RBSE Solutions Class 12 Biology Chapter 35 Mendel’s Law of Inheritance

RBSE Class 12 Biology Chapter 35 Multiple Choice Questions

 

Question 1. The main reason for the success of Mendel was that he:
(a) Selected pea plant
(b) Studied one character at a time in hybrid
(c) Kept record of pedigree
(d) All of the options
Answer: (d) All of the options
In simple words: Mendel was successful because he carefully chose pea plants, focused on one trait at a time, and kept detailed records. This systematic approach helped him discover the laws of inheritance clearly.

🎯 Exam Tip: Remember Mendel's methodical approach – choosing suitable organisms, working with distinct characters, and using statistical analysis – which were key to his groundbreaking discoveries.

 

Question 2. Law of Independent assortment is proved by:
(a) All offspring of the F₁ generation is long
(b) Long and dwarf plants produce in 3:1 ratio
(c) Loading [MathJax]/extensions/MathZoom.js & wrinkled seeds in the F2 generation.

 

Question 3. A monohybrid cross gives an F2 generation is a characteristic phenotypic ratio of:
(a) 9:3:3:1
(b) 3:1
(c) 1:1
(d) 2:1
Answer: (b) 3:1
In simple words: When you cross two different types of parents and then breed their first generation together, the next generation (F2) will show the traits in a 3:1 ratio for a single characteristic. This means three parts will show the dominant trait and one part will show the recessive trait.

🎯 Exam Tip: The 3:1 phenotypic ratio is a hallmark of a monohybrid cross, showing simple dominance, while the genotypic ratio is 1:2:1.

 

Question 4. Red and White cross give pink progeny. In this R gene represents:
(a) Hybrid
(b) Recessive
(c) Incomplete dominance
(d) Mutation
Answer: (c) Incomplete dominance
In simple words: When red and white flowers are crossed, and they produce pink flowers, it means neither red nor white is fully dominant. They blend together to show a new, in-between trait, which is called incomplete dominance.

🎯 Exam Tip: Incomplete dominance results in a blended phenotype, like pink flowers from red and white parents, which is different from codominance where both traits are fully expressed.

 

Question 5. The effect of the genotype of group AB in human being shows:
(a) Dominant-Recessive
(b) Incomplete dominance
(c) Co-dominance
(d) Complimentary
Answer: (c) Co-dominance
In simple words: If a person has blood group AB, it means both the A and B genes are fully expressed at the same time. Neither gene hides the other, and they don't blend. Both traits show up together.

🎯 Exam Tip: In codominance, both alleles are expressed equally and distinctly in the phenotype, as seen in ABO blood groups where both A and B antigens are present in type AB blood.

 

Question 6. What is the genotype ratio in a lethal gene?
(a) 1:2:1
(b) Loading [MathJax]/extensions/MathZoom.js

RBSE Class 12 Biology Chapter 35 Very Short Answer Type Questions

 

Question 1. Who is called the father of heredity and why?
Answer: Gregor Johann Mendel (1822 to 1884) is known as the "father of genetics." He earned this title because he was the first to explain how traits are passed from one generation to the next. His detailed experiments laid the foundation for understanding heredity.
In simple words: Gregor Mendel is called the father of heredity because he was the first person to explain how features are passed down from parents to children.

🎯 Exam Tip: Always include both Mendel's name and the core reason (explaining trait transmission) when asked about the "father of genetics."

 

Question 2. What are the monohybrid and dihybrid phenotypic ratio of the F2 generation?
Answer:
Monohybrid \( \implies \) 3:1 (Dominant: Recessive)
Dihybrid \( \implies \) 9:3:3:1
In a monohybrid cross, only one trait is studied, while a dihybrid cross looks at two traits at once, showing how they pass on.
In simple words: For one trait, the F2 generation shows a 3:1 ratio. For two traits, it shows a 9:3:3:1 ratio.

🎯 Exam Tip: Clearly distinguish between monohybrid and dihybrid ratios. Remember that monohybrid focuses on one character and dihybrid on two characters inherited independently.

 

Question 3. What are the multiple gene characters?
Answer: Multiple gene characters, also known as multiple alleles, occur when a single trait is controlled by three or more different forms of a gene. Normally, a trait is controlled by just a pair of alleles. However, in these cases, many alleles are responsible for one specific character. The ABO blood system in humans is a common example of this.

