Linkage and Recombination MCQ Quiz - Objective Question with Answer for Linkage and Recombination - Download Free PDF

The correct answer is: 0.025

Explanation: To find the expected frequency of double crossovers between genes P and R, we will use the formula for the probability of double crossovers, which is the product of the recombination frequencies between adjacent genes. Double crossovers occur when both recombination events happen independently.

Given:

• Recombination frequency between P and Q \((( r_{P-Q} )) \)= 0.18
• Recombination frequency between Q and R \((( r_{Q-R} ))\) = 0.14

The double crossover frequency \((( r_{P-Q} )) \) ×  \((( r_{Q-R} ))\) is: \([ \text{Double crossover frequency} = r_{P-Q} × r_{Q-R} = 0.18 × 0.14 ]\)

• Calculating this: \([ 0.18 × 0.14 = 0.0252 ]\)

Double crossovers are relatively rare events compared to single crossovers. The result 0.025, or 2.5%, indicates the frequency at which double crossovers will be observed in the progeny due to independent recombination events between these genes. In practical terms, understanding double crossover frequency is important for accurate genetic mapping and linkage analysis.

The correct answer is An ideal mapping population for a self-pollinating species is generated using polymorphic parents that are inbred lines.

Explanation:

1. Genetic markers would always show higher recombination frequencies when they are closer to each other than if they are far apart: This statement is incorrect. In fact, genetic markers that are closer together tend to have lower recombination frequencies because they are more likely to be inherited together. Farther apart markers have higher recombination frequencies due to the likelihood of crossing over occurring between them.

2. The genetic distance between two markers is a true representation of the physical distance between them: This statement is incorrect. Genetic distance is measured in centiMorgans (cM) and reflects recombination frequencies, not direct physical distances on the chromosome. The physical distance can vary significantly among different regions of the genome.

3. An ideal mapping population for a self-pollinating species is generated using polymorphic parents that are inbred lines: This statement is correct. Inbred lines are homozygous and provide a stable genetic background for mapping traits. Using polymorphic parents helps to create a mapping population that retains genetic variation, making it easier to identify linkage between markers and traits.

4. An F2 mapping population would segregate in a 1:2:1 ratio for a dominant marker: This statement is incorrect in the context of dominant markers. An F2 generation resulting from a monohybrid cross typically segregates in a 3:1 ratio for dominant to recessive phenotypes, not 1:2:1.

Thus, the correct statement regarding principles of linkage mapping in plants is An ideal mapping population for a self-pollinating species is generated using polymorphic parents that are inbred lines.

Key Points

• T. H. Morgan experimented with Drosophila melanogaster to establish that the basis of variation was caused by sexual reproduction. 
• Drosophila melanogaster or fruit-fly was used as a model organism because of the following reasons:
  • Their size (3mm) is small enough to keep millions in a laboratory, but not so small that cannot be seen without a microscope.
  • They can be grown on simple synthetic media in the laboratory.
  • They complete their life cycles in about 2 weeks, which allows to study multiple generations.
  • A single mating produces a large number of offspring.
  • Male and female individuals are morphologically distinct.
  • Many of the hereditary variations can be studied under low power microscopes.
  • About 75% of the genes causing diseases in humans are also found in Drosophila.

Explanation:

Statement (A) They could be cultured easily in the monastery - INCORRECT

• Morgan experimented with Drosophila in the laboratory, not in monastery.
• It was Gregor Mendel who performed the genetic experiments on pea plant in monastery garden.

Statement (B) They showed many contrasting traits - CORRECT

• Contrasting characters are important for studying genetics.
• Drosophila showed many such characters like miniature wings, eye colour, body colour, etc.

Statement (C) The generation time was one year - INCORRECT

• Their generation time was only 2 weeks, which allowed study of multiple generations in short time.

Statement (D) There was clear differentiation of sexes - CORRECT

• The male and female individuals had distinct morphological characteristics.
• For example, yellow-bodied white-eyed females were crossed with brown-bodied red-eyed males.

