ICSE 2019: In Mendel's experiments, tall pea plants (T) are dominant over dwarf pea plants (t)

ICSE 2019:

Q. In Mendel's experiments, tall pea plants (T) are dominant over dwarf pea plants (t). [5 marks]

(i) What is the phenotype and genotype of the F1 generation if a homozygous tall plant is

crossed with a homozygous dwarf plant?

(ii) Draw a Punnett square board to show the gametes and offspring when both parents

are heterozygous for tallness.

(iii) What is the phenotypic ratio and genotypic ratio of the above cross in (ii)?

(iv) State Mendel’s Law of Dominance.

(v) What is a dihybrid cross?

Solution: 

This question based on the Mendel's Law of Dominance was asked by ICSE board in the year 2019. The question tested the knowledge of students on the Law of Dominance, phenotypic and genotypic ratio of monhybrid and dihybrid cross.

Let's begin with solving the above mentioned questions:

ANSWERS:

(i) If a homozygous tall plant is crossed with a homozygous dwarf plant, the phenotype and genotype of F1 generation will be:

• Phenotype:    Tall
• Genotype:     Tt (Hybrid tall)

(ii) When both parents are heterozygous for tallness, Punnet square can be drawn as:

(iii) In the above cross (ii),

• Phenotypic ratio = 3 : 1 [Tall : Dwarf]

• Genotypic ratio = 1 : 2 : 1  [Pure Tall (TT) : Hybrid tall (Tt) : Dwarf (tt)]

(iv) Mendel's Law of Dominance can be defined as:

"out of a pair of allelomorphic character (contrasting characters), one is dominant over the other".

Mendel found out that characters present as a pair of alleles in one plant dominate over that of the other plants. The factors which get expressed is called dominant factor and unexpressed factor is called recessive factor.

Read in detail about Law of Dominance: Click here.... 

(v) Dihybrid cross can be defined as:

In a Dihybrid cross, inheritance of two pairs of contrasting characters is studied.

In a breeding experiment, when cross between two parents is performed considering alternative forms of two different characters, the cross is called dihybrid cross.

Example: 

Cross bewteen pure homozygous Yellow cotyledon, Round seed pea variety (YYRR) with Green cotyledon, wrinkled seed pea variety (yyrr).

Read in detail about Dihybrid Cross: Click here....

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