Exceptions to Mendel's Law MCQ Quiz - Objective Question with Answer for Exceptions to Mendel's Law - Download Free PDF

The correct answer is Law of Segregation does not apply in this experiment

Concept:

• Incomplete dominance is a type of gene interaction in which the dominant allele is not completely dominant over the recessive allele.
• It forms offspring with intermediate phenotype or different phenotype.

Explanation:

• Antirrhinum is commonly known as snapdragon. Flowers of Antirrhinum follow incomplete dominance.
• When red flowers (RR) are crossed with white flowers (RR), all flowers (Rr) of the F1 generation obtained are pink in color.
• The pink color obtained is due to incomplete dominance as red color is incompletely dominant on the white color.
• When flowers of the F1 generation were crossed, three types of flowers are obtained, red (RR), pink (Rr), and white (RR).
• The genotypic and phenotypic ratio is the same, that is, 1:2:1.
• The Law of segregation is applicable in case of incomplete dominance.
• The Law of segregation says that the alleles do not show any blending and that both the characters are recovered as such in the F2 generation though one of these is not seen at the F1 stage.
• So here in this case, though parental characters red and white are not seen in the F1 generation, but recovered in the F2 generation.
• Hence, the law of segregation is followed here.

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The correct answer is All of these

Explanation:

• Every gene contains the information to express a particular trait.
• Genes code for RNA and proteins that act as enzymes or metabolites.
• These enzymes and metabolites are a part of a biochemical reaction that presents a particular trait.
• DNA contains many genes → transcribed to RNA → translated to protein → RNA and protein act as enzymes and metabolites → biochemical reaction → trait expressed 
• In a diploid organism, there are two copies of each gene, i.e., as a pair of alleles.
• One allele is received from one parent (father) and another from another parent (mother).
• In a heterozygote, one allele is dominant and another is recessive.
• In presence of the dominant allele, the recessive is not able to express.
• Different alleles produce slightly different proteins, which function in different ways.  
• In dominant/recessive relationships, the recessive allele produces a non-functional protein. The dominant allele produces a functioning protein.
• In incomplete dominance, it is thought that in a heterozygote, the dominant allele is functioning properly and the recessive allele is malfunctioning.
• The normal or dominant allele produces the normal enzyme but the modified or recessive allele could be responsible for the production of –
  • the normal/less efficient enzyme, or
  • a non-functional enzyme, or
  • no enzyme at all

The correct answer is Shape of seed

Concept:

• In Gregor Mendel's classic experiments on pea plants, he observed that seeds could be either round or wrinkled. It was later found that this trait is controlled by the gene that regulates the enzyme involved in starch synthesis. When starch is synthesized normally (dominant allele "R"), the seeds are round. When there is a mutation in this gene (recessive allele "r"), it leads to less efficient starch synthesis, resulting in wrinkled seeds

Explanation:

• A single gene product may produce more than one effect. For example, starch synthesis in pea seeds is controlled by one gene.
• It has two alleles (B and b).
• Starch is synthesised effectively by BB homozygotes and therefore, large starch grains are produced.
• In contrast, bb homozygotes have lesser efficiency in starch synthesis and produce smaller starch grains.
• After maturation of the seeds, BB seeds are round and the bb seeds are wrinkled.
• Heterozygotes produce round seeds, and so B seems to be the dominant allele. But, the starch grains produced are of intermediate size in Bb seeds.

The correct answer is TTRr

Explanation:

The correct genotype of the parent plant is TTRr because when this plant is selfed (crossed with itself), it can produce the following combinations of progeny:

Parents: TTRr x TTRr

Gametes: TR, Tr

• Phenotype: Tall plants with round seeds (3):Tall plants with wrinkled seeds (1)
• Genotype: TTRR (1) : TTRr (2): TTrr (1)             

Tall plants with round seeds: These plants can have the genotypes TTRR, TTRr.

