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.
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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.
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