Variation

Cells use genetic information when carrying out protein synthesis.

Give the difference between a gene and a genome.

Describe the process of transcription.

Explain why a change in the sequence of bases in the DNA does not always result in a change in enzyme function.

Some characteristics are determined only by the genes, and some are determined by a combination of the genes and the environment.

Give an example of each type of characteristic in humans.

determined only by the genes

determined by the genes and the environment

Hair colour in cattle is controlled by one gene with two alleles. The allele for white hair, C^W, is codominant with the allele for red hair, C^R.

Heterozygous cattle are a pale red colour called roan.

State what is meant by the term gene.

Complete the table by giving the genotypes of red cattle, white cattle, and roan cattle.

Two roan‑coloured cattle are mated.

Determine the probability that the calf produced is a male calf with roan‑coloured hair.

Include a genetic diagram in your answer.

In a group of cattle, there are cattle with white-, red-, and roan-coloured hair. Height of cattle is also a genetically controlled characteristic. In the same group, there is a greater variation of heights compared with hair colour.

Explain why there is a greater variation in height of the cattle than there is in hair colour.

Describe how a farmer could use selective breeding to increase the height of cattle.

The photograph shows a variety of chicken called a silkie chicken.

(Source: © YVES LANCEAU/NATURE PICTURE LIBRARY/SCIENCE PHOTO LIBRARY)

Silkie chickens have feathers that have a fluffy appearance.

Feather structure is controlled by a single gene.

The allele for producing silkie feathers (f) is recessive to the allele for producing normal feathers (F).

State what is meant by the term gene.

Give the possible genotypes of a chicken with normal feathers.

How many chickens in the family pedigree are heterozygous?

Use a genetic diagram to determine the probability of one of the offspring of individual 6 and individual 7 being a chicken with silkie feathers.

The scientist observes that the chickens have either normal feathers or silkie feathers.

However, the chickens have a wide range of different heights.

Explain why there is a wider range of variation in height than in feather type.

Fur colour in rats is controlled by a gene with two alleles.

One allele codes for black fur colour. The other allele codes for agouti fur colour.

Several female rats with agouti coloured fur are mated with several male rats with black coloured fur.

All of the offspring have agouti coloured fur.

Explain which allele is dominant.

A male and female rat from these offspring are then mated together in a second cross.

Some of the offspring of this second cross have agouti coloured fur and some have black coloured fur.

Draw a genetic diagram to show this second cross. Include the genotypes of the parents, the gametes they produce, and the genotypes and the phenotypes of the offspring.

Calculate the probability of any one offspring from this second cross being male with agouti coloured fur.

Scientists observed that genes that control fur colour in rats can affect rat behaviour, such as how tame they are.

Other genes that control the size of the adrenal glands and the production of neurotransmitters also affect rat behaviour.

Scientists also noticed that coat colour is associated with differences in anatomy and physiology, such as the size of the adrenal glands and the production of neurotransmitters.

State the name of the type of genetic control where many genes control one phenotype.

Explain why the size of the adrenal glands and the production of neurotransmitters would affect rat behaviour.

Some rats with white fur also have pink eyes.

These rats have pink eyes because they do not have pigment in their irises.

This means that their irises let light pass through, unlike the coloured irises found in other rats.

Explain how this difference in the iris affects vision in the rats with pink eyes.

Read the passage below. Use the information in the passage and your own knowledge to answer the questions that follow. Moving plants Flowering plants reproduce using different methods to transfer the pollen from plant to plant. This cross‑pollination enables plants to produce offspring that show genetic variation. The transfer can be done by animals such as insects or by wind. The reliance on animals and wind for reproduction does not end with pollination. Flowering plants also need a mechanism to disperse their fruits or the seeds they contain. This seed dispersal means that seeds producing new young plants will germinate away from the parent plant.

