Complete the following practice problems. There are six steps to completing a monohybrid cross.

  1. Determine the dominant and recessive forms of the allele for the trait being analyzed, and then determine what letters to use to represent the genes.
  2. Determine the genotypes of the parents, and write down the cross.
  3. Determine the alleles of the gametes that would be produced during meiosis.
  4. Draw and set up the Punnett square. Put the genotype of the gametes produced by both parents on the top and left sides of the table.
  5. Fill in the middle of the Punnett square by combining the gametes.
  6. Analyze the results, and determine the genotypes and phenotypes of the offspring.

image of a brown and tan mouse

Source: Brown and tan mice, Scientific American

  1. In mice, brown coat color is dominant to tan. Cross a heterozygous mouse with one that is homozygous recessive.

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    Dominant allele: B=brown
    Recessive allele: b=tan

    Cross: Bb x bb Close Pop Up

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    Dominant allele: B=brown
    Recessive allele: b=tan

    Cross: Bb x bb

    Genotypes:
    50% heterozygous
    50% homozygous recessive

    Phenotypes:
    50% brown
    50% tan
    Close Pop Up
    2. image of radishes that have just been picked

    Source: Radish Roots, Crop Genebank Knowledge base

  2. In radishes, a bent root is the dominant allele while a straight root is recessive. Cross a straight root with a homozygous dominant root.

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    Dominant allele: B=bent
    Recessive allele: b=straight

    The only way to have a straight root is to have the homozygous recessive genotype.

    Cross: BB x bb Close Pop Up

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    Dominant allele: B=bent
    Recessive allele: b=straight

    Cross: BB x bb

    Genotypes:
    100% heterozygous

    Phenotypes:
    100% bent roots
    Close Pop Up
    3. image of a brown and tan mouse

    Source: Hairline shape, groovy post

  3. In humans, a widows peak allele is dominant and a straight hairline allele is recessive. Cross two heterozygous individuals for hairline shape.

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    Dominant allele: W=widows peak
    Recessive allele: w=straight

    Cross: Ww x Ww Close Pop Up

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    Dominant allele: W=widows peak
    Recessive allele: w=straight

    Cross: Ww x Ww

    Genotypes:
    25% homozygous dominant
    50% heterozygous
    25% homozygous recessive

    Phenotypes:
    75% widows peak
    25% straight
    Close Pop Up

    4. Source: earlobe attachment, Human anatomy for the artists

  4. In humans, earlobes are either attached or free. Free earlobes are the dominant allele. Predict the genotypes and phenotypes of two people with attached earlobes.

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    Dominant allele: F=free ear loves
    Recessive allele: f=attached

    If a person has attached earlobes,
    the genotype is homozygous recessive.

    Cross: ff x ff Close Pop Up

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    Dominant allele: F=free earlobes
    Recessive allele: f=attached earlobes

    Cross: ff x ff

    Genotypes:
    100% homozygous recessive

    Phenotypes:
    100% attached earlobes
    Close Pop Up
  5. In humans, the allele for no dimples is recessive while having dimples is dominant. Predict the offspring from the cross of a male with no dimples and a female with dimples whose mother did not have dimples.

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    Dominant allele: D=dimples
    Recessive allele: d=no dimples

    The male must have the homozygous recessive genotype, dd, as he does not have dimples. The female has dimples so she must have the homozygous dominant, DD, or the heterozygous genotype, Dd. The problem states that her mother did not have dimples, so she must have inherited a recessive allele, d, from her mother. The female must have the heterozygous genotype.

    Cross: dd x Dd Close Pop Up

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    Check Your Answer

    Dominant allele: D=dimples
    Recessive allele: d=no dimples

    Cross: dd x Dd

    Genotypes:
    50% heterozygous
    50% homozygous recessive

    Phenotypes:
    50% dimples
    50% no dimples
    Close Pop Up