Tall plants with wrinkled seeds: These plants can have the genotypes Ttrr.

The correct answer is round and yellow

Explanation:

• The Law of Independent Assortment states that when two pairs of traits are combined in a hybrid, the segregation of one pair of characters is independent of the other pair of characters.
• When a round, green-seeded pea plant (RRyy) is crossed with a wrinkled, yellow-seeded pea plant, (rrYY) the seeds produced in the F1  generation are round, yellow (RrYy) seeds. 
• Punnet square can be effectively used to determine the independent segregation of the two pairs of alleles during meiosis and produce eggs and pollen in the F1 generation RrYy plant.
• This is an example of a dihybrid cross.
• In a dihybrid cross, the phenotypes such as round, yellow; wrinkled yellow; round, green; and wrinkled green appeared in the ratio 9:3:3:1.
• Two pea plants one with round green seeds (RRyy) and another with wrinkled yellow (rrYY) seeds produce F1 progeny that have round, yellow (RrYy) seeds.

The cross is in the following,

Therefore, F1 generation: RrYy (Round and yellow)

Concept-

• Gene is the basic unit of inheritance.
• Genes are hereditary markers from which control various characteristics like skin colour, height, etc.
• Gene which controls more than one character is called a pleiotropic gene.
• This gene shows multiple phenotypic effects.

Explanation-

• Phenylketonuria (PKU) is one such disorder that is caused due to a single gene mutation.
• The mutation is caused in the gene encoding phenylalanine hydroxylase enzyme.
• Affected people lack this enzyme, which can convert phenylalanine to tyrosine.
• Lack of the enzyme results in accumulation of phenylalanine, which gets converted to phenylpyruvic acid.
• Phenylpyruvic acid has adverse effects on the body:
  • Mental retardation
  • Reduction of hair
  • Skin pigmentation
• Thus, we can see that a single gene mutation is causing multiple phenotypic expressions.

Additional InformationPolygenic Inheritance -

• Polygenic inheritance was first described by Nilsson Ehle.
• It is the inheritance of characters in which one character is controlled by many genes.
• Here, the intensity of character depends upon the number of the dominant alleles.
• For example, human skin colour which is spread across a gradient and not distinct.
• It is the result of the additive effect of each allele.

• Co-dominance is a form of genetic inheritance where both alleles contribute equally and visibly to the phenotype of the organism. In the F1 generation, this results in offspring that display traits from both parents equally.
• For example, in the case of blood types, if one parent has type A blood and the other has type B blood, their offspring may have type AB blood, exhibiting characteristics of both A and B alleles.

• Rationale: Incomplete dominance occurs when the phenotype of the F1 generation is a blend of the parental traits, rather than a display of both. This means the offspring will show an intermediate phenotype, not clearly resembling either parent fully.

• Rationale: Pleiotropy refers to a single gene influencing multiple, seemingly unrelated phenotypic traits. It does not specifically relate to the resemblance of the F1 generation to the parents in terms of individual traits.

• Rationale: The Law of Dominance, proposed by Gregor Mendel, states that in a heterozygote, one trait will conceal the presence of another trait for the same characteristic. The dominant allele masks the recessive one, meaning the F1 generation will resemble the dominant parent more, not both parents equally.

• Among the given options, co-dominance is the correct answer because it directly describes a situation where the F1 generation shows traits from both parents equally, which is not the case with incomplete dominance, pleiotropy, or the law of dominance.

The correct answer is Half of the heterozygous Ww individuals will be red-flowered.

Concept:

• Mendel's experiments with pea plants laid the foundation for our understanding of genetics. In one of his classic experiments, he crossed plants with different traits to observe how these traits were inherited by the offspring.
• In this particular case, Mendel crossed a pea plant with two dominant alleles for red flowers (WW) with a pea plant with two recessive alleles for white flowers (ww).
• The F1 generation refers to the first generation of offspring resulting from this cross.

