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In a Mendelian test, tall pea plants with violet flowers and short flowers with white flowers were used for breeding. All the flowers of their progeny were violet in color but about half of the plants were small in stature. This shows that the genetic composition of tall plants can be represented as:

In a Mendelian test, tall pea plants with violet flowers and short flowers with white flowers were used for breeding. All the flowers of their progeny were violet in color but about half of the plants were small in stature. This shows that the genetic composition of tall plants can be represented as: Correct Answer TtWW

Concept-

  • Mendel proposed two general rules to consolidate his understanding of inheritance in monohybrid crosses.
  • Today these rules are called the Principles or Laws of Inheritance the First Law or Law of Dominance and the Second Law or Law of Segregation.
  • According to the law of dominance.
    • Characters are controlled by discrete units called factors.
    • Factors occur in pairs.
    • In a dissimilar pair of factors, one member of the pair dominates the other.

Explanation-

  • The dihybrid cross involves two characters in a single plant.
  • The parent pea plants are of two types.
    • Tall plants with violet flowers.
    • Short plants with white flowers.
  • So, the genotype of the parents could be-
    • Tall plants with violet flowers: TTWW or TtWw
    • Short plants with white flowers: ttww
  • Since all the progeny bore violet flowers, it implies that the tall plant with violet flowers has genotype 'WW' for violet flower color. 
  • Since the progeny acquired is both tall and short, the parent plant was not a pure tall plant and carry genes that determine the short height of the plant.
  • Therefore, the genotype of the plant with respect to height would be 'Tt'.

So, if a cross is carried out between a tall parent with violet flowers (TtWW) and a short parent with white flowers (ttww), the progeny obtained is TtWw (8) : ttWw (8).

Mistake Points

  • Please note that in pea plants violet colour of flowers is dominant over white colour.
  • Also, the violet colour character has been denoted here by "W" and not "V" (which is the initial letter of the dominant character).
  • It is important to remember which character is dominant over which, irrespective of the letters that are used to denote it in the cross.

Additional Information

  • Law of Segregation 
    • This law is based on the fact that the alleles do not show any blending and that both the characters are recovered as such in the F2 generation though one of these is not seen at the F1 stage. 
    • Though the parents contain two alleles during gamete formation, the factors or alleles of a pair segregate from each other such that a gamete receives only one of the two factors.
    • Of course, a homozygous parent produces all gametes that are similar while a heterozygous one produces two kinds of gametes each having one allele with equal proportion.

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