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Introduction
It is the transmission of qualities from one generation to the next generation. Traits are transferred or inherited by heredity laws.
Rules for the Inheritance of Traits – Mendel’s Contributions
- According to human genetic inheritance rules, both the father and the mother contribute about equal amounts of genetic material to the child
- So, for each trait, there will be two versions in each child
Mendel’s Experiment
- Pea plants with different characteristics were taken, such as a tall plant (TT) and a short plant (tt)
- Progeny was produced by crossing them, and percentages of tall or short progeny were calculated
- The first generation (F1 progeny) of the parental plants is all tall. However, when Mendel self-pollinated the F1 generation, he found that some plants were dwarfed in the second generation or F2 generation. Tall and dwarf plants were in the ratio of 3:1, 3 tall plants with trait homozygous tall (TT) and 1 with homozygous dwarf (tt)
- Two copies of a trait-controlling component (called a gene) are thought to be present in sexually reproducing organisms. Depending on the paternity, these two could be identical or different
- The dominant trait is expressed when the copies are not identical, and the recessive trait is expressed when the copies are not identical
Traits and their Expression
- The cellular DNA carries the information source for making proteins in the cell
- The gene for a protein is a piece of DNA that contains information for that protein
- Genes regulate an organism’s characteristics or attributes
- Each gene set is present as separate independent bits called chromosomes rather than as a single lengthy thread of DNA
- Each chromosome is duplicated in each cell, one from each of the male and female parents
- Each germ cell will take a pair of chromosomes, which could be either maternal or paternal in origin
- When two germ cells fuse with each other, the child acquires the correct number of chromosomes, hence ensuring the integrity of species DNA is maintained
Sex Determination
Both environmental and genetic factors determine the sex of an organism.
- Environmental factors: Physical factors such as temperature, light, etc. may determine the sex of an organism
- The temperature at which fertilised eggs are stored impacts whether the animals developing in the eggs will be male or female in a small number of reptiles
- Snails may alter their gender, suggesting that sex is not defined by genetics
- Genetic Factors: In human beings, the sex of the individual is largely genetically determined
- There are 22 pairs of maternal and paternal chromosomes in the human genome
- One set of chromosomes, known as the sex chromosomes, is unusual in that it is not always a perfect match
- Women have a perfect set of sex chromosomes, both of which are designated as X. Women, then, are XX
- Men have a mismatched set of Xs and Ys, with one being a normal-sized X and the other being a short Y. As a result, men are XY
- All children inherit an X chromosome from their mother regardless of their sex
- The child’s gender is determined by whether the paternal chromosome is X (for females) or Y (for males)
Conclusion
From the above discussion it can be concluded that Mendel’s contribution in the field of genetics was unknown to humankind unless the Law of genetics was rediscovered by the scientists Hugo Marie de Vries, Karl Franz Joseph Correns, and Erich Tschermak. Mendel on his observation on monohybrid crosses, he gave the following 3 rules. These rules are called the Principle of Inheritance. These 3 rules are as follows: the First Law or Law of Dominance, the Second Law or Law of Segregation, and the Third Law or Law of Independent Assortment. Mendel introduces us with various terms like traits, alleles, homozygous, heterozygous, dominant allele, recessive allele, etc. With the help of Punnett Square different combinations of gametes can be drawn. Mendel’s third law i.e., Law of Independent Assortment was modified into Chromosomal Theory of Inheritance. Mendel gave the concept for the linked genes. With this concept, sex-linked genes were studied. Chromosomes in two different sexes were called sex chromosome while the remaining was called autosomes. In human, a female has 22 pairs of autosomes and a pair of sex chromosomes (XX) and a male has 22 pairs of autosomes and a pair of sex chromosomes (XY). In Chicken, sex chromosomes in male are ZZ, and in females are ZW. Mutation in the arrangement of a set of chromosomes can lead to genetic disorders and change in the whole set of chromosomes(polyploidy) or change in subset of chromosome number (aneuploidy).
