M6:S2 Patterns of inheritance

• Continuous variation


  Individuals in a population vary within a range, there are no distinct categories
• Continuous variation


  • Height of humans
  • Waist circumference of humans
  • Fur length of dogs
• Discontinuous variation


  There are two or more distinct categories, each individual falls into only one category, there are no intermediates
• Discontinuous variation


  • Blood group of humans
  • Violet flower colour
• Genotype


  The genes and alleles an organism has
• Phenotype


  The characteristics displayed by an organism
• Sexual reproduction leads to variation in genotypes within a species
• Meiosis makes gametes with a unique assortment of alleles through crossing-over and the independent assortment of chromosomes
• The random fusion of gametes during fertilisation increases genetic variation in the offspring
• Polygenic


  Inherited characteristics that show continuous variation are usually influenced by many genes
• Polygenic characteristic


  • Human skin colour
• Monogenic


  Inherited characteristics that show discontinuous variation are usually influenced by only one gene (or a small number of genes)
• Monogenic characteristic


  • Violet flower colour
• Etiolation


  Plants grow abnormally long and spindly because they're not getting enough light
• Chlorosis

  Plants don't produce enough chlorophyll and turn yellow, caused by several environmental factors e.g. lack of magnesium in soil
• Most phenotypic variation is caused by the combination of genotype and environmental factors
• Phenotypic variation influenced by both genotype and environment

  • Height of pea plants
  • Body mass in animals
• Monogenic characteristic

  A characteristic controlled by only one gene
• Phenotypic variation in plants caused only by environment

  • Etiolation
  • Chlorosis
• Continuous variation is usually influenced by many genes, discontinuous variation is usually influenced by only one or a few genes
• Gene

  A sequence of bases on a DNA molecule that codes for a protein (polypeptide)
• Allele

  A different version of a gene
• Genotype

  The alleles an organism has
• Phenotype

  The characteristics the alleles produce
• Dominant

  An allele whose characteristic appears in the phenotype even when there's only one copy
• Recessive

  An allele whose characteristic only appears in the phenotype if two copies are present
• Codominant

  Alleles that are both expressed in the phenotype, neither one is recessive
• Homozygote

  An organism that carries two copies of the same allele
• Heterozygote

  An organism that carries two different alleles
• Carrier

  A person carrying an allele which is not expressed in the phenotype but that can be passed on to offspring
• Gametes contain only one allele for each gene
• When gametes from two parents fuse together, the alleles they contain form the genotype of the offspring produced
• Monogenic inheritance is the inheritance of a characteristic controlled by a single gene
• Whenever you do a monogenic cross with two heterozygous parents you get a 3:1 ratio of dominant: recessive characteristic
• Codominant alleles are both expressed in the phenotype, neither one is recessive
• Codominant alleles

  • Alleles for sickle-cell anaemia in humans
• In the ABO blood group system in humans there are three alleles for blood type
• Allele I is the allele for blood group A, allele I^B is the allele for blood group B, and allele i is the allele for blood group O
• Allele i is recessive, alleles I and I^B are codominant
• Multiple alleles

  More than two alleles of the same gene