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OCR Categories Archives: B1: You and Your Genes

B1.4 How is a clone made?

B1.4 How is a clone made?

Clones are genetically identical individuals. Bacteria, plants and some animals can reproduce ASEXUALLY to form clones that are genetically identical to their parent. ASEXUAL REPRODUCTION only requires one parent, unlike sexual reproduction, which needs two. Since there is only one parent, there is no fusion of gametes and no mixing of genetic information. As a result are genetically identical to the parent and to each other.

Identical human twins are also clones – ANY DIFFERENCES BETWEEN THEM ARE DUE TO ENVIRONMENTAL FACTORS.

Plants- asexual reproduction in plants can take a number of forms:

Some plants such as strawberries produce shoots called RUNNERS. These eventually break off and become new strawberry plants, clones of the original.

Other plants grow BULBS. When bulbs are planted they grow into genetically identical plants. Again the environment will alter them. No two organisms can occupy the same space in the universe so the environment will always be different for individuals, even if they are clones.

Animals – clones in animals can occur naturally and artificially:

Clones of animals occur naturally when, during the earliest stages after fertilisation, the developing embryo splits into two, they have the same genes. As the genes came from both parents they are not clones of either parent, but they are natural clones of each other.

It is now possible to make clones artificially by taking the nucleus from an adult body cell and transferring it into an empty, unfertilised egg cell.

Cloning depends on cells that have the potential to become any cell type in the body. These are called STEM CELLS.

ADULT STEM CELLS are unspecialised cells that can develop into many, but not all types of cell.

EMBRYONIC STEM CELLS are unspecialised cells that can develop into ANY type of cell, including more embryonic stem cells.

As a result of being unspecialised, stem cells from embryos and adults offer the potential to treat some illnesses.

For example – skin can grow as a treatment for serious burns and sight can now be restored to people who are blind due to damage of their corneas.

The majority of cells of multicellular organisms become specialised during the early development of the organism. This is because after the zygote has divided four times to reach the 16 cell stage, the majority of cells in the embryo start to become specialised.

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B1.3 How can and should genetic information be used? How can we use our knowledge to prevent disease?

B1.3 How can and should genetic information be used? How can we use our knowledge to prevent disease?

Different forms of the same gene are called ALLELES. You inherit one allele for each gene from your father and one allele for each gene from your mother. For example, the gene for eye colour has alleles for blue eye colour and alleles for brown eye colour. Your eye colour will depend on the combination of alleles you have inherited from your parents.

Some diseases are inherited from our parents though our genes: they are called GENETIC DISORDERS. They occur because of faulty or defective alleles.

E.g. CYSTIC FIBROSIS

Cystic fibrosis is caused by a recessive allele. You need to inherit two copies of the faulty allele to be born with cystic fibrosis. If you have just one copy, you are a CARRIER

Cystic fibrosis affects the cell membranes causing a THICK MUCUS to be produced in the lungs, gut and pancreas.

SYMPTOMS – Thick and sticky mucus

Breathing problems

Chest infections

Difficulty digesting food

HUNTINGTON’S DISEASE is another genetic that affects the central nervous system. However is caused by a dominant allele – the presence of just one dominant allele can cause the disease. You only need to inherit one copy of the faulty allele to have Huntington’s disorder, unlike cystic fibrosis, where you need to inherit both copies. You can inherit the disorder if one or both of your parents carry the faulty allele, because it is DOMINANT.

SYMPTOMS – Uncontrollable shaking

Clumsiness

Memory Loss

Inability to concentrate

Mood changes

 

 

GENETIC TESTING

 

  • It is now possible to test adults, children and embryos for a faulty allele if there is family history of a genetic disorder. If the test turns out positive, the individual will have to decide whether or not to have children and risk passing on the disorder. This is called PREDICTIVE TESTING FOR GENETIC DISEASES.
  • Genetic testing can also be carried out to determine whether an adult or child can be prescribed a particular drug without suffering from serious side effects. (TESTING AN INDIVIDUAL BEFORE PRESCRIBING DRUGS). E.g. certain people are highly prone to getting liver damage while taking COX-2 inhibitor drugs. A genetic test would ensure that only those patients who do NOT have the prone gene are prescribed the drug.
  • Embryos can be tested for embryo selection. The healthy embryos that do not have the faulty allele are then implanted. This process is called in vitro fertilisation (IVF).

The process for embryo selection is called PRE-IMPLANTATION GENETIC DIAGNOSIS (PGD). After fertilisation, the embryos are allowed to divide into eight cells before a single cell is removed from each one for testing. The selected cell is then tested to see if it carries the allele for a specific genetic disease.

PGD has risks including inaccuracy in results-healthy embryo not being implanted and it may also decrease the chance of the embryo surviving once it has been implanted.

