Chapter 4: Principles of Inheritance and Variation

Class 12 Chapter 4: Principles of Inheritance and Variation Exercise Question and Answers :

1. Mention the advantages of selecting pea plant for experiment by Mendel.

Answer :  Mendel chose pea plants for his experiments in principles of inheritance and variation due to several advantages:

Distinct Traits: Pea plants have easily distinguishable traits like seed color and flower position, simplifying data collection.

Controlled Pollination: Their hermaphroditic nature allows precise cross-pollination, ensuring controlled parentage.

True Breeding Lines: Mendel could use pure-breeding plants, guaranteeing consistent trait expression.

Short Generation Time: Pea plants have a short life cycle, enabling rapid experimentation.

Abundant Offspring: They produce many offspring, yielding statistically significant results.

These advantages facilitated Mendel's groundbreaking work in genetics.

2. Differentiate between the following –

(a) Dominance and Recessive

(b) Homozygous and Heterozygous

(c) Monohybrid and Dihybrid.

Answer : (a) Differentiating between Dominance and Recessive traits:

Dominant Traits	Recessive Traits
Dominant traits are expressed when an individual carries one dominant allele and one recessive allele (e.g., Aa).	Recessive traits are only expressed when an individual carries two copies of the recessive allele (e.g., aa).
Represented by an uppercase letter (e.g., A).	Represented by a lowercase letter (e.g., a).
Dominant alleles mask the expression of recessive alleles in heterozygous individuals.	Recessive alleles are only expressed in individuals who are homozygous for the recessive trait.
Typically, dominant traits are less common in the population compared to recessive traits.	Recessive traits are often more common in the population

(b) Differentiating between homozygous and heterozygous:

Homozygous	Heterozygous
Both alleles for a gene are identical	Two alleles for a gene are different
For a specific gene, both alleles are the same (e.g., AA or aa)	For a specific gene, alleles are different (e.g., Aa)
Limited genetic diversity and uniformity in the trait	Greater genetic diversity and variation in the trait
If both alleles carry a recessive trait, it will be expressed	Recessive trait is masked by the dominant allele

(c)  Differentiating between monohybrid and dihybrid crosses:

Monohybrid Cross	Dihybrid Cross
Examines the inheritance of a single gene with two alleles	Examines the inheritance of two different genes with two alleles each
Considers one trait at a time	Considers two traits simultaneously
Requires a 2 x 2 Punnett square	Requires a 4 x 4 Punnett square
Example : Cross involving a single gene, like Mendel's pea color (Rr x Rr)	Example : Cross involving two different genes, like pea color and pea shape (RrYy x RrYy)

3. A diploid organism is heterozygous for 4 loci, how many types of gametes can be produced?

Answer : In a diploid organism heterozygous for 4 loci, each locus has two different alleles. To determine the number of gamete combinations, you can use the multiplication rule of probability. For each locus, there are 2 possible alleles, and since there are 4 loci, you multiply the possibilities together: 2 x 2 x 2 x 2 = 16. Therefore, this diploid organism can produce 16 different types of gametes, each with a unique combination of alleles at these four loci.

4. Explain the Law of Dominance using a monohybrid cross.

Answer :  The Law of Dominance is a fundamental principle in genetics that can be illustrated using a monohybrid cross, where two different alleles for a single gene are examined. This law explains why, in a monohybrid cross, only one of the parental characters is expressed in the  generation, both parental characters are expressed in the  generation, and a 3:1 phenotypic ratio is obtained in the .

Expression in  Generation: In a monohybrid cross between a homozygous dominant parent (e.g., PP for purple flowers) and a homozygous recessive parent (e.g., pp for white flowers), the dominant allele (P) masks the expression of the recessive allele (p) in the  generation. Consequently, all offspring in the  generation will exhibit the dominant trait (purple flowers), suppressing the expression of the recessive trait (white flowers).

Expression in Generation: In the generation, the heterozygous individuals produced in the generation (Pp) are allowed to interbreed. Here, the Law of Dominance still applies, but the genetic makeup becomes more evident. Both the dominant and recessive alleles segregate independently, leading to the expression of both parental characters (purple and white flowers) in the generation.

Proportion of 3:1 in  : Mendel observed that in the generation of a monohybrid cross, approximately 75% of the offspring displayed the dominant trait (e.g., purple flowers), while about 25% displayed the recessive trait (e.g., white flowers). This 3:1 phenotypic ratio results from the Law of Dominance, as it reflects the segregation and random combination of alleles during gamete formation and fertilization. This ratio indicates that for every four offspring, three will have the dominant phenotype, and one will have the recessive phenotype, confirming Mendel's law.

