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Kerala Plus Two Biology (DHSE) study materials
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Kerala Plus Two Biology (DHSE) practice questions

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Question 1Reproduction (sexual reproduction in flowering plants, human reproduction, reproductive health)

In a typical angiosperm ovule, the embryo sac is described as monosporic and eight-nucleate. Which cell of the mature embryo sac is binucleate at the time of fertilisation?

  1. The synergid
  2. The egg cell
  3. The antipodal cell
  4. The central cell Correct answer.

Explanation

The central cell carries two polar nuclei that may fuse to form the diploid secondary nucleus before fertilisation, so it has two haploid nuclei. The egg and each synergid are uninucleate, and antipodals are typically uninucleate, though their number varies. After triple fusion the central cell becomes the triploid primary endosperm cell.

Question 2Reproduction (sexual reproduction in flowering plants, human reproduction, reproductive health)

A pollen grain of a flowering plant is shed at the two-celled stage. Which event must occur before the pollen tube can discharge its contents into the embryo sac?

  1. Division of the generative cell to form two male gametes Correct answer.
  2. Formation of the secondary nucleus
  3. Formation of a third male gamete
  4. Fusion of the vegetative nucleus with the egg cell

Explanation

In a two-celled pollen grain the generative cell has not yet divided, so it divides in the pollen tube to produce two male gametes. The vegetative cell does not divide further and only directs tube growth. The secondary nucleus belongs to the ovule, not the pollen grain.

Question 3Reproduction (sexual reproduction in flowering plants, human reproduction, reproductive health)

Double fertilisation is a characteristic feature of angiosperms. Which statement correctly describes the two fusion events it involves?

  1. The vegetative nucleus fuses with the egg and a male gamete fuses with the polar nuclei
  2. Both male gametes fuse with the egg, producing a tetraploid zygote
  3. One male gamete fuses with the egg and the other fuses with a synergid
  4. One male gamete fuses with the egg and the other fuses with the two polar nuclei Correct answer.

Explanation

Syngamy is the fusion of one male gamete with the egg to give the diploid zygote, while triple fusion is the fusion of the second male gamete with the two polar nuclei to give the triploid primary endosperm nucleus. These two fusions occurring in the same embryo sac constitute double fertilisation.

Question 4Reproduction (sexual reproduction in flowering plants, human reproduction, reproductive health)

After fertilisation in a dicotyledonous plant, the ovule and the ovary develop into which structures respectively?

  1. Endosperm and pericarp
  2. Seed and fruit Correct answer.
  3. Fruit and seed
  4. Seed and endosperm

Explanation

The ovule develops into the seed, containing the embryo, endosperm and seed coat, while the ovary wall develops into the pericarp of the fruit. This transformation is the standard post-fertilisation fate in angiosperms.

Question 5Reproduction (sexual reproduction in flowering plants, human reproduction, reproductive health)

Which structure of the human male reproductive system stores spermatozoa and allows them to mature before ejaculation?

  1. Seminal vesicle
  2. Epididymis Correct answer.
  3. Vas deferens
  4. Rete testis

Explanation

Spermatozoa leaving the testis pass through the rete testis into the epididymis, a coiled duct where they undergo maturation and are stored. The vas deferens only transports sperm during ejaculation, and the seminal vesicle contributes secretion to semen.

Question 6Reproduction (sexual reproduction in flowering plants, human reproduction, reproductive health)

The acrosome of a human spermatozoon is derived from which cell organelle?

  1. Ribosome
  2. Centriole
  3. Golgi complex Correct answer.
  4. Mitochondrion

Explanation

During spermiogenesis the Golgi complex gives rise to the acrosome, a cap-like vesicle over the nucleus that carries hydrolytic enzymes for penetrating the egg coverings. Mitochondria instead form the middle-piece sheath, and the centriole contributes to the tail apparatus.

Question 7Reproduction (sexual reproduction in flowering plants, human reproduction, reproductive health)

Which hormone surge is directly responsible for triggering ovulation in the human menstrual cycle?

