Do most of the complex phenotypes we observe come from a single gene?
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No, people did not understand that DNA is our hereditary material until long after Mendel's time. Our modern understanding of DNA and chromosomes helped to explain how Mendel's rules worked. Mendel laid the foundation for modern genetics, but there were still a lot of questions he left unanswered. What exactly are the dominant and recessive factors that determine how all organisms look?
And how do these factors work? Since Mendel’s time, scientists have discovered the answers to these questions. Genetic material is made out of DNA. It is the DNA that makes up the hereditary factors that Mendel identified. By applying our modern knowledge of DNA and chromosomes, we can explain Mendel’s findings and build on them. In this concept, we will explore the connections between Mendel’s work and modern
genetics. Recall that our DNA is wound into chromosomes. Each of our chromosomes contains a long chain of DNA that encodes hundreds, if not
thousands, of genes. Each of these genes can have slightly different versions from individual to individual. These variants of genes are called alleles. Each parent only donates one allele for each gene to an offspring. For example, remember that for the height gene in pea plants there are two possible factors. These factors are alleles. There is a dominant allele for tallness (T) and a recessive allele for shortness (t). Genotype and PhenotypeGenotype is a way to describe the combination of alleles that an individual has for a certain gene (Table below). For each gene, an organism has two alleles, one on each chromosome of a homologous pair of chromosomes (think of it as one allele from Mom, one allele from Dad). The genotype is represented by letter combinations, such as TT, Tt, and tt. When an organism has two of the same alleles for a specific gene, it is homozygous (homo means "same") for that gene. An organism can be either homozygous dominant (TT) or homozygous recessive (tt). If an organism has two different alleles (Tt) for a certain gene, it is known as heterozygous (hetero means different).
Phenotype is a way to describe the traits you can see. The genotype is like a recipe for a cake, while the phenotype is like the cake made from the recipe. The genotype expresses the phenotype. For example, the phenotypes of Mendel’s pea plants were either tall or short, or they were purple-flowered or white-flowered. Can organisms with different genotypes have the same phenotypes? Let’s see. What is the phenotype of a pea plant that is homozygous dominant (TT) for the tall trait? Tall. What is the phenotype of a pea plant that is heterozygous (Tt)? It is also tall. The answer is yes, two different genotypes can result in the same phenotype. Remember, the recessive phenotype will be expressed only when the dominant allele is absent, or when an individual is homozygous recessive (tt) (Figure below). Figure \(\PageIndex{1}\): Different genotypes (AA, Aa, aa or TT, Tt, tt) will lead to different phenotypes, or different appearances of the organism.Summary
Explore MoreUse the resources below to answer the questions that follow. Explore More I
Explore More II
Review
Are most traits determined by a single gene?Most traits are determined by more than one gene. For example, skin color and height are determined by many genes. Some phenotypes however, are determined by a single gene. We will explore some of these single gene traits in the laboratory.
How many phenotypes result from a single gene trait?With one gene controlling a trait we have three possible genotypes, AA, Aa and aa and depending on the allelic interactions (dominance or incomplete dominance) we can have two or three phenotypes.
How are different phenotypes produced from a single gene?In a number of cases, identical mutations have resulted in significantly different phenotypes. This is due to somatic mosaicism. The co-expression of wild allele shifts the AIS subtype to a higher degree of virilization than expected from the mutant allele alone.
What are complex phenotypes?Thus, a complex phenotype is the consequence of complex interactions of a large number of genetic and non-genetic determining factors (Figure 1). The review primarily focuses on the commonly used approaches to identify the DSVs that are associated with phenotypic effects.
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