• Normally, phenotype expression is controlled by a pair of alleles.
• But when 3 or more alleles are responsible for one character, it is called multiple alleles.
• Example: ABO blood system.

In simple words: When one trait is controlled by more than two different types of genes, it's called a multiple gene character. Human blood groups are an example.

🎯 Exam Tip: Explain that multiple alleles involve more than two forms of a gene, not just multiple genes, and provide a clear example like the ABO blood system.

 

Question 4. Where and in which organization Mendel read the research paper of his experiments?
Answer: Mendel presented his research paper on his experiments at the Brunn Society of Natural History. He read it on two dates: February 8th and March 8th in the year 1865. This formal presentation was a crucial step in sharing his findings.
In simple words: Mendel read his research paper at the Brunn Society of Natural History in 1865.

🎯 Exam Tip: For historical questions, accurately recall the specific organization and dates where key scientific findings were presented.

 

Question 5. In which language the original research paper of Mendel was published and what was its title?
Answer: Mendel's original research paper was published in German. The title of his paper was "Versuche über Pflanzen-Hybriden" which translates to "Experiments on Plant Hybridization". This scientific work was the first to explain how traits are inherited.
In simple words: Mendel's paper was published in German and was called "Experiments on Plant Hybridization."

🎯 Exam Tip: When asked for publication details, provide both the language and the exact title of the work for full marks.

RBSE Class 12 Biology Chapter 35 Short Answer Type Questions

 

Question 1. Differentiate the following:
1. Homozygous and Heterozygous
2. Dominant and Recessive trait
3. Genotype and Phenotype
4. Monohybrid Cross and Dihybrid Cross
Answer:
1. Homozygous and Heterozygous:
There are two genes (alleles) for every character. If these two genes for a particular character are identical, the organism is said to be homozygous (e.g., Tall plants (TT) or dwarf plants (tt)). If the two genes for a character are different, the organism is called heterozygous (e.g., Tt for tall plants). This difference in gene pairs leads to different gamete formation.

• If the two genes for a particular character are identical, it is said to be homozygous (G homos \( \implies \) same; zygotes \( \implies \) a pair). For example, tall plants (TT) & dwarf plant (tt) are homozygous.
• If the two genes for a particular character are unlike, it is said to be heterozygous (G. heteros \( \implies \) other, zygotes \( \implies \) a pair). For example, Tt is heterozygous tall.

2. Dominant and Recessive trait:
A dominant character is the trait that appears even when only one copy of its allele is present in a heterozygous condition. A recessive character, on the other hand, only appears when two copies of its allele are present (homozygous recessive). This explains why some traits seem to disappear in one generation and reappear in the next. For example, tallness in pea plants is dominant over dwarfness.

• Dominant character: Amongst two alleles of a character, the character expressed in a heterozygous condition is called the Dominant character. The incidence is known as Dominance and the gene responsible for it is called a dominant gene. For example, the T (tall) condition expresses tallness in a plant.
• Recessive character: Amongst two alleles of a character, the character not expressed in a heterozygous condition is called the Recessive character. The incidence is known as Recessiveness and the gene responsible for it is called a recessive gene. For Example, the tt condition expresses dwarfness of a plant.

3. Genotype and Phenotype:
Genotype refers to the genetic makeup of an organism, like the specific combination of alleles (e.g., RR or Rr). Phenotype refers to the observable physical traits or characteristics that result from the genotype (e.g., round seeds or wrinkled seeds). So, while genotype is internal, phenotype is external and visible. Environmental factors can also influence the phenotype.

• Genotype: The genetic expression of a feature, which is obtained from parents. For Example, a pure round seed producing parent plant has genotype R R.
• Phenotype: The visible expression of the hereditary character possessed by an organism. For example, the round or wrinkled shape of a seed is its phenotype.

4. Monohybrid Cross and Dihybrid Cross:
A monohybrid cross involves studying the inheritance of only one pair of contrasting traits (e.g., tall vs. dwarf plants). A dihybrid cross, however, involves studying the inheritance of two pairs of contrasting traits at the same time (e.g., seed color and seed shape). These two types of crosses reveal different patterns of inheritance. Mendel used both to formulate his laws.

• Monohybrid cross: A cross in which the inheritance of only one pair of contrasting characters is studied.
• Dihybrid cross: A cross in which the inheritance of two pairs of contrasting characters is studied simultaneously.

