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 :
Which genetic cross is represented by these crosses ?
Show Hint
Each graph follows one trait, height. Match the ratios 1 : 0, 3 : 1 and 1 : 1 with the cross types.
Step 1: Recall the types of crosses:
A monohybrid cross follows one pair of contrasting characters, here tall and dwarf height.
A test cross is a cross of an individual of dominant phenotype with a homozygous recessive (tt) individual. It uses 1 : 1 as the sign of a heterozygote.
Step 2: Study crosses I and II:
Both crosses follow only one character, the height of the plant.
Cross I gives all tall plants (TT x TT or TT x Tt). Cross II gives 375 tall : 125 dwarf, a 3 : 1 ratio (Tt x Tt).
So crosses I and II are monohybrid crosses.
Step 3: Study cross III:
Cross III gives 250 tall : 250 dwarf, which is 1 : 1.
This is the ratio of Tt x tt. So cross III is a test cross, which is a back cross with the recessive parent.
Final Answer:
These are monohybrid crosses involving one character (tall or dwarf). Crosses I and II are plain monohybrid crosses (all tall, and 3 : 1), and cross III is a test cross (1 : 1).
