Question

Define and design a test cross.

Answer 1

Test cross is used generally to assess a phenotypically dominant individual's genotype by crossing it with an individual whose genotype is known.

Complete Answer:
- Diploid species have two alleles at each genetic locus, or location, and each parent inherits one allele. Different alleles do not necessarily show the same results or phenotypes. Alleles can be dominant or recessive.
- In a heterozygous condition, one allele will dominate and mask the effect of a second recessive allele that carries two separate alleles at a particular locus. Recessive alleles show their phenotype only if an individual bears two identical copies of the recessive allele, meaning the recessive allele is homozygous. That means an organism's genotype with a dominant phenotype can be either homozygous or heterozygous to the dominant allele.

A test cross is a cross between an organism with unknown genotype and a recessive parent. It can be done to identify if an organism with a dominant trait is homozygous or heterozygous for a specific allele. The organism is crossed with an organism homozygous to the recessive gene, and the test cross offspring is studied. If the test cross contributes to some recessive offspring, the parent organism is heterozygous for the specific allele. If the test cross gives an offspring with only dominant phenotype, the parent organism is homozygous dominant for that particular allele.

To find the genotype, a heterozygous F1 tall flower (Tt) is crossed with a recessive parent (tt). The test gives tall and dwarf progeny in a 1: 1 ratio indicating that the genotype is heterozygous.
Tt x tt (homozygous recessive)

| T| t
---|---|---
t| Tt| tt
t| Tt| tt

Genotype = Tt : tt = 1 : 1

Note: The genetic makeup of an organism is called its genotype, and it reflects all of the alleles, or forms of the gene, that the organism carries. The observable traits that an organism has, example eye color, height, or hair color, make up its phenotype. A test cross may help to decide whether a dominant phenotype for a particular allele is homozygous or heterozygous.