Mendel’s Laws of Inheritance | Zoology Optional Notes for UPSC PDF Download

Introduction

An Austrian monk “Gregor Mendel” laid the foundations of the modern genetics. What if you want to figure out how genetic information is transmitted between generations? For instance, you might be curious how traits can "skip" a generation, or why one child in a family may suffer from a genetic disease while another does not. How could you go about asking these kinds of questions scientifically? Characteristics that prevail in families often have a genetic basis, i.e. they depend on genetic information a person inherits from his or her parents – Inheritance of characters. Here, we'll see how a nineteenth-century monk named Gregor Mendel uncovered the key principles of inheritance using a simple, familiar system: the pea plant.

Johann Gregor Mendel (1822–1884)

Research on heredity
In 1856, Mendel began a decade-long research project to investigate patterns of inheritance. Although he began his research using mice, he later switched to honeybees and plants, ultimately settling on garden-pea (Pissum sativum) as his primary model system.

Mendel's Experiments

Mendel studied the inheritance of seven different features in peas, such as Plant height, flower color, seed color, seed shape, etc. Mendel selected the following 7 contrasting characters in pea-plant for his study. In 1865, Mendel presented the results of his experiments with nearly 30,000 pea plants to the local Natural History Society. Based on the patterns he observed, the counting data he collected, and a mathematical analysis of his results. Mendel proposed a model of inheritance, in which characteristics such as flower color, plant height, and seed shape were controlled by pairs of heritable factors(now called as genes) that appear in different versions.

Mendel's Choice of Pea Plant

Mendel chose pea plant for experiments because of its useful features, such as:

• Several contrasting phenotypic features.
• Rapid life cycle and the production of lots of seeds in each generation.
• Convenient handling (small plants)
• Controlled mating of the plants is possible: Pea is a typically self-fertilizing plant, i.e. the same plant bears both the microspore (sperm) and the ovule (egg) that comes together in fertilization. Mendel took advantage of this property to produce true-breeding or pure lines: he self-fertilized and selected peas for many generations until he got lines that consistently made offspring identical to the parent (e.g., always tall or always short).

Mendel's Experimental Approach

• Once Mendel had established true-breeding lines (or pure lines) of pea plant with different traits for one or more features of interest (such as tall vs. short height), he began to investigate how the traits were inherited by carrying out a series of crosses.
• First, he crossed one true-breeding parent to another. The plants used in this initial cross are called the P generation, or parental generation. Mendel collected the seeds from the P generation-cross and grew them up. These offspring were called the F1 generation or first filial generation. Once Mendel examined the F1 generation plants and recorded their traits, he let them self-fertilize naturally, producing lots of seeds. He then collected and grew the seeds from F1 plants to produce F2 generation or second filial generation. Again, he carefully examined the plants and recorded their traits. Mendel's experiments extended beyond the F2 generation to F3, F4 and later generations, but his model of inheritance was based mostly on the first three generations (P1, F1, and F2). Mendel not only recorded phenotype of the plants in each generation (e.g., tall vs. short), but also, he counted exactly how many plants with each trait were present. This may sound tedious, but by recording numbers and thinking mathematically. Strikingly, he found very similar patterns of inheritance for all seven features he studied. Mendel made discoveries that eluded famous scientists of his time (such as Charles Darwin), who carried out similar experiments but could not grasp the significance of his results.

Mendel's Model of Inheritance

Parents pass along heritable factors (now called as genes), which determine the specific characters (traits) of the offspring. Each individual has two copies of a given gene, called as alleles; for example, alleles for seed-color: yellow (gene Y) and green (gene y). Following a cross (hybridization), one of the two alleles of a given gene (dominant allele) expresses itself in the form of visible character, while the other one (recessive allele) is suppressed, hidden or masked by the dominant allele. In the presented example, the dominant allele is gene Y (yellow-seed) and recessive allele is gene y (green seed). The set of alleles carried by an organism is known as its genotype. Genotype determines phenotype (an organism's observable features). When an organism has two copies of the same allele (say, YY or yy), it is said to be homozygous for that gene. If, instead, it has two different copies (like Yy), we can say it is heterozygous for that gene.

