Q.57 In pea plants, purple color of flowers is determined by the dominant allele while white color is determined by the recessive allele.
A genetic cross between two purple flower-bearing plants results in an offspring with white flowers.
The probability that the third offspring from these parents will have purple flowers is _______ (rounded off to 2 decimal places).
Pea plants showing purple flowers (dominant trait) from a cross that produced at least one white-flowered offspring must both be heterozygous (Pp). Each offspring inherits alleles independently, so the probability of purple flowers (PP or Pp) remains 75% or 0.75 for any specific offspring, including the third.
Genotypes Deduced
Purple flowers result from genotypes PP or Pp, while white requires pp (recessive homozygote). A white offspring proves neither parent is PP, as PP × PP or PP × Pp yields only purple. Thus, both parents are Pp, confirmed by standard Mendelian crosses.
Punnett Square Analysis
The cross Pp × Pp yields:
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PP: 25% (purple)
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Pp: 50% (purple)
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pp: 25% (white)
Purple probability totals 75% (0.75). Offspring events are independent, so the third has identical odds regardless of prior results.
Probability Calculation
Fill in: 0.75.
In pea plants, purple flowers dominate over white in genetic crosses, a classic Mendelian inheritance example. When two purple flower-bearing plants produce a white offspring, both parents carry the recessive allele, making the third offspring’s purple flower probability 0.75. This detailed guide breaks down the genetics for students and researchers tackling pea plants purple white flowers genetic cross probability.
Understanding Alleles and Phenotypes
Purple flower color in pea plants stems from the dominant P allele, while white requires homozygous recessive pp. Two purple parents yielding white offspring confirms both are heterozygous Pp, as PP parents cannot produce pp. This setup aligns with Mendel’s law of segregation.
Punnett Square for Pp × Pp Cross
Visualize the cross:
| P | p | |
|---|---|---|
| P | PP | Pp |
| p | Pp | pp |
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25% PP (purple)
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50% Pp (purple)
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25% pp (white)
Total purple: 75% or 0.75. Independence ensures this holds for every offspring.
Why Probability Stays 0.75 for Third Offspring
No conditioning on prior offspring alters future gametes. The white sibling confirms Pp parents but does not shift ratios—each cross resets. Rounded to two decimals: 0.75.
Common Exam Traps Explained
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Trap 1: Assuming one parent homozygous—ruled out by white offspring.
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Trap 2: Conditional probability (e.g., given one white)—unneeded here.
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Trap 3: Forgetting rounding—0.75 is exact.
This pea plants purple white flowers genetic cross mirrors real biotech research in trait prediction.
