chapter 4 Flashcards
wild type alleles
most common allele in natural populations
-can be dominant or recessive
-Since populations change over time (evolve) wild-type alleles can change
If a genotype is homozygous wild-type, both alleles produce functional protein (ex:1 wildtype allele produces 50 units of enzyme activity, therefore 2 functional alleles produce 100 units of enzyme activity)
Loss of Function - Null Allele
produces no protein. A homozygous null genotype produces no protein while heterozygous genotypes produces 1 copy
(Ex: null allele produces 0 units of enzyme activity.
Haplosufficint: 1 copy is enough for wild-type phenotype. Haploinsuffienct: 1 copy is not enough for wild-type phenotype)
Loss of Function - leaky allele
produces less protein, or a protein with diminished function compared to wild-type alleles
(Ex: Leaky allele produces 25 units of enzyme activity)
dominant negative mutations and what are they relevant for
examples of loss of function mutation
result from impaired interactions between 2 genes
relevant for multimeric protein that forms from protein products of multiple genes ( a mutation at one gene can alter the peptide product, which does not properly interact w/the peptide from a second gene
Gain of Function Mutations
produce more gene activity than wild-type alleles (hypermorphic mutations) ex: gene is duplicated, mutation affects regulation of gene expression etc.
These mutations may also result in protein products with novel function (neomorphic mutation). These typically result in dominant phenotypes.
incomplete/partial dominance and an example
dominance of one allele may not completely mask the recessive allele.
Heterozygous individuals have a phenotype that is intermediate or ‘blended’ to each homozygous phenotype
-Two or more genes may interact, affecting a single trait
-The expression of a trait may also involve an interaction between genes and the environment
red flowers crossed with white produce pink F1 (phenotype literally blends for heterozygotes)
Is it true that some genes may have more than 2 alleles
yes, ex: blood type in humans ABO
allelic series and an example
result from a hierarchy of dominance and partial dominance across multiple alleles
Fur colour in rabbits is an example of an allelic series (more than 2 alleles). Tyrosinase: an enzyme involved in production of melanin. Affects fur pigments.
Co-dominance
co-dominant phenotype is a unique phenotype (not a blended intermediate phenotype in heterozygotes)
-similar to incomplete dominance since heterozygous phenotypes are different from homozygous phenotypes
ex:blood group phenotypes in humans
lethal alleles
recessive alleles (heterozygotes can carry these), found in very low frequencies in populations, detrimental that can cause death
example of lethal allele
in mice wild type fur colour is agouti (combo of yellow and black pigment), the mutant allele A^Y results in yellow fur for heterozygotes (AA^Y)
Mice w/ 2 copies of the A^Y allele (A^Y A^Y) die during gestation
complete penetrance
if a given phenotype always produces the same phenotype
cases where different phenotypes can result from the same genotype:
-sex limited traits
- sex influenced traits
- incomplete penetrance
- variable expressivity
sex-limited traits
cases where a phenotype is only expressed in one sex (but both sexes carry the gene)
(usually a result of hormones interacting with the expression of the gene, resulting the phenotype for one sex)
ex: mammalian lactation is female specific
horn development is male specific in hoofed animals
sex-influenced traits
when both sexes carry a gene, but the phenotype varies between the sexes (unlike sex-linked, sex-influenced can result in phenotypes found in both sexes)
ex: in some goat breeds, a heterozygous genotype (B1B2) results in a bearded chin in males, homozygous phenotypes (B2B2) results in a bearded chin for both sexes
penetrant
when the phenotype of an organism matches its genotype
nonpenetrant
when an individual with a certain genotype does not produce the associated phenotype
fully penetrant
when the genotype always expresses the associated phenotype
incomplete penetrance and how it looks on pedigree
traits that are non penetrant in some individuals but penetrant in others
pedigree: looks like recessive trait but it is seen in every generation
what is polydactyly
autosomal dominant trait that results in extra fingers, 25/30% of individuals who pass the allele have normal hands
example of incomplete penetrance
caused by a dominant but also incompletely penetrant allele
variable expressivity
like incomplete penetrance, however phenotypes are variable rather than being present or absent (variation caused by interactions w/other genes or environmental effects)
Waardenburg syndrome
family members may all have the syndrome (same genotype) but show different combination of symptoms (hearing loss, diff eye colours, white forelock of hair, premature graying of hair)
-autosomal dominant condition
Gene-environment interaction and an example
the influence of the environment on expression of genes and on the phenotypes of organisms
ex: PKU (phenylketonuria) autosomal recessive condition caused by the absence of an enzyme involved in phenylalanine breakdown, infants are normal at birth, over time the phenylalanine builds up and causes serious toxicity to developing neurons (neurological issues)
preventative measure: restrict phenylalanine in child’s diet, this is a environmental solution to help those with the condition live normal lives
pleiotropy and example
when one gene affects many phenotypes
ex: in drosophila a hormone called juvenile hormone (JH) influences various traits in flies including development time, body size, life span
-sickle cell anemia in humans (1 base pair has point mutation (A to T), codes diff amino acid (glutamine to valine), abnormal hemoglobin
