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Most of our characteristics are controlled by more than one pair of alleles and can also be influenced by our environment. Several allelomorphic pairs probably determine our height, it also depends on whether you get enough food when you are growing.

The most striking single factor characteristics are those associated with inherited defects such as sickle cell anaemia and cystic fibrosis. These two diseases are caused by recessive genes, which means they only express, themselves when the partner is recessive. It follows that people who develop the disease must have received both recessive alleles, one from each of their parents. This must also mean that both parents must be heterozygous for the gene. But if the father had been homozygous normal, he would have passed on the dominant gene to all of his children and none will be affected. Only a few genetic diseases are caused by dominant alleles, which means that even heterozygotes will develop the disease.

Gene	Phenotype	Genotype
Dominant	Normal mucus secretion in lung	CC or Cc
Recessive	Thick secretion; will cause severe lung infections	Cc

Cystic Fibrosis

Cystic Fibrosis is the most common inherited disease affecting about one in every 2500 children. The disease occurs when a person inherits two copies of a recessive allele, which controls the production of a protein in cell membranes. The protein normally controls the passage of chloride ions across the membrane but is defective in homozygous recessive individuals. As a consequence, the epithelial cells lining the oesophagus and digestive glands produce a thick, sticky mucus. This will cause serious infections.

Sickle Cell Anaemia

	N	s
N	NN	Ns
	Normal	Carrier
S	NS	Ss
	Carrier	Suffer

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