Question: Describe the process in meiosis that ensures that both maternal and paternal chromosomes are passed on to each spermatozoon.
One of the processes in meiosis that ensures that both maternal and paternal chromosomes are passed on to each spermatozoon is **crossing over**. Crossing over is the exchange of DNA segments between homologous chromosomes, which are the pairs of chromosomes that have the same genes but may have different alleles. Crossing over occurs during prophase I of meiosis, when the homologous chromosomes pair up and form tetrads. A tetrad is a group of four chromatids, each consisting of a duplicated chromosome. During crossing over, a segment of one chromatid breaks and reattaches to the corresponding segment of another chromatid in the same tetrad. This results in a **recombinant chromatid**, which has a mix of maternal and paternal DNA. When the tetrads separate during anaphase I and the chromatids separate during anaphase II, each spermatozoon will receive one recombinant chromatid from each pair of homologous chromosomes. This means that each spermatozoon will have a unique combination of maternal and paternal genes that did not exist before the crossover.
: A single crossover event between homologous nonsister chromatids leads to a reciprocal exchange of equivalent DNA between an egg-derived chromosome and a sperm-derived chromosome. When a recombinant sister chromatid is moved into a gamete cell it will carry a combination of maternal and paternal genes that did not exist before the crossover.