  1. A surge in follicle-stimulating hormone
  2. A surge in luteinising hormone Correct answer.
  3. A surge in prolactin
  4. A surge in progesterone

Explanation

Rising oestrogen from the mature Graafian follicle exerts positive feedback on the hypothalamus and anterior pituitary, producing a sharp mid-cycle surge of luteinising hormone. This LH surge induces rupture of the follicle and release of the oocyte. Progesterone rises only after ovulation.

Question 8Reproduction (sexual reproduction in flowering plants, human reproduction, reproductive health)

Which of the following is a barrier method of contraception that also gives some protection against sexually transmitted infections?

  1. Copper-releasing intrauterine device
  2. Tubectomy
  3. Oral contraceptive pills
  4. Condom Correct answer.

Explanation

Condoms are physical barriers that prevent sperm from meeting the oocyte and also reduce the transmission of pathogens across mucous membranes. Hormonal pills, intrauterine devices and surgical sterilisation prevent conception but do not form a barrier against infection.

Question 9Reproduction (sexual reproduction in flowering plants, human reproduction, reproductive health)

In a mature angiosperm embryo sac, the filiform apparatus is associated with which cells, and what is its role?

  1. Egg cell; preventing polyspermy
  2. Synergids; guiding the pollen tube into the embryo sac Correct answer.
  3. Antipodal cells; storage of food for the embryo
  4. Central cell; formation of the endosperm

Explanation

The filiform apparatus is a specialised thickening of the cell wall at the micropylar end of the synergids. It helps direct the growth of the pollen tube towards the egg cell, after which the synergids degenerate.

Question 10Reproduction (sexual reproduction in flowering plants, human reproduction, reproductive health)

An angiosperm species shows cleistogamous flowers in addition to normal open flowers. Which consequence is expected for the cleistogamous flowers?

  1. They set seed without the need for pollinators Correct answer.
  2. They depend on wind for pollination
  3. They produce larger fruits than chasmogamous flowers
  4. They are always cross-pollinated

Explanation

Cleistogamous flowers never open and are invariably self-pollinated, so they can set seed even when pollinators are scarce. This ensures reproduction but reduces genetic variation because no cross-pollination occurs.

Question 11Reproduction (sexual reproduction in flowering plants, human reproduction, reproductive health)

A student observes that a particular plant produces seeds but the seeds are always genetically identical to the maternal parent. Which reproductive feature best explains this?

  1. Double fertilisation followed by normal endosperm formation
  2. Cross-pollination followed by self-incompatibility
  3. Apomixis, in which the embryo develops without fertilisation Correct answer.
  4. Production of a large number of pollen grains

Explanation

Apomixis is asexual seed formation in which the embryo arises from a maternal cell without meiosis and fertilisation, so the offspring are clones of the parent. Normal double fertilisation would produce genetically recombined embryos.

Question 12Reproduction (sexual reproduction in flowering plants, human reproduction, reproductive health)

In a controlled experiment, the style of a flower is removed before pollination. Which outcome is most directly prevented?

  1. Germination of pollen on the stigma
  2. Formation of the endosperm
  3. Growth of the pollen tube towards the ovule Correct answer.
  4. Formation of the seed coat

Explanation

The pollen tube must grow through the style to reach the ovule, so removing the style interrupts this path and prevents fertilisation. Pollen can still germinate on the stigma and begin tube growth, but the tube cannot complete its journey.

Question 13Reproduction (sexual reproduction in flowering plants, human reproduction, reproductive health)

A pollen grain of a plant is described as three-celled at the time of shedding. What does this indicate about its development?

  1. The vegetative cell has divided to form two male gametes
  2. The pollen grain will undergo triple fusion directly on the stigma
  3. The generative cell has already divided to form two male gametes before shedding Correct answer.
  4. The pollen grain is non-viable and cannot germinate

Explanation

In a three-celled pollen grain the generative cell divides within the pollen grain itself, so two male gametes are already present at shedding. The vegetative cell never divides; it only forms the pollen tube after germination.

