The Genetics of Biracial Characteristics

Human variety is immense.
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Race is an imprecise concept. All humans alive today belong to the species Homo sapiens sapiens and the characteristics attributed to “race” have varied historically with cultures and civilizations. Science divides up the study of race into many disciplines, including anthropology, sociology and genetics. The genetic characteristics of so-called biracial individuals often stem from a mix of various genes that together express traits such as skin color and eye shape.

Additive Polygenic Traits

Genes are the small portions of long deoxyribonucleic acid, or DNA, molecules that are located within a cell’s chromosomes. Genes code for all the proteins an individual will manufacture. Humans have 23 pairs of chromosomes, one set from each parent. This means that, except for a few sex-linked genes in men, you have two copies, or alleles, of each gene. Many human traits are polygenic: They arise from the complex interactions of several genes. Often, polygenic traits are additive -- the number of alleles you have for a given characteristic determines the extent to which the trait is expressed.

Single-Nucleotide Polymorphism

Significant variations in traits can often be traced to the mutation of a single nucleotide within a gene, an event that results in a single-nucleotide polymorphism (SNP). The sequence of nucleotides -- ringed molecules containing nitrogen -- within a gene determines the sequence of amino acids in the corresponding protein. An SNP can create a new protein if it is in a protein-coding area and if it results in a codon that encodes a different amino acid. Such a protein change may be evident in a person’s phenotype, or observable characteristics. For example, scientists study SNPs to trace the change in average skin color as humans migrated from Africa to northern climes. A “biracial” individual might have a particular pair of alleles that differ by one SNP.

Skin Color

Several genes are responsible for the amount of melanin found in a person’s skin cells. Melanin creates skin pigment and its quantity and distribution are a polygenic additive trait. Offspring of dark- and light-skinned parents often have skin tones of an intermediate color, reflecting a mix of genes that results in a medium amount of melanin production. However, the additive effect is not always evident, because some combinations of alleles may have dominant or environmentally sensitive interrelationships rather than additive ones.

Eye Fold

Individuals of Asian descent often have eye folds that give their eyes a slanted appearance. The eye fold is one of several traits under the control of a particular gene, making the gene “pleiotropic.” The fold is part of a package that includes differences to the shape of the nose bridge and the amount of fat stored in the eyelid. The offspring of parents with and without eye folds might have a full fold, a reduced fold or no fold at all. Again, this highlights the complexity of ascribing genetic characteristics to the notion of race.

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