7.1&3

• Genotype


  Combination of alleles
• Phenotype


  Observable form of a trait
• Dominant phenotype


  Visible in the heterozygote and the homozygote
• Recessive phenotype


  Only observed in the homozygous with 2 copies of the recessive allele
• Allele


  Different forms of a gene that arise because of mutation
• Gregor Mendel, an Austrian monk, through his breeding of pea plants was able to demonstrate that traits are passed from parents to offspring and that these traits form specific patterns over generations of crossbreeding
• Monohybrid cross


  To determine which trait is dominant, a cross can be carried out
• Monohybrid cross


  1. Where 2 individuals with different alleles at a single locus are crossed

  2. The parent generation (P) breeds to homozygotes together to produce heterozygote offspring in the first filial generation (F1)

  3. The F1 generation is then crossed to produce the F2 generation

  4. The phenotype ratios in the F1 and F2 generations will then indicate which trait is dominant
• Autosome dominant inheritance


  Also called complete dominance, refers to a dominant trait that is passed on to offspring via a gene on an autosome, only 1 copy of the allele is required for the trait to be expressed as the dominant phenotype
• Autosome dominant inheritance


  • Blood types
• Blood group


  Determined by a single gene with multiple alleles
• 1. antigen


  IA
• 1. antigen


  IB
• No antigen


  i
• Genetic relationship


  i is recessive to both IA and IB while IA and IB are co-dominant
• Autosomal recessive inheritance


  Two copies of the recessive allele must be present for the disease or trait to develop
• Punnett square


  A square diagram used to predict the genotypes of a particular cross or breeding experiment
• Dominant allele


  Represented by capital letter
• Recessive allele


  Represented by lowercase letter
• Constructing Punnett squares


  1. Designate letters which will represent the alleles

  2. Write down the genotype (alleles) of each parent

  3. List the alleles that each parent can contribute

  4. Draw a Punnett square – 4 small squares in the shape of a window, write the possible allele(s) of one parent across the top and the allele(s) of the other parent along the side

  5. Fill in each box of the Punnett square by transferring the letter above and in front of each box into each appropriate box, capital letter goes first, lowercase letter follows

  6. List the possible genotype and phenotype of the offspring for this cross
• Mendelian ratio


  E.g.: (dominant 3 : 1 recessive)
• Test cross


  Where the F1 crossed with the pure-breeding (homozygous) strain showing the recessive phenotype, testing the phenotype to see if the trait is dominant/recessive or the number of genes involved
• We can find out how many genes control the phenotype of an individual by carrying out a test cross. We also use it to determine whether an individual is of a dominant phenotype is homozygous or heterozygous
• Blood group is determined by a single gene with multiple alleles
• Incomplete dominance


  Occurs when the phenotype of the heterozygous offspring is a blend between the phenotypes of the homozygous parents, this type of relationship between alleles, with a heterozygote phenotype, is called incomplete dominance
• Incomplete dominance


  • In the snapdragon, Antirrhinum majus, a cross between homozygous white-flowered plant (CW CW) and a homozygous red-flowered plant (CR CR) will produce offspring with pink flowers (CR CW)
• We can still use Mendel's model to predict the results of crosses for alleles that show incomplete dominance
• Incomplete dominance


  • Self-fertilisation of a pink plant would produce a genotype ratio of 1 CRCR:2CRCW:1CWCW and a phenotype ratio of 1:2:1 red:pink:white
• Alleles are still inherited according to Mendel's basic rule, even when they show incomplete dominance