Inheritance

• The chromosomes you inherit carry your genetic information in the form of genes
• Many of these genes have different forms, or alleles (sometimes called variants)
• Each allele codes for a different protein
• The combination of alleles you inherit will determine your characteristics
• We can make biological models that help us predict the outcome of any genetic cross
• Homozygote


  An individual with two identical alleles for a characteristic, for example, BB or bb
• Heterozygote


  An individual with different alleles for a characteristic, for example, Bb
• Genotype


  The alleles present or genetic makeup of an individual regarding a particular characteristic, for example, Bb or bb
• Phenotype


  The physical appearance of an individual regarding a particular characteristic, for example, black fur or brown fur in a mouse
• A gene is a position on a chromosome, and an allele is the particular form of information in that position on an individual chromosome
• The mouse inherits one allele from each parent, so the mouse will have two alleles controlling whether it is black or brown
• The alleles present in an individual, known as the genotype, work at the level of the DNA molecules to control the proteins made
• These proteins result in characteristics - such as coat colour or the presence of dimples-that are expressed as the phenotype of the organism
• Dominant allele


  Some alleles are expressed in the phenotype even when they are only present on one of the chromosomes
• Use a capital letter to represent the alleles for dominant phenotypes, for example, B for black coat in mice
• If a mouse inherits BB or Bb from its parents, it will have a black coat
• Recessive allele


  Some alleles only control the development of a characteristic if they are present on both chromosomes - in other words, when no dominant allele is present
• Use a lower case letter to represent recessive alleles, for example, b, and they are only expressed if the organism is homozygous
• A mouse would only have a brown coat if it inherited two recessive alleles, bb