RowQ
The Vault
RowQ
The Vault
CBSE Class 10 Biology · 11 questions · 26 marks
Why do children resemble their parents, yet no two siblings look exactly alike (identical twins aside)? Gregor Mendel answered this using pea plants in his monastery garden, uncovering the rules of heredity long before genes or DNA were known. This chapter builds on his laws to explain how small heritable variations, acted on by natural selection over vast timescales, produce the incredible diversity of life through evolution.
In Mendel's monohybrid cross between pure tall (TT) and pure dwarf (tt) pea plants, what phenotypic ratio is observed in the F2 generation?
Answer
3:1 is correct — self-pollination of the all-tall F1 (Tt) plants produces about 3 tall plants for every 1 dwarf plant in the F2 generation.
In humans, which parent's contribution determines the sex of the child?
Answer
The father is correct — the mother's egg always carries an X chromosome, but the father's sperm may carry an X (giving a girl, XX) or a Y (giving a boy, XY), so the sperm determines the child's sex.
Which of the following is an example of analogous organs?
Answer
Wings of a bird and a butterfly is correct — they perform the same function (flight) but have completely different structures and origins, unlike homologous organs, which share structure and ancestry.
Who proposed the theory of evolution by natural selection?
Answer
Charles Darwin is correct — he proposed that individuals with heritable variations better suited to their environment are more likely to survive and reproduce, gradually leading to evolution.
Assertion (A): Two individuals can carry the gene for a particular trait without that trait being visibly expressed in them. Reason (R): A trait controlled by a recessive allele is only expressed in the phenotype when the dominant allele for that trait is absent.
Answer
Both A and R are true and R is the correct explanation of A — a heterozygous individual (e.g. Tt) carries a recessive allele silently, since it is masked by the dominant allele, but can still pass it on to offspring.
State Mendel's law of dominance with a suitable example.
Answer
The law of dominance states that when two pure-breeding organisms with contrasting traits are crossed, only one of the two traits (the dominant one) appears in the offspring of the first filial (F1) generation, while the other (recessive) trait is masked though still present. For example, crossing pure tall pea plants (TT) with pure dwarf pea plants (tt) produces F1 offspring (Tt) that are all tall, since the tallness allele is dominant over the dwarfness allele.
Differentiate between homologous organs and analogous organs, giving one example of each.
Answer
Homologous organs have the same basic structural plan and originate from a common ancestor, but may perform different functions in different organisms due to divergent evolution — for example, the forelimbs of humans, whales, and bats have the same bone arrangement but are used for grasping, swimming, and flying respectively. Analogous organs have different structures and origins but perform a similar function because of convergent evolution in similar environments — for example, the wings of a bird (modified forelimb bones) and the wings of a butterfly (a membranous outgrowth) both enable flight but are structurally unrelated.
Why did Mendel choose the garden pea plant for his hybridisation experiments on heredity?
Answer
Mendel chose the garden pea because it has several easily distinguishable, contrasting traits (such as tall/dwarf stems or round/wrinkled seeds), it is normally self-pollinating, which allowed him to maintain pure-breeding lines, yet it can also be easily cross-pollinated by hand, and it has a short life cycle and produces many offspring, making it convenient for observing inheritance patterns over several generations.
Describe Mendel's dihybrid cross experiment and explain the ratio obtained in the F2 generation.
Answer
Mendel crossed pea plants that were pure-breeding for two traits at once — round, yellow seeds (RRYY) with wrinkled, green seeds (rryy). All F1 offspring were round and yellow (RrYy), showing that round and yellow are dominant traits. When these F1 plants were self-pollinated, the F2 generation showed four phenotypes in the ratio 9 round yellow : 3 round green : 3 wrinkled yellow : 1 wrinkled green. This 9:3:3:1 ratio revealed that the two genes (for seed shape and seed colour) segregate and assort into gametes independently of each other, which Mendel formulated as the law of independent assortment — the inheritance of one trait does not affect the inheritance of the other.
Explain, with the help of a cross, how the sex of a child is determined in humans.
Answer
Humans have 23 pairs of chromosomes, one pair of which are the sex chromosomes: females have two X chromosomes (XX) and males have one X and one Y chromosome (XY). During gamete formation, every egg produced by the mother carries a single X chromosome, since she is XX. The father, being XY, produces two types of sperm in equal numbers — half carry an X chromosome and half carry a Y chromosome. If an X-carrying sperm fertilises the egg, the resulting zygote is XX and develops into a girl; if a Y-carrying sperm fertilises the egg, the zygote is XY and develops into a boy. Since both types of sperm are produced in equal numbers, there is roughly an equal probability of having a boy or a girl at each pregnancy, and it is the father's chromosome contribution that determines the child's sex.
Paleontologists discovered a fossil of Archaeopteryx, an ancient organism with feathers and wings like a bird, but also teeth, a long bony tail, and clawed fingers like a reptile. Explain what such a fossil suggests about evolution, and define the term used for organisms like this.
Answer
A fossil like Archaeopteryx, which shows a mixture of reptilian features (teeth, clawed fingers, long bony tail) and avian features (feathers, wings), provides strong evidence that birds evolved gradually from reptilian ancestors over millions of years, rather than appearing suddenly. Such an organism is called a connecting link (or transitional fossil) — it displays characteristics of two different groups and helps establish an evolutionary relationship between them, supporting the idea that present-day species descended with modification from earlier forms.
RowQ generates fresh questions on Heredity and Evolution, marks your answers, and explains every step.
Start free