Inheritance

• Genotypes - the genetic makeup of an organism, in terms of the alleles it contains
• Homozygous - An organism with two identical alleles for a gene
• Heterozygous - An organism with two different alleles of a gene
• Phenotype - characteristics expressed in the organism. These are determined by its genotype and the environment it develops in.
• Dominant - an allele always expressed in the phenotype, even if a different allele for the same gene is present in the genotype
• Recessive - an allele only expressed in the phenotype in the presence of another identical allele. 
• Codominant - both alleles are expressed in the phenotype of a heterozygote
• Linkage - two or more genes located on the same chromosome. The linked alleles are normally inherited together because they do not segregate independently at meiosis, unless chiasmata have been formed between them. Alleles from one chromatid become linked to alleles on the other chromatid - linkage reduces the number of phenotypes resulting from a cross
• Sex Linkage - a characteristic that's gene is found on one of the sex chromosomes

...a form of gene interaction where one gene surpresses the phenotypic expression of another gene at a different locus - no new phenotypes are produced

• Epistatic Alleles - the alleles masking the effect of another allele
• Hypostatic Alleles - the alleles whose effect is being masked

Dominant Epistasis - if an epistatic allele is present, it masks the effect of the other allele

Recessive Epistasis - it requires two epistatic alleles to mask the effect of the other allele

Duchenne Muscular Dystrophy:- on the x chromosome

• Dystrophin is a protein needed for muscular contraction. Mutations result in a severly truncated version or none at all. 
• Boys with the disease develop muscle weakness and are often wheelchair bound by 10. Death often occurs due to complications of muscle degeneration by their early 20's

Cystic Fibrosis:- caused by a mutation to a gene that disrupts the transport of CL- ions and water across membranes. It changed the shape fo the transmembrane Cl- ion channels so they no longer function correctly.

In the lungs, cilia are not properly hydrated and cannot shift the mucus. this accumulates, dehydrates and bacterial infections may occur and lead to lung failure

• Heterozygotes - some abnormal channels but enough for the lungs not to function (Carriers)
• Homozygous dominant - normal alleles
• Homozygous recessive - sufferer cfcf 

Haemophilia A:- the recessive allele expresses an altered protein that does not function, which leads to an increase in blood clotting time. Internal bleeding and into joints is particularly harmful

males have only one x chromosome and if it has the allele for haemophilia A, they will be sufferers. These males are hemizygous - they have only one allele for the characteristic

• 50% of males will suffer from a carrier mother and normal father

Sickle Cell Anaemia:- mutation of the beta strand of haemoglobin by valine instead of glutamic acid at codon 6.

When abnormal haemoglobin is deoxygenated, it becomes crystallin and more linear. This deforms the red blood cell making them inflexible and unable to squeeze through capillaries. After many cycles, cells become irreversibly sickled/destroyed. if they becomes ladged in capillaries, blood flow is impeded. Organs and bones don't recieve enoguh oxygen leading to painful crisis. THe heart, lungs and kidneys get damaged particularly.

It is a recessive disease but codominant allele - in heterozygotes, red blood cells are made half sickled, half normal so it prevents the cell sickling