Passage of information from parents to offspring — practice question

At times during meiosis, homologous chromosomes do not separate at anaphase; this is called non-disjunction. Turner’s syndrome is the most common chromosome mutation in females. It may arise because non-disjunction occurs in meiosis during gametogenesis. Some of the gametes formed will therefore lack an X chromosome. In some cases of Turner’s syndrome, one of the pair of X chromosomes is not absent but has been damaged. The damaged X chromosome may have broken and then rejoined so that part of its structure is lost. Fig. 7.1 shows a normal X chromosome and two types of ‘damaged’ X chromosomes, $X_1$ and $X_2$: • In $X_1$, part of the ‘p’ arm of the chromosome is absent. This deletion causes reduced height in the female and defects such as narrowing of the aorta. • In $X_2$, part of the ‘q’ arm of the chromosome is absent. This deletion causes little or no development of the ovaries.