Fig. 1 shows a karyotype prepared from cells of a newborn baby. The chromosomes have been arranged in pairs according to size and banding pattern. The docto...
Question 1 Report
Fig. 1 shows a karyotype prepared from cells of a newborn baby. The chromosomes have been arranged in pairs according to size and banding pattern. The doctor requested the test because an ultrasound scan suggested that the baby might have an extra chromosome. The parents ask how a chromosome number change can arise. The doctor explains that chromosomes normally separate during meiosis to form eggs and sperm. A chromosome-number mutation can affect many genes at the same time because each chromosome contains many genes.
(a) Use Fig. 1 to state the number of chromosomes in the extra set shown. [1]
(b) Describe how an egg cell with an extra chromosome can be produced during meiosis. [2]
(c) Suggest why a chromosome-number mutation can have a larger effect than a mutation in one base of DNA. [3]
Answer Details
Labelled answer diagram:
(a) The extra set shown contains three chromosomes [1]. Chromosome 21 has three copies rather than the usual pair.
(b) During meiosis, a pair of chromosomes may fail to separate [1]. Both chromosomes then enter the same egg cell or gamete [1], leaving that egg with an extra chromosome.
(c) A whole chromosome contains many genes [1]. Having an extra or missing chromosome changes gene dosage, meaning the amount of genetic information or number of copies of many genes [1]. Consequently, production of many proteins, and therefore many characteristics, can be affected [1]. A mutation in one DNA base may affect only one gene, whereas chromosome-number changes can affect many genes at once.
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