Question 14Reproduction (sexual reproduction in flowering plants, human reproduction, reproductive health)

Which statement about the human seminiferous tubules is correct?

  1. They are the site of sperm storage before ejaculation
  2. They connect the epididymis to the urethra
  3. They contain Sertoli cells that nourish developing germ cells Correct answer.
  4. They secrete testosterone into the blood

Explanation

The seminiferous tubules are the site of spermatogenesis, and their Sertoli (nurse) cells support and nourish the developing spermatogenic cells. Testosterone is secreted by Leydig cells in the interstitial tissue between the tubules.

Question 15Reproduction (sexual reproduction in flowering plants, human reproduction, reproductive health)

During oogenesis in the human female, the first meiotic division is completed at which stage?

  1. Only after fertilisation by a spermatozoon
  2. Before birth, during fetal development
  3. At the time of puberty
  4. Just before ovulation, completing the first division Correct answer.

Explanation

Primary oocytes remain arrested in prophase I from fetal life until puberty, and the first meiotic division is completed just before ovulation, producing a secondary oocyte and the first polar body. The second division is completed only after sperm entry.

Question 16Reproduction (sexual reproduction in flowering plants, human reproduction, reproductive health)

A woman with a regular 28-day menstrual cycle ovulates on day 14. On which days is the probability of conception highest?

  1. Days 10 to 16 Correct answer.
  2. Days 26 to 28
  3. Days 1 to 5
  4. Days 20 to 24

Explanation

The oocyte survives only about 24 hours after ovulation, whereas spermatozoa can survive in the female tract for a few days. Conception is therefore most likely around the time of ovulation, so days 10 to 16 cover the fertile window.

Question 17Reproduction (sexual reproduction in flowering plants, human reproduction, reproductive health)

Which sequence correctly describes the path of spermatozoa from their site of formation to the outside of the body?

  1. Seminiferous tubules, vas deferens, epididymis, urethra
  2. Vas deferens, epididymis, seminiferous tubules, urethra
  3. Epididymis, seminiferous tubules, urethra, vas deferens
  4. Seminiferous tubules, epididymis, vas deferens, urethra Correct answer.

Explanation

Spermatozoa are formed in the seminiferous tubules, mature and are stored in the epididymis, and are then carried by the vas deferens to the urethra for ejaculation. This order reflects the anatomical continuity of the male ducts.

Question 18Reproduction (sexual reproduction in flowering plants, human reproduction, reproductive health)

A couple is advised to use a copper-releasing intrauterine device. Which statement best explains how this device prevents pregnancy?

  1. It thickens cervical mucus by releasing progesterone
  2. It suppresses ovulation by inhibiting pituitary gonadotropins
  3. It blocks the fallopian tube mechanically
  4. It makes the uterine environment unfavourable to sperm and prevents implantation Correct answer.

Explanation

Copper ions released from the device are spermicidal and also create a local inflammatory environment in the uterus that is unfavourable for implantation. The device does not act by suppressing ovulation, which is the mechanism of hormonal pills.

Question 19Reproduction (sexual reproduction in flowering plants, human reproduction, reproductive health)

Which assisted reproductive technique involves transferring an embryo into the uterus after fertilisation has occurred outside the body?

  1. Gamete intrafallopian transfer
  2. In-vitro fertilisation with embryo transfer Correct answer.
  3. Intrauterine insemination
  4. Lactational amenorrhoea method

Explanation

In IVF, oocytes are collected and fertilised with sperm in a culture dish, and the resulting embryo is transferred into the uterus. Intrauterine insemination places semen in the uterus without external fertilisation, and GIFT transfers gametes rather than an embryo.

Question 20Reproduction (sexual reproduction in flowering plants, human reproduction, reproductive health)

Which of the following is a sexually transmitted infection caused by a bacterium and is effectively treated with antibiotics?

