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Mendel and the Gene Idea

Mendel’s law of segregation claims that the two alleles for each trait of a diploid organism split in the process of gamete formation and that during the formation of new zygotes, the alleles will randomly combine with other alleles.

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Genotype is the totality of all the genes of an organism, which compose its hereditary basis. A phenotype is a set of all the characteristics of an organism that are revealed in the process of individual development in a certain internal and external environment. While homozygote is an organism that has allelic genes of one molecular form (AA or aa), heterozygote has allelic genes of different molecular forms (dominant and recessive). A recessive gene is an allele that determines the development of a trait only in the homozygous state, and an allele that determines the development in both homozygous and heterozygous states is dominant.

A testcross used an experimental mating test to identify whether an allele is homozygous or heterozygous. Namely, an organism to be tested is crossed with another one whose recessive trait is homozygous, and their offspring are examined. Recessive offspring indicates that the parental organism is heterozygous.

According to Mendel’s law of independent assortment, every pair of alternative traits is inherited in several generations self-reliantly each other during meiosis. As a result, organisms with new combinations of traits appear among the descendants of the second generation.

The crossing of white snapdragons with red snapdragons produces all F1 hybrids having a pink color. When they reproduce, it leads to F2 hybrids with one red to two pinks to one white ratio, which does not confirm the blending theory. It means that the hybrids maintain their identity since pink gametes produce red and white flowers.

In complete dominance, the phenotypes of the dominant homozygote and heterozygote cannot be distinguished as they are identical, while incomplete dominance presents seemingly blending phenotypic expression. In co-dominance, both alleles impact the phenotype in distinct ways.

The ABO blood system is associated with glycoproteins that can be present it absent in red blood cells. An allele and B allele is responsible for producing and inserting into A and B membranes, respectively, but the O allele is not characterized by the production of glycoprotein. An individual can be homozygous for O, and he or she can have AO or AB genotypes. People with B blood can be BB or BO, people with A blood can be AA or AO, and those with AB have both types of glycoprotein.

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Pleiotropy is multiple gene action, influencing the development of several traits. The pleiotropic effect is possessed by genes that control the synthesis of broadly acting enzymes involved in several biochemical processes. An example of pleiotropy is the gene for white coat color in cats. The dominant allele W of this gene determines not only the white color of the coat but also the color of the eyes and deafness of blue-eyed cats.

Epistasis is the interaction of non-allelic genes when the gene of one allelic pair suppresses the action of the dominant gene of the other allelic pair. Instead of Mendelian splitting during dihybrid crossing of 9: 3: 3: 1, splitting of 15: 1, 9: 7, and others were recorded. This indicated that there are certain relationships between non-allelic genes, leading to other types of splitting of traits in offspring. In a dihybrid cross, there will be fewer than four common phenotypic classes.

A simple polygenic inheritance refers to the interaction of several genes to generate a particular trait. This process is usually described in quantitative terms because there are multiple genes, and their characteristics may vary, which cannot be accurately identified using qualitative methods.

Environmental conditions affect the phenotype through temperature, nutrition patterns, stress, as well as exposure to toxins, radiation, and pathogens. For example, smoking and environmental pollution may be triggers for developing a predisposition to asthma and lung cancer. In Siamese cats, higher temperature leads to lighter coats.

A lethal recessive gene can only be homozygous, but it is transferred by heterozygous carriers since they possess normal phenotypes. If the lethal allele did not cause the death of its carrier until his or her maturity and reproduction, it is maintained in the population and conveyed to future generations.

Consanguinity occurs when people with the “same blood” (cousins, siblings, and so on) meet and mate, and they are more likely to carry the same recessive disease-causing alleles compared to unrelated persons. Therefore, it raises the ratio of homozygous offspring for recessive traits.

Huntington’s disease is one of the most prominent hereditary neurodegenerative diseases that are characterized by the presence of a lethal receive allele. Because of the almost complete penetrance of the mutant BG gene, its carrier will inevitably fall ill. Until the onset of the disease, a carrier of this gene remains healthy and produces children, who have a 50% probability of inheriting the mentioned disease. Since the disease’s phenotypic effect is expressed only at 35-45 ages, it can easily escape elimination.

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Morgan discovered the mutation of “white eyes”, which laid the foundation for experimental genetics of Drosophila. Morgan also confirmed that the mutant gene is on the X chromosome, which was the first specific proof of the leading role of chromosomes in heredity. Sutton suggested the theory that the transmission of hereditary information in a series of generations is carried out by the transfer of chromosomes, in which genes are located in a certain linear sequence. Sturtevant proposed to strip genetic maps by plotting dots on a straight line corresponding to specific genes following the frequency of crossing over between them, and it was the first recombination genetic map.

Its small size and the ease of cultivation made it possible to use several Drosophila species as model objects for genetic research. Important characteristics of D. melanogaster as the object of experiments are the small number of chromosomes (2n = 8), a wide variety of visible manifestations of mutations, and the presence of polytene chromosomes in several organs. In addition, the advantages of Drosophila melanogaster over other biological objects are a short development cycle and high fertility.

Linkage is the state when two traits do not assort fully autonomously. Genes are arranged likewise pearls on chromosomes, and they do not assort independently. Depending on the closeness of the genes, meiosis may or may not interfere with them.

Parental phenotypes are offspring phenotypes that resemble those of the parents, and recombinant phenotypes are offspring phenotypes that differ from the parent’s phenotypes. Thus, it is the similarity between offspring and parental phenotypes that helps in distinguishing between parental and recombinant phenotypes. Recombinant phenotypes appear from crossing over during meiosis.

The presence or absence of the Y chromosome and SRY gene defines human sex, whereas females do not possess this chromosome. The activation of SRY leads to the formation of the anti-mullerian hormone and testosterone, which develop a male reproductive system. In humans, sex is also determined by the environment, and the differences begin at the seventh week of gestation.

Wilson explained the inheritance of color-blindness, suggesting that it is localized on the X chromosome and that in humans, heterogametic (XY) is the male sex. It becomes clear that in a marriage of a homozygous normal woman (XD XD) with a color-blind man (XDY), all children are born normal. However, at the same time, all daughters become hidden carriers of color-blindness, which may manifest in subsequent generations. If a woman suffering from color-blindness marries a man with normal vision, their daughters will receive the sign of a father, and all sons will receive the mother’s color-blindness.

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