Mendelian Genetics

MENDELIAN GENETICS

Mendelian genetics is a way of understanding inheritance according to laws of genetics outlined by Gregor Mendel. They serve as the main model for how genes get transferred from parents to offspring in both plants and animals. As mentioned, Mendel studied pea plants and hybridized them as part of his studies on inheritance. His work determined that inheritance is discontinuous rather than blended or continuous. Mendelian genetics is also referred to as classical genetics. Out of his studies came Mendel s laws.

He bred purebred purple and white pea plants to make what was called the F1 generation. Because these were purebred, they carried two copies of the same gene for flower color. The end result was that all flowers in the F1 generation were purple. He determined that purple color was a dominant trait. Figure 5 shows a Punnett square:

Figure 5.

He then looked at the progeny or offspring of the F1 generation to make an F2 generation. Because the F1 generation consisted entirely of heterozygous alleles, the end result was a ratio of purple to white flowers that was three to one. A homozygous organism has identical copies of the same allele, while a heterozygous organism has two different copies of an allele.

Remember that Mendel did not know anything about genes or alleles. Instead, he called the genes factors” and alleles were forms of the different factors. According to Mendel, these pairs of alleles separated independently and inherited one from each parent. This observation led to the Law of Segregation and the Law of Independent Assortment. According to the Law of Dominance, some traits are considered dominant over others.

Recessive traits only get expressed if they are homozygous. If the dominant allele or trait is at all a part of the organism s makeup, the phenotype will be that of the dominant trait. An organism with a dominant trait may be homozygous or heterozygous for the trait but one would not be able to tell the difference because the recessive trait is effectively hidden.

Mendel s first law is the law of segregation. It states that each organism has two alleles for each trait that completely separate during the making of sex cells or gametes. The second law is the law of independent assortment. It means that the different gametes are equally likely to be fertilized. Two separate alleles inherited by the offspring will be inherited separately. This is the law most likely to be violated because of the phenomenon of genetic linkage that we just discussed. The third law or the law of dominance says that a recessive trait will always be hidden by a dominant trait. This law may be broken but this is uncommon.

Are there instances of inheritance that can be non-Mendelian? As it turns out, there are. In the case of complete dominance, the offspring are of the dominant phenotype, even if they are heterozygous. In some cases, there can be incomplete dominance, where there is an intermediate to being dominant or recessive. There is codominance, where both the alleles get equally expressed. The ABO blood type in humans is an example of that. There are also multiple alleles. There will still be one allele per parent but the choices of alleles are more than two. In polygenic inheritance, many genes play a role in the inheritance pattern.