Mendel’s Laws of Heredity
Despite the fact that people often have a difficult picture of Huntington’s disease and Phenylketonuria (PKU), two examples of hereditary psychological disorders, both disorders can be explained by a number of simple heredity rules. The essence of these rules was worked out more than a century ago by Gregor Mendel (1866). However, the rules have some exceptions.
Mendel’s Laws of Heredity
Mendel concluded that there are two elements of heredity. are present in every individual for every trait. These elements are separated from each other during the production of germ cells. The offspring receives one of the two elements, one from each parent. Mendel further concluded that one of these elements dominated the other, so that an individual has only one dominant element will display the property. The non-dominant, recessive, element is only voiced if both elements are recessive. This conclusion is the essence of Mendel’s first law: The law of divorce (The law of Segregation).
Mendel’s elements are now known as genes: the basic units of heredity. The alternative forms of genes become alleles mentioned. An individual’s combination of alleles becomes genotype mentioned. The observed properties become phenotype mentioned. Mendel systematically experimented with crosses between different types of pea plants. These races differed from each other in two or more characteristics. He discovered that the alleles for the two traits are inherited separately. In other words, the inheritance of one gene is not affected by the inheritance of the other gene. This is Mendels Law of independent assortment (The Law of independent assortment).
Most important about Mendel’s second law are the exceptions. We now know that genes do not float around in an egg or sperm cell. They are worn by chromosomes (literal meaning: ?? colored body ??: colored body). Genes are located in places called loci (simplicity: locus) on chromosomes. Egg cells contain one chromosome from each pair of the mother’s chromosomes and sperm cells contain one chromosome from each pair of the father’s chromosomes. However, Mendel’s law of independent assortment is violated when the genes for two traits are close to each other on the same chromosome. The two properties are then not inherited independently. This phenomenon becomes linkagementioned. Unless genes are close together on a chromosome, they will be recombined by a process of exchanging parts of chromosomes: crossing-over. Recombination occurs when germ cells are formed (meiosis) in the ovaries and testes. Such violations of Mendel’s laws make it possible to map genes on chromosomes using linkage analysis. For Huntington’s disease and Phenylketonuria (PKU), the connections between genes have been established and the genes responsible for the disorder have been identified.
Other Exceptions to Mendel’s Laws
The most common exception to Mendel’s laws pertains to DNA mutations that do not affect the parents because they arise during the formation of the egg or sperm cells. This situation is not entirely a violation of Mendel’s laws because the new mutations are further inherited according to Mendel’s laws, despite the affected individuals having no affected parents. Many mental disorders arise from such spontaneous mutations.
Changes in chromosomes
Chromosome changes are a major cause of mental retardation. At the end of 1950 it was discovered that Down syndrome (Trisomy 21) is caused by the presence of a complete extra chromosome (chromosome 21). In this case, Mendel’s laws are being violated because the number of chromosomes that is inherited is different. 2 chromosomes are inherited from the mother and one chromosome is inherited from the father. This results in a fertilized egg with three times chromosome 21 instead of two times chromosome 21.
Multiple Triplet Repetitions
A special form of mutation involves the repetition of pieces of DNA. Short stretches of DNA (two, three or four nucleotide bases in length) are repeated a number of times or more than a dozen times. It is still unknown why this happens. Sometimes the number of repetitions at a particular locus is increased, higher than the normal ?? number of repetitions. This causes problems. This extensive number of triplet repeats is unstable and can increase in later generations. Ultimately, this leads to illnesses. For Huntington’s disease, longer repetitions lead to earlier onset and higher severity of the disease. The extended repetition of the triplet is CAG. This triplet codes for the amino acid glutamine and results in a protein with an extensive number of glutamines in the center of the protein. The extra glutamines change the three-dimensional structural organization of the protein and add new and toxic properties to the protein. This leads to the death of neurons in the brain. Despite the fact that this mutation goes against Mendel’s Laws, Huntington’s disease is otherwise inherited according to Mendel’s Laws.
Another example of an exception to Mendel’s laws is genomic imprenting. For the expression of a gene, it depends on whether the gene originates from the father or the mother. The precise mechanism by which the allele of one parent is expressed is unknown. We do know that it relates to the inactivity of part of the gene through a process called methylation.