Genotype and phenotype, though perhaps sounding a little like cartoon siblings, are both central concepts in basic genetics. They are related in the same fundamental way as "blueprint" and "building" or "recipe" and "meal": An organism's genotype supplies specific instructions to carry out an assembly job of some sort, whereas its phenotype represent the visible, tangible results of that assembly job.
To understand how the genotypes and phenotypes of human traits are related, a basic overview of inheritance patterns at the molecular level is in order.
Any complete trip though the world of modern genetics starts with Gregor Mendel, the monk whose painstaking experiments with breeding pea plants in the 19th century paved the way for the understanding of the discipline even before anyone knew what DNA or genes even were. Mendel bred plants with different phenotypes each other until only plants that looked identical with respect to specific traits were produced – for example, he created a "family" of plants that all had yellow round pods, and a different "family" that all had green wrinkled pods. He assumed that phenotypically identical plants in these families must have had the same molecular composition with respect their genetic material.
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When he mated these lines of plants with each other, he noticed that some traits were more prevalent than others after several generations, and that no blending of certain traits would occur. Mendel realized that some traits would mask the presence of others but not obliterate them because they could emerge in subsequent generations, and that this had to do with variants of the material that produced a given trait (e.g., tall vs. short plants) known today as alleles. Each parent carried two copies of a given allele for each trait: Both could be dominant or both could be recessive, or one of each might be present. This genotype would determine the plant's phenotype.
Genotype and Phenotype Examples
To represent dominant and recessive alleles symbolically, and thus create a system for linking phenotypes and genotypes, geneticists assign alleles for a given trait a letter, with the dominant allele represented by a capital letter and the recessive allele given a lowercase letter. So if tall pea plants proved to be dominant over short plants, the letter "T" could represent the allele for tallness and "t" the allele for shortness. Each plant has two alleles for the trait of height, one from each parent plant; if a single "T" is present, the plant will grow tall, but two "t" alleles must be present for the plant to remain short.
Thus the four possible genotypes for this plant are TT, tT, Tt and tt; the phenotype for the first three is "tall," while the phenotype for the last combination is "short." Importantly, as you can see, certain tall plants can contribute to shortness in later generations by passing along a "t" allele that has been masked in the case of its own life by a "T" allele. Phenotypes and genotypes of human traits work in the same essential way.
Sickle Cell Anemia
Sickle cell anemia is a disease of red blood cells in human beings in which the disordered condition results from the recessive genotype. The allele for a normally shaped red blood cell is usually labeled "A," and that for the malformed kind that tends to get stuck in capillaries and cannot carry oxygen properly is "a." The genotypes AA, Aa and aA do not result in clinical issues, but the Aa and aA genotypes are considered "carriers" of disease, whereas the aa genotype causes sickle cell anemia. Symptoms of the aa genotype include anemia (a low red blood cell count), frequent infections, chest pain and spleen problems. The disease can be managed but not cured. People with the phenotype aa, should they have children, can only pass along the damaging allele for this red blood cell trait, meaning that any offspring will either be carriers or have outright sickle-cell disease.