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Gene interaction refers to a situation where two or more genes influence the  same trait.  In the case of structural proteins, sometimes the final functional protein is not made from a single gene product but is the result of the combination of different polypeptides produced by different genes. Such interaction is especially important in the formation of complex structural proteins that require the association of multiple chains to become functional. Two good examples of this kind of gene interaction are seen in  haemoglobin and MHC (Major Histocompatibility Complex) molecules. 1. Haemoglobin (HbA) Haemoglobin is the oxygen-carrying protein found in red blood cells. The adult type of haemoglobin, called  HbA,  is a tetramer made up of two  alpha-globin chains and two beta-globin chains.  These chains are coded by different genes: The alpha-globin gene is located on  chromosome 16. The beta-globin gene is located on  chromosome 11. Both these...

The discovery of  X-linkage  was one of the most important milestones in classical genetics. This concept was first established by  Thomas Hunt Morgan  in 1910 during his genetic experiments on the  fruit fly, Drosophila melanogaster. Morgan's  work not only confirmed  Mendel's laws  but also extended them by explaining how certain traits are inherited through the sex chromosomes, especially the X chromosome. How did Morgan discover X-linkage in Drosophila? Thomas Hunt Morgan discovered X-linkage in Drosophila melanogaster through careful breeding experiments that studied the  inheritance of eye color.  This discovery was very important because it showed that genes are located on chromosomes, supporting the  chromosome theory of inheritance. Morgan started his research with fruit flies that  normally had red eyes.  One day, he found a  male fly with white eyes,  which was a rare mutation. To understand how this t...

In genetics, traits can be influenced or expressed differently depending on the sex of the individual. Some traits are linked to sex chromosomes, while others are affected by hormonal or physiological differences between males and females. To describe these traits more precisely, geneticists use three main terms:  sex-linked, sex-limited and sex-influenced traits.  Although these terms may sound similar, they refer to different types of genetic expression related to sex. Understanding the distinction among these three is important for grasping how certain traits are inherited and expressed differently in males and females. 1. Sex-linked characters: These are traits controlled by genes that are located on the  sex chromosomes,  usually on the X chromosome in humans. Because males have only one X chromosome (XY) and females have two (XX), the pattern of inheritance and expression is different in both sexes. Most sex-linked traits are X-linked, and very few are Y-linked...

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...