Class 12 Principles of Inheritance and Variation Mock Test (CBSE)

Topics in Principles of Inheritance and Variation

• Mendel's laws of inheritance
• Inheritance of one gene
• Incomplete dominance and co-dominance
• Multiple alleles
• Inheritance of two genes
• Chromosomal theory of inheritance
• Linkage and recombination
• Polygenic inheritance
• Pleiotropy
• Sex determination
• Mutation
• Pedigree analysis
• Mendelian disorders
• Chromosomal disorders

Principles of Inheritance and Variation practice questions with answers

1. Q1Multiple choice·1 mark
   In a human pedigree chart, what does a shaded square symbol represent?

   1. (a)Unspecified sex and affected
   2. (b)Unaffected female
   3. (c)Affected male
   4. (d)Affected female
   Show answer and step marks

   1. Correct option: (c) Affected male+1

   Answer: (c)
2. Q2Multiple choice·1 mark
   A point mutation involving a single base pair change that results in the replacement of glutamic acid by valine in the $\beta$-globin chain of haemoglobin is responsible for:

   1. (a)Down's syndrome
   2. (b)Sickle-cell anaemia
   3. (c)Phenylketonuria
   4. (d)Thalassemia
   Show answer and step marks

   1. Correct option: (b) Sickle-cell anaemia+1

   Answer: (b)
3. Q3Short answer·2 marks
   A child is diagnosed with phenylketonuria, an inborn error of metabolism. State the genetic cause of this disorder and mention two symptoms observed in affected individuals.
   Show answer and step marks

   1. Genetic cause: an autosomal recessive disorder; the affected person is homozygous for a mutated gene coding for the enzyme phenylalanine hydroxylase, so phenylalanine is not converted to tyrosine and accumulates (as phenylalanine and phenylpyruvic acid).+1
   2. First symptom, any one of: mental retardation, as phenylalanine and its derivatives accumulate in the brain; reduced pigmentation of hair and skin, as less tyrosine is available to make melanin; phenylpyruvic acid and other derivatives are excreted in the urine; or any other valid symptom.+0.5
   3. Second symptom, a different one from the same list.+0.5

   Answer: Autosomal recessive mutation (homozygous recessive) in the gene for phenylalanine hydroxylase, so phenylalanine is not converted to tyrosine and accumulates. Any two symptoms of: mental retardation; reduced hair and skin pigmentation; phenylpyruvic acid excreted in urine.
4. Q4Short answer·3 marks
   State Mendel's Law of Segregation. Explain this law with the help of a monohybrid cross between pure tall and pure dwarf pea plants up to the $F_2$ generation.
   Show answer and step marks

   1. Statement of Law of Segregation: the two alleles (factors) of a pair do not blend; they segregate during gamete formation so that each gamete receives only one of the two.+1
   2. Cross: pure tall ($TT$) $\times$ pure dwarf ($tt$) gives an $F_1$ generation that is all tall ($Tt$).+0.5
   3. $F_1$ plants ($Tt$) form $T$ and $t$ gametes in equal proportion; on selfing these recombine, so tall and dwarf plants both appear in $F_2$.+0.5
   4. $F_2$ phenotypic ratio: $3$ tall $: 1$ dwarf.+0.5
   5. $F_2$ genotypic ratio: $1\,TT : 2\,Tt : 1\,tt$.+0.5

   Answer: Law of Segregation: the two alleles of a pair segregate during gamete formation, so each gamete gets only one. $TT$ (tall) $\times$ $tt$ (dwarf) gives all tall $Tt$ in $F_1$; $F_1$ forms $T$ and $t$ gametes equally, and selfing gives $F_2$ in the phenotypic ratio $3$ tall $: 1$ dwarf and genotypic ratio $1\,TT : 2\,Tt : 1\,tt$.
5. Q5Short answer·3 marks
   Name the chromosomal disorder caused by the presence of an additional copy of chromosome 21. State any two characteristic phenotypic symptoms of individuals suffering from this disorder.
   Show answer and step marks

   1. Naming the disorder: Down's syndrome (Trisomy of chromosome 21).+1
   2. First symptom, any one of: short stature; small, round head; furrowed tongue; partially open mouth; broad palm with a characteristic palm crease; retarded physical, psychomotor and mental development; or any other valid symptom.+1
   3. Second symptom, a different one from the same list.+1

   Answer: Down's syndrome (trisomy 21). Any two of: short stature; small round head; furrowed tongue; partially open mouth; broad palm with a characteristic palm crease; retarded physical, psychomotor and mental development.

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