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Atoms are the basic units of matter and are made up of smaller components called subatomic particles. There are many types of subatomic particles known to science, but in the context of basic atomic structure, only three are considered most important: electrons, protons and neutrons. These three particles differ in their location, charge and mass. Together, electrons, protons and neutrons form the complete structure of atoms. Their arrangement and interaction define the atom's properties, chemical behavior, and participation in physical and chemical processes. These subatomic particles laid the foundation of modern atomic theory and quantum chemistry. 1. Electron Electrons are negatively charged subatomic particles. They are extremely small in mass and are found outside the nucleus of the atom in specific regions called orbitals or shells. The charge of an electron is −1  and its mass is approximately 1/1836 of a proton or neutron, which is around 9.1 × 10⁻³¹ kg, meaning it is n...

Question: Iᴬ, Iᴮ, i, H genes govern blood group antigens A and B. Using Punnett square, show the ratio of different phenotypes obtained on crossing two heterozygotes, IᴬIᴮHh × IᴬIᴮHh. Which factors contribute to the typical phenotypic ratio?  Cross Involving Two Genes: IᴬIᴮ Hh × IᴬIᴮ Hh Genes Involved This cross involves two independently assorting genes: ABO gene (Iᴬ, Iᴮ, i): Determines A, B, AB or O blood group. Iᴬ and Iᴮ are codominant, while 'i' is recessive. H gene (H, h): Governs expression of ABO antigens. The dominant H allele allows surface expression of A or B antigens. The hh genotype leads to the Bombay phenotype, where antigens are not expressed regardless of ABO genotype. Note : Since both parents are IᴬIᴮ, only Iᴬ and Iᴮ alleles will segregate, not 'i'. So 'O' genotype does not appear in this case, but Bombay phenotype may mimic 'O'. Step 1: Gametes Formation Each parent (IᴬIᴮ Hh) can produce four types of gametes due to independent se...

In classical Mendelian genetics, one gene controls one trait. But in real life, many traits are controlled by two or more genes and sometimes one gene can affect or interfere with the expression of another gene. This interaction is called  epistasis.  Here, we will compare two types:  recessive epistasis and duplicate recessive epistasis.  Both involve two genes, but the way they interact is different. Recessive Epistasis Recessive epistasis occurs when the recessive alleles of one gene (in homozygous form) can mask or suppress the effect of another gene. In this case, the second gene can be dominant or recessive, but its expression will not appear if the first gene is homozygous recessive. This means one gene (called the epistatic gene) is stronger and can block the phenotype controlled by the second gene (called the hypostatic gene), but only when present in recessive form. Example: A good example is coat colour in mice. One gene (A) controls pigment production. If...

Question: A person has met with an accident and needs blood transfusion. However, when tested for blood group, his blood was found to agglutinate blood of types A, B as well as O. His both parents have blood group A. What is the genotype of this person for this trait? How can we explain these observations? Normally, humans have four main blood groups:  A, B, AB and O.  These are decided by a gene with three alleles:  Iᴬ, Iᴮ and i.  The alleles  Iᴬ and Iᴮ are dominant  and code for A and B antigens on red blood cells, while the  i allele is recessive  and does not produce any antigen. Blood group O people have  genotype ii  and their red blood cells show no A or B antigens. But these A or B antigens are not formed directly. They are actually built upon a base molecule called the  H antigen.  This H antigen works like a platform or foundation where A or B sugars attach to form the A or B antigen. The H antigen itself is made by a...

Albinism is a genetic condition where a person lacks melanin, the pigment responsible for the colour of skin, hair and eyes. It is usually inherited in an autosomal recessive manner. The most common forms of albinism, such as oculocutaneous albinism (OCA), are caused by mutations in genes like TYR, OCA2, TYRP1 etc. For a person to show albinism, they must inherit two copies of the defective allele, one from each parent. This means the genotype of an albino individual is generally homozygous recessive (aa). If both parents are albino due to mutations in the same gene, then their genotype would be aa, and the only gametes they can produce will carry the a allele. When we perform a cross: Parent 1: aa × Parent 2: aa Gametes: a a All children will also be aa, hence all children will be albino. Therefore, if both parents are albino due to the same gene, they cannot produce a normally pigmented child. However, in rare cases, albinism can result from mutations in different genes in e...

