Introduction
Genetic transmission is the driving force behind evolution. Therefore, the fundamental laws of inheritance are essential for understanding disease transmission patterns. The inheritance patterns of single-gene disorders are referred to as Mendelian since Gregor Mendel was the first to notice the various patterns of gene segregation for particular traits in garden peas and was able to calculate the likelihood of a trait recurring in later generations. A complete family history will be necessary to determine a pattern of transmission if a family is affected by a disease. Family history might also aid in ruling out hereditary diseases, particularly for prevalent diseases when environment and behavior are major contributors.
What Are Transmission Genetics?
Transmission genetics is a branch of genetics dealing with the study of inheritance in people. It is the transmission of genetic information from one generation to the next (from parents to children) or from one part of a cell to another, nearly identical to heredity. It is otherwise known as classical genetics. In transmission genetics, cross-sexual reproduction occurs between individuals, and the development of traits in children is studied. The basis of classical or transmission genetics is Gregor Mendel's analysis of the hereditary behavior of seven genes.
What Is the Focus of Transmission Genetics?
Transmission genetics is the field of genetics that studies the processes involved in transferring genes from parents to their offspring. Cell division, cell cycle, reproduction, replication, transcription, and signal transduction are some of the fundamental biological processes examined under it. The basis of classical or transmission genetics is cell division, including chromosomes, which comprise inheritance-containing units called genes. Mitosis is the division of somatic cells that contain 46 chromosomes (22 pairs of autosomes and two sex chromosomes, either XX or YY). A single somatic cell undergoes mitosis, resulting in two daughter cells, each having two diploid chromosomes (46). Only germ cells undergo meiosis, producing two gametes, each with a haploid number of chromosomes (23).
What Is Mendel's Concept of a Gene?
Gregor Mendel is the father of genetics. He was the first to demonstrate the inheritance pattern of traits from one generation to the next. In 1866, he developed the fundamental laws of genetics based on his research on the trait inheritance in the garden pea, Pisum sativum. Mendel started with pure-bred pea plants because they consistently produced offspring with the same trait as the parent plant. Then, he cross-bred pea plants to discover the inheritance pattern over many generations.
He followed the inheritance of the seven characteristics of the pea plant, including:
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Pea color (green or yellow).
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Pea shape (wrinkled or round).
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Plant height (short or tall).
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Flower color (white or purple).
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Flower position (axial or terminal).
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Pod color (green or yellow).
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Pod shape (constricted or inflated).
Mendel's experiments would not have been successful unless he had chosen traits that were entirely dominant or recessive (contrasting traits) or traits that were all found on distinct chromosomes (pea plants have seven chromosomes). Some basics of Mendel's genetics include the following:
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Alleles refer to each gene's two different forms.
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An organism is said to be homozygous if it has two copies of the same allele.
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An organism is said to be heterozygous if it has various alleles of the same gene.
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If an allele is dominant, it is expressed anytime that allele is present.
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Recessive traits are those that are hidden by one another.
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A recessive allele is covered or hidden by a dominant allele.
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The genotype is all an individual's genetic information about a specific trait. In a diploid organism, this indicates two copies.
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The phenotype of an individual comprises those features that are manifested and may be observed (physical traits) or measured (chemical characteristics). A heterozygous individual for quality might only exhibit the effects of one allele (a dominant one) and not the other recessive one.
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Cross-breeding or crossing is the process of combining two genetically distinct species.
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Hybrids are the offspring of such crosses.
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The P or paternal generation refers to the parents.
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The F1 generation refers to the children of these parents (first filial).
Why Did Mendel Select the Pea Plant for His Experiment?
Mendel chooses Pisum sativa (pea plant) as it has the following characteristics:
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There are many variants with contrasting traits.
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True-breeding, self-pollinating varieties.
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It is simple to cut portions to cross-pollinate.
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Requires little space.
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Produces many offspring.
What Are the Principles of Inheritance?
Two principles behind inheritance include the following:
1. Principle of Unit Factors: In every organism, single factors cause their features. It is called unit factors. Mendel did not know how these attributes were passed down, and now Mendel's unit factors are called genes.
2. Principle of Dominance: In an organism, some traits are visible or pronounced, and some are hidden, i.e., dominant and recessive.
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Dominant Genes - Dominant Genes are called functional alleles. Functional genes or dominant genes are written in uppercase letters.
