1. Define homozygous and heterozygous. 5. Under what conditions does a female acquire an X-linked recessive disorder? 10. Describe briefly amniocentesis and its purpose. 21-5 Differentiate a chromosomal disorder from a single-gene disorder. (Autosomal recessive, autosomal dominant, and x-linked dominant and x-linked recessive disorders.) 21-6 a. At which stage of pregnancy does the highest risk or central nervous system damage occur? b. State 5 substances that are thought to be teratogenic. 21-8 Explain the meaning of trisomy 21..