Sample 1Easy
Down's syndrome in humans is caused by
- A
trisomy of chromosome 21
- B
the karyotype XO
- C
the karyotype XXY
- D
a single base substitution in the beta globin gene
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The answer is A.Down's syndrome is caused by the presence of an additional copy of chromosome number 21, that is, trisomy of 21. It was first described by Langdon Down in 1866.
- B.
XO (45 chromosomes) causes Turner's syndrome.
- C.
XXY (47 chromosomes) causes Klinefelter's syndrome.
- D.
A single base substitution in the beta globin gene causes sickle-cell anaemia.
NCERT: Class 12 Biology, Chapter 4 (Principles of Inheritance and Variation), Genetic Disorders, Chromosomal Disorders
Sample 2Medium
A pedigree shows, in generation I, an unaffected man (open square) married to an unaffected woman (open circle). In generation II they have an affected daughter (shaded circle), an unaffected son and an unaffected daughter. The trait could be phenylketonuria. What does the pedigree show?
- A
Autosomal dominant inheritance
- B
Autosomal recessive inheritance, with both parents heterozygous carriers
- C
X-linked recessive inheritance with a carrier father
- D
Y-linked inheritance
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The answer is B.An affected child of two unaffected parents means the trait is recessive, and an affected daughter of an unaffected father rules out X-linkage. Phenylketonuria is an autosomal recessive disorder.
- A.
A dominant trait cannot appear in a child of two unaffected parents.
- C.
A daughter affected by an X-linked recessive trait needs an affected father; males cannot be carriers.
- D.
A Y-linked trait cannot appear in a daughter.
NCERT: Class 12 Biology, Chapter 4, 4.8