Statement (E) Very few progeny were produced in a single mating - INCORRECT

• They produce large number of offspring in every single mating.

Hence, the correct answer is option 1: (B) and (D) only.

The correct answer is higher number of parental types

Concept:

• When two genes are located very close to each other on the same chromosome, they are said to be tightly linked. This close proximity reduces the likelihood of recombination occurring between them during meiosis.
• As a result, the parental combinations of alleles (genes) are more likely to be inherited together, thus producing a higher number of parental types in the offspring.

Explanation:

• Higher number of the recombinant types: This is not correct. Tight linkage results in fewer recombinant types, not more.
• Segregation is expected in a 9:3:3:1 ratio: This is typical for genes that assort independently, which is not the case here due to the close linkage.
• Segregation in 3:1 ratio: This ratio is expected for a single gene pair showing complete dominance, not for two closely linked genes.
• Higher number of parental types: This is the correct answer. Due to the close linkage between genes R and Y, the recombination rate is low, resulting in a higher frequency of parental types in the F₂ generation.

The correct answer is a direct relationship

Concept:

• The distance between genes on a chromosome is directly related to the percentage of recombination. This is because genes that are farther apart are more likely to experience crossover events during meiosis, leading to higher recombination frequencies.
• Recombination frequency is used as a measure of genetic distance between two genes, typically expressed in map units or centimorgans (cM). One map unit corresponds to a 1% recombination frequency.

Explanation:

• Direct relationship: The farther apart two genes are on a chromosome, the higher the percentage of recombination, as there is a greater chance of crossover occurring between them.
• Recombination frequency: A recombination frequency of 1% corresponds to one map unit, or one centimorgan (cM), which is used to estimate the distance between genes.
• Inverse relationship: This does not apply in the case of gene distance and recombination. An inverse relationship would suggest that greater distance leads to lower recombination, which is incorrect.

Key Points

• T. H. Morgan experimented with Drosophila melanogaster to establish that the basis of variation was caused by sexual reproduction. 
• Drosophila melanogaster or fruit-fly was used as a model organism because of the following reasons:
  • Their size (3mm) is small enough to keep millions in a laboratory, but not so small that cannot be seen without a microscope.
  • They can be grown on simple synthetic media in the laboratory.
  • They complete their life cycles in about 2 weeks, which allows to study multiple generations.
  • A single mating produces a large number of offspring.
  • Male and female individuals are morphologically distinct.
  • Many of the hereditary variations can be studied under low power microscopes.
  • About 75% of the genes causing diseases in humans are also found in Drosophila.

Explanation:

Statement (A) They could be cultured easily in the monastery - INCORRECT

• Morgan experimented with Drosophila in the laboratory, not in monastery.
• It was Gregor Mendel who performed the genetic experiments on pea plant in monastery garden.

Statement (B) They showed many contrasting traits - CORRECT

• Contrasting characters are important for studying genetics.
• Drosophila showed many such characters like miniature wings, eye colour, body colour, etc.

Statement (C) The generation time was one year - INCORRECT

• Their generation time was only 2 weeks, which allowed study of multiple generations in short time.

Statement (D) There was clear differentiation of sexes - CORRECT

• The male and female individuals had distinct morphological characteristics.
• For example, yellow-bodied white-eyed females were crossed with brown-bodied red-eyed males.

Statement (E) Very few progeny were produced in a single mating - INCORRECT

• They produce large number of offspring in every single mating.

Hence, the correct answer is option 1: (B) and (D) only.

The correct answer is option 2.

Concept:

• Chromosomes are defined as the genetic material present in all cells. 
• It is a  threadlike structure of nucleic acids and proteins found in the nucleus of most living cells, carrying genetic information.
• Every chromosome (visible only in dividing cells) essentially has a primary constriction or the centromere.
• Genes are units of heredity.
• They carry information from one generation to the next generation.