wings on a sycamore fluff on a dandelion spines on burdock (Source: © joanna wnuk/Shutterstock) (Source: © Viktorija Reuta/Shutterstock) (Source: © Paulpixs/Shutterstock) The simplest method of seed dispersal uses gravity. The seeds are within heavy fruit, which fall from the tree when they are ripe. Although some of the fruit may roll away from the parent plant, most remain close to the parent plant. Animal dispersal is when plants rely on animals to transport their seeds to a different area. This may be because the seeds are surrounded by a brightly‑coloured and sweet‑tasting fruit. Examples of this are soft fruits such as raspberry and hard fruits such as apple. Animals may also carry seeds in a different way. Many plants produce fruits or individual seeds covered in hooks or spines that attach the seeds to the animals’ fur. The seeds are then carried away from the parent plant. Eventually, the seeds may fall off, or be rubbed off by the animal. Examples of plants using this form of dispersal are burdock and sea holly. Some plants provide seed pods with a mechanism that ejects the seeds from the pod by force. All of these rely on the effect of evaporation of water in the seed pod, so this method of seed dispersal usually takes place in sunlight. Examples of plants using this form of dispersal are gorse bushes and lupins.

Wind is one of the main methods of seed dispersal. Some tall trees produce seed pods that have wings, which allow the seeds to travel long distances. Some seed pods have two wings such as the sycamore while others have one wing such as the ash. There are also lightweight adaptations that help seeds to be blown by the wind. These include various sorts of fluff that increase the surface area of the seed, so that it can be picked up by the slightest breeze. Examples of plants using this form of dispersal are thistle and dandelion. The last method uses water. Trees found on tropical beaches often have their seeds carried away by the sea. The seeds have woody, waterproof coverings enabling them to float in the water for long periods. Coconuts are a well‑known example.

Explain how cross‑pollination can lead to an increase in genetic variation. (Lines 2–3)

Explain the advantages of the seeds germinating away from the parent plant. (Line 8)

Give one advantage of a seed germinating close to the parent plant. (Line 11)

Explain the conditions needed for seed germination.

Explain why some seeds are surrounded by a brightly‑coloured and sweet‑tasting fruit. (Lines 13–14)

Large numbers of tomato plants are often found growing along the sides of drains and settling beds on sewage farms. Suggest a reason for this observation.

Give the reason why sunlight is required for lupin seeds to be dispersed. (Lines 21–24)

Describe an experiment you could carry out to investigate how the presence of fluff on dandelion seeds affects how fast they fall. (Lines 30–31)

Cell division can be by meiosis or by mitosis.

Where are cells dividing by meiosis found in a human?

Which part of a flowering plant is usually used to demonstrate cells dividing by mitosis?

The table lists features comparing the processes of meiosis and mitosis in human cells.

Complete the table by giving the missing information.

Cell division can cause variation in offspring.

Describe other causes of variation in offspring.

Scientists investigating a drug treatment use rats that are homozygous for many genes.

Suggest the advantages of using rats that are homozygous for many genes.

Cleft chin is a phenotype believed to be controlled by a single gene that has two alleles. The dominant allele, N, codes for cleft chin and the recessive, n, allele codes for the absence of the cleft chin.

With reference to the example of cleft chin, state what is meant by the following terms

phenotype

gene

allele.

A woman with a cleft chin has a child with a man who also has a cleft chin. The child does not have a cleft chin.

Use a genetic diagram to show the genotypes of the parents, the gametes they can produce and all the possible genotypes and phenotypes of their children.

The parents then have a second child. Calculate the probability that this child will be female and not have a cleft chin.

Sometimes a cleft chin does not develop even if the individual inherits the dominant allele. Suggest what might cause this.

Most inherited conditions are not controlled by a single gene, but by many genes. Describe how a scientist could distinguish between a genetic condition in rats controlled by a single gene and one controlled by many genes.

Read the passage below.

Use the information in the passage and your own knowledge to answer the questions that follow.

Toystory

Toystory is a bull who was born in 2001. The photograph shows Toystory.

