Mendelian inheritance describes three laws or basic principles of genetic inheritance documented by creationist Gregor Mendel. The laws specified deal with the transmission of hereditary characteristics from parent organisms to their children and are a fundamental to genetics, making Mendel the father of genetics.
In 1866, Gregor Mendel studied the transmission of seven different pea traits by carefully test-crossing many distinct varieties of peas. Studying garden peas might seem trivial to those of us who live in a modern world of cloned sheep and gene transfer, but Mendel's simple approach led to fundamental insights into genetic inheritance, known today as Mendel's Laws. Mendel did not actually know or understand the cellular mechanisms that produced the results he observed. Nonetheless, he correctly surmised the behavior of traits and the mathematical predictions of their transmission, the independent segregation of alleles during gamete production, and the independent assortment of genes. Perhaps as amazing as Mendel's discoveries was the fact that his work was largely ignored by the scientific community for over 30 years!
While these laws did not account for polygenic traits, linked traits, or genetic recombination, they still hold true today for the limited number of cases they address. The core in changes can be found in the cell nucleus which is made up of several chromosomes. These laws documented in 1865 and 1866 were then later considered quite controversial in 1900. A little bit more than a decade later in 1915, they were integrated into Thomas Morgan's work regarding the chromosome theory of inheritance.
Law of dominance
- Main Article: Genetic dominance
Each trait is determined by two factors (alleles), one inherited from each parent. These factors each exhibit a characteristic that is dominant, co-dominant, or recessive, and those that are dominant will mask the expression of those that are recessive.
Law of segregation
Each of the two inherited factors (alleles) possessed by the parent will segregate and pass during meiosis into separate gametes (eggs or sperm), which will each carry only one of the factors.
This specific law has four parts, which are:
- Alternative versions of genes, or alleles account for variations in inherited characteristics.
- For each characteristic or trait an organism inherits two alternative forms of that gene, one from each parent.
- If the two alleles differ, then one, the dominant allele, is fully expressed in the organism's appearance. The other, the recessive allele has no noticeable effect on the organism's appearance.
- The two genes for each character segregate during gamete production.
Law of independent assortment
In the gametes, alleles of one gene separate independently of those of another gene, and thus all possible combinations of alleles are equally probable.
The emergence of one trait will not affect the emergence of another. Mendel concluded that each organism carries two copies of its expressed phenotype. If one differs from the other on the same phenotype, one will inevitably dominate the other.
Exceptions to Mendel's Laws
There are many examples of inheritance that appear to be exceptions to Mendel's laws. Usually, they turn out to represent complex interactions among various allelic conditions.
Pleiotropism (or pleiotropy), refers to the phenomenon in which a single gene is responsible for producing multiple, distinct, and apparently unrelated phenotypic traits. That is to say, an individual can exhibit many different phenotypic outcomes. This is because the gene product is active in many places in the body. An example is Marfan's syndrome, where there is a defect in the gene coding for a connective tissue protein. Individuals with Marfan's syndrome exhibit abnormalities in their eyes, skeletal system, and cardiovascular system.
Some genes mask the expression of other genes just as a fully dominant allele masks the expression of its recessive counterpart. A gene that masks the phenotypic effect of another gene is called an epistatic gene; the gene it subordinates is the hypostatic gene. The gene for albinism in humans is an epistatic gene. It is not part of the interacting skin-color genes. Rather, its dominant allele is necessary for the development of any skin pigment, and its recessive homozygous state results in the albino condition, regardless of how many other pigment genes may be present. Because of the effects of an epistatic gene, some individuals who inherit the dominant, disease-causing gene show only partial symptoms of the disease. Some, in fact, may show no expression of the disease-causing gene, a condition referred to as nonpenetrance. The individual in whom such a nonpenetrant mutant gene exists will be phenotypically normal but still capable of passing the deleterious gene on to offspring, who may exhibit the full-blown disease.
Multigenic traits result from the expression of several different genes. This is true for human eye color, in which at least three different genes are responsible for determining eye color. A brown/blue gene and a central brown gene are both found on chromosome 15, whereas a green/blue gene is found on chromosome 19. The interaction between these genes is not well understood. It is speculated that there may be other genes that control other factors, such as the amount of pigment deposited in the iris. This multigenic system explains why two blue-eyed individuals can have a brown-eyed child.
A somatic mosaic expresses two or more different phenotypes in different parts of his body. Somatic mosaicism might produce eyes with two different eye colors (i.e. brown and green). In multicellular organisms, every cell in the adult is ultimately derived from the single-cell fertilized egg. Therefore, every cell in the adult normally carries the same genetic information. But sometimes a mutation occurs in only one cell at the two-cell stage of development. The adult then consists of two types of cells: cells with the mutation and cells without. If a mutation affecting melanin production occurred in one of the cells in the cell lineage of one eye but not the other, then the eyes would have different genetic potential for melanin synthesis. This could produce eyes of two different colors.
Penetrance refers to the degree to which a particular allele is expressed in a population phenotype. If every individual carrying a dominant mutant gene demonstrates the mutant phenotype, the gene is said to show complete penetrance.
A Mendelian mutation (or trait) is controlled by change in alleles at a single locus and is inherited in a Mendelian fashion or according to Mendel's laws. In most cases these are single gene mutations such as those that cause sickle-cell anemia contrasted to those that are located on several loci such as arthritis.
- What is a genome? by the National Center for Biotechnology Information, National Institute of Health, March 31. 2004. Accessed August 21, 2008.