301. Which statement about strength of linkage is correct?
ⓐ. Genes that are farther apart on the same chromosome generally show stronger linkage.
ⓑ. Genes on different chromosomes always show strong linkage.
ⓒ. Genes that are closer together on the same chromosome generally show stronger linkage.
ⓓ. Strength of linkage is unrelated to gene position on a chromosome.
Correct Answer: Genes that are closer together on the same chromosome generally show stronger linkage.
Explanation: Genes located close to each other on the same chromosome are less likely to be separated by crossing over. Because of this, they tend to pass together more often and show stronger linkage. Genes farther apart are more easily separated, so recombinants become more frequent. Linkage strength therefore depends on distance between genes.
302. Which statement best describes recombination?
ⓐ. Formation of only parental combinations in the offspring
ⓑ. Appearance of non-parental combinations due to exchange between homologous chromosomes
ⓒ. Permanent fusion of linked genes into one new gene
ⓓ. Suppression of all recessive alleles during gamete formation
Correct Answer: Appearance of non-parental combinations due to exchange between homologous chromosomes
Explanation: Recombination refers to the formation of new allele combinations that were not present together in the parents. It commonly arises because homologous chromosomes exchange corresponding segments during crossing over. This reshuffles linked genes and produces recombinant offspring. Recombination therefore contributes to genetic variation.
303. Which event is the immediate source of recombination between linked genes?
ⓐ. Independent assortment of homologous chromosomes only
ⓑ. Mutation in both alleles of the same gene
ⓒ. Fusion of male and female gametes during fertilisation
ⓓ. Crossing over between homologous chromosomes
Correct Answer: Crossing over between homologous chromosomes
Explanation: Linked genes lie on the same chromosome, so they tend to travel together unless a physical exchange occurs between homologous chromosomes. Crossing over provides that exchange. When a crossover happens between the positions of two linked genes, recombinant combinations can result. This makes crossing over the direct source of recombination for linked genes.
304. If two genes are very tightly linked, which result is most likely?
ⓐ. Recombinant types will greatly exceed parental types.
ⓑ. Recombinant and parental types will appear equally often.
ⓒ. Parental types will greatly exceed recombinant types.
ⓓ. Only recombinant types will be formed.
Correct Answer: Parental types will greatly exceed recombinant types.
Explanation: Tight linkage means the genes are very close together on the same chromosome. Such genes are less often separated by crossing over, so they are transmitted together most of the time. As a result, parental combinations are much more frequent than recombinant ones. This unequal distribution is a strong clue for tight linkage.
305. Which statement correctly connects linkage and recombination?
ⓐ. Linked genes can show recombination, but usually less often than independently assorting genes.
ⓑ. Linked genes and recombination are mutually exclusive ideas.
ⓒ. Recombination occurs only between genes on different chromosomes.
ⓓ. Linkage always produces a perfect $1:1:1:1$ test-cross ratio.
Correct Answer: Linked genes can show recombination, but usually less often than independently assorting genes.
Explanation: Linkage does not mean that recombinant types are impossible. It means that genes on the same chromosome tend to stay together more often than genes that assort independently. Crossing over can still separate them and create recombinants. Therefore recombinant classes exist, but they are usually fewer than parental classes for linked genes.
306. What does a higher recombination frequency between two genes usually indicate?
ⓐ. The genes are identical alleles of the same locus.
ⓑ. The genes are farther apart on the chromosome.
ⓒ. The genes are more strongly linked and very close together.
ⓓ. The genes are present only in recessive form.
Correct Answer: The genes are farther apart on the chromosome.
Explanation: Recombination frequency is related to how often crossing over occurs between two gene loci. The farther apart the genes are, the greater the chance that a crossover will occur between them. This produces more recombinant offspring. A higher recombination frequency therefore usually means a greater distance between the genes.
307. Alfred Sturtevant is best known for showing that
ⓐ. genes are made only of protein
ⓑ. sex is determined entirely by environment
ⓒ. recombination has no relation to chromosome behavior
ⓓ. gene positions on chromosomes can be estimated using recombination frequency
Correct Answer: gene positions on chromosomes can be estimated using recombination frequency
Explanation: Sturtevant realized that the frequency of recombination between genes could be used as a measure of their relative distance on a chromosome. This idea made it possible to construct genetic maps. Genes with lower recombination frequency are placed closer together, and genes with higher recombination frequency are placed farther apart. His work was a major advance in chromosome mapping.