  1. Genital herpes
  2. AIDS
  3. Gonorrhoea Correct answer.
  4. Hepatitis B

Explanation

Gonorrhoea is caused by the bacterium Neisseria gonorrhoeae and responds to appropriate antibiotic therapy. Genital herpes, AIDS and hepatitis B are viral infections and are not cured by antibiotics.

Question 21Reproduction (sexual reproduction in flowering plants, human reproduction, reproductive health)

A woman is prescribed hormonal contraceptive pills that contain both an oestrogen and a progestogen. Which mechanism chiefly accounts for their contraceptive action?

  1. They inhibit the release of gonadotropins, preventing ovulation Correct answer.
  2. They cause the corpus luteum to secrete more progesterone
  3. They kill spermatozoa in the vagina
  4. They prevent the uterus from developing an endometrium

Explanation

Combined pills act through negative feedback on the hypothalamus and anterior pituitary, suppressing FSH and LH release and thereby preventing follicular maturation and ovulation. They also alter cervical mucus and the endometrium, but ovulation suppression is the principal effect.

Question 22Reproduction (sexual reproduction in flowering plants, human reproduction, reproductive health)

In an angiosperm ovule, the micropyle lies at one end and the chalaza at the other. A pollen tube enters through the micropyle and releases its contents. Which cells does it first encounter, and which fusion gives the triploid nucleus?

  1. It first meets the antipodals, and syngamy gives the triploid nucleus
  2. It first meets the central cell, and syngamy gives the triploid nucleus
  3. It first meets the synergids, and triple fusion gives the triploid nucleus Correct answer.
  4. It first meets the egg cell, and triple fusion gives the diploid nucleus

Explanation

The pollen tube enters through the micropyle and discharges into one of the synergids, which lie adjacent to the egg cell. Triple fusion, the union of one male gamete with the two polar nuclei, produces the triploid primary endosperm nucleus, whereas syngamy gives the diploid zygote.

Question 23Reproduction (sexual reproduction in flowering plants, human reproduction, reproductive health)

Two plant species are studied: species P has small, dull-coloured flowers with exposed stigmas and abundant light pollen, while species Q has brightly coloured flowers with nectar and sticky pollen. Which conclusion is best supported?

  1. Species P is wind-pollinated and species Q is animal-pollinated Correct answer.
  2. Species P is animal-pollinated and species Q is wind-pollinated
  3. Both species are self-pollinated cleistogamous plants
  4. Both species depend exclusively on water for pollination

Explanation

Small dull flowers with exposed stigmas and abundant light pollen are adaptations for wind pollination, whereas bright flowers, nectar and sticky pollen attract and attach to animal pollinators. These suites of traits are classic indicators of the pollination syndrome.

Question 24Reproduction (sexual reproduction in flowering plants, human reproduction, reproductive health)

A man with a blockage in both vas deferens undergoes surgical correction. Which function is most directly restored by removing the blockage?

  1. Maturation of spermatozoa within the epididymis
  2. Production of testosterone by Leydig cells
  3. Passage of spermatozoa from the epididymis to the urethra Correct answer.
  4. Secretion of fructose by the seminal vesicles

Explanation

The vas deferens is the duct that carries spermatozoa from the epididymis to the ejaculatory duct and urethra, so relieving a bilateral block restores this transport. Hormone production, epididymal maturation and accessory gland secretion occur independently of the vas deferens.

Question 25Reproduction (sexual reproduction in flowering plants, human reproduction, reproductive health)

In the human menstrual cycle, the corpus luteum secretes progesterone during the luteal phase. If fertilisation does not occur, which change in hormonal profile is responsible for the onset of menstruation?

  1. A surge in LH that maintains the corpus luteum
  2. A decline in progesterone and oestrogen as the corpus luteum degenerates Correct answer.
  3. A rise in FSH that directly causes endometrial shedding
  4. A rise in progesterone and a fall in oestrogen

Explanation

When no embryo implants, the corpus luteum degenerates into the corpus albicans and progesterone and oestrogen levels fall. This withdrawal of hormonal support causes the endometrium to break down and shed as menstruation.

