Patterns of Single Gene Inheritance Flashcards
Genotype
genetic constitution of an individual or locus
phenotype
outward characteristics of an individual or gene product
Allele
One of both versions of a gene or DNA sequence at a given locus
Locus
Position occupied by a gene on a chromosome
Homozygote
Genotype will identical alleles at a given locus
Hemizygous
Genotype with a single allele for a given chromosome segment. Males are hemizygous for X.
Heterozygote
Genotype with different alleles at a given locus
Compound Heterozygote
genotype with 2 different mutant alleles at one locus
Polymorphism
alternate genotypes present in population greater than 1%
Penetrance
The proportion of mutant individuals manifesting disease. It is an All or None effect.
Expressivity
The extent to which a mutation exhibits a phenotype. The extent to which the disease manifests itself.
Carriers
Have a single mutant allele that is obscured by the normal copy
Genetic Heterogeneity
Can result from different mutations at 1 locus (allelic), or from mutations at different loci. It is where different genes contribute to the same disease or phenotype such as height.
Phenotypic Heterogeneity
Occurs when the same mutations manifests itself differently among individuals.
Sex-Linked
encoded on the X-chromosome (or Y-chromosome) thus conferred together with gender.
Autosomal
encoded on the autosomes or the numerical chromosomes, all chromosomes but X and Y
Recessive
Both alleles must be affected for the trait to be displayed
Dominant
A single mutant allele confers a phenotype
Recessive Inheritance Patterns
For single gene disorders, a phenotype is considered recessive if it manifests itself in the homozygotes. Needs two copies of the recessive allele to manifest disease. Individuals carry 1-5 recessive alleles that are lethal in homozygotes. Recessive disorders often display a clustering of disease among siblings and is absent among ancestors, although consanguinity may be present. Both parents are carriers and the recurrence risk for siblings is 1 in 4. Unaffected siblings of the proband have a 2/3 chance of being a carrier. Males and females are equally affected and parents are asymptomatic carriers. These seem to come out of no where!
Dominance Inheritance Patterns
A characteristic is dominant if it manifests itself in the heterozygote. If the disease is not lethal, the disease phenotype is usually seen in every generation. A child of an affected parent has a 1/2 chance of being affected. Males and females are usually equally affected.
Codominance
Both alleles, the dominant and recessive allele, are expressed equally in the heterozygote. This is seen with A and B blood type to get type AB
Incomplete Dominance
Get an in-between in the heterozygote. It is not red, not white, but pink. Each cell expresses half, not one or the other. The dominant allele is not quite as dominant as we think.
Haploinsufficiency
The single normal copy produces an insufficient quantity of the normal gene product for the requirements of the organism. An example is Familial Hypercholesterolemia in which now only 50% of the normal LDL receptor is present and this level is not enough to maintain blood cholesterol levels in the normal range.
4 things determine that a mutant allele is dominant
1) The single normal copy produces an insufficient quantity of the normal gene product for the requirements of the organism (haploinsufficiency)
2) The product of the inactive mutant gene interferes with the function of the normal gene product (dominant negative effect)
3) The product of the mutant gene acquires a new or enhanced function (simple gain of function)
4) The affected gene is a tumor suppressor resulting in predisposition to cancer that is inherited as a dominant trait because even a single cell losing the function of the other allele by mutation is enough to cause cancer.