Applying Mendel’s Principles

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Gene Noun

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[jeen]

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Gene

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A unit of heredity passed from parents to offspring, which determines a specific biological characteristic of the organism.

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Example: This diagram shows that a gene is a specific section of a DNA molecule, which is tightly coiled into a chromosome found inside a cell's nucleus.

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Allele Noun

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[uh-leel]

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Allele

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One of two or more alternative forms of a gene that are found at the same place on a chromosome.

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Example: This diagram shows that an allele is a version of a gene, like 'A' or 'a', found at a specific spot on a chromosome.

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Dominant Allele Noun

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[dom-uh-nuhnt uh-leel]

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Dominant Allele

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An allele that expresses its phenotypic effect even when paired with a different allele for the same trait.

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Example: This diagram shows a dominant allele (A) paired with a recessive allele (a) in the central 'Heterozygous' example, illustrating how dominance is expressed.

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Recessive Allele Noun

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[ri-ses-iv uh-leel]

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Recessive Allele

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An allele that only expresses its phenotypic effect when two identical copies are present in an organism's genotype.

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Example: This diagram shows a recessive allele (labeled 'a') on a chromosome. It illustrates that for a recessive trait to be expressed, an individual must have two copies of this allele ('aa').

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Genotype Noun

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[jee-noh-tahyp]

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Genotype

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The specific combination of alleles an organism possesses for a particular gene, representing its underlying genetic makeup.

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Example: This image shows how the combination of alleles (like B and b) on a pair of chromosomes determines an organism's genotype (e.g., BB, bb, or Bb).

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Phenotype Noun

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[fee-noh-tahyp]

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Phenotype

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The set of observable physical or biochemical characteristics of an organism, resulting from its genotype and environment.

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Example: This pedigree chart shows how an observable trait (phenotype), like a genetic condition, is expressed (red shapes) and passed down through a family's generations.

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Homozygous Adjective

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[hoh-moh-zahy-guhs]

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Homozygous

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Describes an organism that has two identical alleles for a particular gene, such as GG or gg.

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Example: This diagram shows homozygous genotypes, where an individual has two identical alleles for a gene (like BB or bb) on a pair of chromosomes.

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Heterozygous Adjective

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[het-er-oh-zahy-guhs]

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Heterozygous

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Describes an organism that has two different alleles for a particular gene, such as Gg.

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Example: This diagram shows a pair of homologous chromosomes with different alleles for a gene (B and b), which is the definition of a heterozygous genotype.

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Probability Noun

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[prob-uh-bil-i-tee]

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Probability

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The mathematical likelihood that a particular event will occur, used in genetics to predict the outcomes of crosses.

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Example: This Punnett square shows how parent alleles (P, p) combine, demonstrating the probability of offspring genotypes like homozygous dominant (PP), heterozygous (Pp), or homozygous recessive (pp).

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Gamete Noun

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[gam-eet]

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Gamete

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A specialized reproductive cell, such as a sperm or egg, that contains half the number of chromosomes.

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Example: This diagram shows that gametes (sperm and egg) are haploid cells (n), which combine during fertilization to create a diploid (2n) fertilized egg.