5. Define and design a test-cross.

Answer :  A test-cross, also known as a back-cross or a tester cross, is a genetic cross used to determine the genotype of an organism showing a dominant phenotype. It involves breeding the organism with an individual that is homozygous recessive for the trait of interest. The outcome of the test-cross reveals whether the dominant phenotype is due to a homozygous dominant or heterozygous genotype in the organism with the dominant phenotype.

6. Using a Punnett Square, workout the distribution of phenotypic features in the first filial generation after a cross between a homozygous female and a heterozygous male for a single locus.

Answer : To determine the distribution of phenotypic features in the first filial generation () after a cross between a homozygous female and a heterozygous male for a single locus, you can use a Punnett Square. Let's use the letters "A" and "a" to represent the alleles for this single locus. In this case:

The homozygous female has the genotype AA.

The heterozygous male has the genotype Aa.

Now, create a Punnett Square to show the possible combinations of alleles in their offspring. The Punnett Square will look like this:

	A	a
A	AA	Aa
a	Aa	aa

Each cell in the Punnett Square represents a possible genotype of the offspring, and the letters in each cell represent the alleles inherited from each parent.

Now, let's determine the phenotypic features associated with these genotypes:

"AA" individuals will have the same phenotype as the homozygous female, which is the dominant phenotype.

"Aa" individuals will also have the dominant phenotype because the "A" allele is dominant over "a."

"aa" individuals will have the recessive phenotype.

So, in the  generation, you can expect the following distribution of phenotypic features:

Dominant phenotype (either "AA" or "Aa"): 3 out of 4 individuals (75%)

Recessive phenotype ("aa"): 1 out of 4 individuals (25%)

This distribution is based on Mendelian genetics and assumes complete dominance of the "A" allele over the "a" allele at the single locus we are considering.

7. When a cross in made between tall plant with yellow seeds (TtYy) and tall plant with green seed (Ttyy), what proportions of phenotype in the offspring could be expected to be

(a) tall and green.

(b) dwarf and green.

Answer : To determine the proportions of phenotypes in the offspring when a cross is made between a tall plant with yellow seeds (TtYy) and a tall plant with green seeds (Ttyy), you need to consider the inheritance of two different traits: plant height (tall or dwarf) and seed color (yellow or green). This involves a dihybrid cross.

First, let's break down the genotypes of the parents:

Tall plant with yellow seeds (TtYy): Genotype for height = Tt, Genotype for seed color = Yy

Tall plant with green seeds (Ttyy): Genotype for height = Tt, Genotype for seed color = yy

Now, create a Punnett Square for the dihybrid cross to determine the possible combinations of alleles in the offspring for both height and seed color. 

The Punnett Square :

	TY	Ty	tY	ty
TY	TTYy	TTYy	TtYy	TtYy
Ty	TTYy	TTYy	TtYy	TtYy
tY	TtYy	TtYy	ttyy	ttyy
ty	TtYy	TtYy	ttyy	ttyy

Now, let's determine the phenotypes:

(a) Tall and green individuals (Tall plant with green seeds): These are individuals with the genotype Ttyy. From the Punnett Square, you can see that there are 4 out of 16 possible combinations that result in this phenotype. So, the proportion is , which simplifies to .

(b) Dwarf and green individuals (Dwarf plant with green seeds): These are individuals with the genotype ttyy. Again, from the Punnett Square, you can see that there are 4 out of 16 possible combinations that result in this phenotype, so the proportion is also , which simplifies to .

So, in the offspring, you can expect the following proportions:

(a) Tall and green: (b) Dwarf and green: 

8. Two heterozygous parents are crossed. If the two loci are linked what would be the distribution of phenotypic features in  generation for a dibybrid cross?

Answer :  When two heterozygous parents are crossed for a dihybrid cross and the two loci are linked, the distribution of phenotypic features in the generation will be different from what you would expect in an unlinked dihybrid cross. In a linked dihybrid cross, the phenotypic features will depend on the extent of linkage and the recombination frequency between the two loci.

If the two loci are completely linked (no recombination occurs), then the phenotypic features in the generation will be exactly the same as one of the parents because no new combinations of alleles are formed due to the absence of recombination.

However, if there is some degree of recombination between the two linked loci, you can expect phenotypic features that correspond to the recombinant phenotypes. The proportion of recombinant phenotypes will depend on the recombination frequency, which is a measure of how often recombination occurs between the two loci.

The distribution of phenotypic features in the generation for a linked dihybrid cross will be a combination of parental phenotypes and recombinant phenotypes. The exact proportions will depend on the specific recombination frequency and linkage characteristics of the loci in question. If recombination is rare, the generation will primarily exhibit parental phenotypes, whereas if recombination is frequent, more recombinant phenotypes will be observed.

