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Q4:
Offspring formed due to sexual reproduction have better chances of survival. Why? Is this statement always true?
Solution :
Part 1: The Biological Basis for Enhanced Survival in Sexual Reproduction
Sexual reproduction involves the fusion of heterogametes (male and female) in a process known as syngamy, resulting in the formation of a diploid zygote ($n + n \rightarrow 2n$). The widely accepted biological principle that sexually reproducing offspring have a better chance of survival is fundamentally rooted in the generation of genetic variation. This variation acts as the structural substrate for natural selection and environmental adaptation.
The introduction of genetic variation is driven by highly precise cellular mechanisms during gametogenesis:
- Crossing Over (Homologous Recombination): [Occurring during the Pachytene stage of Prophase I in Meiosis] Non-sister chromatids of homologous chromosomes physically exchange genetic segments at intersection points called chiasmata. This breaks previously linked genes and creates novel recombinant genotypes.
- Independent Assortment of Chromosomes: [Governed by Mendel’s Law of Independent Assortment] During Metaphase I, the random geometric alignment of maternal and paternal bivalents at the equatorial plate ensures that gametes receive a randomized matrix of parental chromosomes. For an organism with $n$ pairs of chromosomes, this creates $2^n$ possible combinations in the gametes.
- Random Fertilization: The probabilistic fusion of two genetically unique gametes exponentially multiplies the genetic diversity of the offspring, further promoting heterozygosity.
Theoretical Justification: In a dynamic and changing ecosystem (characterized by shifting abiotic factors, new predatory pressures, or novel pathogens), an asexually reproducing clonal population risks mass mortality. Conversely, a genetically diverse sexual population possesses a higher statistical probability of containing individuals with specific allelic combinations suited to survive the new stresses. These fit individuals survive, reproduce, and propagate the advantageous traits [Darwinian Fitness and Adaptation].
Part 2: Critical Evaluation - Is This Statement Always True?
The statement is not always true. There are definitive ecological and genetic circumstances where sexual reproduction can actually decrease survival probabilities compared to asexual reproduction:
- Stable, Unchanging Environments: If a parent organism possesses a genotype that is perfectly adapted to a static environment, asexual reproduction (which produces genetically identical clones via mitosis) guarantees that 100% of the offspring will inherit this optimal blueprint. Sexual reproduction disrupts this highly successful gene combination through recombination, generating phenotypic variations that may be less adapted to the established niche.
- Expression of Deleterious Recessive Alleles: Sexual reproduction allows for the segregation and recombination of alleles. This carries the risk of bringing together two deleterious or lethal recessive alleles (e.g., yielding a homozygous recessive condition, $aa$), leading to congenital anomalies or reduced viability in the offspring. Clonal reproduction of a healthy heterozygous parent ($Aa$) entirely circumvents this risk.
- The "Two-Fold Cost of Sex" & Vulnerability: Sexual reproduction requires an immense expenditure of metabolic energy and time to locate a compatible mate. During the mating process, organisms are often highly vulnerable to predation. In sparse populations or highly isolated environments, organisms utilizing asexual reproduction or parthenogenesis have an absolute survival advantage simply by bypassing the necessity of a mate.
Step 3: Comparative Analysis of Survival Potential
| Parameter | Sexual Reproduction Offspring | Asexual Reproduction Offspring |
|---|---|---|
| Genetic Constitution | Heterozygous and genetically unique recombinants. | Genetically identical exact clones of the parent. |
| Environmental Advantage | Highly advantageous in unpredictable, shifting environments. Promotes species evolution. | Highly advantageous in stable, localized, predictable environments. Limits species evolution. |
| Vulnerability to Pathogens | Low. Pathogens targeting specific genotypes cannot wipe out the varied population easily. | High. A single novel pathogen can annihilate the entire clonal population simultaneously. |
Final Solution: Offspring formed via sexual reproduction generally possess a higher probability of survival due to the introduction of genetic variation (via meiosis and syngamy), which provides biological resilience against environmental changes and diseases. However, the statement is NOT always true. In highly stable, unchanging ecological niches, asexually reproducing offspring inherit an already optimized, unbroken parental genotype, thereby yielding a higher localized survival rate without the energy deficits, lethal recessive pairings, or mating risks associated with sexual reproduction.
More Questions from Class 12 Biology Reproduction in Organisms EXERCISES
- Q1: Why is reproduction essential for organisms?
- Q1: Why is reproduction essential for organisms?
- Q2: Which is a better mode of reproduction: sexual or asexual? Why?
- Q2: Which is a better mode of reproduction: sexual or asexual? Why?
- Q3: Why is the offspring formed by asexual reproduction referred to as clone?
- Q3: Why is the offspring formed by asexual reproduction referred to as clone?
