Which of the following could most often account for aneuploidy (Turner’s Syndrome, Klinefelter’s Syndrome) or trisomy (2n + 1) conditions (Trisomy-21/Downs Syndrome, etc) in sexually produced offspring?
a) crossing-over/recombination between homologous chromosomes in prophase I
b) nondisjunction of homologous chromosomes during the first meiosis (meiosis I) cell cycle
c) nondisjunction of sister chromatids during the first meiotic (meiosis I) cell cycle
d) random fertilization
The correct answer and explanation is:
The correct answer is b) nondisjunction of homologous chromosomes during the first meiosis (meiosis I) cell cycle.
Nondisjunction is a process where chromosomes fail to separate properly during cell division, leading to an abnormal number of chromosomes in the resulting cells. In meiosis, which is the process that produces gametes (sperm and eggs), chromosomes are supposed to be divided evenly between two daughter cells. If nondisjunction occurs during meiosis I, the homologous chromosomes (chromosomes that are similar in size, shape, and genetic content) do not separate correctly. This results in one gamete receiving both chromosomes from a pair, while the other gamete receives none.
When the fertilized egg (zygote) forms, the extra chromosome or missing chromosome causes an aneuploidy condition, which is a deviation from the normal chromosome number. For example, Turner’s Syndrome (monosomy X) occurs when a female inherits only one X chromosome instead of two, leading to the absence of one sex chromosome. Klinefelter’s Syndrome (XXY) occurs when a male inherits an extra X chromosome, leading to three sex chromosomes.
Similarly, trisomy conditions like Down Syndrome (trisomy 21) occur when an individual has an extra chromosome in a set. In trisomy 21, the individual has three copies of chromosome 21 instead of the usual two. These conditions are usually the result of nondisjunction during meiosis I, although nondisjunction can also occur in meiosis II, where sister chromatids fail to separate, but the rate of occurrence is less common than in meiosis I.
Crossing-over (a) is a normal process during meiosis that helps to increase genetic variation, but it does not cause aneuploidy. Random fertilization (d) simply refers to the random combination of sperm and egg, but it does not directly cause chromosomal abnormalities.