What map unit (Centimorgan) is adopted in the
construction of genetic maps?
Explanation:
In NCERT, one map unit is a centimorgan, which corresponds to a 1% recombination frequency between two genes on a chromosome. So, the distance between two genes is measured by the percentage of offspring showing recombination. A 1% recombination means a 1 cM distance. The correct option is 4: 1% cross over.
🧠 Did You Know?
Recombination frequency cannot exceed 50% for genes that are linked; 50% or more indicates genes behave as if on different chromosomes (independent assortment).
💡 NCERT Memory Line:
A centimorgan (cM) is the unit of map distance; 1 cM equals 1% recombination between two genes.
Option 2 is incorrect. In domesticated birds (fowls), the sex of the offspring is determined by the egg (the female gamete), which can carry either Z or W. The sperm always carries only a Z chromosome. Thus, sex is determined by the type of egg, not the sperm, according to NCERT biology.
🧠 Did You Know?
In birds, females are the hens (ZW) and males are the cocks (ZZ). The egg determines whether the offspring will be male (ZZ) or female (ZW).
💡 NCERT Memory Line:
In birds, sex determination is by the egg and the W chromosome in eggs decides female offspring, while the sperm contributes only Z.
Under which of the following conditions will there
be no change in the reading frame of following
mRNA?
5'AACAGCGGUGCUAUU3'
Explanation:
The reading frame is set by codons of three bases. The given mRNA has codons: AAC | AGC | GGU | GCU | AUU. Deleting an entire codon (GGU at positions 7–9) removes 3 bases, preserving the frame. Hence, no shift occurs with option 1.
🧠 Did You Know?
Inserting or deleting any number of bases not divisible by three shifts the reading frame (frameshift), changing all downstream codons and potentially altering the protein.
💡 NCERT Memory Line:
A reading frame is read in consecutive triplets; indels not in multiples of three cause frameshifts and alter the amino acid sequence.
