Heredity and Common Genetic Diseases: WBBSE Class 10 Life Science Chapter 3 Solutions & Question Bank

Explanation: Gregor Johann Mendel selected 7 pairs of true-breeding contrasting characters in garden pea (Pisum sativum): stem height, flower colour, flower position, pod shape, pod colour, seed shape, and seed colour.

Explanation: In the $F_2$ generation of a monohybrid cross ($Tt \times Tt$), the genotypic ratio is 1 TT (pure tall) : 2 Tt (hybrid tall) : 1 tt (pure dwarf), i.e., 1 : 2 : 1.

Explanation: This is a classic test cross: heterozygous black ($Bb$) $\times$ homozygous recessive white ($bb$) $\rightarrow$ $1\ Bb\text{ (Black)} : 1\ bb\text{ (White)}$ (50% Black and 50% White, ratio 1 : 1).

Explanation: In $F_2$ generation of a dihybrid cross ($RrYy \times RrYy$), the four phenotypic categories appear in the classic Mendelian ratio of 9 Round-Yellow : 3 Round-Green : 3 Wrinkled-Yellow : 1 Wrinkled-Green.

Explanation: Protanopia is red colour blindness resulting from defective long-wavelength cone opsin photopigment on the X chromosome.

Explanation: A healthy human male possesses 46 chromosomes consisting of 44 autosomes and one heterogametic pair of sex chromosomes ($44A + XY$).

Explanation: In 1902, Walter Sutton and Theodor Boveri independently formulated the Chromosome Theory of Inheritance, demonstrating that Mendelian factors (genes) are carried on chromosomes.

Explanation: The actual genetic makeup or allelic composition of an individual for a particular trait is termed its genotype, whereas the observable physical expression is the phenotype.

Explanation: Self-fertilization or selfing involves the fusion of gametes produced by the same individual or genetically identical clones.

Explanation: In codominance, both alleles in a heterozygote express themselves fully and simultaneously without blending (e.g., $I^A$ and $I^B$ alleles producing AB blood group).

Explanation: Under incomplete dominance, the heterozygous phenotype is an intermediate blending (e.g. Pink in *Mirabilis jalapa*), giving both phenotypic and genotypic ratios of 1 Red : 2 Pink : 1 White (1:2:1).

Explanation: Gregor Johann Mendel conducted pioneering hybridization experiments on garden peas from 1856 to 1863, establishing the basic principles of heredity.

Explanation: Genetics (term coined by William Bateson in 1905) is the biological science studying heredity and variation in living organisms.

Explanation: The specific, fixed physical position occupied by a gene on a chromosome is called its locus.

Explanation: Selfing or crossing of two monohybrid heterozygotes ($Aa \times Aa$ or here designated $Ab \times Ab$) yields the classical $1 : 2 : 1$ genotypic ratio.

Explanation: Mendel formulated the Law of Segregation and the Law of Independent Assortment.

Explanation: Crossing over and random alignment of non-homologous chromosomes at metaphase-I ensure the independent assortment of non-allelic genes.

Explanation: In garden pea, Round seed shape ($R$) is dominant over wrinkled seed shape ($r$).

Explanation: Number of gametes $= 2^n$ (where $n = \text{number of heterozygous pairs}$). For $YyRr$, $n = 2$, so $2^2 = 4$ types of gametes: $YR$, $Yr$, $yR$, and $yr$.

Explanation: Cross: Mother ($X^H X^h$) $\times$ Father ($X^H Y$). Female progeny receive the dominant normal $X^H$ allele from the father ($X^H X^H$ normal or $X^H X^h$ carrier). Thus, the probability of a diseased haemophilic girl ($X^h X^h$) is 0%.

Explanation: Haemophilia is an X-linked recessive sex-linked disorder, whereas tongue rolling, thalassemia, and attached ear lobes are governed by autosomal genes.

Explanation: Gregor Johann Mendel is universally honored as the Father of Genetics.

Explanation: The ability to roll the lateral edges of the tongue upward into a tube is controlled by a single dominant autosomal gene.

Explanation: Alternative, contrasting forms of a gene occupying the same locus on homologous chromosomes are known as alleles (e.g. $T$ and $t$).

