Class 12 Biology, Chapter 4 (NCERT). 15 questions by concept. Open “Show answer” for the answer and why.
How to teach this chapter · Common misconceptions & worked visual for this chapter
What happens to the two alleles of a gene during gamete formation?
(c) They separate so each gamete receives one
Separation of homologous chromosomes distributes the allele pair into different gametes.
A heterozygote Aa produces gametes. Which allele content is expected?
(b) Separate A-bearing and a-bearing gametes
Phenotypic masking does not prevent allele segregation.
What does it mean to call an allele dominant?
(b) It determines the heterozygote's phenotype under the stated conditions
Dominance is an expression relationship between alleles in a heterozygote.
A rare allele is expressed in every heterozygote. How should it be described?
(b) Dominant despite being rare
Frequency, fitness effect, and dominance are separate properties.
Which genotype produces blood group AB?
(b)
The alleles and are codominant, so both are expressed in blood group AB. A person normally carries two ABO alleles.
What is the maximum number of distinct ABO alleles in one person, and how many occur in the population, respectively?
(d) 2; 3
Three ABO alleles occur in the population: , and . A person inherits one copy from each parent, allowing at most two distinct alleles.
How many equally frequent gamete types can AaBb produce when the genes assort independently?
(b) Four: AB, Ab, aB, and ab
Independent segregation of each allele pair yields four combinations.
Which F2 phenotypic ratio is characteristic of a standard independently assorting dihybrid cross?
(b) 9:3:3:1
Combining two 3:1 segregations produces four phenotype classes in a 9:3:3:1 ratio.
What observation links Mendel's factors to chromosomes?
(c) Homologous chromosomes separate in meiosis as allele pairs segregate
Parallel behaviour of genes and chromosomes supports the chromosomal theory.
A gene has one allele on each homologous chromosome. What meiotic event explains segregation?
(a) Separation of homologous chromosomes
Homologue separation sends alternative alleles to different gametes.
What is pleiotropy?
(a) One gene influences multiple phenotypic traits
A single gene product may participate in several pathways or tissues.
Which sex produces two types of sex-chromosome-bearing gametes in humans and birds, respectively?
(d) Male; female
Human males produce X-bearing and Y-bearing sperm. Female birds produce Z-bearing and W-bearing eggs.
What range of effects can mutations have?
(b) Neutral, harmful or beneficial effects
Mutation effects depend on the DNA change and its context. Some changes are neutral, while others are harmful or beneficial.
Assume each disease-causing genotype shows the disorder, with no new mutation. Unaffected parents have an affected daughter and son. Which inheritance pattern can account for this?
(c) Autosomal recessive inheritance
Unaffected parents can both carry an autosomal recessive allele. An affected child of either sex can inherit one recessive copy from each parent.
Which syndrome–karyotype pairing is correct?
(b) Down syndrome — trisomy 21
Down syndrome most often results from an extra chromosome 21.
Wrong
A dominant allele must be carried by most members of the population.
Why it feels right. I hear dominant as meaning numerically most prevalent.
Right
Dominance describes the phenotype of a heterozygote. Allele frequency counts copies in a population.
Worked example
