Genetics: Inheritance and Mendel’s Laws
Genetics is a branch of biological sciences that aims to study patterns of inheritance—how traits and characteristics are passed from parents to children. Genes are segments of DNA.
Genotype vs. Phenotype
- Genotype: The set of hereditary genetic constitution of an organism.
- Phenotype: In biology and health sciences, the phenotype is the expression of the genotype in a given environment.
Homozygous and Alleles
- Homozygous: An individual whose genotype has a pair of identical genes (also known as a pure individual).
- Alleles: A pair of genes that determine an individual’s characteristics.
Mendel’s Laws
Mendel’s three laws explain and predict how the physical characteristics (phenotype) of a new individual will be. They have often been described as “laws to explain the transmission of characters (genetic inheritance)” to offspring. From this point of view, character transmission, strictly speaking, does not correspond to Mendel’s first law (Law of Uniformity). It is a widespread misconception to suppose that the uniformity of hybrids that Mendel observed in his experiments is a law of transmission, but dominance has nothing to do with transmission, but with the expression of the genotype. As this observation is Mendelian, it is sometimes not considered a law of Mendel. So, there are three laws that explain the characters of the offspring of two individuals, but there are only two Mendelian transmission laws: the Law of Segregation of independent characters (2nd law, which, if not taking into account the law of uniformity, is described as 1st Law) and the Law of Independent Inheritance of characters (3rd law, sometimes described as 2nd Law).
Mendel’s First Law: Law of Uniformity
The Law of Uniformity states that if two pure breeds are crossed for a given character, the descendants of the first generation are all equal to each other (same genotype and phenotype) and equal (in phenotype) to one of the progenitors. It is not a law of transmission of characters, but of expression of dominance versus non-expression of recessive traits. So, it is sometimes not considered a law of Mendel. It indicates that it gives the same result when broken down into phenotypes.
Mendel’s Second Law: Law of Segregation
The Law of Segregation is also sometimes known as the first law of Mendel, equitable segregation, or disjunction of alleles. This law states that during gamete formation, each allele of a pair is separated from the other member to determine the genetic constitution of the gamete subsidiary. It is very common to represent the pathways of hybridization using a Punnett square. Mendel obtained this law by crossing different varieties of heterozygous individuals (diploid with two allelic variants of the same gene: Aa), and observed in his experiments with peas that he got many features of yellow skin and others (less) with characteristics of green skin. He found that the ratio was 3:4 for yellow and 1:4 for green (3:1). Under the current interpretation, the two alleles that code for each feature are segregated during the production of gametes through meiotic cell division. This means that each gamete will contain a single allele for each gene. This enables the maternal and paternal alleles to be combined in the offspring, ensuring variation. For each characteristic, an organism inherits two alleles, one for each parent. This means that in somatic cells, one allele comes from the mother and one from the father. They can be homozygous or heterozygous.
Mendel’s Third Law: Law of Independent Assortment
The Law of Independent Assortment is sometimes described as the 2nd Law. Mendel concluded that different traits are inherited independently of each other. There is no connection between them; therefore, the pattern of inheritance of one trait does not affect the inheritance pattern of another.
Codominance
Codominance is a state in which a gene expresses its property in a heterozygote in a manner equivalent to its peer. The alleles of the gene are expressed at the same time, giving birth to the male gamete, which is then joined to the female. In short, codominance, or shared dominance, is where the characters give a dominant phenotype from both the father and mother to the next generation.