Test: Genetics - UPSC MCQ

The term that represents a pair of contrasting characters is 4. Allelomorphs. Allelomorphs, also known as alleles, are alternative forms of a gene that occupy the same locus on a specific chromosome and determine different variations of a particular trait. Allelomorphs can be contrasting in terms of their phenotypic expression, such as dominant and recessive alleles.

The correct answer is 3. In the ABO blood group system, there are three alleles: IA, IB, and i. The IA and IB alleles are co-dominant, while the i allele is recessive to both IA and IB.

The possible genotypes and corresponding phenotypes are as follows:

• IAIA or IAi genotype results in blood type A phenotype.
• IBIB or IBi genotype results in blood type B phenotype.
• IAIB genotype results in blood type AB phenotype.
• ii genotype results in blood type O phenotype.

Therefore, there are three possible phenotypes: A, B, and AB. The O blood type is not considered a distinct phenotype but rather a lack of the A or B antigen expression.

A small amount of lethal mutation is always present in the population due to mutation-selection balance.

Mutation-selection balance refers to the equilibrium reached between the introduction of new mutations through genetic variation (mutation) and the elimination of deleterious mutations by natural selection (selection). In any population, mutations continuously arise and can include lethal or severely detrimental mutations.

While natural selection acts to remove harmful mutations from the population, it cannot completely eliminate them because new mutations are constantly occurring. Therefore, a balance is established where the population carries a small number of lethal mutations at any given time.

This concept highlights the ongoing dynamic between the introduction of mutations and the selective pressures that act upon them, resulting in the presence of a baseline level of lethal mutations in the population.

The correct answer is 3. Cystic fibrosis is an autosomal recessive disorder.

Autosomal recessive disorders are caused by the inheritance of two copies of a mutated gene, one from each parent. In the case of cystic fibrosis, it is caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene.

To develop cystic fibrosis, an individual must inherit two copies of the mutated CFTR gene, one from each parent. If a person inherits only one copy of the mutated gene, they are considered carriers of the condition but do not typically exhibit symptoms.

Sex-linked disorders, on the other hand, are caused by mutations in genes located on the sex chromosomes (X or Y). Cystic fibrosis is not a sex-linked disorder and is not associated with the sex chromosomes.

Alleles are alternate forms of genes. They are different versions of the same gene that can produce variations in traits or characteristics. Each individual inherits two alleles for each gene, one from each parent. These alleles may be the same (homozygous) or different (heterozygous). The specific combination of alleles determines an individual's genotype, which can influence their phenotype or observable traits.

A gene is the smallest unit of genetic material that carries information for a specific trait or function. It is a segment of DNA that contains the instructions for producing a particular protein or RNA molecule. Mutations can occur within genes, leading to changes in the genetic code and potentially resulting in phenotypic effects. The term "muton" is not commonly used in genetics and does not refer to the smallest unit of genetic material. "Recon" is not a recognized term in this context, and "nucleic acid" is a broader term that encompasses DNA and RNA, but it is not the smallest unit of genetic material that produces a phenotypic effect on mutation.

The exception to Mendel's law mentioned in the options provided is B: Linkage.

Mendel's laws of inheritance, also known as Mendelian inheritance, describe the patterns by which traits are passed from parents to offspring. These laws include the law of independent assortment, dominance, and the principle of purity of gametes.

The law of independent assortment states that different genes for different traits segregate independently during the formation of gametes. This means that the inheritance of one trait does not influence the inheritance of another trait.

Dominance refers to the concept that one allele of a gene can mask or dominate the expression of another allele, resulting in a phenotype that reflects the dominant allele. The recessive allele is only expressed in the absence of the dominant allele.

The principle of purity of gametes, also known as the law of segregation, states that during gamete formation, the alleles for a trait separate and end up in different gametes. This ensures that each gamete carries only one allele for each gene.

However, the exception to Mendel's laws is linkage. Linkage occurs when two or more genes are located close to each other on the same chromosome. These genes tend to be inherited together as a unit, breaking the law of independent assortment. This happens because the likelihood of recombination, the shuffling of genetic material between homologous chromosomes during meiosis, is reduced when genes are close together. Therefore, linked genes do not assort independently and are more likely to be inherited together as a package.

The correct answer is D: All of the above.

The "law of segregation" is one of the fundamental principles of genetics, proposed by Gregor Mendel. It states that during the formation of gametes (sex cells), the alleles (alternative forms of a gene) separate from each other. This means that each gamete carries only one allele for each gene.

Option A: The law of segregation does not specifically refer to the purity of genes. Instead, it describes the separation of alleles.

Option B: This statement is correct. The law of segregation states that alleles separate from each other during gametogenesis, which is the process of gamete formation (such as sperm and egg cells).

Option C: This statement is also correct. The segregation of factors (alleles) is due to the segregation of chromosomes during meiosis. Meiosis is a specialized type of cell division that produces haploid cells (gametes) with half the number of chromosomes as the parent cell.

