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Biology LibreTexts

8.2: Human Traits Determined by Single Genes

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  • Each human somatic cell has 46 chromosomes in its nucleus. Each chromosome contains thousands of individual genes. There are thought to be about 20,000 genes in human DNA. Interestingly, the number of genes in human DNA is not appreciably different from the number of genes in chimpanzees or mice.

    Most traits are determined by more than one gene. For example, skin color and height are determined by many genes. Some phenotypes however, are determined by a single gene. We will explore some of these single gene traits in the laboratory.

    1. Use a lab partner to help you determine your phenotype for the traits listed.
    2. Complete the table. Use two alleles per trait for the genotype. If you exhibit the dominant phenotype, use a dash to represent the second allele.

    Example \(\PageIndex{1}\)

    B_    genotype for the phenotype of brown eyes (dash indicates second allele could be B or b which means a genotype of BB or Bb)
    bb    genotype for the phenotype of blue eyes

    • Interlocking fingers
      Interlock fingers. Observe which thumb is on top (right or left). The tendency to place the left thumb on top is due to a dominant allele (I) and the genotype is I- (either II or I-). The right thumb on top is determined by the ii genotype.
    • Ear lobes
      The dominant allele (E) results in the phenotype of free earlobes. The recessive allele (e) is for attached earlobes.
    • Widow’s peak
      Widow’s peak occurs when the hairline forms a distinct point in the center of the forehead. Widow’s peak is controlled by a dominant allele (W).
    • Tongue curling
      A dominant allele (T) gives the individual the ability to curl the tongue in a U-shape.
    • Hitch hiker’s thumb
      A person homozygous recessive for this trait (hh) can bend the last (distal) thumb joint back to about a 90 degree angle. Those with the H allele cannot.
    • Pigmented iris
      A person with the B allele has brown eyes. The recessive allele (b) encodes blue eyes.
    • PTC tasting
      If you can taste PTC, you have the dominant allele (P). Place the PTC paper on your tongue for a few seconds. If you cannot taste anything, you do not possess the dominant allele.
    TRAIT Describe Phenotype Genotype
    Interlocking fingers    
    Widow’s peak    
    Tongue curling    
    Hitch hiker’s thumb    
    Eye color    
    PTC tasting