Gregor Mendel’s experimental work with garden peas established principles of heredity. He observed that certain traits segregated in predictable ways. Each diploid organism, like the peas Mendel studied, carries two copies of each inherited factor (now called genes). These factors influence a particular phenotypic trait. All these and many Kore are what we shall examine in what is known as Mendelian inheritance.
The Law of Independent Assortment
The Law of Independent Assortment states that alleles for separate traits are passed independently of one another. This is a fundamental principle of genetics.
The Concept Of Dominant Allele In Mendelian Inheritance
In the pea plants Mendel studied, each gene came in just two versions, or alleles. Mendel’s simple experiments made it easy to see how these alleles acted together to determine traits like flower color and plant height. He developed the principle of dominance from his observations, which says that one allele for a trait dominates over another allele in determining the phenotype of an individual. He also learned that when one allele is dominant over the other, it produces its phenotype in individuals who are homozygous for that gene — they have two copies of the dominant allele and none of the recessive allele.
Mendel’s rule of independent assortment states that genes for separate traits are randomly mixed in each gamete — an egg or sperm cell that contains only half the DNA of its parent. If the alleles for two different traits are located on opposite chromosomes, their effects will be separated as well. If the alleles for two different genes are closely linked to each other on a single chromosome, however, they will be passed on together in each gamete. This is called linkage.
A good example of this is the MN blood group in humans. Individuals with this blood type have an M allele on one chromosome and a N allele on the other. The M allele dominates over the N allele, and this causes the production of a marker displayed on the surface of red blood cells. MN blood types are common, but people who have a N allele also have the disease called sickle-cell disease. Their long, pointy red blood cells get caught in capillaries and block the flow of oxygen and nutrients to muscle and organ cells, which can cause painful, dangerous complications.
In Mendelian inheritance, the Mendel’s rule of independent assortment was validated by his dihybrid crosses — experiments in which he crossed two individuals carrying the same pair of alleles for a certain trait. He found that the alleles for each trait were inherited independently of each other. This was important evidence because it meant that a person’s choice of parents had no impact on his or her genetic makeup. This is the foundation of modern genetics, and it has led to many medical advances.
The Concept of Recessive Allele in Mendelian Inheritance
In Mendelian inheritance studies, the recessive allele is a form of a gene that masks or suppresses the effect of another allele at the same locus. For a recessive allele to manifest, it must be present in two copies (homozygous) in a person. In a homozygous person, the trait is dominant over any other allele at that locus. Gregor Mendel discovered this phenomenon with his pea plants and established the laws of dominance and segregation.
In diploid organisms, each gene is coded for by one of two alleles at a specific locus or position. During sexual reproduction, the normal complement of 46 chromosomes in a sperm or egg cell needs to be halved to 23 so that the two haploid gametes can fuse and form a diploid zygote. Each zygote is then equipped with two sets of the genes encoded by each parent, one from the biological mother and the other from the biological father. Different alleles at a given locus may produce identical outward characteristics, called phenotypes. In order for a recessive allele to express, it must be present in two identical copies (homozygous).
Mendel used the Punnett square to illustrate these principles by following only a single characteristic, such as pod color, in his pea plants. Each plant had two alleles at the particular locus, with one from the biological mother and the other from its biological father. Each genotype, or set of genes, could be either dominant or recessive.
Dominant alleles always produce their phenotype, whether the person is heterozygous for those alleles or homozygous. This is what Mendel referred to as the law of dominant over dominant. A recessive allele, on the other hand, is not expressed unless two copies of the gene are present. In this case, the gene is masked by the dominant allele and does not manifest its phenotype.
It is important to note that although many of the traits and diseases that we classify as recessive or dominant are actually Mendelian, this is not true of all. In fact, many genetic traits and diseases, including complex ones like blood group A and phenylketonuria, are controlled by multiple genes with additive effects, so that they do not follow the simple Mendelian inheritance model.
Codominance in Mendelian Inheritance
When a dominant trait and a recessive trait appear together in a offspring, it is referred to as codominance. A common example is the color of a flower’s petals. A pink carnation can be produced from parents with red and white coat colors, demonstrating that both the dominant and recessive alleles for this trait are expressed in the progeny. Other examples of codominance are found in dogs, where a dog with black and white fur can produce offspring with both the black and the white coat color.
Gregor Mendel was the first to observe patterns of inheritance in different families. These general patterns of inheritance are known as Mendelian inheritance, and they are the basis for much of modern genetics. Mendel discovered that hereditary factors exist in alternative forms, which are now called alleles. Each gene has two alleles, and an individual who inherits two identical alleles for a trait is said to be homozygous for that trait (AA). Heterozygotes for a trait possess one dominant allele and one recessive allele, which results in a phenotype intermediate between the homozygous dominant and heterozygous individuals.
Many traits have more complex inheritance patterns than those studied by Mendel. The complexity is usually due to the fact that a single gene can have more than two alleles. These are referred to as polygenic traits.
The Law of Independent Assortment states that alleles for different traits are passed on independently of each other and do not influence the appearance of the other allele in offspring. Mendel’s experiments with pea plants thus demonstrated this law in monohybrid and dihybrid crosses. In both types of cross, an idealized 3:1 ratio of dominant to recessive phenotypes resulted.
Incomplete dominance superficially resembles the concept of blending inheritance but is still explained by the principles established by Mendel. Incomplete dominance occurs when a heterozygote for a trait receives both the dominant and the recessive alleles of that gene. The effect of the dominant allele overshadows the effect of the recessive allele, and the offspring resemble a mixture of the two parent phenotypes.
The concept of polygenic inheritance in the Mendelian inheritance studies is vividly captured in the early work of 19th-century Austrian monk Gregor Mendel who helped to revolutionize the understanding of heredity, and the principles he formulated remain central to genetics till today. Specifically, the rules that describe how dominant and recessive alleles interact to determine the phenotype of a trait. However, many traits do not fit the pattern of inheritance described by Mendel. In these cases, the trait is said to be polygenic.
A polygenic trait results from the combined influence of several different genes, and is not subject to the same patterns as a dominant or recessive gene. For example, height is a polygenic trait; it depends on the contribution of genes that each have two alleles (A and B). If both parents are tall, their offspring will be tall as well. However, if one parent has a dominant height gene and the other has a recessive height gene, their offspring will be shorter than either of their parents.
In addition, if the dominant gene has a “b” allele and the recessive gene has an “a” allele, offspring will be intermediate in height. This is because the “b” allele is dominant over the “a” allele.
A number of important genetic disorders exhibit polygenic inheritance. For example, the genetic disorder cystic fibrosis is associated with a gene that has a normal variant and a disease-causing variant. People with a normal variant of the disease-causing gene will not develop the disorder, but those who inherit a single copy of the disease-causing allele from each parent will have a much higher chance of developing the condition.
As a result, polygenic traits do not follow the simple ratios of Mendelian inheritance and are sometimes difficult to predict. However, they are still useful in determining the likelihood that a particular genetic disease will occur within a family and for identifying the specific gene or genes responsible.
The OMIM database is a comprehensive collection of human genes and genetic disorders, and contains information about the molecular interaction between them. It provides a helpful reference for anyone interested in learning about the role of different genes and their alleles in various diseases and conditions, including Mendelian disorders.