In the world of dairy farming Toystory is a famous bull. He fathered 500 000 offspring but did not mate with any cows. He was able to father so many offspring because his semen was collected and then used to fertilise cows using 5 artificial insemination.

Toystory’s mother was a high milk producer and his father was a popular bull. He was sold for $4 000 by his owner to a specialist breeding company called Genex.

Genex started to collect semen from Toystory when he was four years old. The semen is carefully collected, using a teaser animal and an artificial rubber vagina.

The semen is divided into many separate samples. These samples are put in small straws and frozen in liquid nitrogen. The straws can then be sold and sent to dairy farmers around the world to inseminate their cows. A total of 2.4 million samples of semen from Toystory was sold in more than 50 countries around the world.

Toystory’s reputation grew as the offspring he fathered went on to be high milk producers. His semen straws sold for over $60 each. Bull semen can now be sold as sexed or unsexed samples, with sexed samples being more expensive to 20 purchase.

Toystory was highly valued because his offspring produced large quantities of milk of a desired composition. His semen was effective at getting cows pregnant, his daughters gave birth easily and were strong. He had a rare mix of fertility, genetics and appearance.

One of his daughters sold for $300 000 in 2009. His record number of offspring is unlikely to be beaten. This is because bulls are often retired earlier as new genetic advances are discovered.

Suggest why Genex waited until Toystory was four years old before beginning to collect his semen (line 9).

Explain how the semen from the bull is used to fertilise cows using artificial insemination.

Suggest why the semen is stored in liquid nitrogen (line 13).

Sexed semen is guaranteed to produce offspring of one sex.

Suggest why dairy farmers would prefer to used sexed semen (line 19).

Determine the percentage success of Toystory’s semen samples in producing offspring (line 2 and line 15).

Describe how scientists could investigate which of two bulls is the best to use as a father in dairy farming.

Explain why the composition of milk is important to consumers (line 22).

Scientists are now using cloning to produce animals.

Describe the stages that are required to clone a bull.

Give two advantages of using cloning rather than selective breeding to produce offspring.

Variation in a population can have different causes.

Which of these will not lead to an increase in genetic variation in a population of plants?

Explain how a change in the DNA of a microorganism can reduce its ability to digest a substance.

Explain why a change in DNA may not affect the phenotype of an organism.

Syndactyly is a condition where fingers and toes are not separated during development.

The photograph shows the foot of someone with syndactyly.

One type of syndactyly is caused by a dominant allele that is inherited from the parents.

A man without syndactyly and a woman with syndactyly have a male child.

The child does not have syndactyly.

Use a genetic diagram to show the genotypes of the parents, the possible gametes and the genotype and phenotype of their child.

Use D to represent the dominant allele and d to represent the recessive allele.

Determine the probability that the second child of these parents will be a girl with syndactyly.

Another type of syndactyly is caused by a recessive allele.

Explain the differences in the pattern of inheritance between a condition caused by a dominant allele and a condition caused by a recessive allele.

Syndactyly is controlled by a single gene that has two alleles.

Most other phenotypes are the result of a different sort of genetic control.

Describe the genetic control of most phenotypic features.

7 (a) The graph shows the height distribution in an adult population.

Human height is under polygenic control. State the meaning of the term polygenic.

Using the graph, determine the mode and median heights for males and females.

Suggest two reasons why the mean height for males is greater than the mean height for females.

Some genetic characteristics show a different pattern of inheritance to height. One example of this is piebaldism. In this condition, a person has a white patch of hair. The diagram shows a person with piebaldism.

Piebaldism is controlled by a single dominant allele. A man with a white patch of hair and a woman with a white patch of hair have two children. The first child was born without a white patch of hair. The second child was born with a white patch of hair.

Use your knowledge of genetics to explain the phenotypes of these children. Use H to represent the allele for a white patch of hair and h to represent the allele for no white patch of hair.

Another condition, called vitiligo, produces similar symptoms to piebaldism but is not genetically controlled. Suggest how a doctor could diagnose whether a new patient has piebaldism or vitiligo.