RISKS OF GENETIC TESTING:

However testing adults and foetuses for alleles that cause genetic disorders has implications that need to be considered, including:

  • Risk of miscarriage as a result of cell sampling for the genetic test
  • Using results that may not be accurate , including false positives and false negatives

 

 

 

  • Whether or not to have children
  • Whether or not a pregnancy should be terminated
  • Whether other members of the family should be informed

There are ethical considerations that need to be considered very carefully

For example: governments may have the ability to impose genetic tests on individuals by implementing genetic screening programmes, but should they be allowed to do so? There is the potential for genetic testing to be used to produce detailed genetic profiles. These could contain information on everything from ethnicity to whether they are prone to certain conditions (e.g. obesity) or diseases (e.g. cancer).

However how will the information be used?

  • Employers could potentially refuse to employ someone who possessed certain alleles
  • Insurers may not cover a person who had genes that made them more likely to suffer a heart attack.
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B1.2 Why can people look like their parents, brothers and sisters but not identical to them?

1.2 Why can people look like their parents, brothers and sisters but not identical to them?

A human has 23 PAIRS OF CHROMOSOMES

 

 

Parents pass on their genes to their offspring in their sex cells.

A pair of chromosomes carries the same genes in the same place, on each chromosome within the pair. However, there are different versions of a gene called ALLELES. These alleles may be the same (HOMOZYGOUS) on each pair of chromosome, or different (HETROZYGOUS) – For example to give blue eyes or brown eyes.

Sex cells only contain one chromosome from each pair. When an egg cell and sperm cell join together, the fertilised egg cell contains 23 pairs of chromosomes. One chromosome in each pair comes from the mother, the other from the father.

WHICH CHROMOSOME WE GET FROM EACH PAIR IS COMPLETELY RANDOM. THIS MEANS DIFFERENT CHILDREN IN THE SAME FAMILY WILL EACH GET A DIFFERENT COMBINATION. THIS IS WHY CHILDREN IN THE SAME FAMILY LOOK A LITTLE LIKE EACH OTHER AND A LITTLE LIKE EACH PARENT, BUT ARE NOT IDENTICAL TO THEM. THE CHILD WILL SHARE SIMILARITIES WITH ITS PARENTS DEPENDING ON WHICH CHARACTERISTICS HAVE COME FROM THE FATHER AND WHICH HAVE COME FROM THE MOTHER AND WHICH ONES ARE DOMINANT AND RECESSIVE.

An allele can be DOMINANT or RECESSIVE

  1. An individual with one or both DOMINANT alleles (in a pair of alleles) will show the associated DOMINANT
  2. An individual with one RECESSIVE allele (in a pair of alleles) will not show the associated RECESSIVE
  3. An individual with both RECESSIVE alleles (in a pair of alleles) will show the associated RECESSIVE

GENETIC DIAGRAMS

It is easiest to follow what is happening with the inheritance of gene characteristics by drawing genetic diagrams.

FAMILY TREES can be used to trace the inheritance of a characteristic and to work out who must have been carrying a faulty allele. An example of this:

 

 

 

When looking at the possibilities of inheriting and allele, we use a Punnett square diagram. This shows all the possible pairings of alleles from sperm and egg at fertilisation.

For example if a male with a dominant A allele and recessive a allele was to mate with the same alleles, the following diagram could be drawn:

 

A Punnett square diagram can also be used to represent how sex is determined. This is because one of the 23 pairs of chromosomes in a human cell is the sex chromosome. In females the sex chromosomes are the same – they are both X chromosomes. In males they are different – there is an X chromosome and a Y chromosome.

Sex Determination:

The sex of an embryo is determined by a gene on the Y chromosome called the SRY (sex-determining region Y) gene. If the gene is not present i.e. if there are two X chromosomes present, the embryo will develop into a female and ovaries will grow. If the gene is present i.e. both an X and a Y chromosome are present, then testes will begin to develop.

Six weeks after fertilisation, the undifferentiated gonads start producing a hormone called ADROGEN. Specialised receptors in the developing embryo detect the androgen. This stimulates the male reproductive organs to grow.

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B1.1 What are genes and how do they affect the way that organisms develop?

B1.1 What are genes and how do they affect the way that organisms develop?

A GENE is a short section of DNA. Genes carry instructions that control how you develop and function – they are long molecules of a molecule called DNA. Each gene codes for a specific protein by specifying the order in which AMINO ACIDS must be joined together.

These proteins can be:

STRUCTURAL PROTEIN: Gives the body structure, rigidity and strength E.g. Skin, Hair, Muscles etc

FUNCTIONAL PROTEIN: Enables the body to function E.g. Enzymes, Antibodies etc.

The differences between individuals of the same species are described as VARIATIONS.

Variations may be due to:

  • GENOTYPE – The genetic makeup of an organism. The different characteristics that an individual inherits, E.g. whether you have dimples or not.
  • PHENOTYPE – The observable characteristics the organism has. How the environment changes an individual, E.g. cutting the skin may cause a scar.

IDENTICAL TWINS have the same set of genotype however any differences between them is because of environment.

CONTINOUS VARIATION shows when some characteristics are controlled by several genes working together e.g. eye colours and height. For instance it was originally believed that eye colour was due to a single gene. It is now known that there are a number of genes coding for different pigments in the iris, mainly on chromosome 15 in humans. This means that there is enormous variation in eye colour.

 

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