308. Which statement best describes a genetic map?
ⓐ. It is a map of all body organs affected by a gene.
ⓑ. It shows the exact chemical structure of DNA bases in a chromosome.
ⓒ. It shows the relative positions of genes on a chromosome based on recombination data.
ⓓ. It lists only dominant genes present in a species.
Correct Answer: It shows the relative positions of genes on a chromosome based on recombination data.
Explanation: A genetic map is built from recombination frequencies observed between genes. Because recombination frequency reflects relative distance, genes can be arranged in order along a chromosome. The map shows relative positions rather than a direct microscopic picture of DNA sequence. It is therefore a functional map based on inheritance patterns.
309. Which statement best defines recombination frequency?
ⓐ. It is the percentage of recombinant offspring produced between two genes.
ⓑ. It is the percentage of dominant offspring in a monohybrid cross.
ⓒ. It is the number of chromosomes present in a gamete.
ⓓ. It is the proportion of homozygous individuals in the F2 generation.
Correct Answer: It is the percentage of recombinant offspring produced between two genes.
Explanation: Recombination frequency measures how often new, non-parental combinations appear among the offspring. It is especially useful when studying linked genes on the same chromosome. Because recombinant types arise mainly through crossing over, their frequency reflects how easily the genes are separated. This makes recombination frequency an important genetic tool.
310. If two genes show a low recombination frequency, what is the most appropriate conclusion?
ⓐ. The genes must be on different chromosomes.
ⓑ. The genes are likely to be close together on the same chromosome.
ⓒ. The genes are always inherited independently.
ⓓ. The genes cannot produce parental combinations.
Correct Answer: The genes are likely to be close together on the same chromosome.
Explanation: A low recombination frequency means recombinant types appear rarely. This usually happens when the two genes lie close to each other, so crossing over between them is less likely. Such genes therefore show stronger linkage. Low recombination is thus a clue to close proximity on a chromosome.
311. Which statement correctly explains crossing over?
ⓐ. It is the fusion of male and female gametes during fertilisation.
ⓑ. It is the separation of sister chromatids in mitosis.
ⓒ. It is the exchange of corresponding segments between homologous chromosomes.
ⓓ. It is the permanent joining of two different genes into one.
Correct Answer: It is the exchange of corresponding segments between homologous chromosomes.
Explanation: Crossing over occurs between homologous chromosomes during meiosis. In this process, corresponding chromosomal segments are exchanged. That exchange can separate linked genes and generate recombinant combinations in offspring. Crossing over is therefore a major source of genetic variation.
312. Which statement about parental and recombinant types is correct in the case of linked genes?
ⓐ. Recombinant types always outnumber parental types.
ⓑ. Parental and recombinant types always occur in exactly equal numbers.
ⓒ. Only recombinant types appear because linkage increases variation.
ⓓ. Parental types usually exceed recombinant types.
Correct Answer: Parental types usually exceed recombinant types.
Explanation: Linked genes tend to travel together because they are located on the same chromosome. As a result, offspring often show parental combinations more frequently than recombinant ones. Recombinants still appear if crossing over occurs, but they are usually fewer. This unequal frequency is one of the clearest signs of linkage.
313. Which statement best explains why recombination contributes to variation?
ⓐ. It creates new combinations of alleles that were not present together in the parents.
ⓑ. It ensures that only dominant alleles are transmitted to offspring.
ⓒ. It prevents genes from being inherited together on the same chromosome.
ⓓ. It converts every recessive allele into a dominant allele.
Correct Answer: It creates new combinations of alleles that were not present together in the parents.
Explanation: Recombination reshuffles alleles into new combinations during gamete formation. These new combinations can produce offspring with trait patterns not seen in either parent. That is why recombination increases genetic diversity in a population. It does not remove genes or change recessive alleles into dominant ones.
