Monohybrid Cross - Inheritance of One Gene

Monohybrid Cross - Inheritance of One Gene

Crosses involving inheritance of only one pair of contrasting characters are called monohybrid crosses. For example it is a cross between two forms of a single trait like a cross between tall and dwarf plant.

Mendel’s Explanation of Monohybrid Cross

Parental generation: Pure breeding tall plant and a pure breeding dwarf plant.

F₁ generation: Plants raised from the seeds of pure breeding parental cross in F₁ generation were tall and monohybrids.

F₂ generation: Selfing of the F₁ monohybrids resulted in tall and dwarf plants respectively in the ratio of 3:1. The actual number of tall and dwarf plants obtained by Mendel was 787 tall and 277 dwarf. External expression of a particular trait is known as phenotype. So the phenotypic ratio is 3:1.

In the F₂ generation 3 different types were obtained:

Tall Homozygous – TT (Pure) – 1

Tall Heterozygous – Tt – 2

Dwarf Homozygous – tt – 1

So the genotypic ratio 1:2:1. A genotype is the genetic expression of an organism

Mendel’s Interpretation on Monohybrid cross

Based on these observations it was confirmed by Mendel that ‘factors’ are passed on from one generation to another, now refered to as genes . Tallness and Dwarfness are determined by a pair of contrasting factors tall plant possess a pair of factors (represented by T- taking the first letter of the dominant character) and a plant is dwarf because it possess factors for dwarfness (represented as t- recessive character). 

These factors occur in pairs and may be alike as in pure breeding tall plants (TT) and dwarf plants (tt). This is referred to as homozygous . If they are unlike (Tt) they are referred to as heterozygous.

1. Two factors making up a pair of contrasting characters are called alleles or allelomorphs. One member of each pair is contributed by one parent.

2. When two factors for alternative expression of a trait are brought together by fertilization only one expresses itself, (tallness) masking the expression of the other (dwarfness). The character which expresses itself is called dominant condition and that which is masked is called recessive condition.

3. The factors are always pure and when gametes are formed, the unit factors segregate so that each gamete gets one of the two alternative factors. It means that factors for tallness(T) and dwarfness(t) are separate entities and in a gamete either T or t is present. When F₁ hybrids are self crossed the two entities separate and then unite independently, forming tall and dwarf plants.