- Q4: Offspring formed due to sexual reproduction have better chances of survival. Why? Is this statement always true?
- Q5: How does the progeny formed from asexual reproduction differ from those formed by sexual reproduction?
- Q5: How does the progeny formed from asexual reproduction differ from those formed by sexual reproduction?
- Q6: Distinguish between asexual and sexual reproduction. Why is vegetative reproduction also considered as a type of asexual reproduction?
- Q6: Distinguish between asexual and sexual reproduction. Why is vegetative reproduction also considered as a type of asexual reproduction?
- Q7: What is vegetative propagation? Give two suitable examples.
- Q7: What is vegetative propagation? Give two suitable examples.
- Q8(a): Define (a) Juvenile phase,
- Q8(a): Define (a) Juvenile phase,
- Q8(b): Define (b) Reproductive phase,
- Q8(b): Define (b) Reproductive phase,
- Q8(c): Define (c) Senescent phase.
- Q8(c): Define (c) Senescent phase.
- Q9: Higher organisms have resorted to sexual reproduction in spite of its complexity. Why?
- Q9: Higher organisms have resorted to sexual reproduction in spite of its complexity. Why?
- Q10: Explain why meiosis and gametogenesis are always interlinked?
- Q10: Explain why meiosis and gametogenesis are always interlinked?
- Q11(a): Identify each part in a flowering plant and write whether it is haploid (\(n\)) or diploid (\(2n\)). (a) Ovary ———————————
- Q11(a): Identify each part in a flowering plant and write whether it is haploid ((n)) or diploid ((2n)). (a) Ovary ———————————
- Q11(b): Identify each part in a flowering plant and write whether it is haploid (\(n\)) or diploid (\(2n\)). (b) Anther ———————————
- Q11(b): Identify each part in a flowering plant and write whether it is haploid ((n)) or diploid ((2n)). (b) Anther ———————————
- Q11(c): Identify each part in a flowering plant and write whether it is haploid (\(n\)) or diploid (\(2n\)). (c) Egg ———————————
- Q11(c): Identify each part in a flowering plant and write whether it is haploid ((n)) or diploid ((2n)). (c) Egg ———————————
- Q11(d): Identify each part in a flowering plant and write whether it is haploid (\(n\)) or diploid (\(2n\)). (d) Pollen ———————————
- Q11(d): Identify each part in a flowering plant and write whether it is haploid ((n)) or diploid ((2n)). (d) Pollen ———————————
- Q11(e): Identify each part in a flowering plant and write whether it is haploid (\(n\)) or diploid (\(2n\)). (e) Male gamete ———————————
- Q11(e): Identify each part in a flowering plant and write whether it is haploid ((n)) or diploid ((2n)). (e) Male gamete ———————————
- Q11(f): Identify each part in a flowering plant and write whether it is haploid (\(n\)) or diploid (\(2n\)). (f) Zygote ———————————
- Q11(f): Identify each part in a flowering plant and write whether it is haploid ((n)) or diploid ((2n)). (f ) Zygote ———————————
- Q12: Define external fertilisation. Mention its disadvantages.
- Q12: Define external fertilisation. Mention its disadvantages.
- Q13: Differentiate between a zoospore and a zygote.
- Q13: Differentiate between a zoospore and a zygote.
- Q14: Differentiate between gametogenesis from embryogenesis.
- Q14: Differentiate between gametogenesis from embryogenesis.
- Q15: Describe the post-fertilisation changes in a flower.
- Q15: Describe the post-fertilisation changes in a flower.
- Q16: What is a bisexual flower? Collect five bisexual flowers from your neighbourhood and with the help of your teacher find out their common and scientific names.
- Q16: What is a bisexual flower? Collect five bisexual flowers from your neighbourhood and with the help of your teacher find out their common and scientific names.
- Q17: Examine a few flowers of any cucurbit plant and try to identify the staminate and pistillate flowers. Do you know any other plant that bears unisexual flowers?
- Q17: Examine a few flowers of any cucurbit plant and try to identify the staminate and pistillate flowers. Do you know any other plant that bears unisexual flowers?
- Q18: Why are offspring of oviparous animals at a greater risk as compared to offspring of viviparous animals?
- Q18: Why are offspring of oviparous animals at a greater risk as compared to offspring of viviparous animals?
CBSE Solutions for Class 12 Biology Reproduction in Organisms
Chapters in CBSE - Class 12 Biology
- Human Reproduction
- Reproductive Health
- Principles of Inheritance and Variation
- Molecular Basis of Inheritance
- Evolution
- Human Health and Disease
- Strategies for Enhancement in Food Production
- Microbes in Human Welfare
- Biotechnology: Principles and Processes
- Biotechnology and Its Applications
- Organisms and Populations
- Ecosystem
- Biodiversity and Conservation
- Environmental Issues
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