Chap 5: Beyond Mendel's Law Flashcards
Lethal Alleles
When one has a double dose of a dominant allele it may cause death before the individual can reproduce.
Example: Achondroplastic Dwarfism
Multiple Alleles
Individuals carry two alleles for each autosomal gene but a gene can have multiple alleles. The combinations can produce variations in the phenotype.
Example: ABO type blood
Incomplete Dominance
This is when the heterozygous phenotype is between those of the two homozygotes.
Example: Familial Hypercholesterolemia
A heterozygous individual affected by this allele has half the normal number of receptors in the liver for LDL cholesterol because the dominant allele has incomplete dominance. However, if he/she were homozygous for the mutant allele, he/she would totally lack the receptors so their serum cholesterol levels are very high.
Codominance
The heterozygous phenotype is the result of the expression of both alleles.
Example: ABO blood type. IA allele produces A antigen, IB allele produces B antigen, iO allele does not produce an antigen. IA and IB are codominant and to i.
Epistasis
One gene masks or affects the expression of a second gene. This is not between alleles of the same gene.
Example: The H gene which places a molecule at the cell surface to which the A or B antigens are attached, is epistatic to the I gene. W/O it (hh genotype), the A or B antigens cannot be attached to the surface of the RBC. Therefore, the hh phenotype have the phenotype of type O, although the ABO blood group can be anything.
Penetrance
The all or nothing expression of a single gene. An allele combination that expresses a phenotype in everyone who inherits it is completely penetrant.
Example:
Huntington Disease is nearly completely penetrant meaning that almost everyone who inherits that mutant allele will develop symptoms if they live long enough.
Incomplete Penetrance
Some individuals do not express the phenotype or show no symptoms.
Example:
Some individuals with the dominant polydactyly allele have extra digits, whereas some individuals with that allele do not express the extra digits but we know they have the allele b/c they have an affected parent or child.
Expressivity
The severity and extent of the phenotype.
Example:
One individual with polydactyly may have extra digits on both hands and both feet. Another individual may have just one extra fingertip.
Pleiotropy
One gene controls several functions or has more than one effect.
Example:
One individual with MS has issues with muscle weakness and GI complications while another individual has loss of vision.
Porphyrias
Diseases that result from deficiencies of any of several enzymes required to make heme. An intermediate biochemical builds up and may be excreted in urine or accumulated in tissues causing symptoms.
Symptoms include reddish teeth and photosensitivity. They may have inspired vampire and werewolf legends.
Genetic Heterogeneity
Different genes can produce identical phenotypes. Different forms of the disease can develop b/c there are many ways that the mutation can disrupt normal function.
There are 132 recessive genes that cause hearing loss. There are at least 2 different genes involved in Osteogenesis imperfecta and at least 4 different genes involved in Alzheimers Disease.
Phenocopy
When a gene appears to be inherited but is caused by the environment.
Example:
Fetal alcohol syndrome being caused by alcohol consumption during pregnancy. PKU affected mother not following diet will pass elevated levels of phenylalanine to fetus.
Mitochondrion
Organelle which provides cellular energy. Contains small circular DNA called mtDNA, 37 genes w/o noncoding sequences.
There is no crossing over and little DNA repair, high exposure to free radicals and a greater mutation rate than nuclear DNA.
Transmitted from mother to all offspring.
Mitochondrial Disorders
These genes encode proteins that participate in protein synthesis and energy production. Several diseases result from mutations in tDNA.
Ooplastic transfer technique can prevent women from transmitting a mitochondrial disorder.
Examples:
Mitochondrial myopathies- weak and flaccid muscles
MERRF- (myoclonic epilepsy w/ ragged red fibers)
Leber Optical Atrophy- impaired vision
Heteroplasmy
When the mtDNA sequence is no the same in all copies of the genome, causing the mitochondrion to have different alleles for the same gene. When the cell divides, the mitochondria are distributed at random into daughter cells at each division. Heteroplasmic oocytes transmit a differing number of copies of a mutant mtDNA and the phenotype reflects the proportion of mitochondria bearing the mutation.
Linkage
Genes which are close on the same chromosome do not assort independently in meiosis. They are usually inherited together in a gamete. They do not produce typical Mendelian ratios.
Recombination
Chromosomes recombine during crossing over in prophase I of meiosis.
Crossing over disrupts linkage.
Parental Chromosomes
Have the original configuration.
Recombinant Chromosomes
Have new combinations of alleles.
The correlation between crossover frequency and gene distance is used to construct linkage maps.
