Final answer:
Parthenogenesis is a type of uniparental reproduction where offspring are developed from an unfertilized gamete, usually resulting in female progeny, as seen in whiptail lizards. In humans, genetic variation is achieved via meiosis I through crossover events and random assortment of chromosomes. Uniparental disomy arises from meiotic nondisjunction, leading to genetic disorders like Down syndrome.
Step-by-step explanation:
Parthenogenesis is a form of uniparental reproduction that occurs in some insects and a few vertebrates where progeny develop from a gamete without fertilization. Notably, this method allows for reproductive success in the absence of a mate, ensuring genetic propagation. Typically, only female progeny are produced in species such as whiptail lizards, due to the nutrients stored in eggs. Meanwhile, in haplodiploid systems such as some insect species, unfertilized eggs can develop into new male offspring, with females being diploid and males haploid.
In the context of human reproduction, each parent contributes 23 chromosomes to form a diploid organism through sexual reproduction. The process of meiosis I is crucial for generating genetic diversity. During this stage, homologous chromosomes undergo crossover events and randomly assort at the metaphase plate, leading to unique combinations of maternal and paternal chromosomes in the gametes.
Uniparental disomy (UPD) occurs when both copies of a chromosome are inherited from one parent. This can happen because of nondisjunction during meiosis, leading to aneuploidy, which can have serious consequences such as genetic disorders. For instance, Down syndrome is often the result of trisomy 21, where three copies of chromosome 21 are present due to nondisjunction.