Explanation:

• Every individual in the F1 generation will receive a W allele from the red-flowered parent: This is correct. Each offspring will inherit one allele from each parent. Since the red-flowered parent is homozygous dominant (WW), it can only pass on the W allele.
• In the course of meiosis, a heterozygous individual will form two kinds of gametes: This is correct. A heterozygous individual (Ww) will produce two types of gametes - one with the W allele and one with the w allele.
• Half of the heterozygous Ww individuals will be red-flowered: This is incorrect. All individuals in the F1 generation will be heterozygous (Ww) and will display the dominant red flower phenotype. Therefore, 100% of the F1 generation will be red-flowered, not just half.
• Ww individuals are red-flowered and are indistinguishable from the homozygous WW individuals: This is correct. The presence of even one dominant allele (W) will result in the red flower phenotype, making Ww individuals phenotypically identical to WW individuals.

The correct answer is O blood group.

Key Points

• Blood groups show codominance inheritance.
• When both the alleles combine that both express and produce their effect.
• It does not blend like incomplete dominance to show a different phenotype.
• The genotypic ratio is 1:2:1
• The phenotypic ratio is also 1:2:1.

• In blood group A=B > O
• A and B blood groups show codominance and it is dominant over o blood group.
• From the above cross, we can see that the child can have A, B, and AB blood groups.
• If a person having blood group A is married to a female with blood group AB their child's blood group can not be the ''O'' blood group.

Additional Information 

Concept:

• A test cross is done to find out the genotype of an individual whose genotype is unknown.
• It is usually done to find the kind alleles of a gene present in the genome of an individual for only one or two characters.
• In a test cross, one of the individuals is the one whose genotype for the character is unknown.
• For example, we want to know the genotype for height, which is expressed by one gene with two alleles, T and t.
• The recessive phenotype is expressed only in homozygous condition.
• But dominant phenotype may be homozygous or heterozygous.
• Thus, test cross helps in determining the unknown genotype of the dominant individual.
• There can be two scenarios: the individual with an unknown genotype can be
  1. TT (homozygous dominant)
  2. Tt (heterozygous)
• The individual with an unknown genotype will be crossed with one with a homozygous recessive genotype, i.e., tt.

Explanation

• In scenario 1 where the individual with an unknown genotype is homozygous dominant with genotype TT, all the progeny will be tall.
• The phenotype of all the progeny is tall.

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• In scenario 2 where the individual with an unknown genotype is heterozygous with genotype Tt, half of the progeny are tall and half are dwarf.
• The phenotypic ratio is 2:2, 50% of the progeny is tall and 50% is short.

• This clear identification is because the use of a homozygous recessive individual in a test cross allows the expression of the unknown genotype.
• The recessive allele will not express until another allele also expresses dwarfism, so, it is easy to identify.

Explanation:

• Blood types are determined by the presence or absence of certain antigens on the surface of red blood cells.
• There are four main blood types: A, B, AB, and O. Blood type is inherited, with each parent contributing one of two ABO genes to their child.
• If the father's blood type is A and the mother's blood type is B, their child can inherit any combination of these two blood types, depending on the specific ABO genes that the parents carry.
• This means their child could potentially have any of the four main blood types: A, B, AB, or O. So, the correct answer is 4) A, B, AB, or O.

 Additional Information

• It's important to note that each blood type also has a positive or negative Rh factor, which is another antigen found on the surface of red blood cells.
• The Rh factor is inherited separately from the ABO blood types, so a child's Rh factor could be positive or negative, regardless of their ABO blood type.

Concept-

• Gene is the basic unit of inheritance.
• Genes are hereditary markers from which control various characteristics like skin colour, height, etc.
• Gene which controls more than one character is called a pleiotropic gene.
• This gene shows multiple phenotypic effects.

