Determining sex and disorders: Inheritance, variation and evolution: Biology: GCSE (9:1)

• Genetic disorder


  A disease caused by errors in a DNA sequence, disorders are heritable so can potentially be passed on to offspring

  
• Alleles


  Different versions or form of the same type of gene, the interaction of two alleles determines the overall displayed characteristic

  
• Genotype


  The combination of two alleles that an organism has for a particular characteristic

  
• Punnett square


  A diagram used to predict an offspring's possible genotypes, following the cross of a male parent with a known genotype and a female parent with a known genotype

  
• Dominant


  An allele that is always expressed, even if only one copy of the allele is present, for example "A" in AA or Aa

  
• Recessive


  An allele that is only expressed if two copies of the allele are present, for example "a" in aa

  
• Homozygous


  When the two alleles present in an organism are the same, for example AA or aa

  
• Heterozygous


  When the two alleles present in an organism are different, for example Aa

  
• Cystic fibrosis


  A genetic disorder of the cell membranes that leads to excess mucus that damages organ systems, caused by a recessive allele

  
• Inheritance of cystic fibrosis

  Cystic fibrosis is a recessive disorder that will only occur if two recessive alleles are present, heterozygous individuals are carriers

  
• Carrier

  An individual who carries a recessive allele for a disorder but does not express the disorder's phenotype

  
• Polydactyly

  A genetic disorder where individuals have extra fingers or toes, caused by a dominant allele

  
• Inheritance of polydactyly

  Polydactyly is a dominant disorder so it only takes one allele for the disorder to occur, it is impossible to be a carrier for polydactyly

  
• Embryo screening

  The extraction of a cell from an embryo made by IVF which can be analysed to identify any genes that link to genetic disorders

  
• Gene therapy

  An experimental technique that involves inserting new genes or inhibiting existing defective genes to treat genetic disorders

  
• X and Y chromosomes

  These chromosomes are the 23rd pair of chromosomes in a human, the combination of the two chromosomes determines the sex of an offspring

  
• XX


  A human that is biologically female has two X chromosomes, there is a probability of 50% that a zygote will be XX

  
• XY

  A human that is biologically male will have X and Y chromosomes, there is a probability of 50% that a zygote will be XY

  
• Human body cells


  Typical cells that contain 46 chromosomes or 23 pairs of chromosomes

  
• Gamete cells


  Reproductive cells that contain 23 chromosomes, half the number of chromosomes in a human body cell

  
• Ethical issues

  Moral considerations need to be made if using embryo screening or if considering having a child that has a chance of inheriting a disorder