Το back cross an individual heterozygous for tallness (Tt), it will be crossed with the parent having genotype:

Question

Accepted Answer

The correct option is 4: Both TT & tt

The term **back cross** in genetics is a breeding method defined as the mating of a hybrid individual (the $\text{F}\_{1}$ generation) with one of its two original parental genotypes. In this scenario, the heterozygous individual for tallness ($\text{Tt}$) is the hybrid, and the two original parents were the homozygous dominant plant ($\text{TT}$) and the homozygous recessive plant ($\text{tt}$). Therefore, to perform a back cross, the $\text{Tt}$ individual must be crossed with either the $\text{TT}$ parent ($\text{Tt} \times \text{TT}$) or the $\text{tt}$ parent ($\text{Tt} \times \text{tt}$). This technique is widely used by breeders and geneticists to efficiently introduce a specific trait into a pure-bred line or, conversely, to rapidly recover the original genetic background while retaining a desired trait. Since the definition of a back cross includes both possibilities, the only correct and comprehensive answer is the one that lists both $\text{TT}$ and $\text{tt}$.

The incorrect option is 1: TT only

This option is incorrect because it represents only one possible form of the back cross. Crossing the heterozygous hybrid ($\text{Tt}$) with the homozygous dominant parent ($\text{TT}$) is indeed a valid back cross, sometimes referred to as an *outcross* or *backcross to the dominant parent*. However, the classic definition of a back cross is not restricted to the dominant parent alone. By excluding the homozygous recessive parent ($\text{tt}$), this option fails to fully encapsulate the conceptual meaning of a back cross, which is a cross with *either* parent from the initial generation.

The incorrect option is 2: tt only

This option is also incorrect because it excludes the homozygous dominant parent ($\text{TT}$). The cross between the $\text{Tt}$ hybrid and the homozygous recessive parent ($\text{tt}$) is a highly specialized and important type of back cross known as a **Test Cross**. Its unique purpose is to test or determine the unknown genotype ($\text{TT}$ or $\text{Tt}$) of an organism that exhibits the dominant phenotype. While all Test Crosses are a subset of Back Crosses, the term "back cross" itself is broader and must include the option of crossing back to the $\text{TT}$ parent. Therefore, "$\text{tt}$ only" is an insufficient answer to the general question of what a back cross involves.

The incorrect option is 3: Tt

This option describes the cross of the heterozygous individual with another individual of the identical genotype ($\text{Tt} \times \text{Tt}$). This particular mating is known as a **self-cross** (if the plant is self-pollinated) or an **intercross**. It is the cross used to generate the $\text{F}\_{2}$ generation, which famously produces the $3:1$ phenotypic ratio as predicted by Mendel's Law of Segregation. However, the $\text{Tt}$ individual is the *offspring* of the original parents, not one of the parents itself. A back cross specifically requires crossing the hybrid *back* to the original, parental genotypes ($\text{TT}$ or $\text{tt}$), which this option fails to do.