Therefore, the correct answer is D, as all of the statements are true regarding the "law of segregation."

Genotypic Ratio of a Monohybrid Cross:
A monohybrid cross is a genetic cross between two individuals that differ in only one trait. The genotypic ratio refers to the ratio of different genotypes that result from this cross. In a monohybrid cross, there are three possible genotypes for the offspring:
1. Homozygous Dominant (AA): Offspring with two dominant alleles for the trait.
2. Heterozygous (Aa): Offspring with one dominant and one recessive allele for the trait.
3. Homozygous Recessive (aa): Offspring with two recessive alleles for the trait.
The genotypic ratio is determined by the combination of these genotypes in the offspring.
Options for the Genotypic Ratio:
A: 1:2:1
- This ratio suggests that there is an equal chance of obtaining each of the three genotypes in the offspring. It implies that there is a 25% chance for homozygous dominant, 50% chance for heterozygous, and 25% chance for homozygous recessive genotypes. This ratio is characteristic of a monohybrid cross where the dominant allele is not completely dominant over the recessive allele.
B: 3:1
- This ratio suggests that there is a higher chance of obtaining the dominant genotype (homozygous dominant or heterozygous) compared to the recessive genotype. It implies that there is a 75% chance for the dominant genotype and a 25% chance for the recessive genotype. This ratio is characteristic of a monohybrid cross where the dominant allele is completely dominant over the recessive allele.
C: 2:1:1
- This ratio suggests that there is a higher chance of obtaining the dominant genotype (homozygous dominant or heterozygous) compared to the recessive genotype. It implies that there is a 50% chance for the dominant genotype and a 25% chance for each of the two recessive genotypes. This ratio is characteristic of a monohybrid cross where the dominant allele is completely dominant over the recessive allele.
D: 9:3:3:1
- This ratio suggests that there is a higher chance of obtaining specific combinations of genotypes. It implies that there is a 9/16 chance for the dominant phenotype, a 3/16 chance for each of the two heterozygous genotypes, and a 1/16 chance for the recessive phenotype. This ratio is characteristic of a dihybrid cross, which involves the inheritance of two different traits.
Conclusion:
Based on the options given, the correct genotypic ratio for a monohybrid cross is A: 1:2:1. This ratio indicates an equal chance of obtaining each of the three possible genotypes in the offspring.

Gregor Johann Mendel, an Austrian scientist and Augustinian friar, is commonly referred to as the "Father of Genetics." He conducted groundbreaking experiments with pea plants in the mid-19th century and formulated fundamental laws and principles of heredity. His work laid the foundation for modern genetics and the understanding of how traits are inherited from one generation to the next. Mendel's discoveries were initially overlooked but were rediscovered and recognized as significant contributions to the field of genetics in the early 20th century.

A Punnett square is a graphical tool used in genetics to predict the potential outcomes of a cross between two individuals. It allows us to determine the possible combinations of alleles that can be inherited from each parent and predict the probability of specific traits or genetic disorders in offspring. The Punnett square is named after Reginald Punnett, a British geneticist who developed this method in the early 20th century. It is widely used in genetics to illustrate and understand patterns of inheritance.

To determine the probability of obtaining a specific genotype from a self-fertilization cross, we need to consider the principles of Mendelian genetics and the laws of segregation and independent assortment.

In this case, the plant has the genotype AaBb, which means it carries one copy of the A allele and one copy of the B allele. During gamete formation, these alleles will segregate independently, meaning that each allele has an equal chance of being passed on to the offspring.

To calculate the probability of obtaining the AABB genotype, we need to determine the probability of each allele combination coming together. Since the alleles segregate independently, the probability of obtaining the AABB genotype would be the product of the probabilities of getting the A allele twice and the B allele twice.

The probability of getting the A allele twice is 1/2 * 1/2 = 1/4 (assuming A is dominant and a is recessive). Similarly, the probability of getting the B allele twice is also 1/4 (assuming B is dominant and b is recessive).

To find the probability of both events occurring together (AABB genotype), we multiply the probabilities:

Probability = (1/4) * (1/4) = 1/16

Therefore, the correct answer is D: 1/16.

To understand the significance of the 9:7 ratio in the F2 generation, we need to review the different types of genetic interactions and their corresponding phenotypic ratios.
1. Incomplete dominance:
- In incomplete dominance, the heterozygous phenotype is intermediate between the two homozygous phenotypes.
- The phenotypic ratio in the F2 generation is 1:2:1.
2. Co-dominance:
- In co-dominance, both alleles are fully expressed in the heterozygous genotype.
- The phenotypic ratio in the F2 generation is 1:2:1.
3. Epistasis:
- Epistasis occurs when one gene masks or modifies the phenotypic expression of another gene.
- The phenotypic ratio in the F2 generation can vary depending on the type of epistatic interaction.
- The 9:7 ratio specifically represents a recessive epistasis, where the presence of a recessive allele at one gene locus masks the expression of alleles at another gene locus.
- The ratio is derived from the combination of two dihybrid crosses, where the phenotypic ratio for each individual dihybrid cross is 9:3:3:1.
4. Complementary interaction:
- In complementary interaction, two genes work together to produce a specific phenotype.
- The phenotypic ratio in the F2 generation is 9:7, which indicates that both genes must be present in a specific combination to produce the desired phenotype.
Therefore, the 9:7 ratio in the F2 generation represents C: Epistasis, specifically a recessive epistasis where one gene masks the expression of another gene.

