Gregor Johann Mendel (1822-1884), an Austrian monk, laid the setting stone in the path to the evolution of genetics as we know it today. His famous hybridization experiments with the pea plant laid the foundations for the laws of inheritance, which are the basis for all sexually reproducing eukaryotic diploid organisms. Mendel carefully carried out certain sets of experiments using different qualitative traits of pea plants and crossed them with each other. He formulated his observations into three principles of inheritance. He discovered that traits are inherited in the form of certain discrete “factors”. A diploid organism carries two versions of the same factor, known as its alleles, which retain their physical identity, even in a hybrid. It was only later on that these discrete factors were discovered to be genes. These paired copies of a gene carry information for the expression of the same traits but produce different effects.
Mendel performed several experiments by crossing true-breeding pea plants with each other, to produce the first generation (F1 generation aka filial generation 1). The selfing of F1 progeny resulted in forming the F2 generation. Each time Mendel performed a cross he carefully recorded his observations and collected statistical data. He focused mainly on seven different contrasting traits and conducted several monohybrid crosses and dihybrid crosses. Based on his experiments with Pisum sativum (i.e., pea plant) he laid down three basic principles of heredity, namely −
In a heterozygous organism, the expression of one allele encoding a particular trait is always repressed by the presence of the other allele encoding the contrasting characteristic.
In a cross between true-breeding plants for contrasting traits, the F1 generation exhibits only one of the two traits, which is termed the dominant trait.
This, however, does not imply that the F1 generation doesn't inherit the other trait. It inherits both traits, but only one that is dominant is expressed phenotypically.
This law states that in a diploid organism that possesses two alleles for a particular characteristic, each allele segregates during the process of gametogenesis, one allele going into each gamete. Thus, gametes are always pure for a certain trait.
This law states that in a dihybrid cross, the assortment of one gene of one pair is independent of the other pair at the time of gamete formation. This means that each pair of contrasting characters bears no association with a particular trait.
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The law of independent assortment was deduced through the dihybrid cross of pea plants. A dihybrid cross occurs between organisms that differ in two characteristics or traits.
The Law of Independent Assortment states that in a dihybrid cross, the assortment of one gene occurs independently of the other genes during the process of gametogenesis.
What this implies, in simple terms, is that the inheritance of one character by the offspring is independent of the presence of other characters in a sexually reproducing, diploid organism.
During meiosis, the homologous pairs of chromosomes separate, and the members of the pair are segregated into a different daughter nucleus, resulting in haploid gametes.
What each gamete receives is random and independent of the other pairs of chromosomes. This implies that chromosomes from the same source (i.e., maternal or paternal) can assort into different gametes.
Hence, the allele for a gene received by a gamete does not show linkage to other genes. In Mendel’s experiment, for example, the segregation of seed color is independent of the segregation of seed shape, and both characters appear in the F2 generation.
Examples
In Mendel’s experiment, he crossed a true-breeding pea plant producing round and yellow seeds (RRYY) with a true-breeding plant that produces wrinkled and green seeds (rryy).
The F1 generation thus produced all plants with round and yellow seeds (RrYy).
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However, upon self-crossing the F1 generation, he obtained striking results in the F2 generation.
While the F1 generation produced round and yellow seeds (RrYy), the F2 generation produced new combinations, which were very different from the parental combination of RrYy.
Hence, the alleles R, r and Y, and y from the parents can assort independently of each other, into separate gametes in the F1 generation, and result in new combinations observed in the F2 generation.
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The genotypic ratio in such dihybrid crosses is 1: 2: 2: 4: 1: 2: 1: 2: 1.
The phenotypic ratio obtained is 9: 3: 3: 1 (round, yellow: wrinkled, yellow: round, green: wrinkled, green).
Gregor Johann Mendel performed a series of breeding experiments with pea plants, to understand the inheritance of characters in offspring. Based on his experiments, he formulated three laws.
According to the law of dominance, only one of the two contrasting traits of the same character is exhibited in the offspring. The trait which is expressed is the dominant trait.
The law of segregation states that two alleles encode the same character in a diploid organism, and each allele segregates (separates) into a separate gamete during gametogenesis.
According to the law of independent assortment, the genes encoding different characters in a diploid organism are assorted into gametes independently of each other.
The Law of independent assortment was formulated via observations of a dihybrid cross between two pea plants. The phenotypic ratio of such a cross is 9: 3: 3: 1.
Q1. Is the law of dominance universal?
Ans. No. Dominance is not a universal concept. Sometimes, a heterozygote may exhibit a phenotype that is intermediate between the two homozygotes, a condition known as incomplete dominance. In other cases, a heterozygote may express both the homozygotic phenotypes simultaneously. This is known as co-dominance.
Q2. What is meant by the penetrance of genes?
Ans. Penetrance is the extent to which a gene is expressed in the phenotypes of individuals carrying it.
Q3. Are both parents always involved in the inheritance of all characters?
Ans. No. There are some traits, especially some diseases that are sex-linked and can be present on the sex chromosomes. Moreover, we know that mitochondria have their DNA. This mitochondrial DNA is in fact, inherited from the mother ONLY. In the inheritance of all other nuclear traits, both parents are involved.
Q4. What is gene interaction?
Ans. Gene interaction refers to when two or more non-allelic genes (i.e., genes at different loci) influence the expression of a single trait. The expression of the alleles at one locus depends on the products of another gene, situated at another locus on the chromosome.
Q5. What are the limitations of the law of independent assortment?
Ans. The law of independent assortment is not applicable in the cases of linked genes, non-allelic gene interactions, and under the cases of codominance or partial dominance.
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