Biology: B1

• nucleus contains genetic material
• nucleus contains 23 pairs of chromosomes
• chromosomes carry genes -> genes control the development of characteristics e.g. hair colour
• A gene is a short length of a chromosome which is 1 molecule of DNA
• DNA is the coiled arms of the chromosome
• genes have different versions; with different characteristics. 
• Different versions of the SAME gene are called ALLELES.

      Genes are instructions for cells:

• gene is a code for making a certain protein
• different versions of proteins means we have different characteristics
• structural protein = skin, hair, blood and cytopasm in our cells
• enzymes = control chemical reactions in an organism e.g respiration & digestion 

• Alleles are different forms of a gene. They can be dominant or recessive.
•  Huntington’s disease is a disorder of the nervous system that is caused by a dominant allele. 
• Cystic fibrosis is a disorder of the cell membranes caused by a recessive allele.
• characteristic controlled by a dominant allele develops if the allele is present on one or both chromosomes in a pair
•  the characteristic controlled by a recessive allele develops only if the allele is present on both chromosomes in a pair

• For example, the allele for brown eyes is dominant, while the allele for blue eyes is recessive. An individual who inherits one or two alleles for brown eyes will have brown eyes. An individual will only have blue eyes if they inherit two copies of the allele for blue eyes.