Question 4 of 15intermediate💡 UnderstandLong Answer5 marks

Explain the Law of Dominance using a monohybrid cross.

Correct Answer

The Law of Dominance was proposed by Mendel based on his observations on monohybrid crosses. According to this law: (i) characters are controlled by discrete units called factors, (ii) factors occur in pairs, and (iii) in a dissimilar pair of factors, one member of the pair dominates (dominant) the other (recessive).

This law explains the expression of only one of the parental characters in a monohybrid cross in the F1 generation. When a cross is made between two parents differing in a single trait, the F1 progeny shows only the dominant character while the recessive character remains unexpressed.

The law also explains the expression of both parental characters in the F2 generation. The recessive character that was not expressed in heterozygous condition may be expressed again when it becomes homozygous. This results in the characteristic phenotypic ratio of 3:1 obtained at the F2 generation.

Importantly, the dominant characters are expressed when factors are in heterozygous condition, while the recessive characters are only expressed in homozygous conditions. The characters never blend in heterozygous condition.

Thus, the Law of Dominance successfully explains why only one trait appears in F1 and why both traits reappear in specific proportions in F2.

Exercise: EXERCISES | Q: 4 | (Page 28)
For More Understanding

Explanation

The answer draws directly from Section 4.2.1 of the textbook which outlines Mendel's Law of Dominance with three key points about factors. The context explains how this law accounts for F1 showing only the dominant trait and F2 showing both traits in a 3:1 ratio. The textbook also clarifies that dominant characters express in heterozygous conditions while recessive characters require homozygous conditions, and importantly, characters do not blend in heterozygotes.

Solution Steps

  1. Step 1: State the three postulates of Law of Dominance from the textbook

  2. Step 2: Explain how the law accounts for F1 generation showing only dominant parental character

  3. Step 3: Explain how the law accounts for F2 generation showing both characters in 3:1 ratio

  4. Step 4: Clarify the conditions for dominant (heterozygous) and recessive (homozygous) expression

  5. Step 5: Conclude with the significance of non-blending of characters

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