In a linked dihybrid cross, it is essential to know the specific recombination frequency and the linkage characteristics to determine the distribution of phenotypic features accurately. The distribution will vary based on the specific genetic context of the cross.

9. Briefly mention the contribution of T.H. Morgan in genetics.

Answer : T.H. Morgan made significant contributions to the field of genetics, particularly in understanding the linkage and recombination of genes on chromosomes. His work with Drosophila (fruit flies) provided valuable insights into the inheritance patterns of genes located on the X chromosome. Some of his key contributions are :

(a) Morgan conducted dihybrid crosses in Drosophila, similar to Mendel's work with peas, to study the inheritance of genes. He observed that certain genes located on the same chromosome did not segregate independently, leading to deviations from the expected Mendelian 9:3:3:1 ratio. This deviation indicated that these genes were linked on the same chromosome.

(b) Morgan introduced the terms "linkage" to describe the physical association of genes on a chromosome and "recombination" to describe the generation of non-parental gene combinations resulting from crossing over events during meiosis. This laid the foundation for understanding the genetic basis of gene linkage and recombination.

(c) Morgan and his group observed that genes on the same chromosome could be tightly linked (showing low recombination) or loosely linked (showing higher recombination). This variation in linkage helped establish the concept of genetic mapping.

(d) Morgan's student, Alfred Sturtevant, used the frequency of recombination between gene pairs on the same chromosome as a measure of the distance between them. This approach allowed for the creation of genetic maps that indicated the relative positions of genes on chromosomes. Genetic mapping has since become a crucial tool in genetics and was instrumental in various genome sequencing projects, including the Human Genome Project.

Overall, Morgan's pioneering work in the early 20th century laid the foundation for our understanding of gene linkage, recombination, and the physical arrangement of genes on chromosomes. His contributions significantly advanced the field of genetics and paved the way for further research in this area

10. What is pedigree analysis? Suggest how such an analysis, can be useful.

Answer : The idea that disorders are inherited has been prevailing in the human society since long. This was based on the heritability of certain characteristic features in families. After the rediscovery of Mendel’s work the practice of analysing inheritance pattern of traits in human beings began. Since it is evident that control crosses that can be performed in pea plant or some other organisms, are not possible in case of human beings, study of the family history about inheritance of a particular trait provides an alternative. Such an analysis of traits in a several of generations of a family is called the pedigree analysis.

Pedigree analysis is a crucial tool in human genetics and has several important uses :

(i) Pedigree analysis helps determine whether a trait or genetic disorder is inherited in an autosomal dominant, autosomal recessive, X-linked dominant, or X-linked recessive manner. By examining the pattern of trait transmission within a family, geneticists can deduce the mode of inheritance.

(ii) Pedigree analysis is a key component of genetic counseling. Genetic counselors use pedigree charts to assess the risk of genetic disorders in families, provide information about inheritance patterns, and guide family planning decisions.

(iii) Pedigree analysis can identify carriers of recessive genetic disorders. In a pedigree, carriers are often represented as individuals without the disorder but who have a family history of it. This information can be vital for family members considering having children.

(iv) Pedigree analysis can help identify individuals at risk for genetic diseases, allowing for early diagnosis and treatment. For example, it can reveal a family history of conditions like Huntington's disease or cystic fibrosis.

(v) By analyzing a pedigree, geneticists can estimate the probability of an individual inheriting a particular genetic condition. This information is valuable for risk assessment and making informed healthcare decisions.

(vi) Pedigree analysis plays a crucial role in genetics research. It helps scientists identify families with a high incidence of a specific trait or condition, enabling them to study the genetic basis and potential treatments for these conditions.

11. How is sex determined in human beings?

Answer :  Sex determination in human beings is based on the XY chromosome system. Humans typically have 23 pairs of chromosomes, with 22 pairs being autosomes (chromosomes that are the same in males and females) and one pair determining an individual's sex.

In males:

(i) Males have one X chromosome and one Y chromosome, making their sex chromosomes XY.

(ii) During spermatogenesis (the process of sperm formation), two types of gametes (sperm) are produced.

(iii) Approximately 50% of the sperm carry an X chromosome, and the other 50% carry a Y chromosome, in addition to the autosomes.

(iv) When fertilization occurs, and an X-carrying sperm fertilizes an egg, the resulting zygote will have an XX chromosome pair and develop into a female offspring.

(v) If a Y-carrying sperm fertilizes the egg, the zygote will have an XY chromosome pair and develop into a male offspring.

In females:

(i) Females have two X chromosomes (XX).

