The genetic factors value in phenotype formation

Formation of phenotype is a complicated process, which takes a time. The phenotype - is the observable expression of trait (affecting an individual і structure, physiology or behavior) that results from the biological activity of the DNA molecules. It is the realized expression of genotype. Genes provide only a possibility of traits expression. It depends on genetic factors, environmental factors, and individual development and so on. That means that formation of phenotype is under direction of many factors.

Among genetic factors affecting phenotype formation are interactions of genes from one allele (dominance, recessing, incomplete dominance, codominance) and from different alleles (dominant and recessive epistasis, hypostasis, complementarity), from many alleles, pleiotropic gene action, gene dose.

The dominance is such interaction when one dominant allele (A) is expressed independently from others recessive alleles (a). Heterozygotes (Aa) phenotypicaly are the same as homozygotes (AA). This allele is dominant in heterozygous organism. The example is eyes color inheritance. Heterozygous organisms have brown eyes, so that mean that brown eyes color is dominant where blue is recessive.

The incomplete dominance occurs when recessive allele is not fully suppressed. Some human and animal traits are subject to such gene interaction. In incomplete dominance, the heterozygous individuals express neither dominated phenotype nor recessive phenotype. Heterozygous individuals express an intermediate phenotype with slight deviance to dominant or recessive one. The example of incomplete dominance in human is inheritance of anophtalmia (aa) and normal eyes development (AA). Heterozygous individuals (Aa) have reduced eye size. The similar examples are inheritance of sickle cell anemia, acatalasia (absence of catalase enzyme) and others.

Alleles of one gene may work together in heterozygous organism. Such event was named codominance. It can be traced by assessing proteins, which are encoded by both of the genes. If both proteins are present in a blood, it is a codominance. This method is used in genetic counseling to determine heterozygous individuals having recessive alleles of hereditary diseases. The IV (AB) blood group has codominant pattern of inheritance.

The modified proportions may be due to interaction of non allelic genes. It can be two types complementary and epistasis (dominant and recessive).

The complementary or accessory genes are the genes, which can give a new trait when they are both in genotype (A-B-). If they are along (aaB- or A-bb) they encode only usual traits. In human complementary interaction occurs in heredity of normal hearing and deafness. Complementary usually leads to new traits formation, which were absent in parents.

In dominance, one gene is suppressed by another from same allele: A>a, B>b and so on. However, there is another type of interaction when one gene is suppressed by another from different allele: C>D, A>b, c>d and so on. Such event was named epistasis. The gene suppressing expression of another gene is called epistatic gene. The gene, which is suppressed by epistatic gene, is called hypostatic. Epistatic genes also called gene supressors. There are dominant and recessive epistases accordinary to epistatic genes.

The recessive epistasis can be studied on example of human “Bombey phenomenon”. If a person has dominant allele of blood group (A or B), these alleles aren’t expressed. As result of this person have I blood group. Such effect is explained by suppression effect of “Bombay phenomenon” gene in recessive homozygous state (hh). In cross of diheterozygotes of these genes we will have 25% of persons having I blood group, because of their homozygous genotype in H gene (hh).

All what was said above is correct if one locus of homologous chromosomes has only two alleles: A and a, В and b. But really we may have modified genes having several alleles such as a1, a2, a3, a4 and so on. Such alleles are called multiple alleles. Almost all genes that have been studied exhibit several different alleles. The alleles that determine the human ABO blood group, for example, comprise three common alleles.

The existence of ABO blood system was suggested by K. Landshteiner in 1900. He observed that blood coagulation occur in some cases blood mixture, but in some does not. In blood transfusion, it can lead to death. It was stated that erythrocytes contain two antigens A and B, whereas plasma contain two antibodies, б and в. In a population, there are all four blood groups (Table 8.1): A (having antigen A and antibody в), В (having antigen В and antibody б), AB (Having both A and В antigens and none antibody), and О (having only antibodies б and в without antigens). Group AB always has heterozygous genotype (Ialb). Group A may be homozygous (Iala) or heterozygous (Ialo) in genotype.

The same to В blood group. Group О always has homozygous recessive genotype (Iolo). Also genes of human HLA histocompatibility system, which are localized in 6th chromosome, are multiple genes (Pic. 14.1).

Often, an individual allele will have more than one effect on phenotype. Such an allele is said to be pleiotropic. The pleiotropic gene action may be primary and secondary. Primary pleiotropic gene expresses its effects simultaneously. For example, Marphan’s syndrome is encoded by one gene (pic 8.2).

Pic. 8.2. The arachnodactilia in Marphan’s syndrome(by E.Verschuer, 1938)

It has following traits: big height, thin fingers (arachnodactilia), eye lens dislocation, heart defect, high catecholamine level in blood. Another example is sickle cell anemia. The mutation in normal allele leads to defective hemoglobin formation. The erythrocytes loose their ability to transport oxygen and acquire sphere shape. Homozygote dies right after birth, but heterozygotes survive and are more resistant to malaria. The dominant mutation causing brahidactilia (short fingers) in homozygous state leads to embryo death before delivery (pic 8.3).

Ріс. 8.3. The herediting of dominant brachidactilia gene in human: Black circles are heterozygotes (Bb), light circles are recessive homozygotes (bb), BB - lethal dominant homozygote (by O. Mohr, C. Wieidt, 1919)

The gene mutation causing Hartnep’s disease leads to breaking tryptophan amino acid absorption in small intestine and reabsorbtion in kidney tubules. That result in simultaneous damage of two organs. In secondary pleiotropic gene action, we may see one gene effect that causes expression of several others. In particular, abnormal hemoglobin s in homozygous state leads to sickle cell anemia, which in turn leads to secondary phenotypic traits as malaria tolerance, anemia, hepatolienar syndrome, affection of heart and brain.

The gene action depends on gene dose. Normally each trait is controlled by two allelic genes, which may be homoallelic (dosage 2) or heteroallelic (dosage 1). In some cases gene dosage may be more than 2 (trisomia) or even less than 1 (monosomia). Gene dose is necessary for normal organism formation. For example, in female inactivation of one X-chromosome occurs after 16 days of embryonic development.

It is more complicated to determine variant of heredity in case of genocopies — are the cases when same trait is developed under control of different genes. For example, phenilketonuria is developed either with deficiency of dehydropteridinreductase or with deficiency dehydropholatereductase.