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  1. Gene: The sequence of nucleotides (usually DNA, bacteria is often RNA) that encodes a functioning protein or RNA sequence.  
  2. Allele: One of two or more alternative states of a gene.
  3. Dominance: One allele is said to be dominant with respect to an alternative allele if a heterozygous individual bearing the two alleles is indistinguishable from an individual homozygous for the dominant allele; the alternative not detected in the heterozygote is said to be recessive with respect to the dominant allele.  An allele that is dominant with respect to one alternative allele may be recessive with respect to another.
  4. Incomplete dominance: The ability of two alleles to produce a heterozygous phenotype that is different from either homozygous phenotype.
  5. Mitosis: Nuclear division in which the duplicated chromosomes separate to form two genetically identical daughter nuclei.
  6. Meiosis: Two successive nuclear divisions in which a single diploid cell forms four haploid nuclei, having the same chromosome number.
  7. Heterozygous: A diploid individual that carries two different alleles on homologous chromosomes at one of more genetic loci.
  8. Homozygous: A diploid individual that carries identical alleles at one or more genetic loci on its two homologous chromosomes. 
  9. Hybrid: An individual which is heterozygous for one or more alleles.
  10. Monohybrid cross: The study of the results of crossing only one characteristic. 
  11. Dihybrid cross: A cross that investigates two characteristics.  
  12. Sex-linked trait: Certain recessive inherited traits are more common in males than females.  For example, hemophilia, colour blindness, and myopia.  In a male, for example, if an allele, say h, on the X chromosome has no corresponding partner on the Y chromosome, that single recessive gene will be expresssed.