Question 26Reproduction (sexual reproduction in flowering plants, human reproduction, reproductive health)

In a typical angiosperm ovule, which region gives rise to the megaspore mother cell that undergoes meiosis?

  1. The micropyle, the small pore at the apex of the integuments
  2. The nucellus, the mass of parenchymatous tissue enclosed within the integuments Correct answer.
  3. The outer integument, the protective jacket surrounding the nucellus
  4. The chalaza, the basal region where the funicle joins the ovule

Explanation

The megaspore mother cell differentiates in the nucellus, the nutritive parenchymatous tissue inside the integuments, and then undergoes meiosis to form a linear tetrad of megaspores. The chalaza and micropyle are positional landmarks of the ovule, and the integuments form the seed coat rather than producing the spore mother cell.

Question 27Reproduction (sexual reproduction in flowering plants, human reproduction, reproductive health)

A pollen grain lands on a stigma and the pollen tube grows through the style. Which structure ultimately receives the two male gametes inside the embryo sac?

  1. The egg cell, which alone receives both male gametes
  2. The egg apparatus, where one gamete fuses with the egg and the other with the two polar nuclei Correct answer.
  3. The synergids, which absorb both male gametes
  4. The central cell, which receives both male gametes

Explanation

Double fertilisation is the rule in angiosperms: the pollen tube discharges its two male gametes into the embryo sac, one fusing with the egg (syngamy) and the other with the two polar nuclei (triple fusion). Both gametes are used, so no single cell receives them both.

Question 28Reproduction (sexual reproduction in flowering plants, human reproduction, reproductive health)

A plant breeder wants to produce a large number of genetically uniform plants that are true to type from a single selected superior hybrid. Which approach is most appropriate?

  1. Raising the hybrid through repeated self-pollination for several generations
  2. Crossing the hybrid again with one of its original parental lines
  3. Collecting open-pollinated seeds from the hybrid grown in an isolated field
  4. Vegetative propagation of the hybrid by cuttings or tissue culture Correct answer.

Explanation

A hybrid is heterozygous, so sexual reproduction (selfing or open pollination) produces segregating, non-uniform progeny. Asexual methods such as cuttings or micropropagation clone the parental genotype, giving many uniform plants true to type.

Question 29Reproduction (sexual reproduction in flowering plants, human reproduction, reproductive health)

A sample of human semen is analysed and found to contain a normal sperm count but a high proportion of spermatozoa with poor motility. Which region of the spermatozoon is most likely defective?

  1. The mitochondrial sheath of the middle piece Correct answer.
  2. The haploid nucleus in the head
  3. The distal centriole embedded in the neck
  4. The acrosome covering the anterior nucleus

Explanation

Motility depends on ATP produced by the numerous mitochondria packed in the middle piece, which power the axoneme of the tail. A normal count with poor motility therefore points to an energy-supply or flagellar defect rather than to nuclear or acrosomal problems.

Question 30Reproduction (sexual reproduction in flowering plants, human reproduction, reproductive health)

A woman has a regular 28-day cycle and ovulates on day 14. Which statement about the hormonal events of her cycle is correct?

  1. A rising level of progesterone secreted by the corpus luteum maintains the endometrium during the luteal phase Correct answer.
  2. Progesterone secreted by the developing follicle thickens the endometrium before ovulation
  3. A surge of FSH from the anterior pituitary directly triggers rupture of the mature Graafian follicle
  4. Oestrogen remains low throughout the follicular phase and rises only after ovulation

Explanation

After ovulation the ruptured follicle becomes the corpus luteum, which secretes progesterone; this hormone prepares and maintains the endometrium for possible implantation. If fertilisation does not occur, the corpus luteum regresses, progesterone falls and menstruation follows.

Question 31Reproduction (sexual reproduction in flowering plants, human reproduction, reproductive health)

A male is diagnosed with a condition in which the testes remain in the abdominal cavity and have not descended into the scrotum. Which consequence is most directly explained by this condition?

