Heredity and Evolution

Heredity is the biological process through which characteristics are transmitted from parents to their offspring. Genes carry the information for many such traits, so children may resemble their parents while still showing differences. Therefore option A states the correct meaning. Digestion, breathing, and blood flow are life processes, but they do not describe the transmission of inherited traits between generations.

The governing concept is variation. Although offspring inherit genetic information from their parents and often resemble them, they are not exact copies; differences may arise from genetic recombination, mutation, or environmental influence. These differences in traits are called variation, so option A is correct. Digestion, respiration, and excretion are physiological processes and do not name differences among inherited characteristics.

A gene is a functional segment of DNA that carries hereditary information associated with a characteristic or biological function. By providing instructions for particular proteins or functional RNA, it can influence traits, and this information may pass from parents to offspring. Therefore option A is correct. Breaking food, pumping blood, and cooling the body are functions of organs or physiological systems, not genes themselves.

In human cells, hereditary information is encoded in DNA, and most nuclear DNA is organised into chromosomes. Chromosomes contain genes that carry instructions influencing inherited traits. Thus option A is the best answer at this school level. Lungs and the stomach perform respiration and digestion-related functions, respectively, while the outside of the skin is not a location for the body’s hereditary material.

A normal human body cell is diploid, meaning its chromosomes occur in 23 homologous pairs. The calculation is 23 pairs × 2 chromosomes per pair = 46 chromosomes. Therefore option A is correct. The number 23 applies to a typical human gamete, which is haploid, while 12 and 2 do not represent the complete chromosome number of a normal human body cell.

Human gametes, such as sperm and ova, are haploid cells. During meiosis, the chromosome number is reduced from the diploid body-cell number of 46 to half: 46 ÷ 2 = 23. When two gametes fuse, their 23 chromosomes combine to restore 46 in the zygote. Therefore option B is correct; 46 describes body cells, while 44 and 200 are incorrect totals.

The governing concept is that a single inherited character may occur in two opposite forms. In Mendel’s pea plants, stem height was represented by tallness and dwarfness. These forms differ from each other for the same character, so they are called contrasting traits. “Same traits” is incorrect because the forms are not identical, while food and disease traits do not describe stem height.

Mendel’s principle of dominance explains that, when two contrasting forms are brought together, one may be expressed in the first filial generation (F1). The expressed form is called the dominant trait. A recessive trait is masked in F1, while “invisible” and “mixed” are not the scientific terms used for this inheritance pattern.

The governing concept is recessive inheritance. In a typical Mendelian cross, the recessive form is not expressed in the F1 generation when a dominant allele is present, although its allele is still carried by the offspring. It may reappear in a later generation when two recessive alleles come together. Therefore, option B is correct; the other choices are not names of hidden inherited traits.

Let pure tallness be TT and pure dwarfness be tt. Each F1 offspring receives T from the tall parent and t from the dwarf parent, so every offspring has genotype Tt. Because T is dominant over t in this Mendelian example, all F1 plants show the tall phenotype. Thus option A is correct; dwarf plants would be expected after a suitable later-generation cross, not in this F1 cross.

In the usual monohybrid experiment, F1 plants are heterozygous Tt. Self-pollination gives Tt × Tt, producing TT, Tt, Tt and tt in a 1:2:1 genotypic ratio. Since TT and Tt are tall and only tt is dwarf, the observable phenotypic ratio is three tall plants to one dwarf plant, or 3:1. Therefore option A is correct.

Sex determination is the biological process by which the sex of offspring is established. In humans, it is associated with the sex-chromosome combination formed at fertilisation: XX is typically female and XY is typically male. Therefore option A gives the correct meaning. Food choice, breathing rate and eye-colour change are unrelated biological processes, not definitions of sex determination.

In the standard human chromosome pattern, females possess two X sex chromosomes, written as XX. During reproduction, a female’s egg normally carries one X chromosome. An XY combination is typically associated with a male, YY cannot normally form a viable human individual because an X chromosome is required, and XO represents a different chromosomal condition rather than the usual female complement. Hence option A is correct.

