Basic Principles of Inheritance
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Gregor Mendel conducted experiments with pea plants in the mid-19th century to study inheritance of traits. He found that traits are passed from parents to offspring through discrete units called genes, and that these genes segregate and assort independently during reproduction. Mendel identified laws of inheritance, including that each organism inherits two alleles for each gene, and that alleles segregate so each gamete receives one allele. His work formed the basis of classical genetics.
Basic Principles of Inheritance
- 1. BIT-303 Fundamentals of Genetics 4(3-1) by Amjad Masih Gull 1 Basic principles of Inheritance (Mendelism) Genetics It is a branch of biology concerned with the study of genes, genetic variation, and heredity in organisms. Genomics It is a branch of biology concerned with the study of whole set of chromosomes in an organism/species. Heredity had been observed by Gregor Mendel, a scientist working in the 19th century, was the first to study genetics scientifically. Mendel studied "trait inheritance", patterns in the way traits are handed down from parents to offspring. He observed that organisms (pea plants) inherit traits by way of discrete "units of inheritance". This term, still used today, is a somewhat ambiguous definition of what is referred to as a gene. Mendelian and classical genetics The observation that living things inherit traits from their parents has been used since prehistoric times to improve crop plants and animals through selective breeding. The modern science of genetics, seeking to understand this process, began with the work of the Gregor Mendel in the mid-19th century. Prior to Mendel, Imre Festetics, a Hungarian noble, who lived in Kőszeg before Mendel, was the first who used the word "genetics." He described several rules of genetic inheritance in his work “The genetic law of the Nature”. His second law is the same as what Mendel published. In his third law, he developed the basic principles of mutation.
- 2. BIT-303 Fundamentals of Genetics 4(3-1) by Amjad Masih Gull 2 Other theories of inheritance preceded Mendel's work. A popular theory during the 19th century, and implied by Charles Darwin's 1859 On the Origin of Species, was blending inheritance: the idea that individuals inherit a smooth blend of traits from their parents. Blending inheritance leads to the averaging out of every characteristic, which as the engineer Fleeming Jenkin pointed out, blending inheritance makes evolution by natural selection impossible. Mendel's work provided examples where traits were definitely not blended after hybridization, showing that traits are produced by combinations of distinct genes rather than a continuous blend. Blending of traits in the progeny is now explained by the action of multiple genes with quantitative effects. Another theory that had some support at that time was the inheritance of acquired characteristics: the belief that individuals inherit traits strengthened by their parents. The inheritance of such a characteristic means its reappearance in one or more individuals in the next or in succeeding generations. This theory (commonly associated with Jean-Baptiste Lamarck) is now known to be wrong—the experiences of individuals do not affect the genes they pass to their children, although evidence in the field of epigenetics has revived some aspects of Lamarck's theory. Other theories included the pangenesis of Charles Darwin (which had both acquired and inherited aspects) and Francis Galton's reformulation of pangenesis as both particulate and
- 3. BIT-303 Fundamentals of Genetics 4(3-1) by Amjad Masih Gull 3 inherited. It describes that each part of the body continually emitted its own type of small organic particles called gemmules that aggregated in the gonads, contributing heritable information to the gametes Morgan's observation of sex-linked inheritance of a mutation causing white eyes in Drosophila led him to the hypothesis that genes are located on chromosomes. Mendel Studied seven traits of pea plant Our understanding of how inherited traits are passed between generations comes from principles first proposed by Gregor John Mendel in 1866. Mendel worked on pea plants, but his principles apply to traits in plants and animals – they can explain how we inherit our eye color, hair color and even tongue-rolling ability etc. Mendel followed the inheritance of 7 traits in pea plants (Pisum sativum). He chose traits that had 2 forms: Pea shape (round or wrinkled) Pea colour (yellow or green) Flower colour (purple or white) Flower position (terminal or axial) Plant height (tall or short)
- 4. BIT-303 Fundamentals of Genetics 4(3-1) by Amjad Masih Gull 4 Pod shape (inflated or constricted) Pod colour (yellow or green). Why Mendel preferred Pea plant? 1. It is easy to cultivate 2. Its flowers are hermaphrodite 3. It is normally self-fertilizing but can be cross fertilized 4. Time gap between generations is very short 5. Pea has many distinct traits Gene: Basic unit of biological information Locus: Position of a Gene on chromosome Alleles: Partners of the gene pair on homologous chromosomes Homologous Chromosomes: A pair of chromosomes having alleles of a gene
- 5. BIT-303 Fundamentals of Genetics 4(3-1) by Amjad Masih Gull 5 Phenotype: Form of appearance of a trait. Genotype: Genetic compliment of an individual Gene Pool: All the genes / alleles found in a breeding population at a given time are collectively called gene pool Dominant Allele: Allele of a gene that shows effect of expression on its partner allele on homologous chromosome Recessive Allele: Allele of a gene whose expression is affected by its partner allele on homologous chromosome Mendel’s Law of Segregation “Two coexisting alleles for each trait in an individual segregate from each other during meiosis so that each gamete receives only one of two alleles” Mendel proposed that, during reproduction, the inherited factors must separate into reproductive cells. He had observed that allowing hybrid pea plants to self- pollinate resulted in progeny that looked different from their parents. Separation
- 6. BIT-303 Fundamentals of Genetics 4(3-1) by Amjad Masih Gull 6 occurs during meiosis when the alleles of each gene segregate into individual reproductive cells (eggs and sperm in animals, or pollen and ova in plants). Mendel’s Law of Independent Assortment “When two contrasting pairs of traits followed in the same cross, their alleles assort independently into the two gametes” Mendel observed that, when peas with more than one trait were crossed, the progeny did not always match the parents. This is because different traits are inherited independently – this is the principle of independent assortment. For example, he cross-bred pea plants with round, yellow seeds and plants with wrinkled, green seeds. Only the dominant traits (yellow and round) appeared in the F1 progeny, but all combinations of trait were seen in the self-pollinated F2 progeny. The traits were present in a 9:3:3:1 ratio (round, yellow: round, green: wrinkled, yellow: wrinkled, green).