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 :
What can you infer about the genotype of parents in crosses I and II ?
Show Hint
Cross I shows only tall plants and cross II shows a 3 : 1 ratio. Work out which parental genotypes can give each result.
Step 1: Read the graphs:
Let T stand for the tall allele and t for the dwarf allele.
Cross I gives 500 tall plants and no dwarf plants. Cross II gives 375 tall and 125 dwarf plants.
The ratio in cross II is 375 : 125, which simplifies to 3 : 1.
Step 2: Infer the parents of cross I:
Every progeny plant is tall, so no dwarf (tt) plant appears.
A dwarf offspring needs a t allele from each parent. None appeared, so at least one parent cannot give t.
That parent must be homozygous tall (TT). The simplest inference is TT x TT, giving all TT tall plants. A cross TT x Tt would also give all tall plants.
Step 3: Infer the parents of cross II:
A 3 tall : 1 dwarf ratio is the monohybrid ratio from two heterozygous parents.
Dwarf plants appear, so both parents carry a t allele. Tall plants are 3 times as many, so tall is dominant.
Both parents are therefore heterozygous tall: Tt x Tt.
Step 4: Check with a Punnett square for cross II:
Gametes of each parent: T and t in equal numbers.
Offspring: 1 TT : 2 Tt : 1 tt, which is 3 tall : 1 dwarf. This matches 375 : 125.
Final Answer:
Cross I parents: homozygous tall (TT x TT, or at least one TT parent). Cross II parents: both heterozygous tall (Tt x Tt).