Explanation: When neither allele is completely dominant over the other, the heterozygous condition produces a blending intermediate phenotype (e.g. Pink flowers in Snapdragon).

Explanation: In Mendel's monohybrid cross ($Tt \times Tt$), 3/4 show the dominant phenotype and 1/4 show the recessive phenotype, giving a ratio of 3 : 1.

Explanation: Haemophilia is an inherited congenital bleeding disorder caused by an X-linked recessive gene mutation.

Explanation: A carrier mother ($X^H X^h$) transmits either $X^H$ or $X^h$ to her sons with equal probability. Since sons receive their Y chromosome from the father, 50% of sons ($X^h Y$) inherit the defective allele and suffer from haemophilia.

Explanation: A test cross (crossing an individual showing dominant phenotype with a homozygous recessive parent) is specifically designed to determine whether the dominant organism is homozygous ($TT$) or heterozygous ($Tt$).

Explanation: Heterozygous black ($Bb$) crossed with pure white ($bb$): $$\text{Gametes: } B, b \times b \rightarrow 50\% Bb\text{ (Black)} : 50\% bb\text{ (White)}$$

Explanation: This is the statement of Mendel's First Law (Law of Segregation / Law of Purity of Gametes), derived from monohybrid crosses.

Protanopia (Red colour blindness).

Mirabilis jalapa (Four O'clock plant) / Antirrhinum majus (Snapdragon).

7 pairs (seven pairs).

Haemophilia (or Colour blindness).

Genotypes (e.g. $TT$ and $Tt$ both produce Tall phenotype).

Hybridization (or Cross-breeding).

Mendel's Laws of Inheritance.

Variations.

Deuteranopia (Green colour blindness).

Bisexual (self-pollinating).

Segregation.

Sex chromosome (Heterosome / Allosome).

Y chromosome (specifically SRY gene of father).

Haemophilia B (Factor IX clotting deficiency) is termed Christmas disease after Stephen Christmas, the first patient diagnosed.

'TT' represents homozygous dominant pure tall.

Correction: Normal man is 44A + XY. $44A + XXY$ is Klinefelter's Syndrome.

Correction: Protanopia is a type of red colour blindness, not haemophilia.

Pea flowers are naturally self-pollinating but can easily be artificially cross-pollinated via emasculation and dusting.

Correction: In the first filial ($F_1$) generation, Mendel obtained 100% hybrid tall ($Tt$) plants. In $F_2$, only 25% were pure tall ($TT$).

Correction: Mendel used the term 'Factor' (Elemente). The word 'gene' was coined later by Wilhelm Johannsen in 1909.

Classical $F_2$ dihybrid phenotypic ratio is 9:3:3:1.

Correction: It causes Klinefelter's Syndrome with sterile hypogonadism.

Mendel analyzed 7 morphological traits.

The father produces heterogametic sperm (50% X-bearing and 50% Y-bearing). Fertilization by a Y sperm produces a son, while an X sperm produces a daughter.

Correction: Failure of blood coagulation occurs in Haemophilia. Thalassemia is characterized by defective hemoglobin chain synthesis causing microcytic anemia.

Gregor Johann Mendel.

Heterozygous (contains two different alleles: dominant $T$ and recessive $t$).

The numerical ratio representing the exact genetic and allelic constitution of offspring in a generation (e.g., $1\ TT : 2\ Tt : 1\ tt = \mathbf{1 : 2 : 1}$ in monohybrid $F_2$).

3 : 1 (3 Dominant : 1 Recessive).

A trait is a distinct phenotypic manifestation of an inherited characteristic (e.g., tallness or dwarfness for the character plant height).

3 Black : 1 White (3 : 1) (Cross: $Bb \times Bb \rightarrow 1\ BB, 2\ Bb, 1\ bb$).

First Filial Generation ($F_1$) is the immediate generation of hybrid offspring produced by cross-pollinating or mating two genetically distinct parental varieties.

It is an X-linked recessive allele that shows criss-cross inheritance.

Selfing (self-fertilization) is the fusion of male and female gametes originating from the same individual or genetically identical individuals.

The initial generation of pure-breeding homozygous individuals chosen to initiate a cross-breeding experiment, designated as 'P' generation.

Mirabilis jalapa (Four O'clock plant) or Antirrhinum majus (Snapdragon).

