Heredity and Evolution

Human somatic, or body, cells are diploid and normally contain 46 chromosomes arranged in 23 pairs. Human reproductive cells, or gametes, are haploid and contain 23 chromosomes, one from each pair. This reduced number allows sperm and egg to fuse and restore 46 chromosomes in the zygote. Therefore option B is correct; options A, C and D contradict the normal human chromosome counts.

An inherited trait is determined substantially by genetic information received from parents through reproductive cells. Human blood group is an example because it is genetically controlled. Learning to swim results from practice, training and experience during life, so it is an acquired trait rather than a directly inherited one. Thus option A correctly pairs the examples; the other options reverse or misclassify them.

A gene is a segment of DNA that carries information for a particular characteristic or functional product. Genes are arranged at specific positions, called loci, on chromosomes. Chromosomes are long DNA-protein structures found mainly in the cell nucleus. Hence B correctly states the relationship. A reverses the relationship, while C and D are unrelated statements about biological structures.

The governing concept is that an acquired physical change is different from a heritable genetic change. Cutting the mouse’s tail affects its body after birth, but it does not normally change the genes in its sperm or eggs. The offspring therefore receive the usual genetic instructions for tail development and can grow tails. Option A is correct; B reverses the genetic effect, C is false because offspring have chromosomes, and D incorrectly treats development as purely environmental.

The governing concept is the inheritance of sex chromosomes in the XX–XY system. The egg from the mother contributes X, because human eggs normally carry X. A sperm from the father contributes either X or Y. An XY child must therefore receive X from the mother and Y from the father. Option A is correct; the mother does not normally provide Y, and neither parent can provide both chromosomes through one gamete.

For a gene, the two alternative forms are called alleles. When the alleles at a particular locus are identical, such as TT or tt, the individual is homozygous. Homozygous describes the genetic combination, not whether the trait is dominant or recessive: TT is homozygous dominant and tt is homozygous recessive. Therefore A is correct, whereas heterozygous means two different alleles.

The governing concept is recessive inheritance. Represent the tall allele by T and the dwarf allele by t. Since T masks t in a heterozygous Tt plant, a plant can show dwarfness only when it lacks a dominant T allele. Its genotype must therefore be tt, meaning that it has received a recessive copy from each parent. Option B is correct. A dominant copy would produce tallness, while the other choices deny the genetic basis of the trait.

The governing concept is the XX–XY pattern of human sex determination. A human egg normally contributes an X chromosome, whereas a sperm contributes either X or Y. When an X-bearing egg fuses with a Y-bearing sperm, the zygote has XY chromosomes, the usual male chromosomal combination. Therefore B is correct. The mother does not normally supply Y, so C and D are impossible, while A gives XX.

The governing concept is chromosomal sex determination in humans. The mother’s egg contributes an X chromosome, and the father’s sperm contributes either X or Y. If an X-bearing sperm fertilises the X-bearing egg, the resulting zygote has the XX combination, which is the usual chromosomal pattern associated with female development. Thus A is correct. XY is the usual male combination, and YY or a single Y cannot produce the required zygote.

The governing concept is the inheritance of sex chromosomes and the equal genetic contribution of the parents. A human egg normally carries X, while sperm cells are of two types: X-bearing or Y-bearing. The sperm that fertilises the egg therefore determines whether the zygote is XX or XY. Option A is correct. The mother does not provide Y, the father contributes a chromosome, and food does not determine chromosomal sex.

Heredity is the transmission of genetic information from parents to offspring, so it produces resemblance and continuity between generations. Variation refers to differences among offspring or individuals; it supplies the raw material on which natural selection can act. The two ideas work together: heredity preserves successful patterns, while variation introduces differences that may improve adaptation. Therefore option A gives the correct balance, unlike the absolute claims in the other options.

Use T for the dominant tall allele and t for the recessive dwarf allele. A pure tall plant has genotype TT, while a dwarf plant must be tt. The cross TT × tt gives only Tt offspring because the first parent can provide only T and the second can provide only t. Every F1 plant is therefore heterozygous and phenotypically tall. A and C incorrectly predict dwarf offspring, while D confuses heterozygous Tt with homozygous TT.

A test cross is used to determine the genotype of an organism showing a dominant phenotype. The organism with the unknown genotype is crossed with a homozygous recessive organism. If all offspring show the dominant phenotype, the unknown parent is likely homozygous dominant; if recessive offspring appear, it is heterozygous. Therefore option B is correct. A involves two traits, C is reproduction within the same plant, and D is selection rather than a genotype-testing cross.

For a trait controlled by a dominant and recessive allele, the dominant allele masks the recessive allele in a heterozygote. The recessive phenotype appears only when the individual receives a recessive allele from each parent, giving a homozygous recessive genotype such as aa. Therefore option B is correct. One recessive allele with a dominant allele produces the dominant phenotype, and the other alternatives do not describe a valid allele condition for expression.

The governing concept is independent probability in sex determination. In the simplified human XX–XY model, each pregnancy involves a new fertilisation event: an X-bearing or Y-bearing sperm may fertilise the X-bearing egg. Therefore, earlier daughters do not alter the next probability, which is approximately 1/2 for a boy. Option B is correct; options A, C and D incorrectly treat births as predetermined or dependent.

An acquired trait develops during an individual’s lifetime because of activity, environment or experience and is not ordinarily encoded for transmission in reproductive cells. Exercise can enlarge muscles through training, so option B is the acquired example. Blood group, natural eye colour and pea seed colour are primarily determined by inherited genetic information. They are therefore distractors representing inherited characteristics rather than acquired changes.

For a diploid organism, the two factors controlling a trait are commonly represented as alleles at the same gene locus. When these two alleles are different, such as Tt, the organism is heterozygous for that trait. When they are identical, such as TT or tt, it is homozygous. Therefore, option B is the correct term.

A pedigree chart is a diagram that records the occurrence of a particular trait or disorder among members of a family over several generations. By comparing affected and unaffected individuals, scientists can infer patterns such as dominant, recessive, or sex-linked inheritance. It is not designed to measure fossil age, digestion, or rainfall. Thus, option A is correct.