A genotype is the specific genetic makeup of an individual, representing the combination of genes they possess. A human female’s genetic sex is determined by possessing two X chromosomes, while a male has one X and one Y chromosome. However, this is just one aspect of a person’s complete genetic identity, which consists of thousands of individual genotypes that influence a wide array of traits. Determining the specific genes a woman has for a particular characteristic requires applying the concepts of heredity.
The Fundamental Concepts of Genotypes
An organism’s genotype is its unique DNA sequence, while its phenotype is the observable physical expression of that genetic code. For instance, the genes for eye color constitute the genotype, whereas the actual brown or blue color is the phenotype. This genetic information is passed from parents to offspring, with each parent contributing one copy of every gene.
These different versions of a gene are called alleles. For any given gene, a woman inherits two alleles, one from each parent. Alleles can be dominant or recessive; a dominant allele is expressed if at least one copy is present, while a recessive allele is only expressed when two copies are present.
This leads to three possible genetic combinations. If a woman has two identical alleles for a gene, she is homozygous for that trait (AA or aa). If she possesses two different alleles for a gene (Aa), she is heterozygous. In the case of eye color, a woman with either a BB or Bb genotype will have brown eyes, while only a woman with a bb genotype will have blue eyes.
Deducing a Genotype from Known Information
Observing a woman’s phenotype is a direct way to infer her genotype. If she displays a recessive phenotype, like blue eyes, her genotype must be homozygous recessive (bb). However, if she exhibits a dominant phenotype, like brown eyes, her genotype could be either homozygous dominant (BB) or heterozygous (Bb).
Information about her parents can resolve this ambiguity. If a brown-eyed woman has a blue-eyed parent (bb), that parent could only pass on a recessive allele (b). The woman must have inherited this recessive allele, making her genotype heterozygous (Bb).
A woman’s children can also provide definitive clues. If a woman with a dominant phenotype has a child who expresses the recessive phenotype, she must be heterozygous. For example, if a brown-eyed woman has a blue-eyed child (bb), the child inherited a recessive allele from each parent. The woman must have contributed one of those alleles, confirming her genotype is heterozygous (Bb).
The Role of Sex-Linked Traits
Some traits are sex-linked, meaning the responsible gene is on a sex chromosome (X or Y). Most sex-linked traits are X-linked because the X chromosome is larger and contains more genes than the Y chromosome. These traits follow different inheritance rules that impact how a woman’s genotype is determined.
In females (XX), a recessive allele on one X chromosome can be masked by a dominant allele on the other X chromosome. Such an individual is a “carrier”; she does not express the trait herself but can pass the recessive allele to her offspring. Males (XY) have only one X chromosome, so any recessive allele on their X chromosome will be expressed because there is no corresponding allele to mask it.
Red-green color blindness is an example of an X-linked recessive trait. If a woman is colorblind, her genotype must be XbXb, where ‘b’ is the recessive allele. If she has normal vision, her genotype could be either XBXB or XBXb. If she has a colorblind son (XbY), he inherited the Xb chromosome from her, proving her genotype is heterozygous (XBXb) and she is a carrier.
Analyzing Pedigree Charts
Pedigree charts trace the inheritance of a trait through multiple generations. These charts use standardized symbols: circles for females, squares for males, and shading for individuals expressing the trait. Analyzing these relationships helps determine genotypes and inheritance patterns.
Examining a pedigree can reveal the inheritance pattern. A trait appearing in every generation is likely dominant, while one that skips generations suggests a recessive pattern. If a trait affects males more frequently than females, it points toward an X-linked recessive condition.
The chart integrates family history to determine an individual’s genotype. For example, consider a woman who does not express a recessive trait. If the chart shows her father expressed it, he must have a homozygous recessive genotype. This means he passed a recessive allele to her, making her heterozygous.