Meiosis and fertilization

Sexual reproduction is the union of two gametes, one from each parent. The sperm comes from the father while the egg from the mother. Each gamete has a haploid chromosome number, that is, half of the total number. This is due to meiosis, where one cell (that contains a chromosome having 2 strands) divides two times. The first division occurs to a duplication of the cell, resulting to two cells having the same chromosome. The second division further divides these cells, but this time the chromosome is split, leaving one strand of each on the gametes.

Fertilization then takes place, and the single stranded chromosomes from each gamete fuses together, creating an offspring that has combined features from both parents, and not genetically similar to anyone of them (Campbell and Reece, 2005). One example of an inherited disease is Ichthyosis. It is a disorder resulting from a mutation in the X chromosome which renders the patient unable to correctly synthesize an important enzyme, steroid sulfatase. As such, the mutated enzyme is unable to break down cholesterol sulfate, causing the skin of the patient to clump and form scales.

Since the disease is caused by a disorder in the X chromosome, it is passed on through the offspring, whether male or female. A female, having an XX chromosome, can easily mask this disease since one of its X chromosome is surely safe coming from the mother. A male, however, being XY in chromosome composition, cannot mask the disorder brought about by the deficient X chromosome (Campbell and Recce 2005). References: Neil A. Campbell, Jane B. Reece. 2005. Biology. New Jersey: Pearson, Benjamin Cummings.

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