In simple words: Homozygous means having two identical genes for a trait, while heterozygous means having two different genes. Dominant traits show up even with one gene, while recessive traits need two. Genotype is the hidden genetic code, and phenotype is what you see. A monohybrid cross studies one trait, and a dihybrid cross studies two traits at once.

🎯 Exam Tip: When differentiating, always provide a clear definition for each term, a key distinguishing factor, and an example where applicable.

 

Question 2. Define:
1. Allele
2. Codominance
3. Polygenes
4. Lethal gene
Answer:
1. Allele:
An allele is one of two or more alternative forms of a gene that arise by mutation and are found at the same place on a chromosome. For example, blue eye color and black eye color are two alleles for the eye color gene. Alleles determine variations in a specific trait. For the seed of the pea plant, the round (R) and wrinkled (r) are two alleles.

• Alternative forms of a gene, for example, blue color and black color of the eye are two alleles of the color of the eye gene. An allele is a short form of Allelomorph. Similarly, for the seed of the pea plant, the round (R) and wrinkled (r) are two alleles.

2. Codominance:
Codominance is a genetic relationship where both alleles for a trait are fully and equally expressed in the phenotype of the heterozygous individual. Neither allele is dominant or recessive, and they don't blend. Both traits are clearly visible. For instance, in some cattle, a cross between a red cow (RR) and a white bull (rr) produces offspring with a roan coat (Rr), which has both red and white hairs showing clearly. The results here (red, roan, white in 1:2:1 ratio) do not follow the Mendelian ratio with regard to colors.

• Then a cross between F1 generation gives red, roan and white skin colors in the F2 generation. The ratio of red, roan and white colored skin animals are found 1:2:1 (RR: Rr: RR). The results do not follow Mendelian ratio with regard to colors.

3. Multiple gene or Polygenes:
Polygenic traits are characteristics that are controlled by two or more genes, where each gene contributes a small amount to the overall phenotype. These traits often show a wide range of variation. The more dominant genes an individual has, the more pronounced the trait will be. Human skin color, eye color, and body height are classic examples of polygenic inheritance, as many genes work together to determine these complex traits.

• The phenotype traits which are governed by three or more genes are called polygenic traits. The polygenic traits show a wide range of phenotypes. Each gene of the polygenic trait contributes to the phenotype but to a small degree.
• Presence of more than one dominant gene makes the phenotype more prominent. All the dominant genes add up their effects to produce a full phenotype. Therefore, the polygenic traits are also called as quantitative traits.
• Inheritance of such traits is called polygenic or quantitative inheritance.
• The polygenes may occupy two or more different loci on the same homologous chromosome pair.
• It was first studied by Galton (1883) in man.
• The well-known examples of a polygenic trait are human skin color & kernel color in wheat.
• The other examples of polygenic traits in human beings are - eye color & body height.

Example: 1 - Human skin color

• The human skin color is a polygenic effect and controlled by three pairs of genes Aa, Bb, Cc.
• These genes are located on different chromosomes and are inherited independently.
• Also, the amount of melanin pigment, which causes darkness, depends on the number of dominant alleles present, rather than complete dominance by one gene.

4. Lethal gene:
A lethal gene is an allele that, when present in a homozygous state, causes the death of the organism. These genes can affect not only external characteristics but also the viability of the organism, often before birth or early in development. L. Cuenot's experiments with rat body color in 1905 provided key insights into lethal genes, showing that homozygous yellow rats could not survive. These genes cause a deviation from typical Mendelian ratios.

• Some genes, besides controlling external characteristics, also affect the viability of organisms.
• L. Cuenot in 1905 presented results of his experiment on the body color of rats. These results were different from Mendel's law of segregation.
• According to his experiments, the yellow color of the skin is due to gene Y and grey color is due to gene y, in which Y is dominant over y (grey color).
• When yellow color rats were crossed, yellow and grey color rats in a 2:1 ratio were produced.
• Genotypically, yellow rats were heterozygous (Yy) and grey rats were homozygous (yy).
• The Yy (heterozygous) yellow color rats were unable to survive. Hence the Y gene is responsible for the yellow color of rats, and it also affects survival capacity.
• The yellow homozygous (YY) rats cannot survive; hence, homozygous rats are not found at all.
• Hence YY genes are called lethal genes, and this incidence is known as Lethality.
• Some lethal genes are lethal only in the homozygous recessive condition and are called recessive lethal genes, while a dominant lethal gene may be lethal even in the heterozygous condition.

