Genes & alleles

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

Guinea pigs have either long hair or short hair. The hair length is controlled by one gene with two alleles. The photograph shows a guinea pig with long hair and a guinea pig with short hair.

(Source: © Kallayanee Naloka/Shutterstock)

State what is meant by the term allele.

A breeder did the following crosses. Cross 1 a male long-haired guinea pig with a female short-haired guinea pig, this produces five offspring all with short hair Cross 2 a male short-haired guinea pig with a female long-haired guinea pig, this produces four offspring all with short hair Cross 3 a male short-haired offspring from cross 1 with a female short-haired offspring from cross 2, this produces some offspring with long hair and some offspring with short hair

Draw a genetic diagram to show the parents, gametes and all possible genotypes and phenotypes of the offspring from cross 3.

Calculate the probability that the first offspring born from cross 3 is a male with short hair.

Some short-haired guinea pigs are homozygous and some short-haired guinea pigs are heterozygous. Explain how a breeder could use a cross to determine the genotype of a short‑haired guinea pig.

Guinea pigs stay completely still and do not move when they sense danger. Describe how this behaviour has evolved by natural selection.

The photograph shows a plant called a snapdragon.

(Source: © BRIAN GADSBY/SCIENCE PHOTO LIBRARY) Selective breeding has been used to produce snapdragons with brightly coloured flowers.

Describe how selective breeding can produce snapdragon plants with brightly coloured flowers.

Micropropagation is often used to make copies of a snapdragon plant. Describe the process of micropropagation.

Scientists investigate the effect of exposing snapdragon explants to increasing amounts of ionising radiation. This is the scientists’ method.

• take a snapdragon plant and use micropropagation to produce many explants

• expose groups of explants to different amounts of ionising radiation

• grow the explants into plants and record the number of differences in their phenotypes compared with the original plant

• take samples of each of the plants and measure the number of differences in DNA nucleotides of each plant compared with the original plant

Give the reason why micropropagation is used to produce the plants to be tested.

What term is given to all the DNA in an organism?

The graph shows the scientists’ results.

Number of changes Number of in DNA different

Discuss the effects that increasing the amount of ionising radiation has on the snapdragons.

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 diagram shows part of a molecule of DNA.

State what is meant by the term genome.

Describe the differences between the structure of DNA and the structure of RNA.

Describe the roles of RNA in protein synthesis.

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.

A DNA strand has this sequence of bases.

C A T T C A A T T C A T T T C

How many amino acids does this sequence of bases code for?

Write down the complementary mRNA code for this sequence of bases.

A student reads that DNA codons are non-overlapping.

Suggest what is meant by the term non-overlapping.

You should refer to the sequence of bases in your answer.

Describe what happens in the translation stage of protein synthesis.

State what is meant by the genome of an organism.

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.