There are many types of inheritance that do not follow the Mendelian pattern. Notable ones include: multiple alleles, gene interactions (complementary genes, epistasis and quantitative or polygenic, inheritance), linkage with or without crossing over and sex-linked inheritance.
Pleiotropy, the lack of dominance and lethal genes cannot be classified as variations of inheritance since genes can have these behaviors and at the same time obey Mendelian laws.
Mutations and aneuploidies are abnormalities that alter the Mendelian pattern of inheritance as well as mitochondrial inheritance (the passage of mitochondrial DNA from the mother through the cytoplasm of the egg cell to the offspring).
This condition is called lack of dominance and it can happen in two ways: incomplete dominance or codominance.
In incomplete dominance, the heterozygous individual presents an intermediate phenotype between the two types of homozygous ones, such as in sickle cell anemia, in which the heterozygous individual produces some sick red blood cells and some normal red blood cells. Codominance occurs, for example, in the genetic determination of the MN blood group system, in which the heterozygous individual has a phenotype totally different from the homozygous one, and not an intermediate form.
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Pleiotropy (or pliotropy) is the phenomenon in which a single gene conditions several different phenotypical traits.
Some phenotypical traits may be sensitive to the pleiotropic effects (for example, inhibition) of other genes, even when conditioned by a pair of alleles in simple dominance. A mixture of pleiotropy and gene interaction is characteristic of these cases.
Lethal genes are genes with at least one allele that, when present in the genotype of an individual, cause death. There are recessive lethal alleles and dominant lethal alleles. (There are also genes with alleles that are dominant when in heterozygosity but lethal when in homozygosity, meaning that the dominance related to the phenotype does not correspond to the dominance related to lethality.)
Multiple alleles is the phenomenon in which the same gene has more than two different alleles (in normal Mendelian inheritance, the gene only has two alleles). Obviously, these alleles combine in pairs to form genotypes.
In multiple alleles, relative dominance among the alleles may exist. A typical example of multiple alleles is the inheritance of the ABO blood group system, in which there are three alleles (A, B or O, or IA, IB and i). IA is dominant over i, which is recessive in relation to the other IB allele. IA and IB lack dominance between themselves.
Another example is the color of rabbit fur, which is conditioned by four different alleles (C, Cch, Ch and c). In this case, the dominance relations are C > Cch > Ch > c (the symbol > means “is dominant over”).
Gene interactions are the phenomenon in which a given phenotypic trait is conditioned by two or more genes (do not confuse this with multiple alleles, in which there is a single gene with three or more alleles).
The three main types of gene interaction are: complementary genes, epistasis and polygenic inheritance (or quantitative inheritance).
Complementary genes are different genes that act together to determine a given phenotypic trait.
For example, consider a phenotypical trait conditioned by 2 complementary genes whose alleles are respectively X, x, Y and y. Performing hybridization in F2, 4 different phenotypic forms are obtained: X_Y_ (double dominant), X_yy (dominant for the first pair, recessive for the second), xxY_ (recessive for the first pair, dominant for the second) and xxyy (double recessive). This is what happens, for example, in the color of budgie feathers, in which the double dominant interaction results in green feathers; the interaction that is dominant for the first pair and recessive for the second results in yellow feathers; the interaction that is recessive for the first pair and dominant for the second leads to blue feathers; and the double recessive interaction leads to white feathers.
Each complementary gene segregates independently from the others since they are located in different chromosomes. Therefore, the pattern follows Mendel’s second law (although it does not obey Mendel’s first law).
Epistasis is the gene interaction in which a gene (the epistatic gene) can disallow the phenotypical manifestation of another gene (the hypostatic gene). In dominant epistasis, the inhibitor allele is the dominant allele (for example, I) of the epistatic gene and, as result, inhibition occurs in dominant homozygosity (II) or in heterozygosity (Ii). In recessive epistasis, the inhibitor allele is the recessive allele of the epistatic gene (i) and, as a result, inhibition occurs only in recessive homozygosity (ii).
