Inherited Disorders

Some disorders are inherited. These disorders are caused by the inheritance of certain alleles.

  • Polydactyly (having extra fingers or toes) is caused by a dominant allele.
  • Cystic fibrosis (a disorder of cell membranes) is caused by a recessive allele

Embryo Screening

Embryo Screening is a procedure where people can find out whether there are any problems with the chromosomes in the embryo. However, there are some social economic and ethical issues with embryo screening.


Once an embryo has been screened, parents and scientists may be unsure as to what they would do with the data / the embryo. It is not 100% that the embryos have an inherited illness, or whether it will even develop. People would question whether it is ethical to allow the embryo to develop or whether it would be best to terminate it. Some would also consider it murder if an embryo’s life is terminated, as some believe that life starts in the womb.


If an embryo is screened and has the possibility of developing a range of illnesses, the parents could have the decision to terminate the embryo. This would save the embryo from developing into a baby that will suffer from the illness, as it may be a heartbreak for the parents in having to deal with a child that is suffering.

Sex Determination

There is a 50% chance of obtaining a female from contraception, and there is a 50% chance of obtaining a male. The female gamete would contain ‘X’ chromosome, and the male gamete would contain the ‘Y’ chromosome. The following diagram suggests that there is a equal chance of obtaining a male or a female. Females have a genotype of XX, and males have a genotype of XY.

Variation, figure 1

Causes of Variation

Here are the two main causes of variation that influences the phenotype:

  1. Genetic variation (where different characteristics are obtained due to mutation and sexual reproduction)
  2. Environmental variation (Where different characteristics are caused by an organism’s environment. These characteristics tend to be acquired from their surroundings. E.g. if a person decides to dye their hair, get a tattoo, get a piercing)

The phenotype is most likely affected by both genetics and the environment. There is usually an extensive genetic variation within a population of a species. All variants arise from mutations and that: most have no effect on the phenotype; some in uence phenotype; very few determine phenotype. Mutations occur continuously. Very rarely a mutation will lead to a new phenotype. If the new phenotype is suited to an environmental change it can lead to a relatively rapid change in the species.

Resistant Organisms

The theory of natural selection is demonstrated through bacteria. Bacteria reproduces rapidly, therefore it allows them to evolve rapidly too. When there are mutations of bacteria, new strains are created. When certain bacteria become resistant to antibiotics, they are cannot be destroyed by the antibiotic as they are resistant towards it. The bacteria that is resistant to the antibiotics will reproduce rapidly, as the bacteria without the mutation will die off. This means that the person that is taking the antibiotics will become even more poorly as they have more bacteria that is stronger and does not respond to the first round of antibiotics.

Variation, figure 1

Bacteria Resistance

Bacteria can evolve rapidly because they reproduce at a fast rate. Mutations of bacterial pathogens produce new strains. Some strains might be resistant to antibiotics, and so are not killed. They survive and reproduce, so the population of the resistant strain rises. The resistant strain will then spread because people are not immune to it and there is no effective treatment. MRSA is resistant to antibiotics.

To reduce the rate of development of antibiotic resistant strains:

  • Doctors should not prescribe antibiotics inappropriately, such as treating non-serious or viral infections
  • Patients should complete their course of antibiotics so all bacteria are killed and none survive to mutate and form resistant strains
  • The agricultural use of antibiotics should be restricted.
  • The development of new antibiotics is costly and slow. It is unlikely to keep up with the emergence of new resistant strains.
What is the genotype of a female?
What is embryo screening?
Your answer should include: Procedure / People / Find / Out / Problems / Chromosomes / Embryo
Explanation: Embryo Screening is a procedure where people can find out whether there are any problems with the chromosomes in the embryo.
What are the two main causes of variation?
Your answer should include: Genetics / Environment
Explanation: Genetics and Environment - however they would both interact and affect the phenotype of a person.