Epistasis refers to a genetic phenomenon where the activity of one gene masks or suppresses the expression of another non-allelic gene. In this case, the non-allelic gene is affecting the expression of another gene, leading to the suppression of its activity. It is important to note that epistasis can occur in different ways, including dominant or recessive epistasis.

Pseudo-dominance (option A) refers to a situation where a recessive allele becomes expressed in the phenotype because the dominant allele is absent. It is not specifically related to the suppression of one gene by another.

Hypostasis (option B) is not a term commonly used in genetics and does not describe the described phenomenon.

Incomplete dominance (option D) refers to a situation where the heterozygous genotype results in an intermediate phenotype, blending the traits of both alleles. It does not involve the suppression of one gene by another.

The correct answer is D: All.

Mendel's findings were independently rediscovered by multiple scientists. Three notable scientists who independently rediscovered Mendel's work are:

A. Carl Correns: Correns, a German botanist, independently rediscovered Mendel's laws of inheritance in 1900. He confirmed and expanded upon Mendel's work, providing additional evidence for the principles of segregation and independent assortment.

B. Hugo de Vries: De Vries, a Dutch botanist, also independently rediscovered Mendel's laws around the same time as Correns. De Vries coined the term "mutation" and proposed the concept of genetic mutations playing a role in evolution.

C. Erich von Tschermak: Tschermark, an Austrian botanist, independently rediscovered Mendel's work and published his findings in 1900. He was the third scientist to confirm Mendel's laws and contributed to the understanding of genetics.

These scientists independently arrived at conclusions similar to Mendel's, validating his work and establishing the significance of his discoveries in the field of genetics. Therefore, the correct answer is D: All.

The correct answer is B: They had contrasting characters.

Gregor Mendel, known as the father of modern genetics, conducted his experiments on pea plants (specifically, the garden pea, Pisum sativum) because they exhibited distinct and easily distinguishable traits with clear-cut inheritance patterns. These traits, such as flower color, seed shape, and pod color, showed a clear contrast between alternative forms (e.g., yellow vs. green, round vs. wrinkled).

By studying these pea plant traits, Mendel was able to establish the fundamental principles of inheritance, such as the laws of segregation and independent assortment. The contrasting characters of the pea plants made it easier for Mendel to observe and analyze the patterns of inheritance, leading to his groundbreaking discoveries in genetics.

A test cross is a cross between an individual of unknown genotype and a homozygous recessive individual. By observing the phenotypic ratios of the offspring, it is possible to determine the genotype of the unknown individual.

In a test cross, if the individual being tested is heterozygous (having one dominant and one recessive allele), the offspring will show a 1:1 ratio of the dominant and recessive phenotypes. On the other hand, if the individual being tested is homozygous dominant (having two dominant alleles), all the offspring will display the dominant phenotype.

Therefore, a test cross can be used to determine whether an individual is homozygous (having two identical alleles) or heterozygous (having two different alleles) by examining the phenotypes of the resulting offspring.

In genetics, a backcross involves crossing an F1 hybrid individual with one of its parents or an organism genetically similar to one of its parents. The purpose of a backcross is to reintroduce traits from one parent into the hybrid offspring. It is often used to transfer desirable traits from the parent back to the hybrid or to recover a specific parental genotype.

An allele is an alternate form or variant of a gene. Genes exist in different versions called alleles, which can have slight differences in their DNA sequence. Alleles are responsible for variations in specific traits or characteristics. For example, there may be different alleles for eye color, such as blue, brown, or green. Individuals inherit two alleles for each gene, one from each parent, and the combination of these alleles determines the expression of a specific trait.

The tendency of an offspring to resemble its parent is known as:
1. Heredity:
- Heredity refers to the passing on of traits or characteristics from parents to their offspring.
- It is the process by which genetic information is transmitted from one generation to the next.
2. Variation:
- Variation refers to the differences or diversity that can be observed among individuals within a population.
- It is caused by genetic and environmental factors and leads to differences in traits between individuals.
3. Resemblance:
- Resemblance refers to the similarity or likeness between individuals, particularly in terms of physical appearance or traits.
- Offspring often share certain characteristics with their parents, resulting in a resemblance between them.
4. Inheritance:
- Inheritance refers to the passing on of genetic information or traits from parent organisms to their offspring.
- It involves the transfer of genetic material through genes, which carry the instructions for specific traits.
Conclusion:
The correct answer to the question is B: Heredity. Heredity is the process responsible for the tendency of an offspring to resemble its parent.