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Alleles and Punnett Squares

Genes code for your traits, every human has the same type of traits, for example, everyone has eyes, ears, and skin.  Yet, not everyone has the same eyes, ears, and skin. Furthermore, parents don't look alike (different versions of those genes) and they pass on the traits to their kids.  Sorting through all of these genes will require a little vocabulary.

  • Alleles are different forms of the same type of gene

    • ex: genes that cause brown eyes and blue eyes ​are both genes for eyes

    • Homozygous (homo-same, zygous - zygote) refers to individuals that have two of the same alleles (two blue eye genes "bb", or two brown eye genes "BB")

    • Heterozygous (hetero- other) refers to individuals that have different alleles (Bb)

  • Dominant alleles code for the trait that shows up if you one of each

    • If you are Bb, your eyes will be brown; brown is dominant​

    • Dominant alleles are shown by a Capital Letter

  • Recessive ​alleles can only be shown if they are the only type of gene present

    • Individuals that have the genes, "bb" will have blue eyes​​​

    • Recessive alleles are shown by a lower case letter

  • Genotypes are the genes that you have: BB, Bb, or bb​

    • when asked for a genotype, write letters​

  • Phenotypes are the traits that are caused by the genes:  brown eyes or blue eyes

    • when asked for a phenotype, write words​

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Punnett Squares

Punnett squares show the probability that offspring will inherit and/or show traits from their parents.  They are a simple way of expressing both Meiosis and Fertilization.

  • Alleles (letters) on the outside of the square represent possible gametes

    • Gametes are haploid, they only have one allele each​

  • The inner squares represent potential zygotes​

    • Zygotes are diploid, they have two alleles:  one from each parent​

  • The potential offspring are the "results" of a Punnett square.  The same result can be expressed in different ways:

    • Ratio - 3:1 means out of the 4 possible zygotes, 3 should be dominant and 1 should be recessive​

    • Fraction - 3/4 of the offspring should show the dominant trait, 1/4 should show the recessive trait

    • Percentage - 75% of the offspring should be dominant, 25% should be recessive

    • Remember, Punnett squares show potential offspring; they do not predict the future, only show probability

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Dihybrid Punnett squares show the probability of passing on combinations of two traits.  

  • Outside boxes still represent haploid gametes, but now they have one allele for each trait

    • They should NOT have two of the same letter​

  • Inside boxes still represent potential offspring, but now there are 16 possible combinations.​

Punnett Squares.png
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