(ii) During oogenesis (the process of egg formation), only one type of gamete is produced, which is an egg with an X chromosome.

(iii) When fertilization occurs with a sperm carrying either an X or Y chromosome, the zygote's sex is determined. If fertilized with an X-carrying sperm, it will develop into a female (XX); if fertilized with a Y-carrying sperm, it will develop into a male (XY).

The genetic makeup of the sperm that fertilizes the egg ultimately determines the sex of the child. It's important to note that there is always an equal probability (50%) of having either a male or a female child in each pregnancy, and this probability is not influenced by the mother. The idea of blaming or ill-treating women for giving birth to female children based on this false notion is unfounded and unjust. Sex determination is purely a matter of chance and genetics.

12. A child has blood group O. If the father has blood group A and mother blood group B, work out the genotypes of the parents and the possible genotypes of the other offsprings.

Answer : The ABO blood group system is determined by the presence or absence of specific alleles (gene variants) for the A and B antigens on the surface of red blood cells. The possible genotypes for blood group A are  or  (heterozygous), and for blood group B, they are  or   (heterozygous). Blood group O (Type O) is recessive and is represented by the genotype  .

Given the information provided :

The child has blood group O, which means the child's genotype is .

The father has blood group A, which could be represented by the genotype  (homozygous for A) or  (heterozygous for A).

The mother has blood group B, which could be represented by the genotype  (homozygous for B) or   (heterozygous for B).

Let's consider the possible combinations for the parents:

Father's genotype:    (homozygous for A)

Mother's genotype:   (homozygous for B)

Possible offspring genotypes:    (heterozygous for A),   (heterozygous for B)

Father's genotype:   (heterozygous for A)

Mother's genotype:   (homozygous for B)

Possible offspring genotypes:   (homozygous for A),   (heterozygous for both A and B)

Father's genotype:   (homozygous for A)

Mother's genotype: (heterozygous for B)

Possible offspring genotypes:   (heterozygous for A), (heterozygous for both A and B)

Father's genotype:   (heterozygous for A)

Mother's genotype:   (heterozygous for B)

Possible offspring genotypes:   (homozygous for A),   (heterozygous for A), (homozygous for B),   (heterozygous for B)

So, the possible genotypes of the other offspring could be , , , or depending on the specific genotypes of the parents.

13. Explain the following terms with example

(a) Co-dominance

(b) Incomplete dominance .

Answer : (a) Co-dominance is a genetic phenomenon where both alleles in a heterozygous individual are fully expressed, resulting in a phenotype that shows characteristics of both alleles simultaneously. An example is the ABO blood group system in humans, controlled by the I gene with three alleles: , , and i . Alleles and each produce a distinct sugar on red blood cell surfaces, while allele i does not produce any sugar. When and are present together in an individual, both A and B sugars are expressed, leading to AB blood type, demonstrating co-dominance as both alleles are equally and fully expressed in the phenotype.

(b) Incomplete dominance is a genetic inheritance pattern where neither allele in a heterozygous individual is completely dominant over the other, resulting in an intermediate phenotype. An example is the inheritance of flower color in snapdragons. When true-breeding red-flowered (RR) and white-flowered (rr) plants are crossed, the generation (Rr) exhibits a pink phenotype, demonstrating incomplete dominance. In the generation, the phenotypic ratio deviates from the typical Mendelian 3:1 ratio, showing that R is not completely dominant over r . In this case, the intermediate pink color illustrates incomplete dominance, where neither allele fully masks the other's expression.

14. What is point mutation? Give one example.

Answer : A point mutation is a type of genetic mutation that involves the alteration of a single base pair within the DNA sequence. An example of a point mutation is found in sickle cell anemia. In this disorder, a single base change in the DNA leads to the substitution of adenine for thymine, resulting in the production of abnormal hemoglobin and causing red blood cells to assume a sickle shape, leading to various health issues and characteristic symptoms of the disease.

15. Who had proposed the chromosomal theory of the inheritance?

Answer : Walter Sutton and Theodore Boveri proposed the chromosomal theory of inheritance in 1902 .This theory established the link between chromosomes and the principles of Mendelian inheritance.

16. Mention any two autosomal genetic disorders with their symptoms.

Answer : Two autosomal genetic disorders with their symptoms are :

(a) Cystic Fibrosis: Cystic fibrosis is an autosomal recessive genetic disorder. Symptoms include chronic lung infections, difficulty breathing, coughing with thick mucus, poor growth, and digestive problems.

(b) Huntington's Disease: Huntington's disease is an autosomal dominant genetic disorder. It is characterized by progressive motor dysfunction, cognitive decline, and psychiatric symptoms. These may include involuntary movements, mood changes, and difficulty with coordination.