11.1 - Monohybrid Inheritance

Definition of Monohybrid Cross

Monohybrid inheritance involves one characteristic and contrasting trait that is controlled by a gene.

Mendel Experiments

Gregor Mendel was a scientist who discovered the mechanism of inheritance. In his studies using pea plants, Pisum sativum, he discovered that this plant has 7 characteristics. He came up with a theory stating that a characteristic present in a parent plant will be passed down to the next generation through male and female gametes.

Characteristics and Traits

• Dominant

  • Seed shape: Round

  • Seed color: Yellow

  • Pod shape: Inflated

  • Pod color: Green

  • Flower color: Purple

  • Flower position: Axial

  • Plant height: Tall

• Recessive

  • Seed shape: Constricted

  • Seed color: Green

  • Pod shape: Constricted

  • Pod color: Yellow

  • Flower color: White

  • Flower position: Terminal

  • Plant height: Short

Figure 1 - The 7 characterisitcs studied in Pisum sativum. The pea plant is chosen for the study because it can be planted easily and produce many seedlings. Besides, the pea plant has both the male and female reproductive structures. It also has a short generation interval. The contrasting characteristics and traits are obvious and easily recognizable.

Figure 2 - Mendel's monohybrid inheritance experiment. Only purebreed plants are studied. This means that those plants are produced through self-pollination and possess the same trait as the parent plant. Therefore, all the seedlings produced are tall.

Figure 3 - Mendel's monohybrid cross experiment. Tall purebreed pea plant (TT) and purebreed short pea plant (tt) are studied. Based on the dominance principle, the dominant inheritance factor will suppress the effect of the recessive inheritance factor. As a result, the recessive trait is not visible although it exists together with a dominant inheritance factor.

Terms Related to Inheritance

Terms and Definition

Gene

• Basic unit of inheritance of a DNA segment on a specific locus of a chromosome.
• Control a specific characteristic of an organism.

Allele

• Alternative form of a gene for a specific trait.
• Located on the same locus of a pair of homologous chromosomes.

Characteristics

• Heritable feature such as height, eye color, blood group and presence of dimples.
• Each characteristic is a feature of an organism.

Trait

• Variation of a specific characteristic.
• For instance, height is a characteristic whereas tall or dwarf is a trait.

Phenotype

• Observable characteristic of an organism.
• For instance, height and color.

Genotype

• Genetic composition of an organism that cannot be seen.
• For instance, TT or Tt (genotype for tall) and tt (genotype for short).

Dominant allele

• An allele which always shows its trait when present and suppresses the effect of recessive allele.
• Represented by a capital letter. For example, T.

Recessive allele

• An allele that only shows its trait with paired with another recessive allele.
• The presence of a dominant trait will suppress the effect of a recessive allele.
• Represented by small letters. For example, t.

Homozygote

• Refers to both alleles at loci of a pair of homologous chromosomes are the same.
• For instance, TT or tt.

Heterozygote

• Refers to both alleles at loci of a pair of homologous chromosomes are different.
• For example, Tt.

Parent generation

• The first generation that involves 2 individuals mated to predict the genotypes of their offsprings.

Filial generation

• A generation formed from the mating between individuals of purebreed parental generation.

Dominant trait

• Expressed when both alleles are dominant or one dominant allele is paired with a recessive allele.
• For instance, TT or Tt.

Recessive trait

• Only expressed when a recessive allele is paired with another recessive allele.
• For example, tt.

Purebreed

• Organism that carries two identical alleles for a trait.
• Formed as a result self-cross.

Hybrid

• Product of mating two purebreed varieties.

Figure 4 - Monohybrid cross for flower colour characteristics. The ratio of F2 genotype is 1 BB : 2 Bb : 1 bb. The ratio of F2 phenotype is 3 purple : 1 white. Other than using the schematic diagram above, a Punnett square (Figure 5) can also be used to predict the ratio and frequency of the offsprings' genotype and phenotype.

Figure 5 - Monohybrid cross using a Punnett square.

Mendel's First Law

Mendel's First Law , also known as the Law of Segregation states that a specific characteristic of an organism is controlled by a pair of alleles, however only one of the paired alleles will be passed down in the gametes.

• Summary of Mendel monohybrid experiment
  • Gene controls the characteristics to be passed down from the parent to the next generation.
  • Allele will determine a specific trait to be passed down to the next generation.
  • Segregation of alleles happens at random during the formation gametes. Only one of the paired alleles will be passed down in the gametes.
  • During fertilization, the zygote formed contains two alleles, that is one allele from each parent for a specific characteristic.
  • Fertilization also happens randomly. The combination of genotype which is homozygous dominant will show the dominant trait, whereas a homozygous recessive will exhibit a recessive trait. A heterozygous genotype will also show a dominant trait as the recessive allele is suppressed by the dominant allele.