$\alpha$-Thalassemia (defect on chromosome 16) and $\beta$-Thalassemia (defect on chromosome 11).

Walter Sutton and Theodor Boveri (1902).

Roller tongue (dominant) vs Non-roller tongue (recessive).

Free earlobe (dominant) vs Attached earlobe (recessive).

Autosomes (Chromosome 16 for $\alpha$-globin genes $HBA1, HBA2$; Chromosome 11 for $\beta$-globin gene $HBB$).

Yes, identical: Both express White coat colour with Rough hair, because 'R' (rough) is completely dominant over 'r' (smooth).

Because haemophilia is an X-linked recessive trait. In females with two X chromosomes, a single mutant allele ($X^h$) is masked by the normal dominant allele ($X^H$), expressing disease only when homozygous ($X^h X^h$).

Fate: Severe microcytic anemia, bone deformities, hepatosplenomegaly, and fatal organ failure from iron overload (hemosiderosis).
Eradication measures: Pre-marital genetic screening (Hb electrophoresis) to prevent carrier-to-carrier marriages, and prenatal genetic counselling.

50% (or 0.5). Cross: Mother ($X^C X^c$) $\times$ Father ($X^c Y$). Daughters produced are $X^C X^c$ (Carrier, 50%) and $X^c X^c$ (Colourblind, 50%).

Law of Independent Assortment: When two pairs of contrasting traits are combined in a hybrid, segregation of one pair of characters is completely independent of the other pair during gamete formation.

Heredity is the biological transmission of distinct genetic morphological, physiological, and behavioural characters from parents to offspring across successive generations through germ cells.

A mutation is a sudden, discontinuous, heritable change in the nucleotide sequence of DNA or chromosomal structure that alters genetic information and serves as the ultimate source of new alleles and variation.

Variation refers to the morphological, physiological, or biochemical differences exhibited by individuals of the same species, caused by genetic recombination (crossing over), independent assortment, or environmental influences.

Hybridization is the process of interbreeding or artificial crossing between two genetically dissimilar individuals of the same or closely related species to combine desirable characteristics in the resulting hybrid offspring.

A monohybrid cross is a hybridization experiment where two parent organisms differing in only a single pair of contrasting allelic traits are mated (e.g. pure tall $TT \times$ pure dwarf $tt$).

A dihybrid cross is a genetic cross between two pure-breeding parents considering two pairs of contrasting alleles simultaneously (e.g. Yellow Round $YYRR \times$ Green Wrinkled $yyrr$).

A recessive gene (allele) is an allele whose phenotypic expression is completely suppressed or masked in the presence of its dominant counterpart, and is phenotypically expressed only in homozygous state (e.g. $t$ in $tt$).

Incomplete dominance is a non-Mendelian inheritance pattern where neither allele is dominant, producing an intermediate blending phenotype in heterozygotes (e.g. Red $RR \times$ White $rr \rightarrow$ Pink $Rr$ in Mirabilis jalapa).

A dominant gene is an allele that masks the phenotypic expression of the alternative allele and expresses its trait in both homozygous ($TT$) and heterozygous ($Tt$) states.

An organism that possesses two identical alleles for a particular trait at the same locus on homologous chromosomes (e.g. pure tall $TT$ or pure dwarf $tt$), breeding true for that character.

An organism possessing two different, contrasting alleles for a specific trait at the same locus on homologous chromosomes (e.g. hybrid tall $Tt$), producing two distinct types of gametes upon meiosis.

A pure breeding (true-breeding) variety is a homozygous strain that has undergone repeated self-pollination or inbreeding, consistently producing offspring with the exact same parental phenotype generation after generation.

A test cross is the cross of an individual exhibiting a dominant phenotype with its homozygous recessive parent ($T? \times tt$) to determine whether the dominant organism is homozygous ($TT$) or heterozygous ($Tt$).

Law of Segregation: The two alleles of a contrasting character remain together in a hybrid without blending or altering each other, and segregate (separate) cleanly from each other during gametogenesis so that each gamete receives only one allele in pure form.

• Severe Microcytic Hypochromic Anemia: Pale skin, extreme fatigue, and breathlessness due to destruction of defective erythrocytes.
• Hepatosplenomegaly & Hemosiderosis: Enlarged liver and spleen, accompanied by toxic iron accumulation in heart and endocrine glands from repeated blood transfusions.

