Polygenic Inheritance

Many genes coding for one phenotype. Each copy of the allele (found at loci on the chromosome) has a small effect on the characteristics. The effects of several alleles together contribute to the phenotype of an individual.
• Melanin is made in melanocytes in skin and hair follicles
• The production is activated by melanocyte-stimulating hormone (MSH). People with more receptors to this have darker skin
• Melanin is stored in melanosomes which protect the DNA in the nucleus from damage by UV rays
• UV light increases the amount of MSH and MSH receptors. This causes skin to darken when exposed to light
• The enzyme tyrosinase catalyzes the production of melanin from the amino acid tyrosine
• Some animals have a mutant allele for tyrosinase where the enzyme made is unstable and inactivated at normal body temperature.
• Monoamine oxidase A (MAOA) is an enzyme that catalyzes the breakdown of neurotransmitters in the brain that regulates behavior
• A mutation can lead to no enzyme being produced and these individuals being more aggressive
• The gene for MAOA is carried on the X chromosome, meaning females have double the chance of inheriting the dominant allele for high levels of MAOA.
• Occurs when the rate of cell multiplication is higher than the rate of cell death
• It is essentially a disease of mitosis, the normal ‘checkpoints’ regulating mitosis are ignored by the cancer cell, leading to uncontrolled cell division
• Cell cycle is controlled by enzymes, to either stimulate or repress the next stage
• Cancer cells do not respond to these control mechanisms
• Oncogenes code for the proteins that stimulate the transition from one stage to the next
• Mutations in these genes lead to the cycle being continually active
• Tumor suppressor genes produce proteins that stop the cell cycle
• Mutations of these genes mean the cell cycle does not stop when it should
• Malignant (cancerous) tumors can invade and destroy nearby tissue and spread to other parts of the body

• Benign is not cancerous and grow larger but do not spread to other areas of the body
• Cancerous cells enter the bloodstream and spread to other organs, a process called metastasis