6.2.2 - Monogenic Inheritance

• Monogenic - Determined by a single gene.
• Monohybrid Cross - Crossing homozygous (pure trait) genotypes of a single gene that results in heterozygous offspring.
• In 1865, no one knew about DNA Chromosomes or Meiosis. In 1866, Gregor Mendel published the results of his investigations that would lay the foundation for the branch of biology known as genetics.
• Mendel studied an organism that was easy to grow and, although naturally self-fertilising, was easy to cross-fertilise artificially.

  • He worked with 7 characteristics of the pea plant, each characteristics having two distinctly contrasting traits.
  • He showed that separate 'factors' inherited from parents were dominant or recessive in producing particular traits.
• Mendel obtained true-breeding strains, where the trait had appeared unchanged generation after generation, from local seed merchant.

  • Mendel also kept accurate and quantitative records of data obtained which he analysed.
• Mendel's simplest experiments involved only one characteristic with one pair of contrasting traits:

  • He mated individuals from two parent strains, each of which showed a different phenotype.
• One parent was true-breeding for tall stems and the other was true-breeding for short stems - the P1/Parental Generation:

  • All of the offspring from their cross, the F1 generation, were phenotypically identical to one parent type - all tall-stemmed.
• When Mendel allowed members of the F1 generation to self-fertilise, the resulting F2 generation contained some short plants but 3 times as many tall as short plants. 3/4 tall, 1/4 short.
• When Mendel crossed true-breeding plants showing the other six phenotypic variations, he obtained similar results.
• In pea plants, the characteristics of heigh is monogenic; governed by one gene that has two distinct alleles T/t:

  • One allele, t, when present in a homozygous individual giving the genotype tt, produces phenotypically short plants.
  • The other alleles, T, when present in homozygous (TT) or heterozygous (Tt) individuals, produces phenotypically tall plants
  • The allele, T, is described as dominant (it codes for a dominant characteristic) and the allele t is recessive - coding for a recessive characteristic that will only be visible in the phenotype if there is no dominant allele.
• The genotypes and phenotypes resulting from the possible combinations of gametes during a monohybrid cross, showing the possible outcomes of monogenic inheritance, can be visualised in a Punnett square.
• In a Punnett square, all possible gametes are assigned to a row with those of the female parent in the vertical column, and those of the male parent in the horizontal row:

  • The genotypes of the next generation are predicted by combining the male and female gamete genotypes - a process that represents all possible random fertilisation events.
• • Dominant alleles are capital letters.
  • Recessive alleles are lower case letters.
  • Always same letter used for dominant + recessive.
  • Capital letters always written first.
  • Gametes written in circles when doing genetic crosses to identify them.
• • To determine whether an organism showing the dominant characteristic of a trait is homozygous or heterozygous, a test cross can be performed:
  • This involves crossing the organism with another that is homozygous recessive for the trait.
• • If any of the offspring show homozygous recessive in the phenotype, the parent must be heterozygous.
• The body cells of individuals have 2 alleles for each gene:

  • Gametes (sex cells) contain only one allele for each gene, and when gametes from 2 parents fuse together, the alleles they contain form the genotype of the offspring produced.
  • Genetic diagrams can be used to predict the genotypes and phenotypes of the offspring produced if two parents are crossed.