A typical human male has one X chromosome and one Y chromosome, represented as XY. The X chromosome is present in both sexes, whereas the Y chromosome is usually associated with male development. XX is the usual female combination. YY is not a normal viable human chromosome complement, and OO is not a recognised human sex-chromosome combination. Therefore option B is correct.

Evolution means the gradual change in populations of organisms over many generations. Heritable variations may be passed to offspring, and their frequencies can change over time, sometimes producing forms that differ from earlier populations. Thus, option A gives the correct simple meaning. Cooking, evaporation, and tooth cleaning are unrelated everyday processes, so options B, C, and D are not meanings of evolution.

Variation means differences among individuals of the same population. If a variation is heritable and gives an advantage in a particular environment, its carriers may survive and reproduce more successfully. Natural selection can then make that useful variation more common over generations. Therefore, option A is correct. Variation does not make organisms identical, destroy food, or stop respiration, so B, C, and D are incorrect.

Natural selection is the process in which environmental conditions favour individuals with advantageous, heritable traits. These individuals are more likely to survive and reproduce, so the useful traits may become more common in later generations. Hence option A is the best simple meaning. Cutting plants is a human activity, identical organisms would show no variation, and curing a disease is a medical event; none defines natural selection.

Inherited information is carried by genes located on chromosomes in reproductive cells, or gametes. During sexual reproduction, the sperm and egg combine and pass genetic information from both parents to the zygote and later offspring. Therefore option A is correct. Ordinary skin, hair, and bone cells are somatic cells; although they contain genetic material, they normally do not transmit parental traits directly to the next generation.

A learned skill, such as playing an instrument, is an acquired characteristic developed through practice and experience during an individual's lifetime. It normally does not alter the DNA or hereditary information in that person's gametes. Since offspring receive hereditary information through gametes, the skill is not directly inherited. Thus option A is correct; the other choices are biologically unrelated or false.

Cutting a mouse's tail is a physical change acquired during its lifetime. It affects the body cells and does not normally alter the genes in the sperm or eggs that form the next generation. Consequently, the offspring inherit the genes for normal tail development and are not born tailless merely because parental tails were cut. Therefore option A is correct; B, C, and D do not explain heredity.

An acquired trait develops after birth as a result of an individual's activities, environment, or experiences. Muscles becoming stronger through regular exercise are an example because training changes the body during life and is not normally encoded as a new inherited trait in gametes. Thus option A is correct. Natural eye colour, blood group, and inborn hair colour are primarily determined by inherited genetic information, so B, C, and D are not suitable examples.

An inherited trait is influenced by genetic information received from parents and can be transmitted through reproductive cells. Blood group is determined by alleles inherited from the parents, so option A is correct. Learning to ride a bicycle and learning a language are acquired abilities shaped by practice and environment. Cutting hair is a temporary physical change, not a genetic characteristic; therefore B, C, and D are incorrect.

Fossils are preserved remains, impressions, or other traces of organisms that lived in the past. By studying their structure, age, and position in rock layers, scientists learn which organisms existed earlier and how living forms have changed over geological time. Therefore option A is correct and fossils are important evidence for evolution. Fresh food, modern machines, and weather alone are not what fossils directly document.

The governing concept is fossil preservation as evidence of evolution. When organisms die, their remains may be buried by sediments and, over very long periods, become preserved or leave impressions in sedimentary rock layers. Therefore, option A is correct. Fresh milk, air, and human blood are not normal geological places where fossils are preserved; those choices confuse biological materials with rock deposits.

Homologous organs are structures that share a similar basic plan because of common ancestry, although they may perform different functions in different organisms. For example, the forelimbs of humans, whales, and bats have related skeletal patterns but are used differently. Thus option A is correct. Colour, occurrence only in plants, and digestion do not define homology.