When Mendel crossed a pure tall pea plant ($TT$) with a pure dwarf plant ($tt$), all resulting $F_1$ progeny ($Tt$) were completely tall. The allele for tallness ($T$) completely masked the dwarf allele ($t$), demonstrating the Law of Dominance.

Mendelism encompasses the fundamental principles and laws of heredity discovered by Gregor Johann Mendel (Law of Segregation and Law of Independent Assortment), which laid the foundation of classical genetics.

Cross: Carrier Female ($X^C X^c$) $\times$ Normal Male ($X^C Y$):

This demonstrates criss-cross inheritance, where 50% of sons inherit the disease from their carrier mother.

Cross: Mother ($44A + XX$) $\times$ Father ($44A + XY$):

• Mother produces one type of ovum: $22A + X$ (Homogametic).
• Father produces two types of sperm in equal ratio: $22A + X$ (Gynosperm, 50%) and $22A + Y$ (Androsperm, 50%).

Thus, sex determination is entirely governed by whether the fertilizing sperm carries an X or Y chromosome from the father.

1. Pre-marital Genetic Screening: Undergoing Hemoglobin HPLC or electrophoresis before marriage to detect whether prospective partners are thalassemia carriers (Thalassemia Minor). Marriage between two carriers must be avoided.
2. Genetic Counselling & Prenatal Diagnosis: Providing professional guidance and conducting Chorionic Villus Sampling (CVS) or Amniocentesis in pregnant carrier mothers to identify Thalassemia Major before birth.

1. Distinct Contrasting Characters: Pea plant (Pisum sativum) displays easily observable, sharply contrasting alternative traits (e.g. Tall vs Dwarf, Round vs Wrinkled seeds) with no ambiguous intermediate forms.
2. Bisexual Flowers & Natural Self-Pollination: Flowers are naturally cleistogamous/closed, ensuring strict self-pollination and pure true-breeding lines.
3. Ease of Artificial Cross-Pollination: Large floral structures make manual emasculation (removal of anthers) and controlled hybridization straightforward.
4. Short Life Cycle & High Fecundity: Annual plant that completes its generation in 3-4 months, producing abundant seeds per cross for statistically reliable data.
5. Easy Cultivation: Can easily be grown in small garden plots or pots without complex maintenance.

1. Focus on One or Two Traits at a Time: Unlike previous hybridization researchers, Mendel studied the inheritance of single characters (monohybrid) before analyzing complex combinations (dihybrid).
2. Use of True-Breeding Homozygous Parents: Mendel verified the purity of parental lines through selfing for multiple generations before crossing.
3. Quantitative & Statistical Rigour: Maintained meticulous numerical records of every offspring across $F_1, F_2, F_3$ generations and applied probability ratios.
4. Careful Avoidance of Foreign Pollen: Practiced diligent emasculation and protective bagging to prevent unintended insect pollination.
5. Absence of Gene Linkage: By sheer biological good fortune, the seven characters chosen were situated on different chromosomes (or sufficiently far apart on chromosomes 1, 4, 5, and 7), avoiding complicating linkage.

The seven contrasting pairs of characters studied by Mendel in Pisum sativum are:

Law of Segregation (Mendel's First Law): Alleles of a gene do not blend in a hybrid but remain distinct, and segregate from each other during gamete formation so that each gamete receives only one allele in pure condition.

Monohybrid Cross Description: Pure Tall ($TT$) is crossed with Pure Dwarf ($tt$). The $F_1$ hybrids are 100% Tall ($Tt$). When $F_1$ plants are selfed ($Tt \times Tt$), $F_2$ progeny yield 3 Tall : 1 Dwarf (Phenotypic) and 1 TT : 2 Tt : 1 tt (Genotypic).

1. Unit Factors in Pairs: Each trait is governed by discrete particulate factors (genes) present in pairs in diploid cells.
2. Principle of Dominance: When two unlike unit factors are present in an individual ($Tt$), one factor expresses itself (Dominant) while the other remains unexpressed (Recessive).
3. Law of Segregation: Factors do not blend; during meiosis, homologous chromosome pairs disjoin, sending only one factor of each pair into a gamete.
4. Random Fertilization: Female gametes ($T, t$) fuse with male gametes ($T, t$) with equal probability ($25\%$ chance for each of the 4 zygotic combinations), producing the $3:1$ phenotypic and $1:2:1$ genotypic ratios.

