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Showing posts with the label Mendel's Laws

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?

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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 ...
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Prepare a list of lethal alleles encountered in the human population and their mode of inheritance

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...
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Define lethal allele. Explain with a suitable example the molecular basis of lethality

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 ...
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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

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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...
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Show the parallels between Mendelian laws and chromosomal theory of inheritance

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...
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Describe the molecular basis of multiple alleles giving a suitable example

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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 ...
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What is the difference between codominance and incomplete dominance?

In classical  Mendelian genetics,  traits are often explained through  dominant and recessive alleles.  However, in real biological systems, not all traits follow this simple rule. Two important exceptions to this pattern are  codominance and incomplete dominance.  These types of inheritance help explain how both alleles of a gene may influence the phenotype in different ways. Understanding the difference between them is essential in genetics, especially for interpreting traits like flower colour and human blood groups. Codominance Codominance is a condition where both alleles in the heterozygous condition express themselves  fully and independently. In this case, the phenotype does not show blending or mixture,  but rather both traits appear side by side. A well-known example of codominance is the AB blood group in humans. A person with genotype IAIB will have both A and B antigens on the surface of red blood cells. In this case, neither the...
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What was the criticism of chromosomal theory of Sutton and Boveri? How was it resolved?

The Chromosomal Theory of Inheritance was independently proposed by  Walter Sutton  (1902) and  Theodor Boveri  (1902–1903). This theory stated that genes are physically located on chromosomes and the behavior of chromosomes during meiosis explains Mendel's laws of segregation and independent assortment. Criticism of Chromosomal Theory of Sutton and Boveri Although this theory provided a clear cytological basis for heredity, it initially faced several criticisms, as the scientific community was not fully convinced without direct evidence. There were three major criticisms raised against this theory: 1. Lack of Direct Evidence That Genes Located on Chromosomes When Sutton and Boveri proposed the theory, there was no direct experimental proof showing that genes were located on chromosomes. While chromosomes could be seen under a microscope, genes were hypothetical entities at that time. Critics argued that just because chromosomes and genes behave similarly during meio...
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What phenotypic classes will be obtained from a cross between AaBb × aabb? In what proportions? What is this cross called?

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Parent Genotypes: Parent 1:  AaBb Parent 2:  aabb Gametes Produced: AaBb can produce 4 types of gametes:  AB, Ab, aB, ab aabb can only produce one type of gamete:  ab Punnett Square and Offspring Genotypes: We now combine each gamete from AaBb with the only gamete (ab) from aabb: Phenotypic Classes: The phenotype of each genotype: AaBb →  Dominant for both traits (A- and B-) Aabb →  Dominant for trait A, recessive for trait B aaBb →  Recessive for trait A, dominant for trait B aabb →  Recessive for both traits In what proportions of phenotypic ratio? Phenotypic Ratio (Proportions): Each combination occurs with equal probability (1 out of 4), so the phenotypic ratio is  1 : 1 : 1 : 1 (Each class appears in 25% of the offspring) That is: 25% dominant for both traits (AaBb) 25% dominant A, recessive b (Aabb) 25% recessive a, dominant B (aaBb) 25% recessive for both traits (aabb) What is this cross called? This is a  test cross  between...
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Normal length of fur in rabbits is controlled by the dominant allele R, and a short type of fur called "rex" is determined by the recessive allele r. The dominant allele B is responsible for black fur colour, while the recessive allele b determines brown colour. What are phenotypic ratios resulting from a cross between a homozygous rabbit with normal length of black fur and rex rabbit with brown fur?

Question: Normal length of fur in rabbits is controlled by the dominant allele R, and a short type of fur called "rex" is determined by the recessive allele r. The dominant allele B is responsible for black fur colour, while the recessive allele b determines brown colour. What are phenotypic ratios resulting from a cross between a homozygous rabbit with normal length of black fur and rex rabbit with brown fur? Genetic Information Given: R =  dominant allele for normal fur r =  recessive allele for rex fur (short fur) B =  dominant allele for black fur b =  recessive allele for brown fur Parent Genotypes: Genotype of homozygous normal black fur rabbit Genotype:  RRBB Genotype of rex brown fur rabbit (both recessive traits):  rrbb Cross: RRBB × rrbb By applying Mendel's Law of Independent Assortment, we make a Punnett square using gametes: Gametes from RRBB → Only one type: RB Gametes from rrbb → Only one type: rb When crossed: RB × rb → RrBb So, all offspri...
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What are the salient features of the chromosomal theory of inheritance?

