B1 - NEW 2011 SPEC

Genes: 

• carry instructions that control how we develop and function
• They do this by telling cells to make protiens needed for the body to work.
• Found in the nucleus of the Cell.
• Each gene is a section of a long chemical (DNA) - Coils Of DNA packed into structures are called Chromosomes

  Proteins fall into two groups:

• Functional - enable the body to function. Eg: Enzymes
• Structural - give the body structre. Eg: Keratin in the skin

Human Characteristics are determined by three things:

• Genes (eg: eye colour)
• Environment (eg: scars)
• Or both (eg: weight)

Chromosomes can be arranged into pairs. Human cells have 23 pairs (total of 46).

The sex cells are the eggs and the sperm. They each contain 23 chromosomes.

At fertilisation, the egg and sperm join to produce a zygote- this has a full set of 46 chromosomes. (23 from mother (egg) and 23 from father (sperm)).

The baby that develops has a combination of genes from both its parents. 

Changes to the DNA can sometimes occur causing a mutation. This can happen when the sex cells are being made, or even after fertilisation.

For example, one type of mutation is chromosome mutation - This results in someone having extra chromosomes (eg: someone with downs syndrome will have an extra chromosome 21.)

                                                                                                                                     Sex determination:

• In human males, the sex chromosomes are XY 
• In human females, the sex chromosomes are **

Note- The Y chromosome contains the sex determining Gene, this causes the testes to develop. Without the Y chromosome (in females), ovaries develop

Sexual reproduction causes variation as the offspring has genes from both its parents, hence why the offspring are similar to it's parents because of the genes it inherited from them. Siblings differ, because they inherited different a combination of genes.

We call the complete gene set of an organism, It's Genome.

  The genotype of an organism is its genetic make-up.                                             The phenotype of an organism, is its physical features (things you can see)

Genetic Diagrams such as Punnett Squares and family trees, are used to show genetic crosses. For example:                                                      

Allelles - Different forms of a gene.

An individual is homozygous when two alleles are the same for a characteristic. An individual is heterozygous when two alleles are different for a characteristic.

Alleles can be Dominant - If atleast one of these is present, the trait shown will be dominant (usually represented by a capital letter eg: DD.)

Or alleles can be Recessive - This trait is only seen is there are two recessive alleles present (represented by a lowercase letter eg: dd.)

Body cells have pairs of chromosomes; sex cells have one chromosome from each pair. 

Chromosomes have the same type of genes in the same place on each chromosome of the pair.

Single gene disorders are caused by fault alleles of a gene.

The faulty allele can be dominant or recessive. However, for recessive single gene disorders, a person with a single recessive gene will not have the disorder but will be a carrier (the normal gene is the dominant one).

So, if they marry someone else who is a carrier for the same gene disorder, then their child could have the disorder since two recessive alleles cause a recessive disorder to occur (eg: cystic fibrosis. symptoms for this include:                                                                     the production of thick gluey mucus that affects the lungs and makes digesting food difficult, breathing problems and chest infections)

Huntington's disease, is a dominant dosorder (the presence of a single dominant allele will cause the disease. Symptoms for this include:                                                    tremors, memory loss, mood changes, inability to concentrate

A punnett Square or family tree shows the risk of inheriting a disorder/being a carrier.

Genetic Testing is carried out when genetic disease runs in the family. It is carried out on a large scale (eg: newborns where there is no history of a disease)                                  Embryos produced by IVF (In-vitro fertilisation) can be screened before implantation (Pre-implantation genetic diagnosis -PGD)                                                                    PGD and embryo research are carfully monitered in the UK.                                        Genetic testing suring pregnancy may involve cell sampling by:

• Amniocentesis (collecting cells from developing fetus which are in amniotic fluid)
• chronic villus sampling (testing sample of cells taken from placenta) - both tests carry 1% chance of miscarriage

There are many implications of  testing for a genetic disease, some are ethical whilst others are scientific:

• tests carry risk of miscarriage
• tests not 100% reliable (eg: false positive and false negative)
• decision as to whether to abort the pregnancy or have the child
• should an employer, the family or an insurance company be told?

Natural Clones are individuals with identical genes so any differences between individuals must be due to environmental factors.

Some organisms reproduce asexually to produce clones. (eg: plants:

• using runners
• producting bulbs)

Identical twins are formed when cells of an embryo seperate.

Artificial Clones can be produced when a nucleus from a body cell is transferred to an unfertilised egg. 

Embryonic Stem Cells can develop into any type of specialised cell. Adult stem cells can develop into fewer cell type. 

Being unspecialised, stem cells have potential in the treatment of disease.

Advantages of cloning/ reproducing asexually:

• successful characteristics are seen in offspring
• asexual reproduction is useful where plants and animals live in isolation

Disadvantage of producing clones is that there is no genetic variation. This means that if conditions change or there is disease, the population could be wiped out.