B1.7:Genetic Variation

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B1.7.1:Variation

VARIATION - The differences that exist between individuals

Genetic variation:

An organism's characteristics are determined by genes inherited from the parents. The genes are passed on in sex cells (gametes). The combining of genes from two parents causes genetic variation.

Environmental variation:

The envionment that an organism lives and grows in can cause differences beween members of the same species. Any difference caused by the conditions it lives in is an environmental variation.

Genetic and Environmental variation:

e.g. height - maximum height determined by genes OR how much food it eats

e.g. health - more likely to get certain diseases due to genes OR how much junk food you eat

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B1.7.2:Genes, Chromosomes and DNA

Most cells in the body contain a nucleus which contains genetic material (instructions needed to grow and develop) in the form of chromosomes.

The human cell nucleus has 46 chromosomes or 23 pairs of chromosomes.

DNA (deoxyribonucleic acid) - The molecule in cells that stores genetic information

CHROMOSOME  - A long molecule of DNA found in the nucleus, which carries genes

GENE - A short section of DNA, found on a chromosome, which contains the instructions needed to make a protein

Different genes control the development of different characteristics e.g. hair colour, eye colour. There can also be different versions of the same gene e.g. blue or brown eyes

ALLELE - An alternative version of the same gene

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B.1.7.3:Sexual Reproduction

SEXUAL REPRODUCTION - Where two gametes combine at fertilisation to produce a genetically different new individual

GAMETE - A sex cell in animals e.g. an egg cell or sperm cell

In sexual reproduction the mother and father produce gametes containing 23 chromosomes (half the number of chromosomes in a normal cell).

The egg from the mother and the sperm from the father fuse together to form a cell with the full number of chromosomes (half from the mother, half from the father). The fusion of gametes is called fertilisation.

The mixture of genetic material produces variation in the offspring.

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B.1.7.3:Asexual Reproduction

ASEXUAL REPRODUCTION - Where organisms reproduce by mitosis to produce genetically identical offspring

In asexual reproduction there is only one parent. An ordinary cell makes a new cell by dividing in two.

How does it work?

  • X-shaped chromosomes have two identical halves.
  • Each chromosomes splits down the middle to form two identical sets of 'half chromosomes' (two sets of DNA strands). A membrane forms round each set.
  • The DNA then replicates itself to form two identical cells with complete sets of X-shaped chromosomes.

The offspring are genetically identical to the parent as they are have the same genetic information as the parent cell - they are clones.

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B1.7.4:Cloning

CLONING - Making a genetically identical copy of another organism

PLANT Cloning:

Cuttings - take cuttings from good parent plants and plant them to produce genetically identical clones of the parent plant. They can be produced quickly and cheaply.

Tissue culture - plant cells are put into a growth medium with hormones and grow into new plants which are clones of the parent plant. They can be made very quickly in little space and can be grown all year.

ANIMAL Cloning:

Embryo transplants - sperm fertilises eggs to create an embryo which is then split many times to produce clones before any cells become specialised. The cloned embryos are implanted into host mothers.

Adult cell cloning - a nucleus from an adult body cell is insterted into an 'empty' egg cell. The egg cell is then stimulated to divide by electric shock and the embryo is implanted into the uterus of a surrogate mother.

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B1.7.4:Pros and Cons of Cloning

PROS:

  • quickly gives lots of 'ideal' offspring with known characteristics
  • study of animal clones could lead to greater understanding of the development of the embryo as well as ageing and age-related disorders
  • can help to preserve endangered species

CONS:

  • gives a 'reduced gene pool' - meaning there are fewer different alleles in a population
  • cloned animals might not be as healthy as normal animals
  • if humans were to be cloned in the future it could result in many unsuccesful attempts and you would need to consider the human rights of the clone (clone doesn't have a say)
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B1.7.5:Genetic Engineering

GENETIC ENGINEERING - The process of cutting out a useful gene from one organism's chromosome and inserting it into another organism's chromosome

How does it work?

  • A useful gene is cut from one organim's chromosome using enzymes.
  • Enzymes are used to cut another organism's chromosome.
  • Enzymes are also used to insert the useful gene.

PROS:

  • has potential to solve lots of problems e.g. treating diseases, more efficient food production etc.

CONS:

  • could change a person's genes
  • might accidentally create unplanned problems which may be passed on to future generations
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B1.7.5:Genetic Engineering in Plants and Animals

e.g. sheep have been genetically modified to produce substances, like drugs, in their milk that can be used to treat human diseases

e.g. genetically modified (GM) crops are crops which have had their genes modified to make them resistant to viruse, insects or herbicides (chemicals used to kill weeds)

PROS:

  • can increase the yield (produce) of a crop therefore making more food for people ( e.g. insect-resistant crops won't get eaten by insects)
  • can be engineered to contain nutrients missing in poorer peoples diets

CONS:

  • can affect the number of weeds and flowers therefore the number of insects around the crops reducing farmland biodiversity (number of living organisms on the farmland)
  • may develop allergies to foods that have been genetically modified
  • transplanted genes may get out into the natural environment
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