MCAT Content / Mendelian Concepts / Recessiveness


Topic: Mendelian Concepts

Recessiveness is the expression in which the dominant allele masks the allele; the presence of a recessive gene cannot be determined by observation of the organism because the trait is hidden.

Mendel’s law of dominance states that in a heterozygote, one trait will hide the presence of another trait for the same characteristic. Rather than both alleles contributing to a phenotype, the dominant allele will be expressed completely. The recessive allele will be transmitted to offspring but will not be expressed when paired with a dominant allele. The recessive trait will only be expressed by offspring that have two copies of this allele. An example of this is the disease cystic fibrosis which is only caused when the recessive genotype is homozygous

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Key Points

• Dominant alleles are expressed exclusively in a phenotype, while recessive traits remain latent or hidden but are transmitted to offspring by the same manner in which the dominant allele is transferred.

• The trait corresponding to the hidden allele is the recessive trait. The recessive trait will only be expressed by offspring that have two copies of this allele.

Key Terms

Dominant: a relationship between alleles of a gene, in which one allele masks the expression (phenotype) of another allele at the same locus

Recessive: able to be covered up by a dominant trait

Heterozygote: when two alleles of a genotype are different

Allele: different types of the same gene on a chromosome

Gene: section of a chromosome that codes for a characteristic

Homozygous: of an organism in which both copies of a given gene have the same allele

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