| Admissions | Accreditation | Booksellers | Catalog | Colleges | Contact Us | Continents/States/Districts | Contracts | Examinations | Forms | Grants | Hostels | Honorary Doctorate degree | Instructors | Lecture | Librarians | Membership | Professional Examinations | Programs | Recommendations | Research Grants | Researchers | Students login | Schools | Search | Seminar | Study Center/Centre | Thesis | Universities | Work counseling |
| Epistasis and Other Non-Mendelian Inheritances All Easily Explained Review of Epistasis and Other Non-Mendelian Inheritances 1. According to Mendel’s law phenotypical characteristics would be determined by pair of factors (alleles) that separate independently in gametes. What are the main types of inheritances that are exceptions to Mendel’s rules? There are many types of inheritance that do not follow the mendelian pattern. Notable among them are: multiple alleles, gene interactions (complementary genes, epistasis and quantitative, or polygenic, inheritance), linkage with or without crossing over and sex-linked inheritance. Pleiotropy, lacking of dominance and lethal genes do not fit as variations of inheritance since genes can have these behaviors and at the same time obey mendelian laws. Mutations and aneuploidies are abnormalities that also alter the mendelian pattern of inheritance as well as mitochondrial inheritance (passage of mitochondrial DNA from the mother through the cytoplasm of the egg cell to the offspring). 2. What is the genetic condition in which the heterozygous individual has different phenotype from the homozygous individual? This condition is called lack of dominance and it can happen in two ways: incomplete dominance or codominance. In incomplete dominance the heterozygous presents an intermediate phenotype between the two types of homozygous, as in sickle cell anemia in which the heterozygous 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 has a phenotype totally different from the homozygous, not being an intermediate form. Image Diversity: incomplete dominance codominance 3. What is pleiotropy? Pleiotropy (or pliotropy) is the phenomenon in which a single gene conditions several different phenotypical traits. Some phenotypical traits may be sensitive to pleiotropic effects (for example, inhibition) of other genes, even when conditioned by a pair of alleles in simple dominance. In these cases a mixture of pleiotropy and gene interaction is characterized. Image Diversity: pleiotropy 4. What are lethal genes? Lethal genes are genes having at least one allele that, when present in the genotype of an individual, causes death. There are recessive lethal alleles and dominant lethal alleles. (There are also genes having alleles that are dominant when in heterozygosity but lethal when in homozygosity, i.e., the dominance related to the phenotype does not correspond to the dominance related to lethality.) Image Diversity: lethal genes 5. What are multiple alleles? Is there dominance in multiple alleles? Multiple alleles is the phenomenon in which the same gene has more than two different alleles (in normal mendelian inheritance the gene has only two alleles). Obviously these alleles combine in pairs to form the 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, conditioned by four different alleles (C, Cch, Ch and c). In this case the dominance relations are C > Cch > Ch > c (the symbol > means “dominates over”). Image Diversity: multiple alleles 6. What are gene interactions? What are the three main types of gene interactions? Gene interaction is the phenomenon in which a given phenotypical trait is conditioned by two or more genes (do not confuse with multiple alleles in which there is a single gene having three or more alleles). The three main types of gene interaction are: complementary genes, epistasis and polygenic inheritance (or quantitative inheritance). 7. What are complementary genes? Does this inheritance pattern obey Mendel’s second law? Complementary genes are different genes that act together to determine a given phenotypical 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 phenotypical 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, regarding the color of budgerigar feathers, in which the double dominant interaction results in green feathers, the dominant for the first pair, recessive for the second interaction results in yellow feathers, the recessive for the first pair, dominant for the second interaction 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). Image Diversity: complementary genes 8. What is epistasis? What is the difference between dominant epistasis and recessive epistasis? 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 so 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) so inhibition occurs only in recessive homozygosity (ii). Image Diversity: epistasis 9. In the hybridization of 2 genes (4 different alleles, 2 of each pair) how does epistasis affect the proportion of phenotypical forms in the F2 generation? In dihybridism without epistasis double heterozygous parental individuals cross and in F2 4 phenotypical forms appear. The proportion is 9 double dominant to 3 dominant for the first pair, recessive for the second to 3 recessive for the first pair, dominant for the second to 1 double recessive (9:3:3:1). Considering that the epistatic gene is the second pair and that the recessive genotype of the hypostatic gene means lacking of the characteristic, in the F2 generation of the dominant epistasis the following phenotypical forms would emerge: 13 dominant for the second pair or recessive for the first, i.e., the characteristic does not manifest, 3 dominant for the first pair, recessive for the second, i.e., the characteristic manifests. The phenotypical proportion would be 13:3. In the recessive epistasis in F2 the phenotypical forms that would emerge are: 9 double dominant (the characteristic manifests), 7 recessive for the first pair or recessive for the second, i.e., the characteristic does not manifest. So 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 simply meaning that the dominant allele does not manifest, the number of phenotypical forms in F2 changes.) 10. What is polygenic inheritance? How does it work? Polygenic inheritance, also known as quantitative inheritance, is the gene interaction in which a given trait is conditioned by several different genes having alleles that may or may not contribute to increase the phenotype intensity. The alleles may be contributing or noncontributing and there is no dominance among them. Polygenic inheritance is the type of inheritance, for example, of skin color and of stature in humans. Considering a given species of animal in which the length of the individual is conditioned by polygenic inheritance of three genes, for the genotype having only noncontributing alleles (aabbcc) a basal phenotype, for example, 30 cm, would emerge. Considering also that for each contributing allele a 5 cm increase in the length of the animal is added, so in the genotype having only contributing alleles (AABBCC) the animal would present the basal phenotype (30 cm) plus 30 cm more added by each contributing allele, i.e., 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. Image Diversity: polygenic inheritance 11. What is the most probable inheritance pattern of a trait with gaussian proportional distribution of phenotypical forms? If a trait statistically has a normal (gaussian, bell-shaped 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 add 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, i.e., appearing in larger number, and the homozygous in the extremities. 12. How to find the number of pair of alleles involved in polygenic inheritance using the number of phenotypical forms of the trait they condition? Considering “p” the number of phenotypical forms and “a” the number of involved alleles of the polygenic inheritance. The formula p = 2a + 1 is then applied. (Many times it is not possible to determine precisely the number of phenotypical forms, p, due to the multigenic feature of the inheritance, since often the observed variation of phenotypes seems to be on a continuum or the trait suffers environmental influence.) 13. Why is sex-linked inheritance an example of nonmendelian inheritance? Sex-linked inheritance is a type of nonmendelian inheritance because it opposes Mendel’s first law, which postulates that each trait is always conditioned by two factors (alleles). In nonhomologous regions of the sex chromosomes the genotypes of the genes contain only one allele (even in the case of the XX karyotype, i.e., in women, one of the X chromosomes is inactive). Image Diversity: sex-linked inheritance 14. What is mitochondrial inheritance? Mitochondrial inheritance is the passage of mitochondrial DNA molecules (mtDNA) to the offspring. All stock of mtDNA an individual has have 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 constitutes the cytoplasm of the zygote). There are several genetic diseases caused by mitochondrial inheritance, like Leber's hereditary optic neuropathy, that leads to loss of the central vision of both eyes, and the Kearns-Sayre syndrome, a neuromuscular disease that causes ophthalmoplegia and muscle fatigue. Mitochondrial inheritance is an excellent means of genetic analysis of the maternal lineage (just like the Y chromosome is an excellent means of study of the paternal lineage). Image Diversity: mitochondrial inheritance |