​Explanation:

• All the genes located on the same chromosome form one linkage group.
• A linkage group is defined as all the genes on a chromosome that moves as a single unit during cell division.
• These genes do not tend to separate, until the crossing-over.

Therefore, the correct answer is option 2.

The correct answer is option 1.

Concept: 

• Recombination describes the generation of non-parental gene combinations.
• The frequency of recombination between gene pairs on the same chromosome is a measure of the distance between genes and they are mapped based on their position on the chromosome
• Since recombination frequency is directly proportional to the distance between genes, the values are used to locate genes on a chromosome.

• Alfred Sturtevant explained chromosomal mapping on the basis of recombination frequency which is directly proportional to the distance between two genes on the same chromosome.
• It is to be noted that when the distance between two genes increases, the chances of crossing over increase between the two genes present on non-sister chromatids of the homologous chromosomes.
• Crossing over is directly proportional to the recombination.
• The more the crossing over, the more the chances of exchange of genes,s and the more the new recombination formed. Hence, the distance between genes has a direct relationship with the percentage of recombination
• Therefore, the option is the correct answer.
• The rest of the options an irrelevant.

The correct answer is An ideal mapping population for a self-pollinating species is generated using polymorphic parents that are inbred lines.

Explanation:

1. Genetic markers would always show higher recombination frequencies when they are closer to each other than if they are far apart: This statement is incorrect. In fact, genetic markers that are closer together tend to have lower recombination frequencies because they are more likely to be inherited together. Farther apart markers have higher recombination frequencies due to the likelihood of crossing over occurring between them.

2. The genetic distance between two markers is a true representation of the physical distance between them: This statement is incorrect. Genetic distance is measured in centiMorgans (cM) and reflects recombination frequencies, not direct physical distances on the chromosome. The physical distance can vary significantly among different regions of the genome.

3. An ideal mapping population for a self-pollinating species is generated using polymorphic parents that are inbred lines: This statement is correct. Inbred lines are homozygous and provide a stable genetic background for mapping traits. Using polymorphic parents helps to create a mapping population that retains genetic variation, making it easier to identify linkage between markers and traits.

4. An F2 mapping population would segregate in a 1:2:1 ratio for a dominant marker: This statement is incorrect in the context of dominant markers. An F2 generation resulting from a monohybrid cross typically segregates in a 3:1 ratio for dominant to recessive phenotypes, not 1:2:1.

Thus, the correct statement regarding principles of linkage mapping in plants is An ideal mapping population for a self-pollinating species is generated using polymorphic parents that are inbred lines.

Concept:

• T. H. Morgan experimented with Drosophila melanogaster to establish that the basis of variation was caused by sexual reproduction.
• He further showed that when 2 genes in a dihybrid cross were located on the same chromosome, the proportion of parental gene combinations were much higher than the non-parental type.
• This was attributed to the linkage of genes.
• Morgan found some genes to be tightly linked and some to be loosely linked.

Important Points

• Morgan perfortmed dihybrid crosses between yellow-bodied, white-eyed females to brown-bodied, red-eyed males of Drosophila.
• He found that the F2 ratio differed significantly from the Mendelian ratio 9:3:3:1
• This was because many genes were not assorting independently.
• He attributed this phenomenon to the physical association between two genes and named it 'Linkage'.
• Physical association means that they lie very close to each other and the chance of recombination between them is very less.
• Here, the gene for body colour (yellow or brown) and the gene for eye colour (red or white) are close together.

Additional Information

• Pleiotropy - is the phenomenon of a single gene influencing two or more distinct phenotypic traits. Example - Phenylketonuria.
• Polygenic Inheritance - refers to the inheritance type where one character is controlled by more than one gene. Example - Human skin colour.

Concept: 

• Genetic variations play a very important role in evolution.
• It is also involved in the increase or decrease in the frequency of genes or alleles of chromosomes.
• It is to be noted that there are several reasons for genetic variations.
• It may be caused due to random fertilization and gene combinations or even mutations.

