Monohybrid inheritance

Machado-Joseph disease is a rare genetic condition that affects the nervous system. The condition is caused by a dominant allele and results in lack of control of motor coordination, reflexes and eye movements.

Explain what is meant by the term dominant allele.

The symptoms of Machado-Joseph disease do not become obvious until the person is more than 40 years old. Suggest why the late development of symptoms makes it difficult to reduce the number of people born with the condition.

A man who is heterozygous for this condition has a child with a woman who is homozygous recessive. Use a genetic diagram to show the genotypes of these parents, the gametes they produce and the genotypes and phenotypes of any possible offspring. Use the letter D for the dominant allele and d for the recessive allele.

Genotypes of parents

Gametes

Genotypes of offspring

Phenotypes of offspring

Machado-Joseph disease may have originated from a mutation in one person. Explain what is meant by the term mutation.

The passage describes reproduction in humans and in chickens. Complete the passage by writing a suitable answer in each blank space.

In humans, the hormone called_........ stimulates the repair of the

uterus lining. The ovum is then released from the_........ .

The ovum has a haploid number of 23 chromosomes.

Fertilisation occurs in the_........ tube. Fertilisation results

in a single cell called a_........ with a diploid number

of_........ chromosomes. This cell divides

by_........ to produce an embryo.

Sex inheritance in chickens is different from humans. In chickens the sex chromosomes are

called Z and W. In humans, the sex chromosomes are called_........ .

The sex chromosomes of a male chicken are ZZ and of a female chicken are ZW. This means that

the probability of parent chickens producing a male offspring is_........ .

In pigeons, the inheritance of bars at the end of the wings is controlled by a gene with two alleles. The allele for wing bars (B) is dominant to the allele for no wing bars (b). The diagram shows the phenotype of a pigeon with wing bars and the phenotype of a pigeon with no wing bars.

In a first cross, a heterozygous parent mates with a homozygous recessive parent. Complete the table by giving the genotype of each parent and the possible phenotypes of their offspring.

In a second cross, two heterozygous pigeons mate. Give the possible genotypes of their offspring.

This second cross only produces one offspring. Give the probability that this offspring will have wing bars.

Galactosaemia is an inherited condition. It is caused by a mutation to a gene on chromosome 9. This gene codes for an enzyme that removes the sugar galactose from the body. The normal allele, G, can produce the enzyme and is dominant to the recessive allele, g. The pedigree shows the inheritance of galactosaemia in a family.

How many individuals in the family pedigree have an X and a Y chromosome in each of their body cells?

How many individuals in the family pedigree are homozygous recessive?

Give the genotype of individual 1.

Individuals 8 and 9 are going to have another child. Give the probability of this child having galactosaemia.

What is meant by the term mutation ?

The mutation for galactosaemia is harmful but some mutations can be beneficial. Describe one example of a beneficial mutation.

Some inherited conditions can be fatal but medical treatment is often available. Explain what would happen to the frequency of alleles for these inherited conditions if medical treatment was not available.

The photograph shows a breed of dog called a Border Collie.

Border Collies can inherit an eye defect called CEA (Collie Eye Anomaly). The dominant allele D produces good vision but the recessive allele d produces poor vision.

The diagram shows the possible offspring from parents with different genotypes. Parents Offspring Key

All the offspring from cross R have good vision. Give the letters of the other crosses where all the offspring have good vision.

Give the phenotype of each parent used in cross P.

Which cross has a 50% probability of producing offspring with good vision?

Give the genotype of each of the offspring produced in cross T.

The crosses between the dogs are examples of sexual reproduction.

Name the gametes produced by males in sexual reproduction.

Name the gametes produced by females in sexual reproduction.

Give the term used to describe the fusion of gametes.

In which organ of a female parent do offspring develop?

Polydactyly is a genetic disorder in which people inherit an extra digit. The X-ray image shows the left hand of someone with polydactyly. The person has six digits, five fingers and one thumb.

Polydactyly is caused by a dominant allele (D). The table describes the different genotypes for polydactyly.

Complete the table by giving the correct genotype, alleles of each genotype and the expected number of digits per hand.

The table lists possible matings between parents. Complete the table by writing the probability of each mating producing a child with polydactyly. One has been done for you.

What is meant by the term gene ?

A gene is made from 1000 base pairs. The table shows the percentage of each base found in the gene.

Complete the table by giving the name of the missing base.

Calculate how many cytosine bases you would expect to find in this gene.

The photograph shows a chicken with a mixture of black feathers and white feathers.

In chickens the inheritance of feather colour is controlled by codominant alleles. The allele for black feathers is C^B, and the allele for white feathers is C^W. The diagram shows a parent with all black feathers and a parent with all white feathers. It also shows one of their offspring with a mixture of black and white feathers.

black feathers white feathers

Complete the table by writing the genotype of the chickens shown in the diagram.

Two of the offspring with a mixture of black and white feathers mated. What is the probability that their offspring would also have a mixture of black and white feathers?

In fruit flies, the length of the wings is controlled by a gene with two alleles. The allele for long wings (L) is dominant to the allele for short wings (l). The diagram shows a long-winged fruit fly and a short-winged fruit fly.

Complete the table by writing the genotype, the description of genotype and the phenotype of the fruit flies.

Two heterozygous fruit flies mated. The number of long-winged and short-winged offspring produced are shown below.

Tick one box to show the number of these offspring that is likely to be heterozygous.

Yeast cells feed on rotting fruit. Fruit flies feed on these yeast cells. Use this information to draw a food chain in the space below.

A student wanted to find out if fruit flies are attracted to rotting fruit. He set up this apparatus.

He released 20 fruit flies into the glass tank. After ten minutes, he counted the number of fruit flies found near the fresh fruit and the number found near the rotting fruit. The table shows his results.

Name two factors that the student should control in this investigation.

Do you think the student’s results are reliable? Give a reason for your answer.

(a) Lactose is a sugar found in milk. It is digested by an enzyme called lactase. Some people cannot make lactase. The condition they have is called lactose intolerance. The condition is caused by a recessive allele. This means that only people with the homozygous recessive genotype can be lactose intolerant.

What is meant by the term homozygous ?

State the two possible genotypes of an individual who is not lactose intolerant. Use the symbols D for the dominant allele and d for the recessive allele.

A homozygous dominant mother and a homozygous recessive father have two children. In the boxes below give the genotypes of the parents and their children.

What is the probability that the children are lactose intolerant?

People from different countries were tested to find out if they were lactose intolerant. The table shows the number of people who were tested in each country and the percentage who were lactose intolerant.

The population size of country B is 190 million. Calculate the number of people in this country likely to be lactose intolerant. Show your working.

Suggest a reason why the value of 100% for country H may not be correct.

Human blood group is an example of a phenotype determined by alleles that show codominance. The table shows the different blood groups and their genotypes.

Explain what is meant by the term codominance.

A man with blood group A married a woman with blood group B. They had four children, each with a different blood group. Use a genetic diagram to show this cross.

Genotypes of the parents

Gametes

Genotypes of the offspring