Down's syndrome is caused by an extra copy of chromosome number 21. What percentage of offspring produced by an affected mother and a normal father would be affected by this disorder?
Correct answer: C. 50 %
- A. 100 %
- B. 75 %
- C. 50 %
- D. 25 %
Explanation
Down syndrome is caused by an extra copy of chromosome 21 (trisomy 21). When an affected mother (with three copies of chromosome 21) has a child with a normal father (with two copies of chromosome 21), there is a 50% chance that each offspring will inherit the extra chromosome from the mother, leading to Down syndrome, and a 50% chance that they will inherit the typical two copies of chromosome 21. The genetic transmission is not a simple Mendelian pattern, but this 50% probability reflects the likely outcome.
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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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