In simple words: An allele is a different version of a gene. Codominance means both gene versions show up fully. Polygenes are many genes working together to make one trait. A lethal gene causes death if an organism has two copies of it.

🎯 Exam Tip: For definitions, provide a concise explanation and a relevant example for each term. For codominance, specifically mention that both alleles are expressed equally.

 

Question 3. What are the reasons for Mendel's success?
Answer: Mendel's success in uncovering the principles of inheritance was due to several key factors. He chose the garden pea plant, which was easy to cultivate and had distinct, contrasting traits. He also studied one or two traits at a time and used a large sample size, allowing for statistical analysis of his results. His meticulous record-keeping and mathematical approach were revolutionary for his time.

• Mendel, through his experiments, discarded all previous hypothetical theories from earlier scientists that lacked scientific experimental bases.
• Scientists later discovered dominant and recessive characters in living beings based on Mendel's work.
• Beneficial characters from different genera can be combined into a new genus.
• Mendel's laws helped in discovering immunity against diseases, maintaining continuity in adverse conditions, and developing good quality and quantity of flowers and fruits.
• Similarly, the breed of cow, buffalo, and hen have been improved using these principles.

In simple words: Mendel succeeded because he picked the right plant (pea), studied simple traits, counted his results carefully, and used math to understand them. His findings helped improve plants and animals and led to understanding diseases.

🎯 Exam Tip: When listing reasons for Mendel's success, focus on his scientific methodology: choice of organism, experimental design (one character at a time), large sample size, and quantitative analysis.

 

Question 4. Describe Incomplete dominance with examples?
Answer: Incomplete dominance is a type of inheritance where the heterozygous phenotype is a blend or intermediate of the two homozygous parental phenotypes. Neither allele completely masks the other, leading to a new, mixed trait. This pattern was first observed by Correns in 1903 and is sometimes called blended or partial inheritance. For example, when red-flowered (RR) and white-flowered (rr) four o'clock plants (Mirabilis jalapa) are crossed, the F1 generation produces pink-flowered plants (Rr), showing an intermediate phenotype. The phenotypic and genotypic ratios in the F2 generation are both 1:2:1 for incomplete dominance.

• It was discovered by Correns (1903).
• It is also called as blended or partial or mosaic or 'intermediate inheritance'.
• It is an exception to be an outcome of Mendel's monohybrid cross.
• It is found in both plants & animals.
• In Mendel's pea experiments, dominance was essentially complete & there was no difference between the homozygous & heterozygous plants in the expression of dominant character.
• However, there are characters or alleles that are neither dominant nor recessive. In such cases, both the alleles of the contrasting conditions of a character express as a blend.
• As a result, in the F1 generation, the hybrid produced by crossing two pure individuals does not resemble either of them but is midway between them.
• This expression of an intermediate trait of two pure parents in the F1 hybrids is called as incomplete dominance.
• Example 1: Four O'clock plant (Mirabilis jalapa) & snapdragon plant (Antirrhinum majus). In incomplete dominance in Snapdragon, the phenotypic and genotypic ratios are the same, 1:2:1, which is a characteristic of Incomplete Dominance.

In simple words: Incomplete dominance happens when traits mix together instead of one hiding the other. For example, red and white flowers can make pink flowers. Both the genes and how they look have a 1:2:1 pattern.

🎯 Exam Tip: Define incomplete dominance by emphasizing the "intermediate" or "blended" phenotype in heterozygotes, and provide classic plant examples like Mirabilis jalapa or snapdragons.

 

Question 5. Why Mendel selected Pea plant in his experiments? Explain.
Answer: Mendel chose the garden pea plant (Pisum sativum) for his experiments because it had several advantages that made it an ideal organism for studying inheritance. These advantages included its ease of cultivation, a short life cycle allowing multiple generations to be studied quickly, and its ability to self-pollinate naturally while also being easy to cross-pollinate manually. The plant also displayed many clear, contrasting traits, which simplified the observation of inheritance patterns.

