3.1 Genes & Chromosomes

Cri du chat syndrome is a rare genetic disorder caused by a chromosomal abnormality that occurs very early in embryonic development. Babies born with cri du chat syndrome suffer from a variety of symptoms and have a characteristic cry which sounds like the meowing of a cat.

The karyograms below compare the karyotype of a normal child with one that suffers from cri du chat syndrome.

Contrast the karyotype of a normal child with that of a child suffering from cri du chat syndrome.

Two genes, SEMA5A and CTNND2, are believed to be involved with brain development in a foetus. These genes are missing from children suffering from cri du chat syndrome.

Suggest two possible symptoms of children with cri du chat syndrome as a result of this.

There are some individuals with cri du chat syndrome that do not differ developmentally from their peers in a significant way.

Based on the information provided in the karyograms at part a), explain this occurrence.

Most of the people affected by cri du chat syndrome do not have a family history of the condition.

Suggest what this means in terms of the heritability of the syndrome.

The table below shows the genome size and haploid chromosome number of different organisms.

Calculate the percentage difference in the chromosome number found in the zygotes of chickens compared to those of humans.

Show your working and give your answer to three significant figures.

The diploid number in an organism is always an even number.

Using your knowledge on the behaviour of chromosomes during meiosis, explain the importance of the diploid number in an organism.

Scientists hypothesised that a high chromosome number leads to the development of a more complex organism.

Discuss this hypothesis using the data provided in part a).

Based on your knowledge of chromosomes, suggest a reason why the genome size of a species does not always seem to correlate with the chromosome number.

Wilson's disease is a condition caused by a mutation of gene ATP7B located on chromosome 13, which codes for an ion transport enzyme. This enzyme is responsible for transporting copper ions (Cu²⁺) into bile so that it can be removed from the body through the digestive tract. There are several mutations which may lead to Wilson's disease; one of these mutations involves the replacement of the amino acid histidine by glutamine.

Describe the type of mutation that could have led to this disease.

Based on the information provided in part a), explain the effect this mutation would have on the transport of copper ions.

In most cases of Wilson's disease, a sufferer must have two copies of the mutated ATP7B allele before the disease is present.

Suggest what this may indicate about the dominance of the ATP7B allele.

People suffering from Wilson's disease have high levels of free copper in their bloodstream which have been shown to cause damage to the cell membranes of red blood cells.

Explain the consequences of this to the sufferer.

One mark is available for clarity of communication throughout this question.

Compare and contrast the X and Y chromosome.

The sex of a foetus is determined by the father.

Explain this statement.

Outline how a substitution mutation can alter the amino acid sequence of a polypeptide by using sickle cell anaemia as an example.

The following base sequences represent sections of two different alleles of the gene which determines an individual's ability to roll their tongue. 

Allele A (tongue roller): GCCGTAAC
Allele B (non-tongue roller): GCGCTTAC

Outline why two different alleles result in different expressions of a gene.

Sickle cell anemia is a genetic disorder with symptoms such as dizziness, a rapid heart rate and fatigue. It is caused by an allele that leads to altered haemoglobin proteins. These altered proteins undergo aggregation (sticking together), an event which changes the shape of red blood cells.  This can be seen in the image below.

Suggest how sickled red blood cells may result in the symptoms noted above.

Explain why the shape of white blood cells is not affected by sickle cell anaemia.        

Mutations such as the one seen in sickle cell patients are usually caused by an error during DNA replication. 

Identify the enzyme that is responsible for catalysing the process of DNA replication.

Outline the developments in technology which have enabled the successful sequencing of the genome in the Human Genome Project.

Suggest two medical applications of genome projects like the Human Genomet Project.

It was decided that all data generated for the Human Genome Project would be made publicly available so that access would be free for everyone. 

Suggest one advantage and one disadvantage of making scientific data publicly available.

When carrying out the genome project the DNA from individuals was obtained via blood samples. 

Identify, with a reason, which cell type the DNA is obtained from when using a blood sample.

The human genome is approximately 3 billion, or 3 000 000 000, base pairs long. A DNA sequencing machine allows for 5.5 x 10⁸ base pairs to be sequenced per hour. 

Using this information calculate the number of days it would take to sequence 1500 genomes of hospital patients using this machine. Give your answer to the nearest day.

The table below shows part of the DNA base sequence coding for β-haemoglobin and two mutations of this sequence detected in a sickle cell sufferer.

Complete the table with the DNA sequences that will undergo transcription to produce β-haemoglobin, mutated protein 1, and mutated protein 2.

The table below shows some examples of amino acids, their structures, and the mRNA codons that code for them.

Suggest why mutation 2 from part (b) is of no concern to the scientists studying this patient's DNA

A karyogram, such as the one shown in the image below, can be used to detect some Mutations

State why this karyogram could not be used to detect sickle cell anaemia.

One mark is available for clarity of communication throughout this question.

Outline how a mutation leads to the development of Down syndrome.

Distinguish between prokaryotic and eukaryotic DNA.

Outline the technique developed by John Cairns to measure the length of DNA, and how his methods contributed to further discoveries about DNA.