CUET UG Biology Booster Test 2 Genetic Disorders

To perform an accurate pedigree analysis, a geneticist must follow a logical, step-by-step workflow: 1. Step 2: Begin by raw data collection (Collect the family history regarding a particular abnormality). You cannot chart what you have not recorded. 2. Step 3: Translate this raw textual history into a visual diagram (Construct a family tree using standard symbols). 3. Step 1: Use this visual layout to trace the transmission of the phenotype down the lineage lines (Trace the inheritance of the specific trait over generations). 4. Step 4: Apply genetic rules to determine the trait's behavior (Analyze the pattern to determine if the trait is dominant or recessive). This workflow yields the sequence 2-3-1-4.

In classical genetics, researchers use controlled test crosses and reciprocal matings to determine dominance and linkage. In human populations, controlled crosses cannot be performed due to clear ethical, social, and biological limitations. Additionally, humans produce few offspring and have long generation times. Because scientists cannot design human matings, they look backward at existing family records instead. Pedigree analysis serves as this systematic historical tool.

In pedigree charts, lines indicate relationships: A horizontal line linking a male (square) and female (circle) indicates a standard mating connection. When the mating occurs between close biological relatives (known as a consanguineous mating), it is represented by two horizontal lines between symbols. This double line alerts geneticists to an increased risk of rare autosomal recessive conditions coming together in the offspring.

Let's match each pedigree symbol to its definition: Five offspring are grouped together using a diamond containing the number 5 to save space (1-iv). An Affected male is shown as a solid, filled square (2-i). Mating is indicated by a horizontal marriage line connecting two parents (3-ii). Line of descent is indicated by a vertical line leading down to the children (4-iii). This step-by-step matching aligns with Option A.

Let's evaluate each statement against NCERT guidelines: Statement I: True. Mendelian disorders are caused by an alteration or mutation in a single gene. Statement II: True. Their transmission mirrors classic single-gene inheritance patterns, which can be analyzed using pedigree charts. Option III: False. Mendelian disorders are not restricted to autosomes; conditions like Haemophilia and Color Blindness are well-known sex-linked recessive Mendelian disorders. Because Statements I and II are correct and Statement III is false, Option A is the correct choice.

Autosomal recessive conditions (such as sickle-cell anemia or thalassemia) require two copies of the mutant allele to show symptoms. Heterozygous parents (Aa) are healthy carriers and do not show symptoms, meaning the trait can appear in the children of unaffected parents. It also affects males and females at equal rates because the gene resides on an autosome. However, recessive traits often skip generations when mutant alleles are masked by dominant ones. Therefore, the statement that the trait appears in every generation without fail is false, making C the correct answer for this "NOT" question.

Because color blindness is an X-linked recessive condition, a female needs a homozygous mutant genotype (X^c X^c) to show symptoms. She inherits one X chromosome from her mother and one from her father. Her father must pass on a mutant X chromosome (X^c), meaning the father must be color blind (X^c Y). Her mother must provide at least one mutant X chromosome, meaning the mother must be either a carrier (X^C X^c) or color blind herself (X^c X^c). This matching combination is found in Option B.

Color blindness is an X-linked recessive disorder. A carrier woman has the heterozygous genotype X^C X^c. When she has a son, she passes on one of her two X chromosomes with equal probability (50%). The son receives his Y chromosome from his father (XY). Because males carry only one X chromosome (hemizygous), any allele on that chromosome is expressed directly. If he inherits the mutant X^c allele, he will be color blind, creating a 50% risk.

Haemophilia is an X-linked recessive genetic disorder that affects the blood clotting cascade. A mutation alters a single protein within this cascade, preventing clots from forming and causing an injured individual to experience non-stop bleeding from a simple cut. It is passed down from carrier females (X^H X^h) to their sons. Excessive polymerisation of haemoglobin is not associated with haemophilia; instead, it is the defining molecular feature of Sickle-cell anaemia under low oxygen levels. This makes Option C the correct answer for this "NOT" question.

Haemophilia is an X-linked recessive condition, meaning a female must inherit a homozygous mutant genotype (X^h X^h) to show symptoms. She must inherit one mutant X^h allele from her father, which means the father must be haemophilic (X^h Y). She must inherit the second mutant X^h allele from her mother, meaning the mother must be at least a heterozygous carrier (X^H X^h) or haemophilic herself (X^h X^h). Because haemophilic males often experience severe survival challenges before reproductive age, this combination is extremely rare. This matches Option B.