1. Independent Inheritance: Two separate gene pairs controlling seed shape ($R/r$) and seed colour ($Y/y$) assort independently during meiosis.
2. Gametic Combinations: The dihybrid $F_1$ ($RrYy$) produces four types of pollen and four types of ova in equal numbers ($25\%$ each): $RY$, $Ry$, $rY$, and $ry$.
3. 16-Square Random Syngamy: Unbiased fertilization produces 16 zygotic combinations, giving 9 genotypes and 4 phenotypes in the ratio of 9 Round-Yellow : 3 Round-Green : 3 Wrinkled-Yellow : 1 Wrinkled-Green.
4. Dihybrid as Two Combined Monohybrids: $(3\text{ Round} : 1\text{ Wrinkled}) \times (3\text{ Yellow} : 1\text{ Green}) = 9 : 3 : 3 : 1$.

Definition (2 Marks): A dihybrid cross is a breeding experiment analyzing the inheritance patterns of two distinct pairs of contrasting traits simultaneously.

Events (3 Marks): Parent cross between Pure Round-Yellow ($RRYY$) and Pure Wrinkled-Green ($rryy$). $F_1$ generation produces all Round-Yellow plants ($RrYy$). Self-pollination of $F_1$ yields 16 combinations shown in the Punnett checkerboard below:

Parental Generation (P): Pure homozygous Black guinea pig ($BB$) is mated with pure homozygous White guinea pig ($bb$).

$F_1$ Generation: All progeny are heterozygous Black ($Bb$).

$F_2$ Generation (Inbreeding $Bb \times Bb$): Male gametes ($B, b$) and female gametes ($B, b$) produce 3 phenotypic categories in a 3 Black : 1 White ratio.

Phenotypic Ratio: 3 Black : 1 White • Genotypic Ratio: 1 BB : 2 Bb : 1 bb (1:2:1).

1. Incomplete Dominance: Heterozygote displays an intermediate phenotype (e.g. Mirabilis jalapa flower color: $1\text{ Red} : 2\text{ Pink} : 1\text{ White}$).
2. Codominance: Both alleles express themselves fully in the phenotype (e.g. human ABO blood group allele $I^A I^B \rightarrow \text{AB blood group}$).
3. Multiple Allelism: A gene existing in more than two allelic forms in a population (e.g. $I^A, I^B, i$ alleles controlling ABO blood groups).
4. Linkage: Tendency of genes situated closely on the same chromosome to be inherited together, preventing $9:3:3:1$ independent assortment.
5. Polygenic (Quantitative) Inheritance: Traits controlled by multiple additive genes (e.g. human skin pigmentation and human height).

Law of Independent Assortment (2 Marks): Factors controlling two or more pairs of contrasting characters segregate independently of each other during gamete formation and combine randomly during fertilization.

Dihybrid Cross in Guinea Pig (3 Marks): Pure Black Rough ($BBRR$) $\times$ Pure White Smooth ($bbrr$). $F_1$ is all Black Rough ($BbRr$). When $F_1$ is crossed, gametes $BR, Br, bR, br$ produce 16 combinations resulting in: $$\mathbf{9\text{ Black-Rough} : 3\text{ Black-Smooth} : 3\text{ White-Rough} : 1\text{ White-Smooth}}$$

Test Cross Checkerboard (3 Marks):

Parents: Hybrid Black ($Bb$) $\times$ Pure White ($bb$)

Phenotypic & Genotypic Ratio: 1 Black : 1 White (1:1).

Law of Segregation (2 Marks): Alleles remain unblended in hybrids and separate cleanly during gamete formation so each gamete contains only one allele.

What is Colour Blindness? (2 Marks):
An X-chromosome-linked congenital visual defect where individuals fail to perceive or distinguish between specific primary colours (most commonly Red and Green) due to defective cone cell opsin photopigments.

