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Biology LibreTexts

8.1: Human Genetics - Terms and Concepts

  • Page ID
  • Chromosome One of 46 strands of DNA in a human nucleus.

    A unit of heredity located on a chromosome. Example: eye color gene

    Allele An alternate form of a gene. Example: blue eyes
    Homologous Chromosome Homologous chromosomes contain the same genes at the same location; however, the alleles may differ.
    Dominant allele The dominant allele masks the recessive allele. Example: people with dominant allele, B, make brown eye pigment. Both genotypes BB and Bb give brown eyes. Use an uppercase letter for the dominant allele.
    Recessive allele The recessive allele is masked by the dominant allele. People  bb have blue eyes. Use a lowercase letter for the recessive allele.
    Homozygous The two alleles on homologous chromosomes are the same Example: BB = two brown eye alleles bb = two blue eye alleles
    Heterozygous The alleles on homologous chromosomes are different. Ex: Bb
    Genotype The genetic makeup of an individual. Example: BB, Bb, or bb
    Phenotype The appearance of the individual Ex: brown eyes, blood
    type A
    Punnett Square

    Diagram used to illustrate transmission of traits through generations


    Chromosome pairs 1 – 22 are not different in males and females

    Sex Chromosomes Sex chromosome pairs are shown last in the karyotype. Females possess 2 large X chromosomes while males possess one X chromosome and a smaller Y chromosome.