Genetic diagrams

• Some characteristics are controlled by single genes
• What genes you inherit control what characteristics you develop
• Different genes control different characteristics. Some characteristics are controlled by a single gene e.g. mouse fur colour and red-green colour blindness in humans
• Most characteristics are controlled by several genes interacting
• All genes exist in different versions called alleles (which are represented by letters in genetic diagrams)
• You have 2 versions (alleles) of every gene in your body - one on each chromosome in a pair
• If an organism has 2 alleles for a particular gene that are the same, then it's homozygous for that trait
• If its two alleles for a particular gene are different, then it's heterozygous
• If the two alleles are different, only one can determine what characteristic is present. The allele for the characteristic that's shown is called the dominant allele (use a capital letter for dominant alleles). The other one is called recessive (and you show these with a lower case letter)
• For an organism to display a recessive characteristic, both its alleles must be recessive. But to display a dominant characteristic the organism can be either CC or Cc, because the dominant allele overrules the recessive one if the plant/animal/other organism is heterozygous
• Your genotype is the combination of alleles you have. Your alleles work at a molecular level to determine what characteristics you have - your phenotype
• Genetic diagrams show the possible alleles of offspring
• Suppose you start breeding cats with tabby marks. The allele which causes hamsters to have tabby marks is recessive ("b") whilst other marks is due to a dominant allele ("B")
• A cat with tabby marks must have the genotype bb. But a cat with other marks could be BB or Bb
• The ratio of cats with other marks to cats with tabby marks is 3:1 in this generation (a 1 in 4 or 25% probability of tabby marks)

  
• Genetic diagrams only tell you probabilities. They don't say definitely what'll happen
• (1st image)

  In this cross, a homozygous dominant hamster (BB) is crossed with a homozygnous recessive hamser (bb). All the offspring are normal

  (2nd image)

  If you crossed a homozygous dominant hamster (BB) with a heterozygous hamster (Bb) you would get all normal offspring

  
• Knowing how inheritance works can help you to interpret a family tree - this is one for cystic fibrosis

  
• From the family tree, you can tell that the allele for cystic fibrosis isn't dominant because plenty of the family carry the allele but don't have the disorder

  
• There is a 25% chance that the new baby will have the disorder and 50% chance that it will be a carrier, as both of its parents are carriers but are unaffected. The baby could be unaffected (FF), a carrier (Ff) or have cystic fibrosis (ff)