.
  • The sex determining gene on the Y chromosome triggers the development of testes and in the absence of a Y chromosome ovaries develop.
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B1.2 Part 2

  • The term genotype describes the genetic make-up of an organism (the combination of alleles), and the term phenotype describes the observable characteristics that the organism have.
  • a) The inheritance of single gene characteristics with a dominant and recessive alleleH is the Huntingdon's allele. Father has two h alleles, mother has one H and one h allele. The four combinations are: Hh, Hh, hh, hh.  (
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B1.3 How can and should genetic information be use

  • a small number of disorders are caused by faulty alleles of a single gene, including Huntington’s disease and cystic fibrosis.
  • disorders may be caused by dominant alleles (eg Huntington’s disease) or recessive alleles (eg cystic fibrosis)
  • the symptoms of Huntington’s disease and cystic fibrosis, to include:

    • a) Huntington’s disease – late onset, tremor, clumsiness, memory loss, inability to concentrate, mood changes

      b) cystic fibrosis – thick mucus, difficulty breathing, chest infections, difficulty in digesting food

  • a person with one recessive allele (in a pair of alleles) will not show the symptoms of the disorder, but is a carrier and can pass the recessive allele to their children.
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B1.3 How can and should genetic information be use

  • uses of genetic testing for screening adults, children and embryos, limited to:

    a) testing embryos for embryo selection ( pre-implantation genetic diagnosis) 

    b) predictive testing for genetic diseases testing an individual before prescribing drugs

  • testing adults and fetuses (foetuses) for alleles that cause genetic disorders has implications that need to be considered, including:

    • a.) risk of miscarriage as a result of cell sampling for the genetic test

    • b) using results that may not be accurate, including false positives and false negatives

    • c) whether or not to have children at all

    • d) whether or not a pregnancy should be terminated

      e) whether other members of the family should be informed

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B1.4 How is a clone made?

  • Bacteria, plants and some animals can reproduce asexually to form clones.
  • Any differences between genes are most likely to be environmental factors.
  • Clones of plants occur naturally when plants produce bulbs or runners
  • Clones of animals occur:

  a) naturally, when cells of an embryo separate (identical twins)

 b)artificially, when the nucleus from an adult body cell is transferred to an empty unfertilized cell.

  • There are different types of stem cell:

  a) adult stem cells which are unspecialised cells that can develop into many, but not all types of cells.

 b)embryonic stem cells which are unspecialised cells that can develop into any type of cell.

  • As a result of being unspecialised, stem cells from embryos and adults offer the potential to treat some illnesses.
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Gene Therapy

Gene therapy involves inserting copies of a normal allele into the chromosomes of an individual who carries a faulty allele. It is not always successful, and research is continuing.

  1. Doing research to find the gene involved in the genetic disorder.
  2. Cutting out the normal allele. Specialenzymesare used to do this.
  3. Making many copies of the allele.
  4. Putting copies of the normal allele into the cells of a person who has the genetic disorder
  5. The main difficulty is usually the last step. Here are some of the problems:

  -the alleles may not go into every target cell

  -the alleles may join with the chromosomes in random places, so they do not work properly

  -treated cells may be replaced naturally by the patient’s own untreated cells

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Pre-implantation genetic diagnosis

Pre-implantation genetic diagnosis (PGD) is a procedure used on embryos before implantation. Fertility drugs are taken by the female so that several eggs are released and collected by a doctor. These eggs are then fertilised in a Petri dish by sperm, either from the father or a donor. This is known as in vitro fertilisation (IVF). Once the embryos have reached the eight-cell stage, one cell is removed from each.

The cells are tested for the allele posing a risk (for example the Huntington’s allele). This is known as PGD. Embryos that don’t contain the unwanted allele are then implanted into the uterus to hopefully create a lower risk, full-term pregnancy.

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Key Words

recessive allele only shows if the individual has two copies of it. For example, the allele for blue eyes is recessive. You need two copies of this allele to have blue eyes.

dominant allele always shows, even if the individual only has one copy of it. For example, the allele for brown eyes is dominant. You only need one copy of it to have brown eyes. Two copies will still give you brown eyes.

Genotype describes the genetic make-up of an organism (the combination of alleles).

Phenotype describes the observable, physical characteristics that an organism has. This is often related to a particular gene.

Allele - one form of a gene

Chromosomes - Rod shaped bodies found in the nucleus of cells that contain genetic information.

Gene Therapy - treating or perventing disease by introducing or replacing specific genes inside human cells.

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