Variation and inheritance

• Chlorosis = a condition that causes a plant's leaves to look pale or yellow
• Chlorosis occurs because the cells are not producing the normal amount of chlorophyll. This lack of chlorophyll reduces the ability of the plant to make food by photosynthesis.
• Most plants which show chlorosis have normal genes coding for chlorophyll production. The change in their phenotype is the result of environmental factors. There are many different environmental factors which cause chlorosis, including; lack of light, mineral deficiencies and virus infections.
• Mineral deficiencies:

  If the soil lacks iron and magnesium, the plant simply cannot make chlorophyll and gradually all the leaves will become yellow.
• Lack of light:

  In the absence of light, plants will turn off their chlorophyll production to conserve resources.
• Virus infections:

  When viruses infect plants, they interfere with the metabolism of cells. A common symptom is yellowing in the infected tissues as they can no longer support the synthesis of chlorophyll.
• Discontinuous variation = individuals fall into distinct groups (e.g. blood groups)
• Continuous variation = there are two extremes with every degree of variation possible in between (e.g. height, weight)