Sickle-cell anaemia is controlled by a single pair of alleles, Hb^A (normal) and Hb^S (mutant). Because it is an autosomal recessive disorder, the disease is only expressed in individuals who are homozygous for the mutant gene (Hb^S Hb^S). Heterozygous individuals (Hb^A Hb^S) possess one normal allele which produces enough functional hemoglobin to keep them healthy under normal conditions. These individuals are unaffected carriers who show no clinical symptoms but can pass the mutant gene to their offspring. Individuals with the Hb^A Hb^A genotype are completely normal.

The genetic defect responsible for sickle-cell anaemia is a single base substitution (point mutation) at the sixth codon of the \beta-globin gene. The normal DNA codon translates into the mRNA codon GAG, which directs the ribosome to insert Glutamic acid. The mutation changes this codon to GUG, which codes for Valine instead. This single nucleotide shift changes the properties of the entire hemoglobin molecule, causing it to polymerize under low oxygen levels.

Both conditions are autosomal recessive blood disorders, but they stem from completely different genetic mechanisms: Thalassemia is a quantitative problem where a mutation or deletion results in synthesising too few globin molecules (\alpha or \beta chains). The chains that are made have a normal structure, but their reduced numbers cause an imbalance. Sickle-cell anaemia is a qualitative problem where a point mutation alters the structure of the globin chain (substituting valine for glutamic acid), causing the protein to function incorrectly. This distinction makes Option C correct.

Beta Thalassemia is an autosomal recessive condition that reduces the synthesis of \beta-globin chains. This process is controlled by a single gene, HBB, which is located on Chromosome 11. Because this is an autosome, every individual carries two copies of the gene, inheriting one copy on Chromosome 11 from each parent. A mutation in both inherited copies causes the disease to manifest in the offspring.

During normal cell division, nuclear division (karyokinesis) is immediately followed by cytoplasmic division (cytokinesis). If a cell replicates its DNA but fails to complete cytokinesis, it does not divide into two separate cells. Instead, all the duplicated chromosomes remain trapped within a single cell, resulting in an increase in a whole set of chromosomes. This condition is known as Polyploidy (3n, 4n, 5n, etc.) and is often seen in plant genetics.

Let's match each genetic term to its algebraic description: Trisomy means having an extra copy of an individual chromosome, raising the normal diploid count by one (2n + 1) (1-ii). Monosomy means missing a chromosome from a normal pair, lowering the diploid count by one (2n - 1) (2-i). Polyploidy describes an increase in entire sets of chromosomes, changing the ploidy level to triploid or tetraploid (3n or 4n) (3-iii). This matching sequence is found in Option A.

Down's syndrome is a chromosomal disorder caused by an error in cell division called non-disjunction. This happens when chromosome pair 21 fails to separate during meiosis, causing a gamete to retain both copies. When this gamete combines with a normal gamete during fertilization, the resulting zygote inherits an extra copy of chromosome 21 (Trisomy 21). The condition is caused by an extra chromosome, not a change to the chromosome's internal structure.

Down's syndrome affects genes across an entire autosome, which alters development throughout the body. According to NCERT guidelines, clinical features include delayed physical growth, impaired coordination (psychomotor retardation), and cognitive delays (mental retardation). Because the extra chromosome affects multiple systems, the condition is characterized by delays in all three areas: physical, psychomotor, and mental development.

The provided reading text states that chromosomal disorders happen when there is an absence, excess, or abnormal arrangement of chromosomes, and that a failure of chromatids to separate causes a gain or loss of a chromosome, known as aneuploidy. Klinefelter's syndrome occurs when a male inherits an extra X chromosome (47, XXY). Because it involves an extra individual chromosome on the sex pair, it is a classic example of aneuploidy of sex chromosomes.

Turner's syndrome is a sex chromosome disorder seen in females who are missing an X chromosome (45, XO). This condition results in underdeveloped (rudimentary) ovaries, a lack of secondary sexual characteristics, and sterility. Gynaecomastia refers to the development of breast tissue in biological males who carry an extra X chromosome (47, XXY, Klinefelter's syndrome). Because Gynecomastia occurs in males with Klinefelter's syndrome rather than females with Turner's syndrome, statement C is incorrect and is the correct choice for this "NOT" question.