  1. Spermatogenesis is impaired because the abdominal temperature is higher than the scrotal temperature Correct answer.
  2. Testosterone secretion stops completely, so secondary sexual characters fail to develop
  3. The epididymis cannot mature spermatozoa because it remains in the abdomen
  4. Spermatozoa are produced normally but cannot be stored before ejaculation

Explanation

The scrotum keeps the testes about 2-2.5 degrees Celsius below core body temperature, which is required for normal spermatogenesis. In cryptorchidism the undescended testis is exposed to abdominal temperature, so sperm production is impaired and fertility may be reduced.

Question 32Reproduction (sexual reproduction in flowering plants, human reproduction, reproductive health)

A population study reports that a sexually transmitted infection has spread widely but produces no symptoms in most infected people, and that it can also be transmitted from mother to child during delivery. Which infection and feature are described?

  1. Chlamydiasis, caused by Chlamydia trachomatis, which is often asymptomatic and may cause pelvic inflammatory disease Correct answer.
  2. Syphilis, caused by Treponema pallidum, which cannot cross the placenta
  3. Trichomoniasis, caused by Trichomonas vaginalis, which is a fungal infection
  4. Gonorrhoea, caused by Neisseria gonorrhoeae, which always produces painful urination in males

Explanation

Chlamydia trachomatis infections are frequently asymptomatic in both sexes, so they spread unnoticed, and untreated infection can cause pelvic inflammatory disease and infertility; infection of the newborn during delivery is also recognised. This combination of silent carriage and reproductive complications fits the description.

Question 33Genetics and Evolution (principles of inheritance, molecular basis of inheritance, evolution)

In a monohybrid cross between a pure tall pea plant (TT) and a pure dwarf pea plant (tt), what proportion of the F2 generation is expected to be dwarf?

  1. One-fourth Correct answer.
  2. Three-fourths
  3. One-half
  4. Zero

Explanation

The F1 generation is entirely Tt and tall. Selfing F1 gives a 3:1 phenotypic ratio in F2, so the recessive dwarf phenotype appears in one-fourth of the offspring. The dwarf plants are the tt homozygotes.

Question 34Genetics and Evolution (principles of inheritance, molecular basis of inheritance, evolution)

Which of the following is the molecular explanation for the 9:3:3:1 dihybrid ratio observed by Mendel?

  1. Lethal combination of the double recessive genotype
  2. Independent assortment of two pairs of alleles during gamete formation Correct answer.
  3. Incomplete dominance between the two gene pairs
  4. Linkage of the two gene pairs on the same chromosome

Explanation

The 9:3:3:1 ratio appears when two gene pairs assort independently, producing four gamete types in equal frequency. Random fertilisation of these gametes then yields sixteen equally likely combinations. Any linkage or lethality would distort the ratio.

Question 35Genetics and Evolution (principles of inheritance, molecular basis of inheritance, evolution)

A woman with blood group AB marries a man with blood group O. Which blood groups are possible among their children?

  1. A, B, AB and O
  2. AB only
  3. A and B only Correct answer.
  4. AB and O only

Explanation

The AB mother produces gametes carrying either IA or IB, while the O father (ii) contributes only i. Children are therefore IAi (group A) or IBi (group B). Neither AB nor O can appear.

Question 36Genetics and Evolution (principles of inheritance, molecular basis of inheritance, evolution)

In a test cross, an organism showing the dominant phenotype is crossed with a homozygous recessive individual. What is the purpose of this cross?

  1. To establish pure lines with linked genes
  2. To increase the number of dominant offspring produced
  3. To convert the dominant allele into a recessive allele
  4. To determine whether the dominant-phenotype individual is homozygous or heterozygous Correct answer.

Explanation

A homozygous recessive partner contributes only recessive alleles, so the offspring phenotypes directly reveal the gametes of the unknown parent. A 1:1 ratio indicates heterozygosity, while all dominant offspring indicate homozygosity.

