Exercise Solutions

1. Complete the following sentences by choosing the appropriate words from the brackets.

   (Inheritance, sexual reproduction, asexual reproduction, chromosomes, DNA, RNA, gene)

   a. Hereditary characters are transferred from parents to offsprings by genes, hence they are said to be structural and functional units of heredity.

   b. Organisms produced by asexual reproduction show minor variations.

   c. The component which is in the nucleus of cells and carries the hereditary characteristics is called chromosomes.

   d. Chromosomes are mainly made up of DNA.

   e. Organisms produced through sexual reproduction show major variations.

2. Explain the following.

   a. Explain Mendel’s monohybrid progeny with the help of any one cross.

   Answer: A monohybrid cross involves studying the inheritance of a single pair of contrasting characteristics. Let's take Mendel's experiment with pea plant height:

   • Parental Generation (P₁): Mendel crossed a pure tall pea plant (Genotype: TT) with a pure dwarf pea plant (Genotype: tt).
   • Gametes from P₁: The tall plant produces gametes with the 'T' allele, and the dwarf plant produces gametes with the 't' allele.
   • First Filial Generation (F₁): All offspring in the F₁ generation had the genotype Tt and were phenotypically tall. This showed that the allele for tallness (T) is dominant over the allele for dwarfness (t), which is recessive.
   • Self-pollination of F₁ (P₂): Mendel allowed the F₁ plants (Tt) to self-pollinate (Tt x Tt).
   • Gametes from F₁: Each F₁ plant produces two types of gametes: T and t.
   • Second Filial Generation (F₂): The resulting F₂ generation showed both tall and dwarf plants. The genotypes were TT (Tall), Tt (Tall), and tt (Dwarf) in the ratio 1:2:1. The phenotypes were Tall and Dwarf in the ratio 3:1.

   This experiment demonstrated the concepts of dominance and segregation of alleles.

   b. Explain Mendel’s dihybrid ratio with the help of any one cross.

   Answer: A dihybrid cross involves studying the inheritance of two pairs of contrasting characteristics simultaneously. Let's consider Mendel's cross involving seed shape (Round/Wrinkled) and seed colour (Yellow/Green):

   • Parental Generation (P₁): Mendel crossed a plant with pure Round-Yellow seeds (RRYY) with a plant with pure Wrinkled-Green seeds (rryy). (R=Round, r=Wrinkled, Y=Yellow, y=Green).
   • Gametes from P₁: RY from the RRYY parent and ry from the rryy parent.
   • First Filial Generation (F₁): All offspring had the genotype RrYy and phenotypically showed Round-Yellow seeds. This confirmed Round (R) and Yellow (Y) are dominant traits.
   • Self-pollination of F₁ (P₂): Mendel allowed the F₁ plants (RrYy) to self-pollinate (RrYy x RrYy).
   • Gametes from F₁: Each F₁ plant produces four types of gametes due to independent assortment: RY, Ry, rY, and ry.
   • Second Filial Generation (F₂): Using a Punnett square, the F₂ generation showed four different phenotypes in a specific ratio:
     • Round-Yellow: 9 parts
     • Round-Green: 3 parts
     • Wrinkled-Yellow: 3 parts
     • Wrinkled-Green: 1 part
   • Dihybrid Ratio: The phenotypic ratio observed in the F₂ generation of a dihybrid cross is 9:3:3:1. This ratio supports Mendel's Law of Independent Assortment, which states that the alleles for different traits segregate independently during gamete formation.

   c. Distinguish between monohybrid and dihybrid cross.

   Answer:

   Feature	Monohybrid Cross	Dihybrid Cross
   Number of Traits Studied	Inheritance of only one pair of contrasting characteristics is studied.	Inheritance of two pairs of contrasting characteristics is studied simultaneously.
   Example	Cross between tall (TT) and dwarf (tt) pea plants.	Cross between plants with round-yellow seeds (RRYY) and wrinkled-green seeds (rryy).
   F₂ Phenotypic Ratio	Typically 3:1 (Dominant:Recessive).	Typically 9:3:3:1 (Double Dominant : Dominant-Recessive : Recessive-Dominant : Double Recessive).
   Purpose	Used to determine the dominance relationship between two alleles and demonstrate the Law of Segregation.	Used to demonstrate the Law of Independent Assortment.

   d. Is it right to avoid living with a person suffering from a genetic disorder?

