A student performed some crosses in plants and represented the result in the form of bar graphs as shown below. Each graph displays the phenotypic proportion of the progeny. Study the graphs and answer the questions :
Looking at bar graph of cross III, identify the type of cross performed and its importance in genetics.
Show Hint
A 1 : 1 ratio of tall to dwarf is the mark of a test cross. It shows whether a tall plant is TT or Tt.
Step 1: Read cross III:
Cross III gives 250 tall plants and 250 dwarf plants.
The ratio is 1 : 1, with tall and dwarf plants in equal numbers.
Step 2: Identify the cross:
A 1 : 1 ratio comes only from Tt x tt. One parent is a heterozygous tall plant and the other is a homozygous dwarf plant.
Crossing a tall plant of unknown genotype with the homozygous recessive dwarf plant is a test cross.
Step 3: Explain the importance:
The test cross finds the genotype of an individual that shows the dominant phenotype. Tall plants may be TT or Tt, and their looks cannot tell them apart.
If all offspring are tall, the tested plant was homozygous (TT). If tall and dwarf appear in a 1 : 1 ratio, it was heterozygous (Tt), as in cross III.
The test cross is also used to find whether two genes are linked, and to confirm Mendel's laws.
Final Answer:
Cross III is a test cross (Tt x tt). Its importance is that it reveals whether an organism of dominant phenotype is homozygous or heterozygous.