Question 37Genetics and Evolution (principles of inheritance, molecular basis of inheritance, evolution)

The ABO blood group system in humans illustrates which genetic phenomenon?

  1. Two genes showing epistatic interaction
  2. A single allele showing incomplete dominance
  3. Sex-linked inheritance of a single gene
  4. Multiple alleles with codominance between IA and IB Correct answer.

Explanation

Three alleles (IA, IB and i) exist in the population, and IA and IB are both expressed in heterozygotes, giving group AB. This combination of multiple allelism and codominance is the classic example.

Question 38Genetics and Evolution (principles of inheritance, molecular basis of inheritance, evolution)

A colour-blind man marries a woman with no family history of colour blindness who is homozygous normal. What is expected among their children?

  1. Half the sons are colour-blind and half the daughters are carriers
  2. All children are colour-blind
  3. All daughters are carriers and all sons have normal vision Correct answer.
  4. All daughters are colour-blind and all sons are normal

Explanation

Colour blindness is X-linked recessive. The father passes his affected X chromosome to every daughter, making them carriers, and his Y to every son. Since the mother contributes a normal X, no child expresses the trait.

Question 39Genetics and Evolution (principles of inheritance, molecular basis of inheritance, evolution)

Which observation from Morgan's work on Drosophila supported the chromosomal theory of inheritance?

  1. Pea seed shape segregated in a 3:1 ratio
  2. DNA was shown to contain four nitrogenous bases
  3. The white-eye gene showed criss-cross inheritance consistent with the X chromosome Correct answer.
  4. Chromosomes were shown to replicate semiconservatively

Explanation

Morgan found that white eye colour in Drosophila followed the pattern expected of a gene carried on the X chromosome, linking a Mendelian factor to a specific chromosome. This provided direct support for the chromosomal theory.

Question 40Genetics and Evolution (principles of inheritance, molecular basis of inheritance, evolution)

A cross between a plant with genotype AaBb and one with genotype aabb produces offspring in a ratio close to 1:1:1:1. What does this indicate?

  1. One gene shows incomplete dominance
  2. The two genes are completely linked
  3. The two gene pairs assort independently Correct answer.
  4. The genes are present on the X chromosome

Explanation

Equal proportions of the four phenotypic classes indicate that the heterozygous parent produced four gamete types in equal frequency. This is the expected outcome when the two loci assort independently. Close linkage would give an excess of parental combinations.

Question 41Genetics and Evolution (principles of inheritance, molecular basis of inheritance, evolution)

Which of the following best describes the structure of a nucleotide as it occurs in DNA?

  1. A nitrogenous base linked directly to a phosphate without sugar
  2. Two deoxyribose sugars joined by a phosphate and a base
  3. A deoxyribose sugar, a phosphate group and a nitrogenous base Correct answer.
  4. A ribose sugar, a phosphate group and an amino acid

Explanation

A DNA nucleotide consists of deoxyribose, a phosphate group and one of four nitrogenous bases. The sugar and phosphate form the backbone, while the base carries the coding information.

Question 42Genetics and Evolution (principles of inheritance, molecular basis of inheritance, evolution)

According to the Watson-Crick model, which statement about the DNA double helix is correct?

  1. Adenine pairs with guanine and thymine with cytosine
  2. The two strands are antiparallel and held together by hydrogen bonds between complementary bases Correct answer.
  3. The two strands are parallel and held together by covalent bonds between bases
  4. The sugar-phosphate backbones lie in the interior with bases projecting outward

Explanation

In the B-DNA model the two polynucleotide chains run in opposite directions and are stabilised by hydrogen bonds between A-T and G-C pairs. The bases stack in the interior while the backbones lie outside.

Question 43Genetics and Evolution (principles of inheritance, molecular basis of inheritance, evolution)

If a double-stranded DNA molecule contains 30% adenine, what percentage of its bases is guanine?

  1. 30%
  2. 70%
  3. 40%
  4. 20% Correct answer.

