chapter 4 part 1 Flashcards
types of interactions in which genes interact to influence a phenotype
-may be more than two alleles for a given locus within a population
-dominance of one allele over another may be incomplete
-two or more genes may affect a single trait
-the expression of a trait may depend on the interaction of more than one gene &/or the interaction of genes with non-genic factors
two alterative forms that mendel chose
-one form was completely dominant over the other so the heterozygous for the 2 alleles showed the dominant phenotype
-research in the early & mind-20th century was aimed at better understanding the molecular nature of dominance
the terms dominant & recessive have a ________ basis
biological
what is dominance of one allele over another determined by?
the protein product
haplosufficient
-when making one copy of the normal allele is enough for normal function (mutant allele is recessive)
haploinsufficinet
when making one copy of the normal allele is not enough (Tt) for normal function (you need two copies of the normal one to be sufficient) so the mutant allele is dominant
wild-type allele
always produced when an organism has two copes of the wild type allele
what are the categories that mutant alleles can be?
-gain-of-function
-loss-of-function
gain-of-function
when the gene produce aquires a new function or shows an increased level
loss-of-function
when there is a significant decrease or complete loss of functional gene product
null/amorphic mutations
loss of function mutations that produce no functional product
-often are lethal when homozygous
leaky/hypomorphic mutations
-loss-of-function mutations that result in a partial loss of function
-severity of the phenotype of these depends on the level of activity of the leaky mutation
what are particularly subject to dominant negative mutations
multimeric proteins, composed of two or more polypeptides that join together to form a functional protein
why are dominant negative dominant?
due to loss of function of the multimeric protein complex due to an amino acid change of one subunit
why are dominant negative mutations “negative”?
-due to their “spoiler effect” on the protein as a whole
are gain of function mutations usually dominant or recessive?
usually dominant and can be hypermorphic or neomorphic
hypermorphic mutations
produce more gene activity than wild type
neomorphic mutations
aquire novel gene activities not in the wild type
incomplete dominance or partial dominance
when heterozygous individuals display intermediate phenotypes between either homozygous type
-typically the heterozygote will be more similar to one of the homozygous types than the other
how many different ABO blood types resulting in how many different combinations of alleles?
4
3
Blood type A
-RBCs have type A Antigen on their surface IAIA or IAi
Blood type B
-RBCs have type B antigen on their surface
IBIB or Ibi
blood type AB
-RBCs have both A and B antigens on surface
-IbIA
blood type O
-RBCs have neither A or B antigens
-ii
what blood types are codominant?
IA and IB
-this produces heterozygotes with a phenotype different than that of either homozygote
what is completely dominant over the i allele?
IA and IB
-i is fully recessive
the two blood group antigens are what?
glycolipids with the lipid portion anchored in the red blood cell membrane
Bombay Phenotype
-no H antigen (h/h)
-not A, B, or O
-1 in 8000 in Taiwan
-1 in 10000 in India
-1 in a million in Europe
-no adverse effects
what are the blood groups based off of?
a 5 sugar molecule called the H antigen which is modified by the addition of an extra (6th) sugar of one type (A) or a different type (B) or no extra molecule added (O)
what gene is responsible for coat color in mammals?
the C gene
what does the C gene produce?
-enzyme involved in melanin production
-there are dozens of alleles of the gene
-4 form a clear Allelic Series
what does C produce?
a functional enzyme & full coat color
what does Cch produce?
a “dulte” phenotype called chinchilla
-enxyme that is less than 20% active (hypermorphic)
what does ch produce?
a phenotype called Himalayan with little pigment on the body but full color on extremities (termperature sensitive protein)
-enzyme is temperature sensitive
-functional at lower temperatures (extremeties)
-non-functional at higher temperatures (core)
what does c produce
a fulle recessive null allele & produces an albino phenotype
- the c allele produces no functional enzyme (amorphic)
what does the C allele produce
a tyrosine enzyme that is 100% active & haplosufficient
lethal alleles
-some single-gene mutations are so lethal that they cause death in the organism
-these are caused by lethal mutations, which are inherited as recessive alleles (all homozygous die)
-lethal alleles can be detected as distortions in segregation ratios caused by one or more missing classes of progeny
in mice what is wild type coat color called
agouti
-produced by a combination of yellow & black pigments along each hair
-a dominant allele, AY, causes yellow pigment to be deposited along the entire hair, resulting in a yellow coat
in mice, the AY produces what?
fully yellow hair in heterozygotes
this allele is lethal in the homozygous state
-all yellow mice are heterzygous
all yellow mice are what?
heterozygous
the outcomes of the corss A/AY x A/AY
-1/4 A/A agouti
-1/2 A/AY Yellow
-1/4 AY/AY = pre-implantation embryonic lethal
the AY allele is
dominant for yellow coat color
-recessive for survival
what is the AY mutation caused by?
a deletion that affects two genes, agouti and raly
what does raly produce?
a protein essential for mouse development
what does the deletion of raly protein cause
deletion connects the raly promoter to the agouti gene
-this destroys the raly coding sequence
what do heterozygote mice (A/AY) produce?
an excess of yellow pigment
what do homozygote mice produce (AY/AY)
they die to the lack of the raly protein
what can dominant lethal alleles do?
they can sidestep natural selection if they have a delayed age of onset
-abnormalities not expressed until after the affected individual has reproduced
what is a prominent example of dominant lethal alleles?
huntington’s disease
how can the sex of an organism influence gene expression?
due to differing hormone profiles
sex-limited traits
-both sexes carry the genes for such traits, but they are only expressed in one sex
examples of sex-limited traits
-mammalian breast and ability to produce milk
-horn development is limited to males in sheep, cows, and other hoofed animals
-behavioral traits, especially related to mating are influenced by sex
sex-influenced traits
those where the phenotype corresponding to a particular genotype differs depending on the sex of an organism
sex influenced traits example
-chin beard in certain goat breeds
-B2B2 = M & F bearded
-B1B1 = M & F beardless
-B1B2 = phenotype influenced by the presence of male hormones where males are bearded and demales are beardless
pentrance
observed phenotype is consistent with the genotype
non-penetrence
the phenotype associated with the genotype is not visible
fully penetrant
when the expected phenotype is always expressed from a particular genotype
incomplete penetrence
-traits/genotypes that are nonpenetrent in a portion of the population
example of non-penetrence
polydactyly is an autosomal dominant condition where affected people have more than 5 fingers an toes
-the dominant allele is non-penetrant in about 25-30% of the population (not showing, but have the allele)
variable expressivity
seen in individuals who show a phenotype but to a varying degree of severity
variable expressivity example
waardenburg syndrome has four principle features
-each person in the pedigree who has the syndrome has the same phenotype but shoes a different combination of the symptoms
environmental modification example
-PKU is caused by the absence of an enzyme involved in breaking down phenylalanine which is toxic to developing neurons
-key to preventing PKU is restricting phenylalanine in the diet of infants with PKU
