• Chromosomes are the primary carrier of genetic information in organisms (though some organisms contain extrachromosomal DNA).
• In eukaryotes (organisms whose cells have a distinct nucleus), chromosomes are threadlike/rod-shaped. In prokaryotes (single-celled organisms without a distinct nucleus), they are typically ring-shaped.
• Chromosomes are numbered in terms of size with the first being the largest and the last being the smallest.
• While other hominids have 24 pairs of chromosomes, humans have 23 pairs. This is because our second largest chromosome was two separate chromosomes in our ancestors.
• The chromosomes of the first 22 pairs are called “autosomes” because they have the same form as their accompanying chromosome.
• The chromosomes of the last pair are called “allosomes” because they need not have the same form as their accompanying chromosome.
• Having an abnormal number of chromosomes is called “aneuploidy.” Lacking an autosomal chromosome is incompatible with life in nearly all cases. Having an extra autosomal chromosome is incompatible with life in the vast majority of cases, but there are more exceptions.
• Lacking an X chromosome is always incompatible with life. All other combinations of X and Y chromosomes (including having only one allosomal chromosome, so long as it is an X chromosome) are compatible with life.
• A human’s allosomal genotype (e.g., XX, XY, XXY, XXYY) does not fully determine their phenotype (i.e., how their body looks). This means that people judged to be males are not necessarily XY, and people judged to be females are not necessarily XX.
• Many species do not have X or Y chromosomes, with sex determination resulting from entirely different chromosome pairings.
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