Down 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?
Down 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?
The correct option is C 50%
Down syndrome is caused by an extra copy of chromosome 21. Factors such as the mother’s age, and commonly, just a matter of randomness, cause it. In this case, it is given that the mother is affected. A normal woman’s eggs should all have 22 autosomes and one X chromosome. In this case, since the mother is affected by Down syndrome, the eggs she produces, would be two types, of which one type would be normal, with 22 autosomes and one X chromosome, while the other would have 23 autosomes and an X chromosome. Each of the sperms would have 22 autosomes and either an X or a Y chromosome since the father is normal. If either of these sperm types happens to fertilise the normal egg, the progeny would be normal, with the genotypes 44AA+XX or 44AA+XY for male and female offsprings respectively. However, if it fertilises the affected egg, the progeny would end up being affected. The genotypes of the affected progeny would be 45AA+XX (in case of a female) or 45AA+XY (in case of a male). Considering the four possible genotypes of the progeny (44AA+XX, 44AA+XY, 45AA+XX, 45AA+XY), the probability of offspring being affected by the disorder would be would be 2 out of 4, which is 50%.
Down syndrome is caused by an extra copy of chromosome 21. Factors such as the mother’s age, and commonly, just a matter of randomness, cause it. In this case, it is given that the mother is affected. A normal woman’s eggs should all have 22 autosomes and one X chromosome. In this case, since the mother is affected by Down syndrome, the eggs she produces, would be two types, of which one type would be normal, with 22 autosomes and one X chromosome, while the other would have 23 autosomes and an X chromosome. Each of the sperms would have 22 autosomes and either an X or a Y chromosome since the father is normal. If either of these sperm types happens to fertilise the normal egg, the progeny would be normal, with the genotypes 44AA+XX or 44AA+XY for male and female offsprings respectively. However, if it fertilises the affected egg, the progeny would end up being affected. The genotypes of the affected progeny would be 45AA+XX (in case of a female) or 45AA+XY (in case of a male). Considering the four possible genotypes of the progeny (44AA+XX, 44AA+XY, 45AA+XX, 45AA+XY), the probability of offspring being affected by the disorder would be would be 2 out of 4, which is 50%.
