Comprehension: genetics, ecosystems and gene expression

Read the following passage.

Silk is made from the cocoons spun by silkworms, the larvae (caterpillars) of a moth. Male silkworms produce about 20% more silk than females, so silk farmers would like to rear only males. However, male and female larvae look almost identical.

In moths, sex is determined by the Z and W chromosomes. Males have two Z chromosomes (ZZ) and females have one Z and one W chromosome (ZW). The W chromosome carries very few genes.

A gene on the Z chromosome affects the skin of the larvae. The dominant allele, T, gives opaque, white skin. The recessive allele, t, gives translucent skin, through which some of the internal organs can be seen.

Breeders cross males with translucent skin and females with opaque skin. Workers then sort the larvae by their skin and remove the unwanted larvae before they spin cocoons.

In a trial, 600 larvae from this cross were sorted by skin type, and their sex was checked later, when they had become pupae. Of the 304 larvae with opaque skin, 301 were male. Of the 296 larvae with translucent skin, 289 were female.

Use the information in the passage and your own knowledge to answer the following question.

Three of the larvae with opaque skin were female (paragraph 5).

Suggest two explanations for this result.

Read the following passage.

Eurasian beavers were hunted to extinction in Britain about 400 years ago. In recent years they have been reintroduced to several rivers. Beavers fell trees and build dams of branches and mud across streams. The dams hold back water, creating ponds and areas of wetland.

Beavers also feed on the bark and shoots of trees and shrubs near the water. Where beavers fell trees, more light reaches the ground, and dense woodland is replaced by a mosaic of ponds, marsh, open grassland and young trees. If beavers leave an area, their dams eventually collapse and woodland develops again.

Ecologists studied a 2 km stretch of a stream before beavers were released there and five years later. The number of ponds increased from 1 to 11, the number of species of aquatic invertebrate found increased from 28 to 47, and the number of clumps of frog spawn counted each spring increased from 15 to 420. The dams also slowed the flow of water after heavy rain.

Some farmers object to the reintroduction of beavers. Beaver dams can flood fields next to streams, and beavers sometimes fell trees in orchards and gardens.

Use the information in the passage and your own knowledge to answer the following question.

Use paragraphs 1 and 2 and your knowledge of succession to explain how beavers can increase the biodiversity of an area of woodland beside a stream.

Read the following passage.

Silk is made from the cocoons spun by silkworms, the larvae (caterpillars) of a moth. Male silkworms produce about 20% more silk than females, so silk farmers would like to rear only males. However, male and female larvae look almost identical.

In moths, sex is determined by the Z and W chromosomes. Males have two Z chromosomes (ZZ) and females have one Z and one W chromosome (ZW). The W chromosome carries very few genes.

A gene on the Z chromosome affects the skin of the larvae. The dominant allele, T, gives opaque, white skin. The recessive allele, t, gives translucent skin, through which some of the internal organs can be seen.

Breeders cross males with translucent skin and females with opaque skin. Workers then sort the larvae by their skin and remove the unwanted larvae before they spin cocoons.

In a trial, 600 larvae from this cross were sorted by skin type, and their sex was checked later, when they had become pupae. Of the 304 larvae with opaque skin, 301 were male. Of the 296 larvae with translucent skin, 289 were female.

Use the information in the passage and your own knowledge to answer the following question.

Explain why the larvae from this cross can be sorted by sex.

In your answer, give the genotypes of the parents, the genotypes of the gametes they produce, and the genotypes and phenotypes of the offspring. Write a Z chromosome carrying allele T as ZT, a Z chromosome carrying allele t as Zt, and the W chromosome as W.

Read the following passage.

The white-clawed crayfish is the only species of crayfish native to Britain. Its populations have fallen sharply since the American signal crayfish was introduced. Signal crayfish compete with native crayfish for food and shelter, and carry a disease that kills native crayfish.

Crayfish are hard to find because they hide under stones and are active mainly at night. Ecologists now often survey rivers using environmental DNA (eDNA). Animals shed cells and mucus into the water, so a water sample contains small amounts of DNA from the species living nearby.

The ecologists filter a water sample, extract the DNA and carry out the polymerase chain reaction (PCR) with primers that bind only to a short base sequence found in one species. If the sample contains DNA from that species, the target sequence is amplified and detected.

In one survey of 80 river sites, ecologists collected water samples and also searched for crayfish by hand and with baited traps. DNA from white-clawed crayfish was found at 31 sites. White-clawed crayfish were caught or seen at 22 sites, and all of these sites were among the 31 where their DNA was found.

Use the information in the passage and your own knowledge to answer the following question.

Explain how PCR amplifies the target sequence, and why the primers allow the test to detect DNA from only one species of crayfish (paragraph 3).