Symptoms of Thalassemia (3 Marks):

• Severe Chronic Anemia: Pale skin, weakness, lethargy, and stunted somatic growth due to continuous hemolysis.
• Hepatosplenomegaly & Skeletal Deformities: Splenomegaly from overactive RBC removal, along with maxillary bone expansion (chipmunk facies).
• Iron Overload (Hemosiderosis): Excessive toxic accumulation of iron in heart, liver, and pancreas from frequent blood transfusions.

Thalassemia as an Autosomal Disorder (3 Marks):
Thalassemia is an autosomal recessive blood disorder resulting from mutations or deletions in genes coding for hemoglobin polypeptide chains:

• $\alpha$-Thalassemia: Caused by deletion of one or more of the 4 alpha-globin genes on Chromosome 16.
• $\beta$-Thalassemia: Caused by mutation in the $HBB$ gene on Chromosome 11, reducing beta-globin synthesis. When homozygous ($Hb\beta^T Hb\beta^T$), it causes life-threatening Thalassemia Major (Cooley's Anemia) requiring lifelong transfusions.

Role of Genetic Counselling (2 Marks):

1. Pre-marital Carrier Detection: Blood test (HPLC / Hb electrophoresis) identifies asymptomatic heterozygous carriers (Thalassemia Minor). If two carriers marry, their child has a $25\%$ risk of Thalassemia Major.
2. Prenatal Diagnosis: Amniocentesis or chorionic villus sampling (CVS) between 10–16 weeks of gestation detects fetal gene status, guiding medical advice and preventing the birth of diseased children.

Types of Colour Blindness (2 Marks):

1. Protanopia (Red Blindness): Inability to distinguish red light/spectrum; red appears dark grayish.
2. Deuteranopia (Green Blindness): Inability to perceive green light (most common sex-linked form).
3. Tritanopia (Blue-Yellow Blindness): Rare autosomal condition where blue cone pigments are non-functional.

Haemophilia in Brief (3 Marks):
An X-linked recessive hereditary bleeding coagulopathy:

• Haemophilia A (Royal Disease): Deficiency of Anti-Hemophilic Factor (Factor VIII), accounting for ~85% of cases.
• Haemophilia B (Christmas Disease): Deficiency of Plasma Thromboplastin Component (Factor IX).
• Symptoms: Prolonged, uncontrolled bleeding from minor cuts, spontaneous internal joint bleeding (hemarthrosis), and hematomas.

What is Variation? (2 Marks):
Morphological, anatomical, physiological, or behavioural differences observed between individual members of the same biological species, originating through crossing over, independent assortment, and mutations.

Two Inheritable Variable Features in Man (3 Marks):

1. Tongue Rolling: Controlled by a single autosomal gene. The dominant allele ($R$) enables rolling the lateral margins of the tongue upward into a 'U' tube; homozygous recessive individuals ($rr$) are non-rollers.
2. Ear Lobe Attachment: The allele for free/detached earlobes ($E$) is dominant over the allele for attached earlobes ($e$) directly connected to the side of the head.

Definition of Mutation (2 Marks):
A sudden, stable, heritable change in the genetic material (DNA nucleotide sequence or chromosomal count/structure) of an organism that is not caused by genetic segregation or recombination.

Development of Variation (3 Marks):

• Mutational Innovations: Spontaneous errors during DNA replication create new alleles, introducing fresh genetic diversity into a gene pool.
• Recombination during Meiosis: Crossing over during pachytene shuffles linked alleles, while random orientation of maternal and paternal chromosomes in metaphase-I generates billions of unique gametic combinations.
• Random Fertilization: Amphimixis of genetically diverse sperm and ova creates novel combinations in offspring, providing raw material for natural selection and speciation.

Symptoms of Thalassemia (2 Marks): Chronic severe anemia, pale complexion, jaundice, bone deformities of skull and face, hepatosplenomegaly, and endocrine failure from iron deposition.

Demonstration that Mothers are NOT Responsible for Female Births (3 Marks):
Human females are homogametic ($44A + XX$) and produce only one type of egg carrying an X chromosome ($22A + X$). Human males are heterogametic ($44A + XY$) and produce two types of sperms in equal numbers ($50\% \text{ with X}$ and $50\% \text{ with Y}$).

Conclusion: The mother always contributes only an X chromosome. It is solely the father's sperm carrying either X or Y that determines the sex of the child. Therefore, blaming mothers for the birth of female children is completely unscientific, false, and socially unjust.