Sex Determination – Long Answer Questions (CBSE Class 10 Science)
Medium Level (Application & Explanation)
Q1. Explain step-by-step how the sex of a human baby is determined during fertilization. Why is the father’s contribution decisive?
Answer:
Humans have 23 pairs of chromosomes: 22 pairs of autosomes and 1 pair of sex chromosomes.
A female has XX and produces eggs that all carry X. A male has XY and produces two types of sperms: X-carrying and Y-carrying, usually in almost equal numbers.
During fertilization, the egg (always X) fuses with either an X sperm (forming XX → female) or a Y sperm (forming XY → male).
Thus, the father’s sperm determines the baby’s sex, not the mother. The probability of having a boy or girl is roughly 50–50 in each pregnancy because sperms carry X and Y with almost equal chance.
This scientific explanation corrects the common myth that the mother decides the baby’s sex. Biology shows the father’s sperm is decisive.
Q2. Differentiate between autosomes and sex chromosomes in humans. How do both together influence traits and development?
Answer:
Autosomes (22 pairs) are chromosomes that carry genes for most body traits, such as height, eye color, metabolism, and immunity. They are the same in males and females and do not directly decide sex.
Sex chromosomes (1 pair) carry genes that determine biological sex and also include genes affecting other traits (for example, some genes on the X chromosome affect vision or blood clotting).
A female is XX, so she has two copies of X-linked genes. A male is XY, so he has only one X; this is why some X-linked traits appear more commonly in males.
Together, autosomes guide general growth and functions, while sex chromosomes decide sex (XX or XY) and influence additional characteristics. Normal development requires the right number of both types.
Q3. How is sex determined in birds, and how is this system different from humans?
Answer:
Birds use a ZW–ZZ system. Here, the female is ZW and the male is ZZ. This is the reverse of humans, where females are XX and males are XY.
A mother bird produces two kinds of eggs: Z and W. The father bird produces only Z sperm.
If the egg carries Z, the chick becomes ZZ (male); if the egg carries W, the chick becomes ZW (female). Therefore, in birds, the mother’s egg decides the sex.
In humans, the mother always contributes X, while the father contributes X or Y, so the father determines sex.
This comparison shows how nature uses different sex-determining systems across groups, yet both typically result in a balanced sex ratio in populations over time.
Q4. Describe sex determination in honey bees and ants. Why are males haploid and females diploid?
Answer:
Honey bees and many ants follow haplodiploidy. Females (workers/queens) develop from fertilized eggs, making them diploid (2n). Males (drones) develop from unfertilized eggs, making them haploid (n).
The queen controls this by choosing whether to fertilize each egg with stored sperm. If she fertilizes it, a female develops; if not, a male develops.
Haploid males carry only one set of chromosomes, so they pass their genes directly to daughters without recombination from a father. This affects relatedness within the colony and supports eusocial behavior.
This system allows colonies to adjust their workforce: more workers (females) when food is abundant, or more males during mating seasons. It is a precise biological strategy for colony success.
Q5. Explain temperature-dependent sex determination (TSD) in reptiles with examples. Why is this biologically important?
Answer:
Some reptiles, like turtles, crocodiles, and certain lizards, do not use sex chromosomes. Instead, incubation temperature during a sensitive period of embryonic development determines sex—this is temperature-dependent sex determination (TSD).
Example: In some turtles, warmer nests produce more females, while cooler nests produce more males. In some crocodiles, intermediate temperatures may yield more males, and higher or lower temperatures more females (species-specific patterns).
TSD may help populations balance sex ratios naturally across seasons and nesting sites, and it can link environmental conditions to the best reproductive strategy.
However, climate change can skew sex ratios, threatening population stability. Understanding TSD is vital for conservation, such as managing nest temperatures or relocating eggs.
High Complexity (Analytical & Scenario-Based)
Q6. A family believes the mother is responsible for having daughters. As a student of biology, how would you explain the science and address the social misconception?
Answer:
Start with clear science: a mother (XX) produces eggs that all carry X. A father (XY) produces two kinds of sperm: X and Y. If an X sperm fertilizes the egg, the baby is XX (female); if a Y sperm fertilizes it, the baby is XY (male). Therefore, the father’s sperm decides the sex with about 50–50 probability each time.
Emphasize that sex determination is a natural, random process, not under voluntary control.
Address the social angle: blaming mothers is unscientific and unfair. Respect for both girls and boys is essential, as both are equally valuable.
Encourage evidence-based thinking and discourage gender bias. Biology promotes equality, showing that sex outcome is chance-based, determined by which sperm reaches the egg first.
Q7. How do sex chromosome disorders like Turner syndrome (XO) and Klinefelter syndrome (XXY) arise? Explain their causes and effects in simple terms.
Answer:
These conditions arise from nondisjunction, an error during the formation of gametes (egg or sperm) when chromosomes fail to separate properly. This can lead to embryos with missing or extra sex chromosomes.
Turner syndrome (XO) occurs when a person has only one X chromosome and no second sex chromosome. Typical features may include short stature, delayed puberty, and some fertility issues, but intelligence is often normal.
Klinefelter syndrome (XXY) occurs in males with an extra X. Common features can include taller height, delayed or reduced testosterone, some learning difficulties, and reduced fertility.
These conditions are not caused by parental actions and are not the same as the normal XX/XY sex determination. They show how accurate chromosome number is crucial for healthy development.
Q8. In a class activity using X–Y slips, a group drew 20 zygotes and got 14 XY and 6 XX. Does this contradict the 50–50 expectation? Analyze the result.
Answer:
The theoretical probability for XX and XY is about 50–50 because sperms carry X and Y in nearly equal numbers. However, in small samples, random variation can cause skewed results like 14 XY vs 6 XX.
This does not contradict the theory; it shows sampling fluctuation. If the activity is repeated many more times (e.g., hundreds of draws), the overall proportion will likely move closer to 50–50.
Possible practical factors: uneven mixing of slips, biased drawing, or too few trials. Improving the activity by increasing the number of trials, shuffling well, and ensuring equal numbers of X and Y slips will reduce bias.
Conclusion: Short-term deviations are normal; long-term averages reflect the 50–50 expectation.
Q9. A turtle nesting beach is getting warmer due to climate change. Predict the likely impact on sex ratios and suggest two conservation actions with scientific reasoning.
Answer:
In many turtles, warmer incubation temperatures produce more females. As beaches warm, nests may yield female-biased hatchlings, risking future mating shortages and population decline.
Scientific prediction: persistent warming will skew sex ratios, reduce genetic diversity, and may collapse local populations if males become too rare.
Conservation actions:
Nest temperature management: Use shading, reflective covers, or relocate nests to cooler microhabitats to bring incubation temperatures back into the male-producing range.
Hatchery management: Collect eggs and incubate them at controlled temperatures to produce a balanced sex ratio (close to 1:1) and then release the hatchlings.
Monitoring nest temperatures and hatchling sex ratios will guide adaptive management for long-term population stability.
Q10. Compare sex determination across humans (XX–XY), birds (ZW–ZZ), honey bees (haplodiploidy), and reptiles (TSD). Propose simple classroom simulations for each and discuss who “decides” the sex.
Answer:
Systems and “deciders”:
Humans (XX–XY): Mother gives X; father gives X or Y. The father decides.
Birds (ZW–ZZ): Father gives Z only; mother gives Z or W. The mother decides.
