Genetics is the study of inheritance, exploring how characteristics are passed from parents to offspring. Every trait, from eye color to blood type, is determined by biological instructions inherited from both parents. Understanding these instructions requires familiarity with the basic terminology used to describe the variations in inheritance.
Understanding Alleles: The Genetic Building Blocks
The fundamental unit of heredity is the gene, a segment of DNA that holds the code for a specific function. Different versions of the same gene are known as alleles, representing the variations that exist for any given trait.
Geneticists use letters as a shorthand to represent these alleles. A capital letter, such as ‘B,’ denotes a dominant allele, which expresses its trait when present. A lowercase letter, like ‘b,’ signifies a recessive allele, whose trait is only observable under specific conditions. Because we inherit one set of chromosomes from each parent, every individual possesses a pair of alleles for almost every trait. This pairing of instructions constitutes the genotype.
Defining Homozygous and Heterozygous
The concepts of homozygous and heterozygous describe the combination of the two alleles inherited for a particular gene. The term “homozygous” means “same pair” and refers to a genotype where both inherited alleles are identical. This occurs when an individual receives the same variation of a gene from both parents.
A genotype is homozygous if it consists of two dominant alleles (BB) or two recessive alleles (bb). In either case, the two genetic instructions are a perfect match.
In contrast, the term “heterozygous” means “different pair” and describes a genotype where the two inherited alleles are different. This means the individual received one dominant allele and one recessive allele (Bb).
Classifying the Genotype bb
To classify the genotype ‘bb,’ we examine the specific pairing of the alleles. Since ‘bb’ consists of two identical alleles (both lowercase ‘b’), it fits the definition of a homozygous state. Specifically, because both alleles are the recessive form, ‘bb’ is referred to as homozygous recessive.
This classification is based purely on the identity of the two letters. The other form of homozygous genotype is ‘BB,’ known as homozygous dominant. Any genotype where the two letters are the same is considered homozygous.
A heterozygous classification only occurs if the two alleles are different, such as ‘Bb.’ In that state, one dominant ‘B’ allele is paired with one recessive ‘b’ allele. Therefore, the genotype ‘bb’ is classified as homozygous.
How Genotype Influences Physical Traits
The classification of a genotype directly determines the resulting physical characteristic, known as the phenotype. The concepts of dominance and recessiveness govern which trait is expressed from the inherited instructions.
In a homozygous recessive genotype like ‘bb,’ the recessive trait is expressed because no dominant allele is present to mask its effect. For example, if ‘B’ coded for the ability to taste a compound and ‘b’ coded for the inability to taste it, a person with the ‘bb’ genotype would be a non-taster. Both inherited instructions are for the recessive trait, ensuring that this characteristic is observed.
In contrast, a heterozygous genotype, such as ‘Bb,’ will express the dominant trait. Even though the recessive ‘b’ allele is present, the dominant ‘B’ allele overrides its instruction. This means a person with the ‘Bb’ genotype would be phenotypically identical to someone with the homozygous dominant ‘BB’ genotype.
Only when an individual inherits two copies of the recessive allele, as in the ‘bb’ state, can the recessive characteristic manifest physically. This mechanism explains why certain traits may skip generations, reappearing when two parents who carry the recessive allele pass that specific instruction to their offspring.