   Answer: No, it is absolutely not right to avoid living with a person suffering from a genetic disorder. Here's why:

   • Not Contagious: Genetic disorders are caused by abnormalities in genes or chromosomes and are inherited; they are not infectious or contagious like diseases caused by pathogens (bacteria, viruses). You cannot 'catch' a genetic disorder by being near someone who has one.
   • Ethical and Moral Reasons: Avoiding or discriminating against individuals with genetic disorders is unethical and inhumane. Everyone deserves compassion, support, and inclusion, regardless of their health condition.
   • Social Responsibility: Society has a responsibility to support individuals with genetic disorders and their families, providing necessary medical care, resources, and understanding. Ostracizing them causes emotional distress and hinders their well-being.
   • Focus on Support: Instead of avoidance, the focus should be on providing support, understanding, and appropriate care to help individuals manage their condition and live fulfilling lives. Many genetic disorders can be managed with medical treatment and lifestyle adjustments.
3. Answers the following questions in your own words.

   a. What is meant by ‘chromosome’? Explain its types.

   Answer: A chromosome is a thread-like structure found within the nucleus of eukaryotic cells (and in the cytoplasm of prokaryotic cells). It is composed mainly of DNA tightly coiled around proteins called histones. Chromosomes carry the genetic information of an organism in the form of genes.

   Chromosomes become clearly visible under a microscope during cell division. Based on the position of the centromere (the primary constriction point that divides the chromosome into arms), chromosomes are classified into four types:

   • Metacentric: The centromere is located exactly in the middle, resulting in two arms of equal length. It appears V-shaped during anaphase.
   • Sub-metacentric: The centromere is located slightly away from the center, resulting in one arm being shorter than the other. It appears L-shaped during anaphase.
   • Acrocentric: The centromere is located very close to one end, resulting in one very short arm and one very long arm. It appears J-shaped during anaphase.
   • Telocentric: The centromere is located at the very end (telomere) of the chromosome, resulting in only one visible arm. It appears i-shaped during anaphase. (Note: True telocentric chromosomes are rare or absent in humans).

   b. Describe the structure of the DNA molecule.

   Answer: The DNA (Deoxyribonucleic Acid) molecule has a structure known as the double helix, famously proposed by Watson and Crick in 1953. Key features include:

   • Double Stranded: DNA consists of two long polynucleotide strands that run parallel to each other but in opposite directions (antiparallel).
   • Helical Structure: These two strands coil around a central axis, forming a right-handed double helix, resembling a twisted ladder.
   • Backbone: The sides or 'rails' of the ladder are made up of alternating molecules of deoxyribose sugar and phosphate groups. This is called the sugar-phosphate backbone.
   • Nitrogenous Bases: The 'rungs' of the ladder are formed by pairs of nitrogenous bases. There are four types of bases in DNA: Adenine (A), Guanine (G), Cytosine (C), and Thymine (T). A and G are purines (double-ring structure), while C and T are pyrimidines (single-ring structure).
   • Base Pairing: The bases pair specifically through hydrogen bonds: Adenine always pairs with Thymine (A=T) via two hydrogen bonds, and Guanine always pairs with Cytosine (C≡G) via three hydrogen bonds. This is called complementary base pairing.
   • Nucleotides: Each building block of DNA is a nucleotide, consisting of one deoxyribose sugar molecule, one phosphate group, and one nitrogenous base.

   c. Express your opinion about the use of DNA fingerprinting.

   Answer: DNA fingerprinting (also known as DNA profiling) is a powerful technology with significant benefits, but also potential ethical concerns.

   Positive Aspects:

   • Forensic Science: It is invaluable in criminal investigations for identifying suspects or victims by matching DNA samples (from blood, hair, saliva, etc.) found at crime scenes with known individuals. It can also exonerate wrongly accused persons.
   • Paternity and Kinship Testing: It provides definitive proof of biological relationships, crucial in paternity disputes, inheritance claims, and identifying missing persons or reuniting families.
   • Medical Applications: It helps in identifying genetic predispositions to certain diseases and in diagnosing inherited disorders.
   • Conservation Biology: It aids in tracking endangered species, understanding population genetics, and combating illegal wildlife trade.

