An organism is diploid if it inherits two variants, or alleles, of each gene, one from each parent. These two alleles constitute the genotype for a given gene. The term genotype is also used to refer to an organism’s complete set of genes. A diploid organism with two identical alleles has a homozygous genotype, whereas two different alleles indicates a heterozygous genotype. Observable traits arising from genotypes are called phenotypes, which can also be influenced by environmental factors. An allele is dominant if only one copy is needed to manifest an associated phenotype and recessive if two copies are required for phenotypic expression.
Diploid organisms, including humans, most other animals, and many plants, have a duplicate set of chromosomes in somatic (non-sex) cells. These chromosome duplicates are homologous, with similar lengths, centromere positions, and gene locations. Diploid organisms inherit two versions of each gene, one from each parent. These two gene variants, or alleles, are situated at the same relative locus, or position, on two homologous chromosomes. Each chromosome contains many genetic loci, and there are often several possible alleles of a given gene.
The two alleles inherited by a diploid organism constitute its genotype at the locus. The term genotype also refers to an organism’s total set of genes. Different genotypes can result in distinct phenotypes, or observable characteristics (e.g., eye color). Phenotypes emerge as a result of genotypes, although multiple genotypes can cause the same phenotype. Phenotypes are also often influenced by environmental factors.
For a given gene, the two inherited alleles may have either identical or distinct nucleotide sequences. An organism with two identical alleles has a homozygous genotype (or is a homozygote). An organism with two different alleles has a heterozygous genotype (i.e., is a heterozygote). If the two alleles are different (heterozygous genotype), but only one affects the phenotype, the allele that influences the phenotype is dominant. The other allele, which does not affect the phenotype, is recessive.
Originally, eye color was thought to be determined by a single gene, and thus it is often used to illustrate genetic dominance (brown eyes) and recessiveness (blue eyes). However, scientists discovered at least eight genes that regulate eye color. Although the OCA2 gene is responsible for nearly three-fourths of the variation in the blue-brown eye color spectrum, other genes occasionally modify or override these effects. Nonetheless, eye color can be a useful example to illustrate dominant and recessive alleles.
In the following, we use the simplified eye color example to illustrate the relationship between genotype and phenotype. The allele for brown eyes is denoted B, and the allele for blue eyes is b. A heterozygote would have genotype Bb as a result of receiving a dominant allele from one parent and a recessive allele from the other. This individual would have brown eyes, the dominant phenotype. The dominant allele (B) overrides or “hides” instructions from the recessive allele for blue eyes. Thus, an individual with blue eyes would necessarily have the genotype bb, whereas an individual with brown eyes could have either genotype BB or Bb.
Although most traits are controlled by multiple genes rather than a single causal gene, these principles can be useful for predicting the probability of important outcomes. For example, Huntington disease (HD) is a progressive, neurodegenerative condition that causes uncontrolled movements and cognitive and emotional disturbances. HD is considered a single gene disorder because it is caused by a mutation in a single gene (HTT), although other genes do modify onset and progression. HD is caused by a dominant mutation, meaning that a single mutated allele leads to expression of the disease. If the parent has a normal and a mutated gene variant, the offspring has a 50% chance of receiving the mutated allele and developing the disease. HD also does not typically manifest until middle to late life. Thus, although the reality is unpleasant, a person with an affected parent can undergo genetic testing to determine if they have the causal allele and in turn inform their family planning.
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