To understand the difference, we need to know that both dominance and epistasis are related to how genes express themselves, but they work at different levels. Dominance Dominance happens between alleles of the same gene. In a pair of alleles (like Aa), the dominant allele masks or hides the effect of the recessive one. For example, in pea plants,  tall (T)  is dominant and  dwarf (t)  is recessive. So in genotype Tt, the plant will be tall, because the dominant T shows its effect and t remains hidden. So, dominance is an interaction between two alleles of the same gene, present at the same locus (same position on homologous chromosomes). Epistasis Epistasis happens between alleles of different genes, located at different loci. In this, one gene can mask or modify the effect of another gene. The gene that does the masking is called the epistatic gene and the gene whose expression is affected is called the  hypostatic gene. For example, in Labrador dogs, one gene...

If Ca²⁺ is removed from the tight junction, it leads to breakdown of the tight junction structure, loss of cell adhesion, increased permeability, and overall disturbance of epithelial and endothelial tissue integrity. Role of Ca²⁺ in Tight Junctions Ca²⁺ plays a critical role in maintaining the structural organization of tight junctions. It helps stabilize the conformation of tight junction proteins and promotes proper adhesion between neighboring cells. Calcium is also necessary for the interaction between  tight junction proteins and the cytoskeleton.  Without calcium, the conformation of these proteins becomes unstable and the adhesion between cells weakens. What Happens When Ca²⁺ is Removed 1. Disruption of Tight Junction Structure When Ca²⁺ is removed from the extracellular environment, the tight junction proteins lose their ability to maintain proper interactions with each other. This leads to disassembly and fragmentation of the tight junction complex. 2. Loss of Cell–C...

Question: In peas, tall (T) is dominant over dwarf (t), yellow (Y) is dominant over green (y) and smooth is dominant (S) over wrinkled (s). What fraction of the offspring in the following cross would be homozygous recessive for all gene pairs in the cross: YyTtSs × Yyttss? To solve this genetics question, we need to find what fraction of the offspring will be homozygous recessive for all three traits, that is:  yy tt ss Let us carefully break down the cross YyTtSs × Yyttss, step by step for each gene. Step 1: Determine the gametes for each gene pair We look at each gene separately and find out how often the homozygous recessive genotype will appear. Gene 1: Yy × Yy Cross: Yy × Yy Possible genotypes: YY, Yy, yY, yy Probability of  yy = 1/4 Gene 2: Tt × tt Cross: Tt × tt Possible genotypes: Tt, tt Probability of  tt = 1/2 Gene 3: Ss × ss Cross: Ss × ss Possible genotypes: Ss, ss Probability of  ss = 1/2 Step 2: Multiply probabilities Now multiply the probabilities of ...

The principle of spectral karyotyping (SKY) is based on  fluorescence in situ hybridization (FISH)  using  chromosome-specific DNA probes,  each labeled with a unique combination of fluorochromes. Although  only five different  fluorescent dyes are used, they are mixed in specific ratios so that each chromosome gets a unique combination of colors. This creates a specific spectral signature for every chromosome. These labeled probes are hybridized to metaphase chromosomes fixed on a glass slide. After hybridization, a fluorescence microscope with a spectral imaging system is used to detect the signals. The spectral imaging system captures the wavelength emission pattern from each chromosome. Then, spectral unmixing algorithms are applied through a computer system to separate and identify the unique color of each chromosome. In short, principle of spectral karyotyping works on the idea that: Each chromosome is labeled with a unique color code using combinatio...

The organisation of genes in prokaryotes and eukaryotes is quite different due to their structural, functional and evolutionary differences. These differences are seen in the way genes are arranged on the DNA, how they are regulated and how they are transcribed and translated. Below is a detailed explanation of their gene organisation based on major points: 1. Arrangement of Genes Prokaryotes: Genes are often arranged in clusters called  operons.  An operon is a group of genes under the control of a single promoter and transcribed together as one mRNA. These genes usually have related functions. For example, the lac operon in E. coli includes genes required for lactose metabolism. Eukaryotes: Genes are usually arranged individually. Each gene has its own promoter, enhancer and regulatory elements. Eukaryotic genes are not usually grouped by function. They are transcribed separately into different mRNAs. 2. Coding and Non-coding Regions Prokaryotes: Their genes are mostly made ...