Explanation-

• Phenylketonuria (PKU) is one such disorder that is caused due to a single gene mutation.
• The mutation is caused in the gene encoding phenylalanine hydroxylase enzyme.
• Affected people lack this enzyme, which can convert phenylalanine to tyrosine.
• Lack of the enzyme results in accumulation of phenylalanine, which gets converted to phenylpyruvic acid.
• Phenylpyruvic acid has adverse effects on the body:
  • Mental retardation
  • Reduction of hair
  • Skin pigmentation
• Thus, we can see that a single gene mutation is causing multiple phenotypic expressions.

Additional InformationPolygenic Inheritance -

• Polygenic inheritance was first described by Nilsson Ehle.
• It is the inheritance of characters in which one character is controlled by many genes.
• Here, the intensity of character depends upon the number of the dominant alleles.
• For example, human skin colour which is spread across a gradient and not distinct.
• It is the result of the additive effect of each allele.

Concept: 

• Mendel studied the inheritance of seven different features in peas, including height, flower color, seed color, and seed shape.

• He established pea lines with two different forms of a feature, such as tall vs. short height.

• He grew these lines for generations until they were pure-breeding (always produced offspring identical to the parent), then bred them to each other and observed how the traits were inherited.

​​Explanation:

Option 1:

• The Law of Dominance states that the offspring always exhibits a dominant trait.
• From the two alleles received from parents, the only dominant allele is expressed.
• Therefore, this is not the correct answer.

Option 2:

• A cross between two pure breeding different varieties of organism taking the alternative traits of one single character is called monohybrid cross or inheritance of one gene.
• Therefore, this is not the correct answer.

​Option 3:

• In codominance, two alleles express themselves independently when present together. Therefore, the offspring resembles both parents.
• Codominance is a type of inheritance in which two versions (alleles) of the same gene are expressed separately to yield different traits in an individual. That is, instead of one trait being dominant over the other, both traits appear, such as in a plant or animal that has more than one pigment color.
• This can be observed in the ABO blood group.
• In the ABO blood group system, both A and B groups are dominant, hence leading to the AB blood group.
• Therefore, an individual inheriting the A allele from the mother and the B allele from the father has a blood type AB.
• Therefore, this is the correct answer.

​Option 4:

• In the case of incomplete dominance, the phenotype of the F1 is an intermediate of the two alleles from the parents in their homozygous state.
• Therefore, the offspring do not resemble either parent.
• Example- Snapdragon (Antirrhinum sp)
• Therefore, this is not the correct answer.

Key Points

• The blood group O is recessive, meaning that a child needs to inherit the O allele from both parents to have this blood group.
• Since the father has the AB blood group, he cannot pass on an O allele.
• The mother with blood group B can pass either B or O alleles to the child.
• Therefore, it is genetically impossible for a child of an AB father and a B mother to have an O blood group.

• Co-dominance is a form of genetic inheritance where both alleles contribute equally and visibly to the phenotype of the organism. In the F1 generation, this results in offspring that display traits from both parents equally.
• For example, in the case of blood types, if one parent has type A blood and the other has type B blood, their offspring may have type AB blood, exhibiting characteristics of both A and B alleles.

• Rationale: Incomplete dominance occurs when the phenotype of the F1 generation is a blend of the parental traits, rather than a display of both. This means the offspring will show an intermediate phenotype, not clearly resembling either parent fully.

• Rationale: Pleiotropy refers to a single gene influencing multiple, seemingly unrelated phenotypic traits. It does not specifically relate to the resemblance of the F1 generation to the parents in terms of individual traits.

• Rationale: The Law of Dominance, proposed by Gregor Mendel, states that in a heterozygote, one trait will conceal the presence of another trait for the same characteristic. The dominant allele masks the recessive one, meaning the F1 generation will resemble the dominant parent more, not both parents equally.

• Among the given options, co-dominance is the correct answer because it directly describes a situation where the F1 generation shows traits from both parents equally, which is not the case with incomplete dominance, pleiotropy, or the law of dominance.