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O-Level Inheritance
What the O-Level syllabus expects for Inheritance, and how to practise it.
What the syllabus expects
- Tell apart the terms gene and allele.
- Account for the terms dominant, recessive, codominant, homozygous, heterozygous, phenotype and genotype.
- Predict how simple crosses turn out, where the ratios expected are 3:1 and 1:1, drawing on the terms homozygous, heterozygous, F1 generation and F2 generation.
- Account for why the ratios observed often depart from those expected, particularly when the number of progeny is small.
- Apply genetic diagrams to tackle problems on monohybrid inheritance.
- Set out how sex is determined in humans through the XX and XY chromosomes.
- Set out mutation as a change in a gene's sequence, as in sickle cell anaemia, or in chromosome number, as in the 47 chromosomes of Down syndrome.
- Name ionising radiation, such as X-rays, and chemical mutagens as factors that can raise the mutation rate.
- Tell continuous variation apart from discontinuous variation, offering examples of both.
- Note that variation and competition bring about differing rates of survival and reproduction among the organisms best suited to their environment.
- Give examples of environmental factors that operate as forces of natural selection.
- Account for the part natural selection may play as a mechanism for evolution, understood as the gradual shift in a population's inheritable characteristics over time.
How it's examined
Questions on this topic most often ask you to identify, describe, explain, outline.
Worked examples
Example 1 (5 marks)
Outline the mutation responsible for sickle cell anaemia. Give one factor that can raise the frequency of mutations.
Show the worked answer
Sickle cell anaemia is caused by a gene (point) mutation in the gene coding for the beta-globin chain of haemoglobin. A single base is substituted (the triplet GAG becomes GTG). This changes one codon so that one amino acid changes: glutamic acid is replaced by valine. The altered haemoglobin (HbS) makes red blood cells become sickle-shaped, especially at low oxygen concentrations. The frequency of such mutations can be raised by exposure to ionising radiation (e.g. X-rays, gamma rays, UV) or to chemical mutagens.
Example 2 (5 marks)
HIV is capable of an extremely high mutation rate. Applying the principles of natural selection, describe how this virus can change into variant forms that the body's white blood cells are no longer able to identify.
Show the worked answer
Because HIV mutates at a very high rate, its genes change frequently and random mutations produce many variants of the virus with different surface proteins (antigens). This creates variation within the virus population. The body's white blood cells (lymphocytes) recognise the virus by its surface antigens; when the antigens change, the existing antibodies/lymphocytes no longer fit or recognise them. When the body's immune system (or a drug) acts as a selection pressure, virus variants whose antigens can still be recognised are destroyed, but any variant with new, unrecognised antigens survives. These surviving variants reproduce/replicate and pass on their altered genes, so the new antigen form becomes more common in the virus population. Over time this repeated selection means the virus population is dominated by forms the white blood cells can no longer identify.
Example 3 (2 marks)
Explain the difference between an organism's phenotype and its genotype.
Show the worked answer
The genotype is the genetic make-up of an organism, that is the alleles/genes it possesses (for example Tt or TT). The phenotype is the observable characteristics or physical/features of the organism (for example tall), which result from the genotype interacting with the environment. In short, the genotype is the set of alleles present, whereas the phenotype is the outward expression of those alleles as observable traits.
More O-Level Pure Biology topics
Cell Structure and Organisation · Movement of Substances · Biological Molecules · Nutrition in Humans · Transport in Humans · Respiration in Humans · all of O-Level Pure Biology