Lethal alleles are  mutations in essential genes  that can cause death either in the embryonic stage, during development, or even later in life. These alleles interfere with important cellular functions like metabolism, neural development and protein synthesis. The concept of lethal alleles was first explained by  Lucien Cuenot  in 1905 in mice while working on coat colour genetics. In human populations, many such lethal alleles are well documented and their modes of inheritance vary, which affects how they are transmitted from parents to offspring. These lethal genes may act in  homozygous or even heterozygous conditions  and can be  autosomal or X-linked,  depending on the location of the gene. In humans, four different types of lethal alleles are encountered, based on how they are inherited and expressed. These are described below with clear human examples. 1. Recessive Lethal Alleles (Autosomal Recessive Inheritance) In this type, the allele c...

A lethal allele is a type of  gene mutation  that causes death to an organism when present in a certain genotype. Usually, lethal alleles are  recessive,  meaning that they cause death only when an individual inherits two copies of the lethal allele (homozygous condition). However, some lethal alleles can also act in a  dominant form  and cause death even when only one copy is present. Lethal alleles generally affect essential biological processes like metabolism, development and organ formation. These alleles usually arise due to mutations that severely disrupt the function of an essential gene. Molecular Basis of Lethality One of the best-known classical examples of a lethal allele is found in mice involving the  yellow coat color gene. This trait is controlled by a single gene with two alleles: A⁺ (normal wild-type allele) Aʸ (mutant yellow allele) The  Aʸ allele  causes  yellow coat color  and is dominant for this trait. But if ...

Question: When a white cow was mated with a red bull, all their offspring were a mottled red and white (roan) colour. If the two roan cattle were mated, what coat colour would the progeny have and in what ratios. This question is based on  codominance,  where both alleles express themselves equally in the  heterozygous state.  In this case, red coat and white coat are codominant traits. Genetic Symbols Used: R =  Allele for red coat W =  Allele for white coat Since both alleles are codominant,  heterozygous condition (RW)  shows a roan coat, which is a mix of red and white patches. Step 1: Initial Cross Parent 1 (Red bull) =  RR Parent 2 (White cow) =  WW When RR is crossed with WW: In F₁ Generation: All offspring: RW (heterozygous) Phenotype: Roan coat (both red and white patches visible) Step 2: Cross Between Two Roan Cattle Now we cross two Roan (RW × RW) individuals. Let's draw a Punnett Square for the cross: Step 3: Resulting Genoty...

Gregor Mendel,  in 1865, performed experiments on pea plants and explained how traits are passed from one generation to the next. He gave three important principles which are now called  Mendel's Laws: Law of Dominancethe, Law of Segregation and Law of Independent Assortment.  He said that traits are controlled by specific  factors,  which we now call  genes,  but he did not know where these genes are located inside the cell. In the early 1900s,  Walter Sutton and Theodor Boveri  gave the  Chromosomal Theory of Inheritance,  which explained that genes are present on chromosomes and chromosomes behave in a special way during meiosis. The movement of chromosomes during meiosis matches exactly with what Mendel had observed in his experiments. So, this theory gave a proper physical explanation to Mendel's laws and showed how inheritance works at the cellular level. Although the Chromosomal Theory confirmed Mendel's principles, it also a...

In classical Mendelian genetics, a gene usually has only two alleles:  one dominant and one recessive.  However, in real-life situations, many genes exist in more than two alternative forms. These are called  multiple alleles.  So, when more than two alleles exist for the same  gene locus  within a population, it is called the  multiple allelic condition. Although a diploid organism, like a human, carries only two alleles at a time, multiple alleles refer to the presence of more than two allelic forms in the population for the same gene.  The molecular basis of multiple alleles  lies in the specific changes in DNA sequence that result in structurally or functionally different proteins from the same gene Molecular Basis of Multiple Alleles The molecular basis of multiple alleles lies in slight differences in the DNA sequence of the gene. These  mutations  in the nucleotide sequence of the same gene lead to the formation of different ...