What life is


Mendelian genetics and patterns of inheritance

Mendelian genetics is based on the observation that each trait or phenotype is the result of two copies of a gene (or set of genes) called alleles. Often, alleles are identical, sometimes they are not. If they are different, the two copies of a gene affect a trait differently, because they both can contribute in different ways, or only one of them has an effect, overriding the effect of the other allele. An allele that overrides the effect of an other version of the same gene is called a dominant allele. An allele that cannot exert an effect in the presence of a dominant allele is called a recessive gene. In this case, only the trait of the dominant version can be seen (=phenotype). The individual is said to be heterozygous because the two alleles differ. If both alleles are identical, the individual is homozygous. A recessive allele can only exert its effect if two copies are present, i.e., in a homozygous individual. Homozygous individuals in animal and plant breeding are known as true-breeders. The combination of two alleles in an individual is called the genotype. The genotype (combination of actual genes) is inherited and determines the phenotype or trait.

The term 'recessive' says that while the effect cannot be seen in some individuals, the allele itself is still present (only in one copy) and can thus be passed on the next generation. If a child inherits two copies of a recessive allele, the dominant version is absent and the trait of the recessive allele appears. Thus, the trait receded for a generation, but reappeared. This observation of two alleles controlling the phenotype of a characteristics and the existence of dominant and recessive alleles was the single most important contribution of Mendel to genetics.

The success of Gregor Mendel is based on selecting suitable characteristics that allowed him to track the inheritance of characteristics with exactly two traits, one being dominant and the other recessive. As we know today, each trait is controlled by one of two alleles. Working with pea plants, Mendel followed seven characteristics that were inherited independently of each other, yet each characteristic had two traits. Independent inheritance of various traits is possible when the genes responsible for them are found on different chromosomes, the 23 individual packages of DNA that make up our Genome.

It is important to see the relevance of having two copies of a gene in one's genome and that these two copies can either be the same or vary slightly. The combination of the two copies we inherit determines not only our own life, but also the life of our children. Two carry two sets of genes allows accumulation of mutations without creating too many detrimental results. If one copy is changed, the other copy still determines the old, healthy form. How often a mutated gene variant is found in a population, is an important consideration. While Mendel artificially enriched alleles for certain characteristics in his breeding experiments, natural population show that certain alleles occur at very low frequency, say 1 in 10,000 individuals. Thus the chance of two individuals with the same genetic defect mating is very small and the allele remains a rare version of a gene. It is important to see that the frequency of an allele does not determine if it is recessive or dominant. A dominant allele that causes a severe disease often remains rare because an afflicted individual has a reduced chance at mating. The reason disease causing alleles exist at all is because they often don't reduce drastically the chance of mating, or sometimes increase the chance of survival if it provides protection against infectious diseases, as the case for sickle cell anemia, a hereditary disease caused by a recessive allele that if heterozygous, does not affect the individual much, but protects those carriers from malaria.

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 Copyright © 1999-2011 Lukas K. Buehler