​​Explanation:

• Option 1:
  • According to Mendel's Law of Independent Assortment, the development of reproductive cells causes various genes to separate from one another.
  • Mendelian dihybrid cross is a good illustration of independent assortment.
  • It states that during gamete formation, each pair of traits segregate into gametes independent of another pair of traits.
  • Thus, this results in variations among siblings. Hence it is an incorrect answer.
• Option 2:
  • When chromosomes of the same kind are paired together during meiosis, an event called crossing over takes place.
  • Parts of the chromosome can be exchanged when two paired homologous chromosomes, i.e., one from the mother and one from the father.
  • This is known as crossing over.
  • Thus, this results in variations among siblings. Hence it is an incorrect answer.
• ​Option 3:
  • Linkage is defined as the physical association of closely related genes or other DNA sequences that are located close to one another on the same chromosome.
  • There is the possibility of two genes or sequences being inherited together increases with their proximity on a chromosome.
  • Linkage reduces the chances of recombination in the offspring so that parental characters are inherited without any variation.
  • Thus, this will not result in variations among siblings. 
  • Hence, this is the correct answer.
• ​Option 4:
  • A mutation is a change in a DNA or gene sequence, which results in a change in the genotype and phenotype of an individual.
  • Mutations are a cause for genetic variation in a population and results in variations among siblings.
  • Hence it is an incorrect answer.

Therefore, the correct answer is an option (3).

Key Points

• T. H. Morgan experimented with Drosophila melanogaster to establish that the basis of variation was caused by sexual reproduction. 
• Drosophila melanogaster or fruit-fly was used as a model organism because of the following reasons:
  • Their size (3mm) is small enough to keep millions in a laboratory, but not so small that cannot be seen without a microscope.
  • They can be grown on simple synthetic media in the laboratory.
  • They complete their life cycles in about 2 weeks, which allows to study multiple generations.
  • A single mating produces a large number of offspring.
  • Male and female individuals are morphologically distinct.
  • Many of the hereditary variations can be studied under low power microscopes.
  • About 75% of the genes causing diseases in humans are also found in Drosophila.

Explanation:

Statement (A) They could be cultured easily in the monastery - INCORRECT

• Morgan experimented with Drosophila in the laboratory, not in monastery.
• It was Gregor Mendel who performed the genetic experiments on pea plant in monastery garden.

Statement (B) They showed many contrasting traits - CORRECT

• Contrasting characters are important for studying genetics.
• Drosophila showed many such characters like miniature wings, eye colour, body colour, etc.

Statement (C) The generation time was one year - INCORRECT

• Their generation time was only 2 weeks, which allowed study of multiple generations in short time.

Statement (D) There was clear differentiation of sexes - CORRECT

• The male and female individuals had distinct morphological characteristics.
• For example, yellow-bodied white-eyed females were crossed with brown-bodied red-eyed males.

Statement (E) Very few progeny were produced in a single mating - INCORRECT

• They produce large number of offspring in every single mating.

Hence, the correct answer is option 1: (B) and (D) only.

The correct answer is option 2.

Concept:

• Chromosomes are defined as the genetic material present in all cells. 
• It is a  threadlike structure of nucleic acids and proteins found in the nucleus of most living cells, carrying genetic information.
• Every chromosome (visible only in dividing cells) essentially has a primary constriction or the centromere.
• Genes are units of heredity.
• They carry information from one generation to the next generation.

​Explanation:

• All the genes located on the same chromosome form one linkage group.
• A linkage group is defined as all the genes on a chromosome that moves as a single unit during cell division.
• These genes do not tend to separate, until the crossing-over.

Therefore, the correct answer is option 2.

Key Points

• The term "linkage" describes the physical closeness of genes on a chromosome and their propensity to be passed down in tandem.
• This indicates that genes on a chromosome that are close to one another are more likely to be passed down as a group from one generation to the next.
• This happens because chromosomes, as opposed to genes, are transferred from one generation to the next in their whole form.
• Meiosis involves a mechanism called crossing over, which can separate connected genes and their inheritance.
• Homologous chromosomes exchange genetic material during crossing over, resulting in novel gene combinations in progeny.
• As a result, the gene combinations of the kids differ from those of either parent, which can boost genetic variety within a community.

