Gregor Johann Mendel (1822-1884) was a biologist and meteorologist in the Austrian Empire. From 1856 to 1863, he conducted his famous pea plant experiments which revealed many of the rules of heredity, such as the law of independent assortment. Analyzing crossbreeding from a new perspective, Mendel established a heredity model that refuted the blending inheritance theory (progeny inherits the intermediate of its parental characteristics). In tribute to his inheritance theories, he was posthumously recognized as the founder of the modern science of genetics.
Basis of Mendelian Inheritance
Trait is a specific characteristic of an individual. It can be determined by genes (such as eye color), environmental factors (such as Darwin’s finches) or a combination of both. Observable traits are called phenotypes.
Relative character is the different expression of one trait of the same organism. For example, relative characters can be the black and white fur of rabbits or the curly and straight hair of humans.
Allele is a variant of the nucleotide sequences on a DNA molecule. Alleles are found in pairs on genomes. Each allele is situated at the same region in two parental homologous chromosomes. The specific regions that two alleles occupy are called loci (plural form of locus).
Genotype is the genetic composition of an organism. Phenotype is determined by the dominant-recessive relationships of the alleles that make up the genotype, along with environmental influences.
Gamete is a haploid cell that fuses with another haploid cell during fertilization. In animals, female gametes are called egg cells, and male gametes are called sperm.
Mendel’s Pea Experiments
Most biology students couldn’t be more familiar with Mendel’s famous story of his pea experiments. We always wonder how such an easy process elicits key theories of heredity. Mendel chose pea plants as his experimental subject, because peas are self-pollinated plants (pollen from the same plant arrives at the stigma of a flower). This helps peas to maintain parental characteristics and avoid external genetic contamination. To begin the experiment, Mendel chose pure-breeding white flowers as female parents (only stigma, no anther) and purple flowers as male parents (only anther, no stigma). Mendel manually cross-pollinated the flowers by transferring the pollen of purple flowers to the stigmas of white flowers. The first generation (F₁) of peas was all purple. Then, F₁ self-pollinated and bred the F₂.
Mendel counted the number of different-colored flowers and found an approximate ratio of 3:1 from purple to white. The result interested Mendel because if explained by blending inheritance theory, F₂ should be pink as the intermediate of the parental traits. From this experiment, Mendel established the law of dominance and uniformity, and the law of segregation.
The Law of Dominance and Uniformity
In Mendel's theories, he classified alleles into two categories “dominant” and “recessive”. We often write dominant alleles with capital letters and recessive alleles with lowercase. An organism with at least one dominant allele will display the effect of it. The table below shows the law of dominance and uniformity.
Whenever, the dominant allele, “B”, is presented in the genotype, the phenotype will be the traits of “B”.
The Law of Segregation
Mendel’s law of segregation states that: “During the formation of gamete, each gene separates from each other so that each gamete carries only one allele from each gene.” This ensures that new genotypes are formed by both female and male parental genes.
Mendel proved his law of segregation by the testcross method. He cross-pollinated F1(Aa) from the previous experiment with a recessive homozygote (aa). According to the Law of Segregation, ‘Aa’ would break into two kinds of gamete A and a, and ‘aa’ will break into one kind of gamete a.
Using the Law of Dominance and Uniformity, it can be predicted that the ratio between purple flowers (AA or Aa) and white flowers (aa) would be 1:1 as shown below. Surprisingly, the experiment results perfectly align with Mendel’s hypothesis which verified the Law of Segregation.
Law of Independent Assortment
When Mendel conducted his dihybrid experiment of yellow round peas (YYRR) and green wrinkled peas (yyrr), he found that F1 were always yellow round peas. This suggested that yellow (Y) and round (R) are two dominant traits. In accordance to the laws above, Mendel expected F2 to be yellow round peas and green wrinkled peas with a ratio of 3:1. However, he found that yellow wrinkled peas and green round peas were produced as well and with a total ratio of 9:3:3:1 for ‘Y_R_’ : ‘Y_rr’ : ‘yyR_’ : ‘yyrr’.
This suggested that each of the two alleles, Yy and Rr, is inherited independently from the other, with a 3:1 phenotypic ratio for each. Their multiple (3:1)(3:1) gives the correct answer of 9:3:3:1. Henceforth, Mendel established the Law of Independent Assortment: The alleles of two (or more) different genes get sorted into gametes independently of one another.
Conclusion
Mendelian inheritance, also named the Law of Dominance and Uniformity, the Law of Segregation, and the Law of Independent Assortment reveals the fundamental logic of heredity. It has revolutionized the understanding and medical treatments of genetic disorders, the crossbreeding of plants and animals for agriculture, and the mechanisms of evolution.
Works Cited
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