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Introduction to medical Genetics Definitions Phenotype- physical description of a character in an individual organism (E.g-green pea) Character- structure/function/attribute determined by a gene or a group of genes Trait- alternate forms of the character Genotype- genes an individual has at a particular locus, it is responsible for the observed phenotype Penetrance- chance that a given genotype will cause a particular phenotype- usually referred to mutations "the chance the anjelina jolie who has the breast cancer gene will get breast cancer" Locus-a location of a gene within the genome of the individual Gene-inherited units of information that specify the phenotype at gross level and at a molecular level particular products such as proteins Genes and disease
-some disease are only due to genes- Achondroplasia is due to mutation of the FGF3 receptor which leads to premature differentiation of chondrocytes leading to shortening of the limbs.
-most diseases are due to the interaction of genes and the environment
-inability to metabolise phenylalanine (amino acid)-the protein accumulates in the brain and leads to mental retardation
-some disease are only due to environment- lead poisoning Modes of inheritance- SINGLE GENE DISORDERS!!!!
-2 copies of a gene- only one needs to be mutated to get the disease, so manifests in the heterozygous state Pattern of inheritance: every generation is affected, male and females are equally affected, both males and females can transmit disease-chances of passing on the mutation is 50%
-heterozygotes are affected but is subject to penetrance of the mutation- in some cases the disease skips a
generation due to the penetrance of the disease which results in some cases non manifestation of the disease
-there is a variable manifestation of the dominant allele due to the expressivity E.g- Achondroplasia (Dwarfism)/Huntington's disease (damage of the nerve cells of the brain)/Marfan syndrome /familial breast cancer
-some autosomal disorders an affected person may have no family historythis could be due to a new mutation arising in the transmission b) Autosomal recessive
-2 copies of the relevant gene-both copies need to be mutated in order to inherit the disease
-heterozygotes are unaffected carriers
-chance of inheriting the disease from 2 carriers is 25%, there is 50% of being an unaffected carrier Pattern of inheritance: there is often a skipping of generation, the males and females are equally affected. It usually affects individuals in one generation in a single sibship and does not occur in previous and subsequent generations. Often people suffering from recessive conditions have parents that are blood linked-this is known as consanguinity- the genes are common by descent
-if a child is born from carriers but is unaffected the probability of being a carrier is 2/3 E.g- Tay-Sachs disease/cystic fibrosis/sickle cell anaemia/albinism
COMMON EXAM QUESTION!!
-Tay-Sachs disease is an autosomal recessive disease that is common in the Jewish populationleads to severe neurological degeneration-unable to walk/communicate normally
-Rachel and Lev are planning to have children-rachel's brother has TaySach's-what is the chance of Rachel inheriting the disease? 2/3 NOT 50% both parents will be carriers of the mtated gene so Rachel's genome could
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