UNIT 1

All oganisms develop following a set of instructions that are coded inside the cell in the nucleus. The instructions control how the organism develops and functions. The basic unit for the instructions is called the gene  

Each DNA molecule cosists of two strands, witch form a double helix.
DNA has four bases a-t and g-c

Genes are sections of DNA that describe how to make protiens. these may be structual- for collagen in skin. Or functional- for enzymes such as amalayse, whitch breaks down starch. some characteristics are coded for by gemes working together e.g. eye colour.  

origanlly it was belived that eye colour was due to a single gene. it is now known that there are a number of genes coding for the diffrent pigmentsin the iris, mainly on chrosome 15. this means their is an emourmas variation in eye colour.

genetic variation- the diffrent characteristics that an individual inherits: dimples or not

enviromental variation- how the enviroment changes an individual: cutting the skin cousing scars, percings, hair style,

Genotype- is tyhe term describing the genetic makeup of an organism( the combination of allels)

phenotype- describes the physical characteristics the organism has.

alleles are described as being dominant or ressesive

a dominant allele is one that controles the development of a characteristic even if it is present on only one chromosome in a pair.

a ressesive allele controls the development of a characteriositc onlt if the dominant allele is not present, i.e. both chrosomoses have the ressesive allele prresent  
 

example the ability to roll ypur tounge is dominant. if your alleles were TT or Tt then you wilol be able to roll your tounge. if you had tt then you would not .
 if you have Tt then you are a carrier of that allele but the dominant allele is expressed.

homozygous- when the two allelles are the same eg TT

heterozygous- when the two alleles are diffrent eg one dominant one ressesive eg Tt

gamates only carry one copy of each chromosome. this is the basis of sexual reproduction

humans have 23 pairs of chromosomes in their body cells.(46 in total) the sex cells have half the amount i.e. 23 single chromosomes.

when fertiolisation has taken place (when the egg and the sperm fuse together) the total number of chromosomes doubles as they pair up again. so they zygote has 23 pairs (46 in total)

as the pairing up of the chromosomes is randon, the new offspirng will differ from its parents. this leads to variation, a major advantage advantage of seual reproduction.

the child will share similatities with it parents depending on which characterisitics have come from the father, which have come from the mother and which ones are dominats and recessive.

The child will also differ drom any brothers and sisters.

the sex of an embryo is determind by a gene on the y chromosome called the sry gene. if the gene is not present meaning there are two x chromosomes present the embryo will develop into a female and ovaries will grow.

if the gene is prenesnt that means there is an x and y chromosome present, the embryo will then develop into a male and testes will beging to develop.

The sperm that fertilises the egg either carries an x or y chromosome that is how the embryo get two chromosomes. if the sperm has a y chromosome it will be a boy if it carries the x chromosome it will be a girl.

           A                a

A        AA            Aa                       genotype ratio 1:2:1

a        Aa               aa                     phenotype ratio 3:1

six weeks after fertilisation, the undifferentiated glands start producing a hormone called androgen. Specilised receptors in the developing embryo detects the andrygen. This stimulates the male reproductive organs grow.

Sometimes the y chromosome is present but the angrogen is not detected. When this happens the embryo develops all the female sex organs exept the uterus. the baby is born with female body but will be infertile. The baby will look like a gril.

Huntington's disease is a genetic disorder that affects the central nervous system. It is caused by a falty dominant allele on chromosome 4.

The allele that causes the disease results in damage to nerve cells in certain areas of the brain. this leads to gradual physical, mentaol and emotional changes that are expressed as symptoms.

the symptoms normally develop in adulthood, which means suffers may have already had children and passed on the gene. syptoms include late onset, a tremor, clumminsess, memory loss, an inability to conectrate and mood changes.

everyone who inherits the huntington;s allele will develop the disease. this is due to the allele being dominant. one one parent needs to pass on the allele for a child to develop the disorder.
if one alult has Hh and the other hh the there will be 50% chance  

CF is the uk's most common life threatening genetic disease. it affects the cell membranes, causing a thick micus to be produced in the lungs, gut and pancreas.

symptoms: difficulty breathing, incread number in chest infections and difficulty digesting food.

CF allele is ressesive. therefor, if an individual is a carrier (has only one ressesive allele) they will not have the disease. They can however pass it on to their children.