314. Which result would indicate stronger linkage between two genes?
ⓐ. Recombinant frequency close to 50%
ⓑ. Very few recombinant offspring compared with parental offspring
ⓒ. A perfect $1:1:1:1$ ratio in a dihybrid test cross
ⓓ. Equal occurrence of all phenotype classes in every generation
Correct Answer: Very few recombinant offspring compared with parental offspring
Explanation: Strong linkage means the genes stay together through inheritance most of the time. That produces many parental combinations and relatively few recombinant ones. If recombinants become very rare, the genes are inferred to be closely placed on the chromosome. Equal classes would instead suggest independent assortment.
315. Which statement best describes the basis of gene mapping by Sturtevant?
ⓐ. Gene positions are determined from visible chromosome colour.
ⓑ. Gene order is inferred only from dominance and recessiveness.
ⓒ. Relative gene distance can be estimated from recombination frequency.
ⓓ. Every gene is mapped by counting the number of gametes formed.
Correct Answer: Relative gene distance can be estimated from recombination frequency.
Explanation: Sturtevant recognized that genes farther apart on a chromosome show higher recombination frequencies than genes closer together. By comparing these frequencies, he could estimate relative distances and arrange genes in order. This made genetic mapping possible without directly seeing the genes themselves. The method depends on inheritance data, not chromosome colour or simple dominance alone.
316. If the recombination frequency between two genes is 10%, the approximate map distance between them is
ⓐ. 1 map unit
ⓑ. 5 map units
ⓒ. 50 map units
ⓓ. 10 map units
Correct Answer: 10 map units
Explanation: In classical gene mapping, 1% recombination is taken as 1 map unit. Therefore a recombination frequency of 10% corresponds to 10 map units between the genes. This indicates relative genetic distance rather than exact physical length. The idea is useful for constructing genetic maps from breeding data.
317. Two pairs of genes are studied in separate experiments. Pair 1 shows 2% recombination, and Pair 2 shows 18% recombination. Which conclusion is most appropriate?
ⓐ. Pair 1 genes are likely to be closer together than Pair 2 genes.
ⓑ. Pair 1 genes are on different chromosomes, while Pair 2 genes must be identical.
ⓒ. Pair 2 genes show stronger linkage than Pair 1 genes.
ⓓ. Both pairs must assort independently because recombination is present.
Correct Answer: Pair 1 genes are likely to be closer together than Pair 2 genes.
Explanation: Lower recombination frequency usually means less chance of crossing over between the genes. That generally indicates that the genes are closer together on the chromosome. By contrast, a higher recombination frequency suggests greater separation. So Pair 1 is inferred to have the shorter genetic distance.
318. Which statement about genetic maps is incorrect?
ⓐ. They show relative positions of genes on chromosomes.
ⓑ. They are based on recombination data.
ⓒ. They help compare distances between linked genes.
ⓓ. They give the exact nucleotide sequence of a chromosome.
Correct Answer: They give the exact nucleotide sequence of a chromosome.
Explanation: A genetic map is not a DNA sequence map. It shows the relative arrangement of genes using recombination frequencies, which reflect how often genes are separated by crossing over. Such maps are useful for comparing gene positions and distances. Exact nucleotide sequence requires different methods and belongs to molecular analysis rather than classical genetic mapping.
319. Which statement best distinguishes parental types from recombinant types in a linkage experiment?
ⓐ. Parental types show the same trait combinations as the original parents, whereas recombinant types show new combinations.
ⓑ. Parental types are always recessive, whereas recombinant types are always dominant.
ⓒ. Parental types appear only in the F1 generation, whereas recombinant types appear only in the F2 generation.
ⓓ. Parental types are formed by mutation, whereas recombinant types are formed without meiosis.
Correct Answer: Parental types show the same trait combinations as the original parents, whereas recombinant types show new combinations.
Explanation: Parental types retain the original combination of alleles present in the parents used in the cross. Recombinant types carry allele combinations that were not present together in the parental generation. These new combinations usually arise because of crossing over between homologous chromosomes. Distinguishing parental and recombinant classes is essential in the study of linkage.
320. In a linkage study, non-parental combinations among offspring are mainly called
ⓐ. dominant types
ⓑ. filial types
ⓒ. homozygous types
ⓓ. recombinants
Correct Answer: recombinants
Explanation: Recombinants are offspring with trait combinations that differ from those seen in the parents. They arise when linked genes are separated by crossing over during meiosis. Their frequency helps geneticists judge how far apart two genes are on a chromosome. This makes recombinants very useful in gene mapping.
