Monohybrid Inheritance

Key definitions

• Monohybrid inheritance concerns the inheritance of a single characteristic, such as plant height or flower color.
  • It involves the inheritance of the alleles of one gene.

Example (Mendel’s peas)

• There is one gene for height in pea plants. The gene for plant height has two alleles: tall, T, and dwarf, t.
  • There are three possible genotypes for plant height:
    • TT = homozygous tall
    • Tt = heterozygous tall
    • tt homozygous dwarf

The Punnett square

• Using our previous example, we could produce a Punnett square to show the combinations of gametes.

The parental cross

• In Figure 16.7.2,, it shows a cross between a homozygous tall plant, TT, and a homozygous dwarf plant, tt.
  • These represent the pure breeding plants that Mendel used in his crosses. This is called the parental cross.
  • The parental cross is the mating of two initial organisms to study how specific genes, traits, and alleles are passed down to their offspring.
• As a result of meiosis, all the gametes from the tall plant contain the dominant allele T, and all the gametes from the dwarf plant contain the recessive allele t.
• When fertilization occurs, the new plants receive one dominant allele and one recessive allele, making it heterozygous. They will also all have the genotype, Tt.
  • Their phenotype will be tall, because the allele T is dominant to the recessive allele t.
• The first generation is known as the $F_1$ generation, and in this particular genetic cross, all the plants in the $F_1$ generation are heterozygous tall (Tt).

The Punnett square for this scenario

Crossing the $F_1$ generation

• The $F_1$ plants are allowed to self-pollinate (review “The Punnett square” at the start of this page).
  • When meiosis occurs, half of the gametes of each plant will have the dominant allele T, and half will have the recessive allele t.
• When fertilization occurs, there are three possible combinations of alleles in the $F_2$, or second generation:
  • TT, homozygous tall
  • Tt, heterozygous tall
  • tt, homozygous dwarf.
• Among this generation:
  • $\frac{1}{4}$ (or 25%) will be homozygous tall.
  • $\frac{1}{2}$ (or 50%) will be heterozygous tall.
  • $\frac{1}{4}$ will be homozygous dwarf.
• Since the phenotypes of TT and Tt are the same, $\frac{3}{4}$ of the $F_2$ plants will be tall and $\frac{1}{4}$ will be dwarf.
  • This can also be written as 3 tall : 1 dwarf, $3:1$.

A test cross (supplement)

• The last two crosses mentioned show that the recessive allele is not expressed in the $F_1$ generation, but is expressed in the $F_2$ generation.
  • This means that we can’t immediately know the genotype of a tall plant only by looking at it, because plants with genotypes TT and Tt have the same phenotype.
  • We already know the genotype of a dwarf plant, because it can only be tt.
• A test cross is used to determine the genotype of an individual to find out whether it is homozygous or heterozygous.

If unknown tall plant is homozygous

• If our tall plant is homozygous (TT), then crossing it with a dwarf plant (tt) gives it all tall plants (Tt).

If unknown tall plant is heterozygous

• If our tall plant is heterozygous (Tt), then crossing it with a dwarf plant (tt) will give half tall plants (Tt) and half dwarf plants (tt) in a ratio of $1:1$.