In dihybridism without epistasis, double heterozygous parents cross-breed and 4 phenotypical forms appear in F2. The proportion is 9 individuals double dominant, 3 individuals dominant for the first pair and recessive for the second pair, 3 individuals recessive for the first pair and dominant for the second pair, and 1 individual double recessive (9:3:3:1).
Considering that the epistatic gene is the second pair and that the recessive genotype of the hypostatic gene implies the lack of the characteristic, in the F2 generation of dominant epistasis, the following phenotypic forms would emerge: 13 individuals dominant for the second pair or recessive for the first pair, meaning that, the characteristic is not manifest; 3 individuals dominant for the first pair and recessive for the second pair, meaning that the characteristic is manifest. The phenotypical proportion would be 13:3. In recessive epistasis, the phenotypical forms that would emerge in F2 are: 9 individuals double dominant (the characteristic is manifest) and 7 individuals recessive for the first pair or recessive for the second pair, meaning that the characteristic is not manifest. Therefore, the phenotypical proportion would be 9:7.
These examples show how epistasis changes phenotypical forms and proportions, from the normal 9:3:3:1 in F2 to 13:3 in dominant epistasis or to 9:7 in recessive epistasis (note that some forms have even disappeared).
(If the recessive genotype of the hypostatic gene is active, not only meaning that the dominant allele is not manifest, the number of phenotypic forms in F2 changes.)
Polygenic inheritance, also known as quantitative inheritance, is the gene interaction in which a given trait is conditioned by several different genes with alleles that may or may not contribute to increasing the intensity of the phenotype. These alleles may be contributing or non-contributing and there is no dominance among them. Polygenic inheritance is the type of inheritance, for example, of skin color and stature in humans.
Considering a given species of animal in which the length of the individual is conditioned by the polygenic inheritance of three genes, for the genotype with only non-contributing alleles (aabbcc), a basal phenotype, for example, 30 cm, would emerge. Also considering that, for each contributing allele, a 5 cm increase in the length of the animal is added, in the genotype with only contributing alleles (AABBCC), the animal would present the basal phenotype (30 cm) plus 30 cm more added for each contributing allele, that is, its length would be 60 cm. In the case of triple heterozygosity, for example, the length of the animal would be 45 cm. That is the way polygenic inheritance works.
If a trait statistically has a normal (Gaussian, bell curve) distribution of its phenotypical forms, it is probable that it is conditioned by polygenic inheritance (quantitative inheritance).
In quantitative inheritance, the effects of several genes are added to others, making it possible to represent the trait variation of a given population in a Gaussian curve with the heterozygous genotypes in the center, that is, those that appear in larger number, and the homozygous ones on the ends.
Considering “p” the number of phenotypic forms and “a” the number of alleles involved in the polygenic inheritance, the formula p = 2a + 1 applies.
(Often, it is not possible to precisely determine the number of phenotypic forms, p, due to the multigenic nature of inheritance, since the observed variation of phenotypes often seems to be a continuum or the trait may suffer from environmental influences.)
Sex-linked inheritance is a type of non-Mendelian inheritance because it opposes Mendel’s first law, which postulates that each trait is always conditioned by two factors (alleles). In non-homologous regions of sex chromosomes, the genotypes of the genes contain only one allele (even in the case of the XX karyotype, in women, one of the X chromosomes is inactive).
Mitochondrial inheritance is the passing down of mitochondrial DNA molecules (mtDNA) to the offspring. An individual's entire stock of mtDNA must come from the mother, the maternal grandmother, the maternal great grandmother and so on, since mitochondria are inherited from the cytoplasm of the egg cell (that later composes the cytoplasm of the zygote).
There are several genetic diseases caused by mitochondrial inheritance, such as Leber's hereditary optic neuropathy, which leads to loss of the central vision of both eyes, and Kearns-Sayre syndrome, a neuromuscular disease that causes ophthalmoplegia and muscle fatigue.
Mitochondrial inheritance is an excellent means for the genetic analysis of maternal lineage (just like the Y chromosome is an excellent means of studying paternal lineage).
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