Explanation

By Chargaff's rule, adenine equals thymine, so A + T = 60%. The remaining 40% is shared equally by guanine and cytosine, giving 20% guanine.

Question 44Genetics and Evolution (principles of inheritance, molecular basis of inheritance, evolution)

In the Meselson-Stahl experiment, DNA grown in heavy nitrogen was transferred to light nitrogen medium. After one generation, what banding pattern was observed?

  1. A single band of intermediate density Correct answer.
  2. Only a heavy band
  3. Only a light band
  4. Two bands, one heavy and one light

Explanation

After one round of replication, each DNA molecule contained one heavy and one light strand, giving a hybrid of intermediate density. The appearance of this single intermediate band ruled out conservative replication.

Question 45Genetics and Evolution (principles of inheritance, molecular basis of inheritance, evolution)

During DNA replication, why is DNA polymerase able to synthesise the leading strand continuously but the lagging strand only in short fragments?

  1. The leading strand template contains no adenine residues
  2. The lagging strand is synthesised by RNA polymerase
  3. DNA polymerase can add nucleotides only in the 5' to 3' direction, while the template strands are antiparallel Correct answer.
  4. DNA polymerase requires a primer only on the lagging strand

Explanation

DNA polymerase extends a new strand only by adding nucleotides to a free 3' end, so synthesis proceeds 5' to 3'. Because the two template strands are antiparallel, one template is copied continuously while the other is copied in short Okazaki fragments.

Question 46Genetics and Evolution (principles of inheritance, molecular basis of inheritance, evolution)

Which enzyme relieves the supercoiling ahead of the replication fork during DNA replication?

  1. Exonuclease
  2. Primase
  3. DNA topoisomerase Correct answer.
  4. DNA ligase

Explanation

As the fork advances, the DNA ahead becomes overwound, and topoisomerase nicks and reseals the strands to relieve this torsional stress. Without it, replication would stall.

Question 47Genetics and Evolution (principles of inheritance, molecular basis of inheritance, evolution)

A mutation changes a codon from UAC to UAA in the coding region of an mRNA. What is the most likely consequence?

  1. No change in the polypeptide sequence
  2. A shift in the reading frame downstream
  3. Substitution of one amino acid by another
  4. Premature termination of translation Correct answer.

Explanation

UAA is a stop codon, so the ribosome will terminate translation at that point, producing a truncated polypeptide. This is a nonsense mutation.

Question 48Genetics and Evolution (principles of inheritance, molecular basis of inheritance, evolution)

Which feature of the genetic code explains why a point mutation does not always change the amino acid sequence of a protein?

  1. The code contains no stop codons
  2. The code is overlapping, so each base is read once
  3. The code is ambiguous, so one codon can specify different amino acids
  4. The code is degenerate, with several codons specifying the same amino acid Correct answer.

Explanation

Degeneracy means that most amino acids are encoded by more than one codon, so a change in the third base often leaves the amino acid unchanged. Such mutations are silent at the protein level.

Question 49Genetics and Evolution (principles of inheritance, molecular basis of inheritance, evolution)

In the lac operon of E. coli, what is the role of the repressor protein produced by the i gene?

  1. It cleaves lactose into glucose and galactose
  2. It codes for the permease enzyme
  3. It activates transcription when lactose is present
  4. It binds the operator and blocks transcription when lactose is absent Correct answer.

Explanation

The i gene product binds the operator region in the absence of lactose, preventing RNA polymerase from transcribing the structural genes. When lactose is present, the inducer binds the repressor and releases it from the operator, allowing transcription.

Question 50Genetics and Evolution (principles of inheritance, molecular basis of inheritance, evolution)

Which type of RNA carries amino acids to the ribosome during translation?

  1. rRNA
  2. snRNA
  3. mRNA
  4. tRNA Correct answer.

Explanation

Transfer RNA molecules recognise codons on mRNA through their anticodons and deliver the corresponding amino acids to the growing polypeptide chain. Messenger RNA carries the coding sequence, and ribosomal RNA forms part of the ribosome.