   Concerns/Ethical Considerations:

   • Privacy: The storage of DNA profiles in large databases raises concerns about potential misuse of sensitive genetic information and individual privacy.
   • Accuracy and Interpretation: While highly accurate, errors in sample collection, handling, or analysis can occur. Interpretation of results, especially partial matches, requires expertise.
   • Potential for Discrimination: There are fears that genetic information could be used to discriminate against individuals in areas like employment or insurance.

   Overall Opinion: DNA fingerprinting is a revolutionary tool with immense potential for good, particularly in law enforcement and medicine. However, its use must be carefully regulated with strong ethical guidelines and robust privacy protections to prevent misuse and ensure fairness.

   d. Explain the structure, function and types of RNA.

   Answer: RNA (Ribonucleic Acid) is another vital nucleic acid involved primarily in protein synthesis.

   Structure:

   • Single Stranded: Unlike DNA, RNA is usually a single-stranded molecule, though it can fold upon itself to form complex structures.
   • Backbone: Similar to DNA, it has a sugar-phosphate backbone, but the sugar is ribose instead of deoxyribose.
   • Nitrogenous Bases: RNA contains Adenine (A), Guanine (G), and Cytosine (C). However, it contains Uracil (U) instead of Thymine (T). Uracil pairs with Adenine (A-U).
   • Nucleotides: The building blocks are ribonucleotides, each containing a ribose sugar, a phosphate group, and one of the four bases (A, U, C, G).

   Function: The primary function of RNA is to act as a messenger carrying instructions from DNA for controlling the synthesis of proteins, although it also has other regulatory and catalytic roles.

   Types of RNA:

   • Messenger RNA (mRNA): Carries the genetic code copied from DNA in the nucleus to the ribosomes in the cytoplasm. It serves as the template for protein synthesis.
   • Ribosomal RNA (rRNA): A major structural component of ribosomes (the cellular machinery for protein synthesis). It also has catalytic activity (as a ribozyme) in linking amino acids together.
   • Transfer RNA (tRNA): Acts as an adapter molecule. It reads the codons (three-base sequences) on the mRNA and brings the corresponding specific amino acid to the ribosome to be added to the growing protein chain.

   e. Why is it necessary for people to have their blood examined before marriage?

   Answer: Examining blood before marriage, often referred to as premarital screening, is necessary for several important health reasons, particularly concerning genetic disorders:

   • Detecting Carriers of Genetic Disorders: Tests can identify if prospective partners are carriers for certain recessive genetic disorders, such as Sickle Cell Anemia or Thalassemia. If both partners are carriers of the same recessive disorder, there is a 25% chance with each pregnancy that their child will inherit the disorder. Knowing this risk allows couples to make informed decisions about family planning, potentially seeking genetic counseling or considering options like prenatal diagnosis or adoption.
   • Preventing Transmission of Infections: Blood tests can screen for sexually transmitted infections (STIs) like HIV, Hepatitis B, and Syphilis. Early detection allows for treatment, preventing transmission to the partner and potential complications during pregnancy or childbirth.
   • Blood Group Compatibility (Rh Factor): Knowing the Rh blood group status of both partners is important. If an Rh-negative mother carries an Rh-positive baby, it can lead to Rh incompatibility, potentially causing serious health problems for subsequent pregnancies. Awareness allows for preventive treatment (RhoGAM injections) during pregnancy.
   • General Health Assessment: Premarital screening often includes tests for general health conditions that might impact future family life or pregnancy outcomes.

   Overall, premarital blood examination empowers couples with crucial health information, enabling them to plan for a healthy future together and minimize the risk of passing on certain conditions to their children.

4. Write a brief note on each.

   a. Down syndrome

   Answer: Down syndrome, also known as Trisomy 21, is a genetic disorder caused by the presence of a third copy of chromosome 21, instead of the usual two. This results in a total of 47 chromosomes per cell instead of 46. It is the most common chromosomal abnormality in humans. Individuals with Down syndrome typically exhibit characteristic physical features such as a flattened facial profile, upward slanting eyes, short neck, small ears, and a single deep crease across the palm (simian crease). They often experience intellectual disability, ranging from mild to moderate, and developmental delays. Associated health problems can include heart defects, hearing and vision problems, and increased susceptibility to infections. While lifespan was once short, medical advancements have significantly increased life expectancy, often into the 60s or beyond with proper care and support.

