Down's syndrome in humans is due to:
Correct answer: B. Three copies of chromosome 21
- A. Three 'X' chromosomes
- B. Three copies of chromosome 21
- C. Monosomy
- D. Two 'Y' chromosomes
Explanation
Down's syndrome is caused by the presence of an extra copy of chromosome 21, a condition known as Trisomy 21. It is characterized by distinct physical and developmental features due to this chromosomal abnormality. The extra chromosome results from a nondisjunction event during meiosis, where chromosomes fail to separate properly, leading to a gamete with an extra chromosome. This condition is different from having three X chromosomes (Triple X syndrome), which affects females, or having two Y chromosomes (47,XYY syndrome), which affects males. Monosomy, the presence of only one chromosome from a pair, is also unrelated to Down's syndrome, which is a trisomy condition.
Last updated
About Inheritance
Mendel's laws explain segregation, independent assortment and the prediction of genetic ratios using alleles, genotypes and phenotypes. The chapter also covers linked genes, recombination and crossing over during meiosis, then applies pedigree reasoning to X-linked recessive traits, which show different inheritance patterns in males and females.
Practise Inheritance
1,238 free Inheritance MCQs from Biology, each with the correct answer and an explanation. Unlimited attempts, no account needed.
Exams that ask Biology questions like this
Biology is on 6 papers prepared for on TestUstad, and all of them draw the same bank, so this question is worth knowing for every one of them.
Related questions
4 Mutations in opsin genes cause_ types of colorblindness.
9 : 3 : 3 : 1 ratio is given by:
A bald man marries to pattern bald woman. The chance of baldness in their daughter is:
A botanist sees that when he breeds a plant with blue flowers and a plant with red flowers, the resulting generation are plants with a 1:1 ratio of blue: red flowers. He knows that the two parents are homozygous for the trait of color. What phenomenon most likely explains the 1:1 ratio in the filial generation?
A carrier female for hemophilia can pass the defective allele to: