Heredity and Evolution

Reproduction connects survival with inheritance during natural selection. Organisms having advantageous variations may survive and reproduce more successfully in a particular environment. Their offspring can inherit the genetic basis of those traits, so the traits may become more common over generations. Option A is correct. Options B, C and D do not describe inheritance or differential reproduction and therefore cannot explain natural selection.

The governing idea of this chapter is the relationship between heredity and evolution. Heredity explains how characteristics are transmitted from parents to offspring, while evolution explains how inherited variations accumulate and populations change over long periods. Thus option A correctly includes both major ideas. Food formation belongs mainly to nutrition, the water cycle to environmental science, and metallic shine to materials chemistry.

Variation means the differences in characteristics observed among individuals of the same species, including offspring of the same parents. These differences may arise from genetic recombination, mutation or environmental influence, although inherited variation is especially important in evolution. Option A is correct. Respiratory actions, digestive juices and bones are biological processes or structures, not names for differences between offspring.

A gene is a functional segment of DNA that carries information related to a particular characteristic or biological function. Genes are located on chromosomes and are passed from parents to offspring, forming the basic units of heredity. Therefore option A is correct. Bone, muscle and blood are tissues or body components; they may contain cells with genes but are not the basic units that transmit hereditary information.

In typical eukaryotic cells, chromosomes are composed of DNA and associated proteins and are located mainly inside the nucleus, which is enclosed by a nuclear membrane. The nucleus protects and organizes the genetic material. Thus option A is correct. The cell wall provides support, the vacuole stores materials, and the cytoplasm contains organelles and supports many reactions, but these are not the normal main locations of chromosomes.

A normal human body cell is diploid, meaning it contains two sets of chromosomes, one set inherited from each parent. Each set has 23 chromosomes, so the total is 23 + 23 = 46 chromosomes, arranged in 23 pairs. Therefore option A is correct. Twenty-three is the usual number in a gamete, forty-four excludes the sex chromosomes, and ninety-two represents chromosome duplication before cell division rather than the normal total.

Human reproductive cells, or gametes, are haploid, so they contain one chromosome set rather than the two sets found in body cells. Meiosis reduces the chromosome number from 46 to 23. During fertilization, the sperm and egg each contribute 23 chromosomes, restoring the diploid number of 46 in the zygote. Therefore option A is correct; 46 describes a typical body cell, not a gamete.

The governing concept is Mendel’s principle of dominance. When contrasting pure traits are crossed, the trait expressed in the first filial generation masks the alternative trait. In the classic pea-plant example, allele T for tallness is dominant over allele t for dwarfness; the hybrid has genotype Tt and is tall. Therefore, tallness is the dominant trait, not an acquired, harmful, or recessive trait.

The governing concept is dominance in inheritance. A recessive allele is expressed only when a dominant allele is absent, usually in a homozygous condition. For example, in a Tt pea plant, the dominant T allele produces tallness and the recessive t allele is not visibly expressed. Thus the hidden trait is called a recessive trait. A dominant trait is expressed, while a fossil or new organism is unrelated.

The governing concept is the human chromosomal basis of sex. A typical human female has two X sex chromosomes, written as XX, while a typical male has one X and one Y, written as XY. The ovum therefore carries an X chromosome. An XY combination is not the usual female arrangement, YY is not a viable normal human combination, and every human normally possesses sex chromosomes. Hence option A is correct.

The governing concept is the XX–XY system of human sex determination. A typical human male has one X chromosome and one Y chromosome, represented as XY. The mother contributes an X-bearing ovum, while the father contributes either an X- or Y-bearing sperm. XX is the usual female combination; YY is not a normal viable human combination, and XXX is not the standard male pattern. Therefore A is correct.

The governing concept is that human females normally have XX chromosomes. During egg formation, each ovum receives one sex chromosome, and because the mother has only X chromosomes, every ovum carries X. The father’s sperm carries either X or Y, so the child receives X from the mother in every case and the paternal chromosome determines whether the combination is XX or XY. Therefore option A is correct.

The governing concept is the XX–XY mechanism of human sex determination. The mother’s ovum contributes X, whereas the father’s sperm may contribute X or Y. If a Y-bearing sperm fertilises the ovum, the zygote has XY chromosomes and is typically male; an X-bearing sperm produces XX, typically female. The mother does not normally supply Y, and sex chromosomes are always present. Thus A is correct.

The governing concept is the normal XX–XY pattern in humans. The mother contributes an X chromosome through the ovum. If the father contributes an X-bearing sperm, the fertilised egg receives X from each parent, giving the combination XX, which is typically female. An XY combination is typically male, YY is not a normal viable human combination, and a single Y alone cannot represent the usual diploid condition. Therefore A is correct.

The governing concept is evolution, the gradual change in inherited characteristics of populations over successive generations. Heritable variations can be passed to offspring, and natural selection or other evolutionary mechanisms can alter the frequency of traits over long periods. Digestion and respiration are life processes occurring within organisms, while evaporation is a physical change of matter. Therefore long-term changes in inherited traits are associated with evolution.

The governing concept is fossil evidence in evolution. Organisms that lived long ago, or marks left by them, can become preserved in layers of sedimentary rock through processes such as burial and mineralisation. These preserved remains or traces are called fossils. Therefore, option A is correct. Modern machines, fresh food and clouds are not biological remains and cannot serve as fossils.

The governing concept is that fossils provide historical evidence for biological evolution. By comparing fossils from different rock layers, scientists can identify organisms that lived in the past, observe changes in body form and study the appearance or disappearance of groups over time. Thus option A is correct. Taste, daily weather and blood pressure are unrelated to what fossil records reveal.

The governing concept is the difference between acquired and inherited variation. A characteristic that develops during an organism’s lifetime because of practice, use, training or environmental influence is called an acquired trait. It generally does not result from a change in the DNA of reproductive cells. Therefore option A is correct; inherited, dominant and recessive describe genetic patterns rather than this life-developed condition.

The governing concept is heredity: genetic information is transmitted from parents to offspring through reproductive cells. A trait controlled by such inherited information can usually appear in the next generation, so option A is correct. Skills learned during life, a surgically or accidentally cut tail, and muscles enlarged through exercise affect the body of the individual but normally do not alter the hereditary information passed to offspring.

The governing concept is that only heritable genetic changes in reproductive cells are normally transmitted to offspring. Cutting a mouse’s tail changes the body during its lifetime, but it does not alter the DNA of the sperm or egg cells that form the next generation. Therefore option A is correct. The cutting does not change genes in the required hereditary sense, a tail is not a chromosome, and offspring are living organisms rather than fossils.