3.4 Genes and the environment Flashcards
(26 cards)
What is an allele?
A different version of a gene.
What does genotype mean?
Genetic makeup.
What does phenotype mean?
Physical characteristics determined by genotype.
What is variation?
Differences in phenotypes within a population.
What is discontinuous variation?
Defined/discrete characteristics. e.g. blood group A, AB, B, O. Usually due to control by one, possibly two, genes. Plot on bar chart.
What is continuous variation?
Characteristics that take any value between two extremes. e.g. height. Due to many genes controlling the characteristic (polygenic inheritance), more influenced by the environment. Plot on frequency histogram - normal distribution.
What is co-dominance?
A pair of alleles are said to be codominant if both are expressed in the phenotype of the heterozygote
What is incomplete dominance?
A mixture of alleles.
What is a dihybrid cross?
Looks at the inheritance of 2 characteristics.
What is polygenic inheritance?
When two or more independent genes control one characteristic.
Why does height show continuous variation?
It is a polygenic characteristic. It’s also determined by interactions of genes with the environment = multifactorial.
What does multifactorial mean?
A condition, e.g. human height, in which several genes and one or more environmental factors are involved
What is melanin?
A dark skin/hair pigment, made in melanocytes.
Melanin is proportional to the number of dominant alleles. The more melanin, the darker the skin.
What are melanocytes?
Special cells found in the skin at the root of the hair in the follicle. They form melanin. Melanocytes are activated by MSH. They place melanin into organelles called melanosomes.
What are melanosomes?
Organelles that are transferred to nearby skin and hair cells where they collect around the nucleus, protecting the DNA from harmful UV light.
Melanin is packaged into melanosomes.
What is MSH?
Melanocyte-stimulating hormone - stimulates the production of melanin in melanocytes. More of it is produced when there is more UV light, making melanocytes more active.
What are MSH receptors?
Receptors found on the cell membrane of melanocytes. People with more receptors have darker skin and hair.
How can a person’s genotype and their environment both affect the colour of their skin?
Genotype - controls the number of MSH receptors in skin cells. If your genotype codes for more MSH receptors, your melanocyte cells will become more active, produce more melanin, and make skin darker.
Environment - more UV light increases the amount of MSH and no. of MSH receptors, making melanocytes more active.
What is MAOA?
An enzyme that breaks down a neurotransmitter in the brain involved in the regulation of behaviour, including the response to stress. Mutation leads to no MOAO.
On which chromosome is MAOA found, and what are the implications of this?
X chromosomes, therefore males only have one allele so are more often affected by the mutation.
What is cancer?
A tumour or swelling resulting from the uncontrolled division of cells - the rate of cell multiplication is greater than the rate of cell death. These abnormal cells rapidly increase in number, invading and destroying the surrounding tissues.
Especially occurs in tissues with increased rates of mitosis.
What causes cancer?
A mutation due to damage of DNA by … physical factors = UV, radiation … chemical factors = carcinogens.
What are the two categories of gene that when mutated are most commonly implicated in causing cancer, and how do mutations in these genes lead to cancer?
1) Oncogenes = code for the proteins that stimulate the transitions of cell cycle stages. b) DNA mutations, such as less methylation, in these genes, can lead to the cell cycle being continually active, causing excessive cell division.
2) Tumour suppressor genes = produce suppressor proteins that stop the cycle. b) DNA mutations mean there is no brake on the cell cycle.
Why is cancer unlikely in any particular cell?
There needs to be damage to more than one part of the cell control system for cancer to occur, because there is a very complex network of signals and inhibitors that control the cell cycle.