• Mendel carried out his work on garden pea Pisum sativum. He selected this plant because of the following reasons:
• It was easy to grow the plants in open ground and in pots.
• The plant had a short life span, and it completes its cycle in one season, so the study of many generations is possible in a few years.
• The pea plant is small, easy to crossbreed artificially.
• The pea flower is bisexual and exhibits self-fertilization in nature, which maintains the purity of characters for many generations. However, it can also be easily cross-pollinated for controlled experiments.

In simple words: Mendel picked pea plants because they were easy to grow, lived for a short time (so he could see many generations quickly), and had clear differences in traits. They could also self-pollinate or be easily cross-pollinated.

🎯 Exam Tip: List at least three distinct reasons for Mendel's choice of the pea plant, focusing on practical advantages for genetic study like ease of control, short generation time, and clear traits.

 

Question 1. Mendel studied which hereditary traits of a Pea. in his experiments, explain.
Answer: Mendel meticulously studied seven pairs of contrasting hereditary traits in the garden pea plant (Pisum sativum). By focusing on these distinct characteristics, he could observe clear patterns of inheritance. These traits included plant height (tall vs. dwarf), flower position (axial vs. terminal), pod shape (inflated vs. constricted), pod color (green vs. yellow), seed shape (round vs. wrinkled), seed coat color (grey vs. white), and cotyledon color (yellow vs. green). His careful observation of these traits led to the formulation of his famous laws of inheritance.

• Mendel conducted a breeding experiment on the garden pea plant, Pisumsativum. He studied the inheritance of seven pairs of contrasting characters in the garden pea & he restricted his experiments to one or two pairs of contrasting traits in each experiment.
• The seven pairs of contrasting characters selected by Mendel are as follows:

• Some authors follow the color of flowers - White and Purple character as a 7th character instead of the color of cotyledons.
• Mendel found all the above 7 traits to be pure genetically in experiments.

In simple words: Mendel looked at seven clear differences in pea plants, like tall or short height, smooth or wrinkled seeds, and different flower colors. These distinct pairs of traits helped him understand how features are passed down.

🎯 Exam Tip: When describing Mendel's traits, remember there are always *pairs* of contrasting characters. Listing at least five of these pairs is usually sufficient for a complete answer.

 

Question 2.

 

Question 3. Explain the deviations of principles of Mendel's laws.
Answer: While Mendel's laws provide a fundamental understanding of inheritance, some genetic patterns do not strictly follow his original principles. These are known as deviations or exceptions to Mendelian inheritance. The main principles of Mendel's laws are: Incomplete Dominance and Codominance. These deviations show that genetic inheritance can be more complex than simple dominant-recessive relationships, sometimes involving blending of traits or simultaneous expression of multiple traits.
Incomplete Dominance:
This occurs when the heterozygous phenotype is a blend of the two homozygous phenotypes, rather than one trait being completely dominant. For instance, a cross between a red flower and a white flower may produce pink offspring. Correns discovered this in 1903. The F1 hybrids do not resemble either parent but are intermediate. The phenotypic and genotypic ratios in the F2 generation are both 1:2:1.

• It was discovered by Correns (1903).
• It is also called as blended or partial or mosaic or 'intermediate inheritance'.
• It is an exception to be an outcome of Mendel's monohybrid cross.
• When crossing two pure parents, the F1 generation shows an intermediate trait, meaning neither parent's trait is fully expressed but instead they blend.
• Example 1: Four O'clock plant (Mirabilis jalapa) & snapdragon plant (Antirrhinum majus). In incomplete dominance in Snapdragon, the phenotypic and genotypic ratios are the same. The ratio 1:2:1 is the characteristic of Incomplete Dominance.

Codominance:
In codominance, both alleles for a trait are fully and equally expressed in the phenotype of the heterozygous individual. This means both traits appear distinctly at the same time, without blending. A classic example is the ABO blood group system in humans, where a person with AB blood type expresses both A and B antigens simultaneously. Another example is the roan coat color in cattle, where both red and white hairs are present.
In simple words: Mendel's laws don't always fully apply. Sometimes, traits mix, like red and white flowers making pink (incomplete dominance). Other times, both traits show up fully at the same time, like in human AB blood type (codominance).

🎯 Exam Tip: Clearly differentiate between incomplete dominance (blending of traits) and codominance (simultaneous full expression of both traits) and provide a distinct example for each.

 

Differences Between Co-Dominance and Incomplete Dominance: