Biology 1

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What Genes do and how they work

  • Genes carry the instructions that control how you develop and function
  • they do this by telling the cells to make the proteins needed for your body to work
  • each gene is a section of DNA, lengths of DNA are coiled and packed into structures called chromosomes, which are found in the nuclei of the bodys cells
  • strands of DNA are made up of four chemicals; ATCG, which are called bases
  • the order of the bases in a DNA strand determines the order of the amino acids in a protein

FUNCTIONAL PROTEIN- enable the body to function, e.g.- enzymes, antibodies, hormones

STRUCTURAL PROTEIN- give the body structure and rigidity, e.g.- collagen

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  • our characteristics are controlled by our genes and the environment
  • we inherit genes from our parents, 23 chromosomes from each, to make 46 pairs
  • genotype- genetic makeup. phenotype- physical appearance
  • differences in genes produce variation in offspring
  • identical twins have identical genotypes because they develop after a fertilised egg splits into two
  • studies of identical twins can help us to understand the effect of the environment on someones genotype
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  • chromosomes are arranged into pairs
  • sex cells are eggs and sperm, which each have 23 chromosomes, one from each pair
  • at fertilisation, an egg and sperm meet together to form a zygote which has a full set of 46 chromosomes
  • changes to our DNA sometimes cause a mutation
  • Downs Syndrome is an example of a mutatuon, where the person will have an extra chromosome 21
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  • Alleles are the different forms in which the genes controlling a characteristic can occur
  • if the two alleles of a gene are identical, the person is said to be homozygous
  • if the two alleles are different, the person is said to be heterozygous
  • in most cases, alleles for a trait can be dominant or recessive
  • dominant alleles are written with upper case letters e.g Hh
  • recessive alleles are written with two lower case letters e.g hh
  • if atleast one dominant allele is present, the trait shown will be the dominant one
  • there must be two recessive alleles present for the trait to be shown

Punnett Squares-

  • used to show genetic crosses
  • find out the probability of two parents producing different types of offspring
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  • the 23rd pair of our chromosomes determines our sex
  • a female has two X chromosomes; **
  • a male has an X and a Y chromosome; XY
  • Each egg cell produced by a fermale will have an X chromosome, half the sperm cells produced by a male with have an X chromosome and half will have a Y chromosome
  • the Y chromosome triggers the development of testes in the embryo, if not ovaries develop
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Genetic Disorders and testing

  • Huntingtons disease is a dominant disorder, the presence of one dominant allele will cause the disease
  • symptoms: dizziness, memory loss, uncontrollable shaking, mood changes
  • Cystic fibrosis is a recessive disorder, the presence of two recessive alleles will cause the disease
  • symptoms: production of thick mucus, digestive problems, breathing problem

Genetic Testing

  • genetic screening is used to check for a particular disorder even when there is no family history of it, e.g. the heel ***** test used on newborn babies
  • genetic testing is carried out when a genetic disorder runs in the family, in order to allow the person to get treatment and plan for the future. it sometimes raises ethical issues
  • genetic testing during pregnancy; amniocentesis- collect cells from amniotic fluid, CVS- cells taken from the placenta. 1% chance of miscarriage
  • PGD- allows doctors to check the genetic makeup of the embryos prior to implantation, aka embryo screening, 
  • Ethical issues- helps decide if the family will keep the baby, allows them to plan ahead, get the best treatment. some tests give false positive/negative results
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Cloning and Stem Cells

  • clones are individuals with identical genes
  • in asexual production; only one parent is involved so the offspring are identical
  • plants can reproduce asexually by using runners (shoots that grow into identical plants) or producing bulbs
  • asexual reproduction is useful where plants and animals live in isolation, however there is no genetic variation so the population could be at risk of being wiped out if there is a disease etc


1- the nucleus from a body cell is extracted

2- it is inserted into an empty egg cell

3- the embryo is implanted into a surrogate mother

Stem Cells

  • embryonic stem cells are produced after 5 days and are unspecialised
  • adult stem cells are used to treat various diseases but have limited uses
  • ethical issues- embryonic stem cells are taken from an unused embryo, they destruct embryos once used
  • the uses of stem cells- testing of new drugs, understanding how cells become specialised, renewing damaged or destroyed cells in spinal injuries, heart disease, alzheimers disease, and parkinsons disease
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