Kuliev and Verlinsky (2004) state that there was a relatively high number of separation errors at meiosis I. In these cases the centromere underwent a premature division, occurring at meiosis I rather than meiosis II. Regarding chromosome 21 what would you expect to be the chromosome 21 complement in the secondary oocyte in which you saw a single chromatid (monad) for chromosome 21 in the first polar body? If this secondary oocyte was involved in fertilization, what would be the expected consequences?

To begin, let's quickly review the basics of meiosis. Meiosis is a type of cell division that produces gametes (eggs and sperm) with half the normal number of chromosomes (haploid). Meiosis I is the first part of the process, where homologous chromosomes pair up, exchange DNA (crossing over), and then separate. Normally, the centromere separates during meiosis II, which results in the formation of four unique haploid gametes from a single germ cell. In the case where premature centromere separation occurs, we have separation errors at meiosis I. This means that chromatids can separate before meiosis II, which can lead to chromosome 21 abnormalities.

To find the chromosome 21 complement in the secondary oocyte, we need to consider the consequences of a separation error at meiosis I. In the question, we are told that the first polar body contains a single chromatid (monad) for chromosome 21. The secondary oocyte must have received the other chromatid for chromosome 21.

Now let's analyze the potential consequences if this secondary oocyte were to undergo fertilization. After fertilization, the secondary oocyte would combine with a sperm cell containing a single copy of chromosome 21. If the secondary oocyte is normal (having 1 chromatid of chromosome 21), the resulting zygote would have two chromatids for chromosome 21, which is the normal complement. However, if the secondary oocyte has an abnormal complement due to the separation error at meiosis I, it will combine with the sperm and result in a situation where we would have a higher risk for trisomy 21 (Down syndrome) or monosomy 21 (extremely rare and usually not compatible with life). Trisomy occurs when a zygote has three chromatids for chromosome 21 instead of two, and monosomy occurs when a zygote has only one chromatid for chromosome 21. In conclusion, the chromosome 21 complement in the secondary oocyte can be estimated based on the presence of a monad in the first polar body, and the consequences of fertilization with this oocyte will have a high risk to result in chromosomal abnormalities such as trisomy 21 or monosomy 21.