In humans, if a non-disjunction event led to an individual with a genotype of XO, they would:

• A. be phenotypically male with normal development
• B. have Turner syndrome
• C. have Down syndrome
• D. have Klinefelter syndrome

Answer: (B)

Verified step by step guidance

1. Understand the meaning of the genotype XO: it indicates the presence of only one X chromosome and no second sex chromosome (no Y chromosome).
2. Recall that Turner syndrome is a condition where an individual has a single X chromosome (XO) instead of two sex chromosomes (XX or XY).
3. Recognize that Down syndrome is caused by trisomy of chromosome 21, which is unrelated to sex chromosomes and the XO genotype.
4. Know that Klinefelter syndrome involves the presence of an extra X chromosome in males (XXY), which is different from XO.
5. Conclude that an individual with the XO genotype would have Turner syndrome, characterized by specific physical and developmental features.