During oogenesis in an animal species with a haploid number of 6, one dyad undergoes nondisjunction during meiosis II. Following the second meiotic division, this dyad ends up intact in the ovum. How many chromosomes are present in (a) the mature ovum and (b) the second polar body? (c) Following fertilization by a normal sperm, what chromosome condition is created?

Contrast spermatogenesis and oogenesis. What is the significance of the formation of polar bodies?

Wolves and coyotes can interbreed in captivity; and now, because of changes in their habitat distribution, they may have the opportunity to interbreed in the wild. To examine this possibility, mitochondrial DNA from wolf and coyote populations throughout North America—including habitats where the two species both reside—was analyzed, and a phylogenetic tree was constructed from the resulting data (see Section 1.4 for details on how this is accomplished). Sequence from a jackal was used as an outgroup and a sequence from a domestic dog was included, demonstrating wolves as the origin of domestic dogs. What do you conclude about the possibility that interspecific hybridization occurred between wolves and coyotes on the basis of this phylogenetic tree?

If one follows 50 primary oocytes in an animal through their various stages of oogenesis, how many secondary oocytes would be formed? How many first polar bodies would be formed? How many ootids would be formed? If one follows 50 primary spermatocytes in an animal through their various stages of spermatogenesis, how many secondary spermatocytes would be formed? How many spermatids would be formed?

An interesting procedure has been applied for assessing the chromosomal balance of potential secondary oocytes for use in human in vitro fertilization. Using fluorescence in situ hybridization (FISH), Kuliev and Verlinsky (2004) were able to identify individual chromosomes in first polar bodies and thereby infer the chromosomal makeup of 'sister' oocytes. Assume that when examining a first polar body you saw that it had one copy (dyad) of each chromosome but two dyads of chromosome 21. What would you expect to be the chromosomal 21 complement in the secondary oocyte? What consequences are likely in the resulting zygote, if the secondary oocyte was fertilized?