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Flashcards in Genetics Deck (90)
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- Deoxyribose nucleic acid - Found in the nucleus - All DNA has a similar structure


Major components of DNA

- Sugars - Phosphates - Bases - These make up the ‘back bone’ of the DNA structure


4 types of bases (an their pairs)

- Adenine always pairs with thymine - Guanine always pairs with cytosine


What is a nucleotide?

- Sub-unit of the DNA molecule - Consists of one sugar, one phosphate and one base molecule


What is a chromosome?

DNA wrapped into a double helix structure around proteins called histones


What is a gene?

- Each section of DNA that controls a trait - Humans have around 25,000 genes in their chromosomes - Genes for a particular trait are controlled at a particular point in the chromosome


Somatic cells

46 chromosomes (23 pairs)



23 single chromosomes


Homologous pair

a pair of chromosomes containing the pair of genes for a particular trait


What are alleles?

- The alternative forms of a gene that are available - The pairs of alleles that each person inherits from their parents control and determine the characteristics of the individual - Many genes have two alleles (dominant and recessive)


What is a genotype?

- The genetic makeup of an individual - It is inherited - E.g. BB/bb/Bb


What is a phenotype

- The observed expression of the alleles for the characteristics - Determined by the genotype and the environment


What is monogenic inheritance?

- Exists when an individual receives just one pair of alleles for a trait - E.g. blood groupings - The traits are discrete (either there or not there) - Only two alleles for the trait - Only two phenotype


What is polygenic inheritance

- Many pairs of genes control the trait - Continuous variation of the trait - Alleles will have an additive effect - E.g. skin colour height and weight - Continuous traits = many phenotypes


Skin colour

- Is a polygenic trait - Many genes contribute - Continuous variation - Depends on the amount of melanin in the skin


What do melanocytes produce?

Melanocytes produce structures called melanosomes that contain melanin and are found in the bottom of the epidermis of the skin


How is melanin produced?

It is made from an amino acid called tyrosine


Amounts of melanocytes and melanosomes in different people

Most people have similar amounts of melanocytes and varying amounts of melanosomes


Darker skinned people

- Can convert more tyrosine into melanin because they have more tyrosinase - More melanosomes containing more melanin - There are also more evenly distributed throughout the skin - This prevents burning


Pale skin

- Less tyrosinase and therefore can’t convert as much tyrosine into melanin - Melanin is not evenly distributed and form clumps - Burns when exposed to UV light


Tyrosinase (enzyme)

- Brings about the production of melanin - Stimulated by UV rays - People with the enzyme will become darker skinned in the sun - Those with less tyrosinase will tan less well - Therefore, the environment affects the phenotype - Darker skin will prevent sun burn


What is a mutation?

- A permanent change to a gene or chromosome leading to a new characteristic in an organism - Due to changes in the base sequences in a section of DNA which alters the types of proteins that will be synthesised by the body - Can be advantageous or disadvantageous


What is a mutant?

An organism with a characteristic resulting from a mutation


What are mutagens?

- Agents that are known to increase the rate at which mutations occur - E.g. UV light, X-rays, radiation, chemicals such as sulphur dioxide or antibiotics


Somatic mutations

- Occur in a body cell - The reproductive cells are not affected and once the organism dies, the mutation will be lost


Germ-line mutations

- Mutation occurring in the gametes - The individual is not usually affected - The individual will produce gametes with changed DNA


Main types of mutations

- Gene mutations - Chromosomal mutations


Gene mutations

- Change in a single gene - They occur during the replication of the DNA molecule before cell division - If the DNA copies incorrectly, the changed DNA will continue to be copied in all future divisions - May code for a new amino acid and a completely different protein


What is a point mutation?

a change in one base



- The result of a missing protein - The protein responsible for the pigment (melanin) of hair, skin and eyes is missing - Patients have whitish blond hair, extremely pale skin and pinkish eyes