The Chromosomal Theory of Inheritance, proposed by  Walter Sutton  (1902) and  Theodor Boveri  (1902–1903), explains that genes are carried on chromosomes and their behaviour during meiosis directly relates to Mendel's laws of inheritance. The following are its main features: 1. Chromosomes are the physical carriers of hereditary information: According to this theory, genes (units of inheritance) are physically located on chromosomes. These chromosomes are passed from parents to offspring during reproduction. Each chromosome carries many genes and this explains how traits are inherited generation after generation. 2. Chromosomes occur in homologous pairs in diploid cells: Every diploid organism has two sets of chromosomes, one set from the mother and one from the father. Each chromosome in one set has a matching partner in the other set, called its  homologous pair.  These homologous chromosomes carry the same type of genes but possibly different alleles. 3...
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What is a testcross and why is it done?

A testcross is a type of genetic cross used in Mendelian genetics to determine the  genotype  of an individual that shows a  dominant phenotype.  The  outward appearance (phenotype)  of an organism may not always reveal its exact  genetic composition (genotype),  especially when dominant traits are involved. An organism showing a dominant trait may be  homozygous dominant (AA) or heterozygous (Aa).  A testcross helps to find out which of the two genotypes is present. In a testcross, the individual showing the dominant phenotype is crossed with a  homozygous recessive (aa)  individual. Since the genotype of the recessive parent is known, the pattern of offspring obtained from this cross can help in determining the unknown genotype of the dominant parent. Gregor Mendel  first used this technique in his  pea plant  experiments. For example, if a  tall pea plant,  which could be  either TT or Tt  ...
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What is the difference between the classical concept and the modern concept of genes?

The concept of the gene has evolved from a simple unit of heredity to a complex molecular entity. During  Mendel's  time, genes were understood only through the inheritance of physical traits. This formed the  classical concept.  With the discovery of DNA, its double helix structure and the development of molecular biology, the gene is now defined by its chemical structure and functional properties. This is known as the  modern concept. For understanding the difference between the classical and modern concept of genes, the following criteria are used: 1. Based on Definition or Concept Classical Concept:  Gene was defined as an abstract unit of heredity responsible for controlling a  single trait.  It was known through breeding experiments and inheritance patterns. Modern Concept:  A gene is a segment of DNA that contains the information to produce a functional product, either a protein or an RNA. It is both a structural and functional unit of...
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Why is the frequency of double crossover overly low?

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What is gene mapping? How do the linked genes help in gene mapping?

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The First Living Molecule

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What is the difference between regulatory gene and structural gene?

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Subatomic Particles

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Differentiate between linked genes and unlinked genes?

Genes are specific sequences of DNA that code for proteins and determine traits in an organism. During the study of chromosomal theory of  inheritance, scientists found that not all genes behave the same way. Some genes tend to be inherited together while others assort independently. Based on this behavior, genes are divided into two types:  linked genes and unlinked genes.  This concept is very important in genetics because it helps in understanding how traits are passed on and how gene positions can be mapped on chromosomes. These differences are explained based on specific criteria: 1. Based on Chromosomal Location Linked genes  are located close to each other on the same chromosome. Because of their close physical proximity, they usually move together during meiosis and are inherited as a group. For example: In Drosophila melanogaster (fruit fly), the genes for eye color and wing shape are located close to each other on the X chromosome. Unlinked genes  are ...
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Can marriage between two albinos produce children with normal pigmentation?

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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...
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Define and distinguish sex-linked, sex-limited and sex-influenced characters

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Give two examples of gene interaction resulting in the formation of structural proteins

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