Patterns of Mendelian I heritance

• Mendel's study of inheritance

  1. Studied one character with two contrasting expressions at a time

  2. Cross-pollinated two pure breeding plants with contrasting expressions

  3. Followed the F1 plants to self-pollinate
• When the F1 plants that expressed only the dominant trait were self-pollinated, some of their offspring (F2) expressed the recessive trait
• The ratio of the plants with dominant traits to the plants with recessive traits is almost 3:1
• F1 plants


  Have a pair of 'unit determiners', one for the dominant trait and another for the recessive trait
• Genes


  The 'unit determiners' of heredity, coined by Walter Sutton in 1903
• Alleles


  Different forms of genes for a character
• Alleles for seed color

  • G (capital letter) for yellow
  • g (small letter) for green
• Homozygous gene pairs


  Made up of identical genes
• Heterozygous gene pairs


  Made up of dissimilar genes
• Dominant and recessive traits


  When the gene pair of an individual is heterozygous, only one of the two genes is physically expressed (dominant), while the other is hidden or not expressed (recessive)
• Mendel's first cross


  1. GG x gg

  2. Gametes: G, G, g, g

  3. F1 generation: all Gg (yellow seeded)
• Mendel's second cross


  1. Gg x Gg

  2. Gametes: G, g, G, g

  3. F1 generation: GG, Gg, Gg, gg (3 yellow: 1 green)
• Mendel's law of segregation


  The two genes of a pair segregate or separate during gamete formation, so that only one gene of a pair is received by each gamete
• Widow's peak (S) is dominant over straight hair-line (s)
• Mother's genotype


  Heterozygous for widow's peak
• Father's genotype


  Straight hairline
• Mother's gametes


  • S
  • s
• Father's gametes


  • s
• Cross between mother and father


  Punnett square: SS, Ss, Ss, ss
• Genotypic ratio of the cross
• Phenotypic ratio of the cross