Correct Answer: Option 1

Concept:

LINKED GENES:

• ​​The genes that are present on the same chromosome tend to get inherited together. Such genes are referred to as linked genes.
• Linkage refers to the tendency of two or more genes present on the same chromosome to get inherited together.
• Linkage groups refer to all the linked genes in a chromosome. The number of linked genes corresponds to the haploid number of chromosomes in that species.
• The works of T.H. Morgan on Drosophila help us understand the linkage in animals.
• There are two types of linkages depending on the distance of the genes in a chromosome - Complete linkage and Incomplete linkage.
  • Complete linkage:
    • These are the linked genes that are closely placed on a chromosome.
    • As a result of this, they do not undergo crossing over (i.e. do not separate) and get inherited together.
    • These genes are referred to as completely linked genes and the linkage shown by them is called complete linkage.
    • As a result of complete linkage, parental traits are seen in their offspring.
  • Incomplete linkage:
    • These are the genes that are placed distantly from each other on the same chromosome.
    • Due to the distance between them, there are chances of crossing over to take place between these genes. As a result, the genes get separated.
    • These genes are referred to as incompletely linked genes and the linkage shown by them is called incomplete linkage.
    • As a result of crossing over, the recombination of genes takes place. This gives rise to new traits in the offspring.
• The genes that are inherited by offspring from their parents that are either X-linked or Y-linked is called sex-linked inheritance.

Explanation:

• To answer the given question, first, we need to understand T.H. Morgan's experiment on Drosophila.
  • Morgan worked on Drosophila melanogaster.
  • He carried out several dihybrid cross experiments on the fruit fly.
  • These experiments were carried out to study sex-linked genes.
  • One such cross involved, a female with white-bodied and yellow eyes with a male with wild type phenotype of brown-bodied and red eyes.
  • The F₁ progeny so obtained was self-crossed.
  • The F₂ progeny obtained showed a deviation in its phenotypic ratio from that of the Mendelian ratio of 9:3:3:1.
  • Morgan concluded from this observation, that the two genes - body color and eye color - did not segregate independently of each other during gamete formation.
  • These genes were present on the X-chromosome and are hence are sex-linked genes.
  • Morgan stated that when genes in a dihybrid cross are linked, the progeny obtained from such a cross shows a higher proportion of parental combination than the non-parental combination. 
  • This is because of the linkage which does not allow the genes to separate from each other.
• Using the above-given explanation, let us now solve the given question,

ASSERTION: When white-eyed, yellow-bodied Drosophila females were hybridized with red-eyed, brown-bodied males; and F₁ progeny was intercrossed, the F₂ ratio deviated from 9 : 3 : 3: 1.

• ​This statement is true.
• According to the above-mentioned experiment conducted by Morgan on Drosophila, the progeny obtained in the F₂ generation when the F₁ progeny is intercrossed shows a deviation in its phenotypic ratio from that of the Mendelian ratio.
• Normally, the phenotypic ratio of F₂ generation in a dihybrid cross is 9:3:3:1. 
• However, the F₂ generation in the above-mentioned cross showed a deviation from this ratio. This is because the genes are linked and tend to get inherited together.

REASON: When two genes in a dihybrid are on the same chromosome, the proportion of parental gene combinations is much higher than in the non-parental type.

• ​This statement is true.
• In the experiment on Drosophila involving genes that give body color and eye color, they tend to get inherited together.
• This is because the two genes are located on the same chromosome - The X chromosome.
• Due to this, they do not get separated during the crossing over.
• As a result the progeny so obtained shows a higher proportion of parental gene combinations than the non-parental type.
• Thus from the above-given explanation, both the assertion and reason are true and the reason is the correct explanation of the assertion.

So the correct answer is option 1.