   b. Monogenic disorders

   Answer: Monogenic disorders are genetic conditions caused by a mutation in a single gene. These mutations can alter the gene's protein product, leading to impaired function or absence of the protein, which in turn causes the disorder. Monogenic disorders follow specific inheritance patterns (autosomal dominant, autosomal recessive, X-linked dominant, X-linked recessive) based on the location of the gene (autosome or sex chromosome) and whether one or two copies of the mutated gene are needed to cause the condition. Examples include Cystic Fibrosis (recessive), Huntington's disease (dominant), Sickle Cell Anemia (recessive), Hemophilia (X-linked recessive), Albinism (usually recessive), and Phenylketonuria (recessive). While individually rare, collectively monogenic disorders affect millions worldwide. Diagnosis often involves genetic testing, and management focuses on treating symptoms and preventing complications.

   c. Sickle cell anaemia: symptoms and treatment.

   Answer: Sickle cell anemia is an inherited monogenic disorder affecting hemoglobin, the protein in red blood cells that carries oxygen. A mutation causes hemoglobin molecules to stick together and form rigid, sickle (crescent) shapes, especially when oxygen levels are low. These sickle-shaped cells are stiff, block blood flow in small vessels, and break down prematurely.

   Symptoms: Symptoms vary but often include:

   • Anemia: Chronic shortage of red blood cells causing fatigue, paleness, and shortness of breath.
   • Pain Crises (Vaso-occlusive crises): Severe, episodic pain in the chest, abdomen, joints, or bones due to blocked blood flow.
   • Swelling: Painful swelling of hands and feet (dactylitis), often an early sign in infants.
   • Frequent Infections: Damage to the spleen increases susceptibility to infections.
   • Delayed Growth/Puberty: Due to chronic lack of oxygen and nutrients.
   • Organ Damage: Long-term complications can include damage to the spleen, liver, kidneys, lungs, heart, and brain (stroke).

   Treatment/Management: There is no universal cure, but treatments focus on managing symptoms, preventing complications, and improving quality of life:

   • Pain Management: Medications (over-the-counter or prescription opioids) for pain crises.
   • Hydroxyurea: Medication that can reduce the frequency of pain crises and the need for blood transfusions by increasing fetal hemoglobin.
   • Blood Transfusions: To treat severe anemia and prevent stroke.
   • Infection Prevention: Vaccinations and prophylactic antibiotics (especially in children).
   • Folic Acid: Daily supplement to help produce new red blood cells.
   • Bone Marrow/Stem Cell Transplant: The only potential cure, but involves risks and requires a suitable donor.
   • Gene Therapy: An emerging area of research offering potential future cures.
5. How are the items in groups A, B and C co-related?

   A (Disorder)	B (Cause/Type/Chromosome)	C (Symptom/Characteristic)
   Leber hereditary optic neuropathy	Mitochondrial disorder	This disorder arises during development of zygote (Inherited via mitochondria from mother)
   Diabetes	Polygenic disorder	Effect on blood-glucose level
   Albinism	Monogenic disorder (Recessive)	Pale skin, white hairs
   Turner syndrome	45+X (or 44+X)	Women are sterile
   Klinefelter syndrome	44+XXY	Men are sterile

   Explanation of Correlation:

   • Group A lists specific genetic disorders.
   • Group B provides the underlying cause, type, or chromosomal abnormality associated with the disorder in Group A.
   • Group C describes a key symptom, characteristic, or consequence related to the disorder in Group A and its cause in Group B.
6. Filling the blanks based on the given relationship.

   a. 44+X: Turner syndrome :: 44+XXY : Klinefelter syndrome

   b. 3:1 Monohybrid :: 9:3:3:1 : Dihybrid

   c. Women: Turner syndrome :: Men : Klinefelter syndrome

7. Complete the tree diagram below based on types of hereditary disorders.
   Hereditary Disorders

   Disorders due to Chromosomal Abnormalities

   Autosomal (e.g., Down Syndrome)

   Sex Chromosomal (e.g., Turner, Klinefelter)

   Disorders due to Gene Mutations

   Monogenic (e.g., Sickle Cell, Albinism)

   Polygenic (e.g., Diabetes, Cleft lip)

   Mitochondrial (e.g., Leber hereditary optic neuropathy)

   (Note: The above diagram represents a possible completion based on the chapter content. Specific categories might vary slightly.)

8. Activity.

   a. Prepare a model of the DNA and give a presentation based on it.

   b. Prepare a power point presentation on awareness about tobacco consumption and cancer and present it in the class.

   (Note: These are practical activities for students to perform.)