Biology Unit 4 Genetics Bitesize

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Karotyping for pre-natal diagnosis

Karotyping is used to determine if a genetic abnormality exists. e.g. Downs Syndrome

 Chorionic villus sampling

- pre-natal test done at 11/13 weeks of pregnancy: involves taking a sample of the chorionic villi to obtain cells from tissue that originally came from the zygote. outer cells of zygote come in contact with mucus lining of uterus (develops into chorionic villi) 

- The cells will have the same genetic composition as the cells of the unborn baby so a karotype can be made. 

Amniocentisis

- done around 16th week of pregnancy:  sample of the amniotic fluid (contains fetal cells) is taken and a culture is made. When enough cells have been obtained, a karotype can be done to detect chromosome abnormalities. 

- Dividing cells are photographed and using pictures, chromosomes are arranged in homologous pairs. 

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Gel electrophoresis

Electrophoresis - technique used to separate large molecules based on their different rates of movement in an electric field caused by a combination of their charge and their size. 

1. a gel is prepared into a thin layer and placed in a container. Holes are made on side where samples will be placed. (often at least one is an unknown sample) 

2. Equipment is is connected to a source of electricity, and the gel will conduct electricity. 

3. Depending on the charge of the molecules they will be more/less attracted to the other side. DNA fragments are slightly negative and will move towards the positive electrode. They will move through the gel, The speed they move depends on the attraction force and size of molecule. Large molecules move slower. 

Gel electrophoresis is used in DNA profiling. It seperates section of DNA according to size and charge. This creates a pattern of stripes and bands determined by the sequence of organic bases. Every person's DNA is unique so it would be highly unlikely that 2 different people have the same pattern of DNA in the gel.

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Polymerase Chain Reaction

PCR is used to amplify minute quantities of DNA. 

1. Desired DNA is heated which breaks the hydrogen bonds between the strands of the double helix so that they separate. 

2. Primers are added to start the process of DNA replication, and as the mixture is cooled, the primers bond to the original but now single stranded DNA molecules (through hydrogen bonding between complementary base pairs) 

3. Nucleotides and a thermostable DNA polymerase is added. Nucleotides will bond with the "exposed" organic bases of the single stranded DNA. (again through H-bonding and CBP). 

4. DNA polymerase will then join them into a DNA strand. Each original strand has formed a new complmentary strand. These strands are heated and seperated and function as a template for more DNA strands. 

5. A large amount identical copies of the original DNA can be made exponentially.

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Outcomes of sequencing the complete human genome

Human Genome Project - undertaken by the scientific community to determine the location and structure of all genes in the human chromosome. 

The project's aims: Map of the sequence of the nucleotides of the human DNA. Mapping the genes - listing and finding the locus of each human gene. 

Outcomes: - an improved understanding of many genetic diseases 

- production of medicines that are based on DNA sequences to cure diseases and/or genetic engineering to remove the genes which cause the disease 

- to determine fully which genetic disease any individual is prone to (genetic screening leading to preventive medicine) 

- research into a particular disease can now focus on only the gene that is relevant to the disease

- it can provide more information about evolutionary paths by comparing similarities and differences ingenes between species. 

 

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Technique for cloning

Technique for cloning using differentiated animal cells: 

Reproductive cloning: creates a new individual (example: Dolly the sheep). First you take a nucleus from a somatic cell. The nucleus is removed from an egg cell and replaced with the nucleus from the somatic cell. A brief electric shock makes the cell start dividing. It grows into a group of cells, and is implanted into a uterus where it grows into a new individual with the same genetic material as the donated somatic cell.

Therapeutic cloning: involves stem cell research. Human embryos are produced and allowed to grow for a few days into a small ball of cells. These cells are not specialised and when SCNT is used the cells can grow into any of a large number different specialised tissues. Other sources of stem cells are cells from the umbilical cord or cells from aborted fetuses. This cloning is often aimed at cell therapy where diseased cells are replaced with healthy ones. Cell therapy is used for people suffering from Parkinson's disease but may also be possible when a patient has a spinal cord injury. Bone marrow transplants for patients with leukemia, new skin cells for burn victims and to grow new corneas for some visual impairments are examples of therapeutic cloning already in use.

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ABO blood groups

ABO blood groups are an example of codominance and multiple alleles.

The ABO blood group system is based on 4 different phenotypes (group A, B, AB and O) caused by different combinations of 3 different alleles (IA, IB and i).

The alleles IA and IB are codominant so both will affect the phenotype.

The allele i is recessive and will only affect the phenotype when homozygous. The following possibilities exist:

Phenotypes

Genotypes

A

IAIA or IAi

B

IBIB or IBi

AB

IAIB

O

ii

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Colour blindess as an example of sex linkage

Colour blindness is a condition that can be caused by genetic factors. Human eyes contain cells with different pigments that that absorb different wavelengths (colours) of light. If this pigment absorbs light a message is sent to the brain and we see a colour. 

The ability to produce different pigments is mainly gound as genes on the X chromosome. This ability is a dominant allele. 

A female has 2 X chromosomes. For a female to be colour blind she would need 2 copies of the recessive allele. A male has only one X chromosome so will need to have only one copy. 

Allele notation XB for normal vision  Xfor colour blindness..

a female can be   a male can be

 XB XB or XB Xb  -  normal vision XY  - normal vision

Xb Xb   -  colour blind  Xb Y - colour blind

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Hemophilia as an example of sex linkage

Hemophilia is a blood disorder. The process of blood clotting involves a number of different proteins, each having their own gene. If even one of these genes has an allele that does not code for the proper protein, the entire process of blood clotting can be disturbed and even very small wounds will not clot.

In humans the locus for the gene that controls the production of a blood clotting factor is on the X chromosome (i.e. and not on the Y chromosome). This means that if a male has one defective allele he will have the hemophilia condition. However a female will need to have two copies of the defective allele in order to have the condition. Statistically this is much less likely and with the advent of menstruation and child birth, a woman will need blood transfusions containing the clotting factors to survive.

a female can be a male can be

XH XH or XH X normal/carrier XXhemophiliac, very rare  ||    XY or normal  Xhemophiliac, rare  

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Sex chromosomes and gender

Humans have 46 chromosomes in somatic cells. These cells are diploid, so there are pairs of homologous chromosomes which carry the same genes and look the same in a karyotype.  Pairs 1-22 are arranged this way.

The remaining 2 chromosomes are sex chromosomes. Males have one X and Y chromosomes while females have 2 X chromosomes.

The X chromosomes is much larger than the Y chromosome. It contains genes that are not found on Y.

A punnet square can be used to predict the gender of a child. 50/50%

            X          X

X          XX       XX

Y          XY       XY 

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Ethics in therapeutic cloning

Arguments in favor of therapeutic cloning focus on:

  • ability to cure serious diseases with cell therapy: currently leukemia an in the future possibly cancer and diabetes

 Concerns raised about therapeutic cloning:

  • fears of it leading to reproductive cloning
  • use of embryonic stem cells involves the creation and destruction of human embryos (although it is possible to use embryos left over from IVF treatment which would be destroyed anyway)
  • embryonic stem cells are capable of many divisions and may turn into tumours
  • Ethical aspects are difficult to discuss because a lot of benefits are not yet realized. They are potential benefits, likewise some disadvantages are still unknown because cloning does not occur at a large scale. 
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