Autosomes and Sex Chromosomes
In the figure above note that most of the chromosomes are paired (same length, centromere location, and banding pattern). These chromosomes are called autosomes. However note that two of the chromosomes, the X and the Y do not look alike. These are sex chromosomes. In humans males have one of each while females have two X chromosomes. Autosomes are those chromosomes present in the same number in males and females while sex chromosomes are those that are not. When sex chromosomes were first discovered their function was unknown and the name X was used to indicate this mystery. The next ones were named Y, then Z, and then W.
The combination of sex chromosomes within a species is associated with either male or female individuals. In mammals, fruit flies, and some flowering plants embryos, those with two X chromosomes develop into females while those with an X and a Y become males. In birds, moths, and butterflies males are ZZ and females are ZW. Because sex chromosomes have arisen multiple times during evolution the molecular mechanism(s) through which they determine sex differs among those organisms. For example, although humans and Drosophila both have X and Y sex chromosomes, they have different mechanisms for determining sex .
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In mammals, the sex chromosomes evolved just after the divergence of the monotreme lineage from the lineage that led to placental and marsupial mammals. Thus nearly every mammal species uses the same sex determination system. During embryogenesis the gonads will develop into either ovaries or testes. A gene present only on the Y chromosome called TDF encodes a protein that makes the gonads mature into testes. XX embryos do not have this gene and their gonads mature into ovaries instead (default). Once formed the testes produce sex hormones that direct the rest of the developing embryo to become male, while the ovaries make different sex hormones that promote female development. The testes and ovaries are also the organs where gametes (sperm or eggs) are produced.
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How do the sex chromosome behave during meiosis? Well, in those individuals with two of the same chromosome (i.e. homogametic sexes: XX females and ZZ males) the chromosomes pair and segregate during meiosis I the same as autosomes do. During meiosis in XY males or ZW females (heterogametic sexes) the sex chromosomes pair with each other (Figure (PageIndex{16})). In mammals the consequence of this is that all egg cells will carry an X chromosome while the sperm cells will carry either an X or a Y chromosome. Half of the offspring will receive two X chromosomes and become female while half will receive an X and a Y and become male.
Figure (PageIndex{16}): Meiosis in an XY mammal. The stages shown are anaphase I, anaphase II, and mature sperm. Note how half of the sperm contain Y chromosomes and half contain X chromosomes. (Original-Harrington-CC:AN)
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