General concepts and single gene disorders Flashcards
What is a phenotype?
The physical description of a character of an individual organism
What is a character?
A structure, function or attribute determined by a gene or group of genes.
What are traits?
Alternate forms of character
What is a gene?
Section of DNA that codes for a specific protein, located on a chromosome
What is a genotype?
The genes an individual has at a particular locus, responsible for the observed phenotype.
What is penetrance?
The chance a given genotype will cause a particular phenotype.
What is an allele?
Different types of the same gene, leading to the same character but a different trait.
Are genetically determined disease phenotypes common in the population?
No, offspring carrying genes are less likely to reach reproductive age and pass on genes
Are (poly)genetically and envrionmentally determined disease phenotypes common in the population?
Yes, symptoms are often less deadly therefore offspring carrying genes contributing to a diseased phenotype are more likely to pass the genes down to the next generation.
Are completely environmental disease phenotypes common in the population?
No, very rare
Describe autosomal dominant inheritance of disease
Two copies of the relevant gene but only one needs to be mutated to express the disease.
Are heterozygotes affected by autosomal dominant diseases?
Yes, depending on the penetrance of the mutation.
Describe autosomal recessive inheritance of disease
Two copies of the relevant gene but both copies must be mutated to get the disease. Heterozygotes are therefore described as unaffected carriers.
What are the chances of two carriers (of autosomal recessive condition) having an affected child, carrier or normal child?
25% - affected
50% - carrier
25% - normal child
Of the children had from two carrier parents (AR), of they are not affected, what is the chance they are a carrier?
2/3
Describe X-linked recessive inheritance of disease
Diseased gene on X chromosome, females can be carriers, can make daughters carriers (50%) but can make sons have disease (50%)
Why is a female carrying a diseased X chromosome (recessive) not normally affected?
The other acts as a backup, X inactivation is random and normally occurs to the diseased chromosome
Describe X-linked dominant inheritance of disease
Mother will be affected, one mutant X and one normal.
If passed onto daughter, will be affected but due to X inactivation will survive. (50%)
If passed onto a son, then the boy only has a mutant X chromosome and a Y chromosome from father. He is unlikely to survive. (50%).
Why can only females usually live with X-linked dominant conditions?
X-inactivation
Describe mitochondrial inheritance of disease?
Only maternally inherited.
The mutation load varies per egg cell and is hard to predict (cells have many mitochondria).
Offspring can therefore either be carriers or affected.
What is uniparental disomy?
Inheritance of two chromosomes from one parent and none from the other parent.
Why is it bad to inherit two autosomes from one parent and none from the other?
Some genes are imprinted meaning that only one copy from one specific parent is used, therefore if that copy is not inherited due to uniparental disomy, we get a condition.
What is a mutation?
Heritable alteration or change in the genetic material. These are usually pathogenic but also drive evolution
What is a polymorphism?
A non-pathogenic alteration in DNA which may alter protein function but is not usually deleterious
