Exam 2 Flashcards
Wild type allele
occurs most frequently in a population
often dominant
standard against which mutations are compared
Loss of function mutation
mutation causes diminution or loss of wild-type function
Null allele
loss of function mutation with complete loss; produces no functional gene product, usually recessive
Gain of function mutation
enhances function of wild-type product, usually by increasing its quantity
usually dominant
Drosophila allele system
allele written uppercase or lowercase depending on whether mutation is dom or recessive
wild-type alleles (nonmutant) are designated with a superscript +
Incomplete / partial dominance
crossing two parents with contrasting traits produces offspring with an intermediate phenotype
for example, red + white flowers = pink flowers
neither allele is dominant
Codominance
two alleles of a single gene are responsible for producing distinct gene products
Joint expression of both alleles in a heterozygote
both are expressed; don’t ‘blend’ like in incomplete
Molecular basis of complete dominance
the heterozygote creates a nonfunctional protein with the recessive gene, and a functional protein with a dominant gene
dominant gene product determines the trait
recessive homo doesn’t produce any functional proteins
Molecular basis of incomplete dominance
caused by “dosage” effect:
- two doses produces the greatest amount of functional protein
- one dose produces less, not fully reaching homo phenotype
- zero doses, no functional protein
i.e, products in a dom homo are sufficient to produce the phenotype, hetero are insufficient to fully reach the phenotype; in a recessive, no products
Cause of lethal alleles
represent interruptions to essential genes, such as deletion mutations
recessive lethal allele
may produce unique mutant phenotypes when heterozygous
kills when homozygous (not necessarily when homo recessive; dominant genes can be recessive for lethal; just means 2 copies are needed)
dominant lethal allele
one copy of the allele is enough to kill
much rarer because harder to pass down
usually develop later in life after offspring have been produced
Pleiotropy
single gene can have many different effects
the gene product has multiple functions, thus affecting many phenotypes
- protein may be used in several different places
- other processes may depend on protein
gene interaction
several genes influence single characteristic
does not imply interaction between products directly, rather: cellular function of gene products contribute to phenotype
Epigenesis
each step of development contributes to the final appearance
gene products may exist in a biochemical pathway dependant on the functioning of several genes
Epistasis
expression of one gene or gene pair masks or changes the expression of another gene or gene pair, due to the implications of one product on the other
Recessive epistasis
recessive genotype masks expression of another dominant gene
Dominant epistasis
dominant allele at one locus masks expression of all alleles at a second locus
Complementary gene expression
Both genes work together to produce a final product
Complementation
Two genotypes may cause the same phenotype
Thus they can produce offspring without the phenotype
Complementation Analysis
an experimental approach used to analyze the cause of a phenotype
IF NORMAL = COMPLEMENTATION
IF ABNORMAL = NO COMPLEMENTATION
[complementation] normal development
mutations are in separate genes, not alleles of each other
following cross, heterozygous for both genes; normal products of both genes are produced by normal copy of each
complementation occurs
[complementation] abnormal development
mutations affect same gene and are alleles, so no normal gene product
complementation does not occur
Complementation groups
All mutations belonging to a single gene
will complement mutations in other groups
with large numbers of complement groups studied, it is possible to predict # of genes involved in determining a trait