Mendel's Monohybrid Cross

Mendel’s Monohybrid Cross deals with the inheritance of a single character (e.g. seed colour). Parental generation (P) has homozygous genes for yellow (YY) and green (yy) seeds. The parents produce gametes Y (for yellow seed) and y (for green seeds). On fertilization (crossing), heterozygous (Yy) F1 hybrid is produced, which produces two types of gametes, viz. Y (yellow seeds) and y (green seeds). Self-pollination of F1 plants results in F2 generation, which shows 3:1 phenotypic ratio (3 yellow : 1 green seeds) and 1:2:1 genotypic ratio (1 YY: 2Yy:1yy). The Punnett Square is a visual representation of Mendelian inheritance. It is a diagram that is used to predict an outcome of a particular cross or breeding experiment. It is named after Reginald C. Punnett, who devised the approach. The diagram is used by biologists to determine the probability of an offspring having a particular genotype. The Punnett square is a tabular summary of possible combinations of maternal alleles with paternal alleles.

Mendel's Law of Dominance

Mendel’s Law of Dominance can be simply stated as: “In a cross of parents that are pure for contrasting traits, only one form of the trait will appear in the next generation. Offspring that are hybrid for a trait will have only the dominant trait in the phenotype.”

Mendel's Law of Segregation

The Law of Segregation states that every individual organism contains two alleles (two forms of a gene) for each trait, and that these alleles segregate (separate) during gamete formation such that each gamete contains only one of the alleles. An offspring thus receives a pair of alleles for a trait by inheriting homologous chromosomes from the parent organisms: one allele for each trait from each parent. In the pea-plant example given below, there are represented 3 generations, viz. P, F1 and F2.

P is parental generation (true-breeding parents): PP (purple) or pp (white) flowers

• F1 generation (Hybrid): Pp (all purple-flower bearing plants)
• F2 generation (3 types of plants):
  • PP (purple)
  • Pp (purple)
  • pp (white)

Mendel's Law of Independent Assortment

Mendel's law of independent assortment states that the alleles of two (or more) different genes get sorted into gametes independently of one another. In other words, the allele a gamete receives for one gene does not influence the allele received for another gene. To solve the problem, Mendel conducted a dihybrid cross, taking into consideration two characters at a time, viz. the seed-color (yellow or green) and seed-shape (round or wrinkled). He found that different genes were inherited independently of one another, following the law of independent assortment.

A Test Cross

Mendel also discovered a way to figure out whether an unknown plant with a dominant phenotype (such as a yellow-seeded pea plant) was a heterozygote (Yy) or a homozygote (YY). This technique is called a test cross and is still used by plant and animal breeders today. Test cross is a back cross of F1 hybrid-plant (Yy) with its parent having recessive genotype (yy). This cross will show a 1:1 ratio between dominant (Yy) and recessive (yy) plants, revealing the genotype of the unknown plant (i.e. it has Yy genotype) – it is a test cross.

Scientific Recognition of Mendel's Work

Mendel's work went largely unnoticed by the scientific community during his lifetime. In some way, Mendel's contemporaries failed to recognize the importance of his work because his findings went against the then prevailing ideas about genetic inheritance. Secondly, although we now see Mendel's mathematical approach to biology as innovative and pioneering, it was new, unfamiliar, and perhaps confusing to other biologists of that time. In the mid-1800s, most biologists used to believe the idea of blending inheritance. Blending inheritance wasn't a formal, scientific hypothesis, but rather, a general model in which inheritance involved the permanent blending characteristics of parents in their offspring. The blending model could not explain Mendel's results, which showed distinct ratios of traits in the offspring of crosses. It was not until around 1900 that Mendel's work was rediscovered, reproduced, and revitalized by the biologists working on the chromosomal basis of heredity.

Conclusion

Mendel's experiments with pea plants and his laws of inheritance laid the foundation for modern genetics. His work was initially overlooked but later rediscovered and recognized for its significance. Mendel's laws of dominance, segregation, and independent assortment explained patterns of inheritance and paved the way for further research in genetics.