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Melanin gene M.
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Pepsin gene P.
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Free earlobe gene = F.
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Polydactyly = P.
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Recessive Genes - Recessive genes are called non-functional alleles. This non-functional or dysfunctional gene is written in lowercase letters.
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mutated melanin gene = m.
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mutated free ear lobe gene = f.
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nonfunctional polydactyly gene = p.
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Recessive traits are those that are hidden by one another. When an allele has two copies and no other alleles, it is said to be homozygous, and only then does it manifest itself. A recessive allele is covered or hidden by a dominant allele.
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Dominance - Functional genes are responsible for dominant conditions. The trait can be caused by a single copy of the gene called polydactyly Pp. Examples: Rolling of the tongue, hitchhiker's thumb.
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Incomplete Dominance - Dominance does not occur in some allele combinations. Instead, the two features are expressed equally. For instance, the color dominance of snapdragon blossoms needs to be completed.
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Codominance or Multiple Alleles - Genes can mutate or alter. There may be various DNA sequences for a trait. When a gene exhibits co-dominance, all its variants are active and equally responsible for the phenotype. It is also referred to as multiple alleles. AB antigen type and sickle cell trait (Hbs) are two examples.
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Polygenic Inheritance - Although alleles influence many traits at a single location on the chromosome, some features are influenced by the interaction of genes on several chromosomes or at different locations on a single chromosome. The Inheritance of this condition is polygenic.
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Sex-Linked Traits - Genes on the X or Y chromatin are responsible for sex-related characteristics. Genes on the X or Y chromatin strands might be dominant or recessive. The likelihood of having a sex-related attribute depends on the person's gender. Examples include hemophilia A and red-green colorblindness.
What Are the Results of Mendel's Experiments?
Mendel tested inheritance patterns through breeding experiments in the garden of his monastery. Over numerous generations, he crossed common pea plants (Pisum sativum) with specified features. After crossing two plants that differed in a single feature, Mendel found that the first generation, or F1, was exclusively composed of individuals expressing one of the traits (short stems vs. tall stems, round vs. wrinkled peas, purple flowers vs. white flowers, etc.). However, when this generation was crossed, its children, the F2 (second filial generation), displayed a 3:1 ratio, meaning that three offspring shared one parent's trait and one offspring shared the characteristic of the other parent.
Mendel then proposed that genes can be composed of three pairs of hereditary units, which he referred to as factors: AA, Aa, and aa. The large A stands for the dominant factor, whereas the small A stands for the recessive factor. In Mendel's crosses, the F1 generation was Aa, the F2 generation was AA, Aa, or aa, and the starting plants were homozygous AA or aa. The relationship between these two determines the physical trait.
What Are Mendel's Laws of Inheritance?
The following three principles, or laws, summarize Mendel's observations and conclusions.
Law of Dominance - Mendel's law of dominance states that when two organisms with different traits combine, each offspring only demonstrates the characteristic of one parent. A person will develop the dominant feature if the dominant factor is present. If both elements are recessive, the recessive trait will only manifest.
Law of Segregation - According to the law of segregation, although the alleles of a character remain together for a long time, they do not mix and separate during gametogenesis, so each gamete acquires just one allele of a trait, which is either dominant or recessive. When homozygous tall and dwarf pea plants from the F1 generation self-fertilize, tall and dwarf plants are produced in a 3:1 ratio.
Law of Independent Assortment - According to the law of independent assortment, when more than two characters are taken, a character's alleles can go through any combination to produce a phenotype distinct from both parents.
What Are the Exceptions to Mendel's Law?
Some exceptions to Mendel's rules or laws have been identified as our understanding of genes, and heredity has grown. For example, the independent assortment principle is not applicable if the genes are close to one another (or connected) on a chromosome. Moreover, alleles may not necessarily interact in a typical recessive or dominant manner, especially if they are codominant or differ in penetrance or expressivity.
Conclusion
Transmission genetics, often known as classical genetics, studies the transmission of genes from parents to children. Mendel's research has provided scientists with the foundation for mathematically forecasting the chances of genotypes and phenotypes in the offspring of a genetic cross. On the other hand, not all genetic observations can be predicted and explained using Mendelian genetics. The concepts of Inheritance and the understanding of unit inheritance were the basis of our modern science of genetics.
