The laws of inheritance were derived by Gregor Mendel, a 19th century Moravian monk conducting plant hybridity experiments.Regardless, the "re-discovery" made Mendelism an important but controversial theory. Its most vigorous promoter in Europe was William Bateson, who coined the term "genetics", "gene", and "allele" to describe many of its tenets.Mendel''s findings allowed other scientists to simplify the emergence of traits to mathematical probability. A large portion of Mendel''s findings can be traced to his choice to start his experiments only with true breeding plants. without his careful attention to procedure and detail, Mendel''s work could not have had the impact it made on the world of genetics.The Law of Segregation, also known as Mendel''s First Law
, essentially has four parts. Alternative versions of genes account for variations in inherited characteristics.
This is the concept of alleles. Alleles are different versions of genes that impart the same characteristic. For example, each human has a gene that controls eye color, but there are variations among these genes in accordance with the specific color for which the gene "codes". For each characteristic, an organism inherits two alleles, one from each parent.
This means that when somatic cells are produced from two alleles, one allele comes from the mother and one from the father. These alleles may be the same (true-breeding organisms/homozygous or different If the two alleles differ, then one, the allele that encodes the dominant trait, is fully expressed in the organism''s appearance; the other, the allele encoding the recessive trait, has no noticeable effect on the organism''s appearance.
In other words, only the dominant trait is seen in the phenotype of the organism. This allows recessive traits to be passed on to offspring even if they are not expressed. Not all traits have a dominant-recessive relationship, however. The petal color of flowers of the "Japanese four o''clock" plant Mirabilis jalapa
illustrate incomplete dominance There is also codominance e.g. human blood types, for which A and B are codominant and O is recessive. The two alleles for each characteristic segregate during gamete production.
This means that each gamete will contain only one allele for each gene. This allows the maternal and paternal alleles to be combined in the offspring, ensuring variation. N.B It is often misconstrued that the gene itself is dominant, recessive, codominant, or incompletely dominant. It is, however, the trait
or gene product that the allele encodes that is dominant, etc.The Law of Independent Assortment, also known as "Inheritance Law" or Mendel''s Second Law
, states that the inheritance pattern of one trait will not affect the inheritance pattern of another. While his experiments with mixing one trait always resulted in a 3:1 ratio between dominant and recessive phenotypes, his experiments with mixing two traits (dihybrid cross) showed 9:3:3:1 ratios.
But the 9:3:3:1 table shows that each of the two genes are independently inherited with a 3:1 ratio. Mendel concluded that different traits are inherited independently of each other, so that there is no relation, for example, between a cat''s color and tail length. This is actually only true for genes that are not linked to each other. Independent assortment occurs during meiosis I in eukaryotic organisms, specifically anaphase I of meiosis
,to produce a gamete with a mixture of the organism''s maternal and paternal chromosomes. Along with chromosomal crossover, this process aids in increasing genetic diversity by producing novel genetic combinations. Of the 46 chromosomes in a normal diploid human cell, half are maternally-derived (from the mother''s egg) and half are paternally-derived (from the father''s sperm). This occurs as sexual reproduction involves tgametes (the egg and sperm) to produce a new organism having the full complement of chromosomes. During gametogenesis - the production of new gametes by an adult - the normal complement of 46 chromosomes needs to be halved to 23 to ensure that the resulting haploid gamete can join with another gamete to produce a diploid organism. An error in the number of chromosomes, such as those caused by a diploid gamete joining with a haploid gamete, is termed aneuploidy. In independent assortment the chromosomes that end up in a newly-formed gamete are randomly sorted from all possible combinations of maternal and paternal chromosomes. Because gametes end up with a random mix instead of a pre-defined "set" from either parent, gametes are therefore considered assorted independently. As such, the gamete can end up with any combination of paternal or maternal chromosomes. Any of the possible combinations of gametes formed from maternal and paternal chromosomes will occur with equal frequency. For human gametes, with 23 pairs of chromosomes, the number of possibilities is 2^23 or 8,388,608 possible combinations. The gametes will normally end up with 23 chromosomes, but the origin of any particular one will be randomly selected from paternal or maternal chromosomes. This contributes to the genetic variability of progeny.