12.1
Genetic Lingo
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 gi…
A gene is a segment of DNA located on a chromosome that carries genetic information. Alleles are different forms of the same gene.
In diploid organisms, one allele for each gene is inherited from each parent. The specific location of a gene or allele on a chromosome is called a locus. The two inherited alleles are located at the same locus on a pair of homologous chromosomes, with one chromosome inherited from the mother and the other from the father.
This pair of alleles is called the genotype for that gene. The observable trait, or phenotype, such as eye color, is influenced by this inherited combination.
Several genes influence human eye color. But for simplicity, consider a one-gene model with two alleles, brown and blue, represented by uppercase B and lowercase b, respectively, to explain how genotype relates to phenotype.
If both alleles are identical, such as two brown alleles or two blue alleles, the genotype is called homozygous. On the other hand, if the alleles are different, such as one brown allele and one blue allele, the genotype is called heterozygous.
The same phenotype, such as brown eyes, can result from different genotypes. For example, both, a homozygous brown genotype and a heterozygous genotype, can produce brown eyes.
This happens because some alleles are dominant, meaning they are expressed even when only one copy is present. Other alleles are recessive, meaning they are expressed only when two copies of the recessive allele are present.
In this example, the brown allele is dominant and is written with an uppercase B, while the blue allele is recessive and is written with a lowercase b. So, both the homozygous brown genotype and the heterozygous genotype result in brown eyes.
The recessive allele affects the phenotype only in a homozygous recessive genotype. So, blue eyes appear only when both alleles are recessive.
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Q1: What is the difference between genotype and phenotype?
Genotype refers to the pair of alleles an organism inherits for a particular gene, while phenotype is the observable physical expression of those alleles, such as eye color. A diploid organism receives one allele from each parent at the same locus on homologous chromosomes. The phenotype results from the genotype, though multiple genotypes can produce the same phenotype, and environmental factors may also influence phenotypic expression.
Q2: What does it mean for an allele to be dominant or recessive?
A dominant allele requires only one copy to be expressed in the phenotype, while a recessive allele requires two copies for expression. When an organism inherits two different alleles, the dominant allele masks or overrides the recessive allele's instructions. For example, the brown eye allele is dominant over the blue eye allele, so an individual with one brown and one blue allele will have brown eyes.
Q3: What is a homozygous versus heterozygous genotype?
A homozygous genotype occurs when an organism inherits two identical alleles for a gene, while a heterozygous genotype results from inheriting two different alleles. For instance, an individual with two brown eye alleles (BB) is homozygous, whereas an individual with one brown and one blue allele (Bb) is heterozygous. Both genotypes are possible for the same phenotype when the dominant allele is present.
Q4: How do alleles relate to inheritance from parents?
Diploid organisms inherit two alleles for each gene—one from each parent—located at the same locus on homologous chromosomes. These two inherited alleles constitute the genotype for that gene. Since each parent contributes one allele, offspring can inherit different combinations of alleles, resulting in variation in genotypes and phenotypes across a population.
Q5: Can the same phenotype result from different genotypes?
Yes, multiple genotypes can produce the same phenotype. For example, both homozygous (BB) and heterozygous (Bb) individuals express brown eyes because the dominant brown allele is present in both cases. Only individuals with the homozygous recessive genotype (bb) express blue eyes. This demonstrates why phenotype alone cannot always determine an organism's exact genotype.
Q6: How does genetic dominance apply to disease inheritance?
Dominant mutations cause disease expression with just one mutated allele, while recessive diseases require two mutated copies. Huntington disease exemplifies dominant inheritance: a parent with one normal and one mutated allele has a 50% chance of passing the mutation to offspring, who will develop the disease. Understanding pedigree analysis and disease inheritance patterns helps predict disease risk and inform family planning decisions.
Q7: Why is eye color used to explain genetic dominance and recessiveness?
Eye color serves as a useful teaching example because it clearly illustrates dominant and recessive alleles, even though scientists discovered at least eight genes regulate eye color. The OCA2 gene accounts for nearly three-fourths of blue-brown variation, making the simplified model of brown dominance over blue effective for explaining how dominant alleles mask recessive ones, despite other genes occasionally modifying these effects.