Genetic Variation

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What is in the base sequence of DNA?
Instructions to make proteins, which determine the characteristics of all living things.
What is a gene?
A length of DNA that holds the instructions for one protein, and can be thousands of base pairs long.
What are base pairs?
Adenine, thymine, cytosine, guanine. (Always A to T and C to G.)
What is a trait?
A characteristic, such as eye or hair colour. (A gene is a section of DNA that holds the code for a specific trait.)
What is an alternate version of a gene?
They're called 'alleles'.
Where does genetic variation come from?
Sexual reproduction: offspring with two parents.
How does genetic variation occur?
Offspring of sexual reproduction are genetically different from their parents due to meiosis, random fertilisation of gametes, mutation, and the environment.
How does sexual reproduction affect genetic variation?
The process of meiosis and random fertilisation result in a unique combination of alleles.
How does fertilisation affect genetic variation?
The unique combination of alleles in sperm and ova and the random chance of them meeting mean that an individual is born with a completely unique genotype and phenotype.
What is a genotype?
The combination of possible alleles.
What is a phenotype?
The physical appearance of a trait.
How does the environment affect genetic variation?
The environment might act to alter the expression of the genotype. For example, food and nutrient availability, competition, light intensity (specifically in plants), disease, etc, may all result in a change in the expected phenotype.
What is meiosis?
The type of cell division that occurs in the male and female gametes. Meiosis is the reason you look similar, but not identical, to you parents and siblings. Though human body cells contain 46 chromosomes, gametes have on 23 chromosomes. During meiosis, these genes are shuffled, thereby variation is produced.
What are mutations?
The sequence of bases on a DNA strand follows the base pair rules. When there is a permanent change to the base sequence in DNA, this is called a mutation.
How does a mutation occur?
Mutations can occur naturally during DNA replication or as a result of exposure to mutagens such as some chemicals (like those in cigarettes), ultraviolet light (from the sun), and radiation (from x-rays or radioactive materials).
What kind of mutations are there?
Neutral/silent mutations, harmful mutations, and beneficial mutations. These mutations can occur either in a body cell (somatic cell), which cannot be passed down to the offspring, or in a reproductive cell (gametic cell), wherefore the mutation CAN be passed down.
What is a neutral/silent mutation?
A mutation with no observable effect on an organism.
What is a harmful mutation?
A mutation which affects the survival of an organism.
What is a beneficial mutation?
A mutation which may give an organism a better chance at survival, and this organism may well continue to live in an environment where others of it's species may die.
What is complete dominance?
A kind of dominance wherein the dominant allele complete masks the effect of the recessive allele in heterozygous conditions.
What is incomplete dominance?
When some alleles are incompletely dominant, meaning that the heterozygous individuals are a combination of both the dominant and the recessive alleles. (In your exam you can tell if you are dealing with a question involving incomplete dominance when the offspring shows a third phenotype which is a BLEND of the parental phenotypes.)
What is co-dominance?
Co-dominance occurs when a single gene has more than one dominant allele. An individual who is heterozygous for two co-dominant alleles will express the phenotypes associated with both alleles. (In your exam you can tell you are dealing with a question involving co-dominance when the off-spring shows a third phenotype which contains characteristics of BOTH the parental phenotypes.)
What are multiple alleles?
Multiple alleles arise due to mutations in a single gene resulting that gene having more that two different alleles. Individuals inherit only two alleles but multiple alleles increase genetic variation.
What are lethal alleles?
Lethal alleles are alleles that produce a phenotypic effect that causes the death of an organism, and usually arise due to a mutation in an essential gene.
What are linked genes?
Linked genes are usually located on the same chromosomes but code for different traits, and are more likely to stay together during meiosis (they don't seperate normally). Linked genes reduce the number of possible combinations of alleles in gametes.
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