Lecture 5 Flashcards
Non-Mendelian patterns include
allelic series where more than two alleles produce a traits (ex. ABO blood groups)
and maternal effect- determination of the phenotype by the genotype of the mother (ex. shell coiling)
Genetic maternal effect often arises because
the maternal parent produces a substance, encoded by her own genotype, that is deposited in the cytoplasm of the egg and that influences early development of the offspring.
shell coiling
in Lymnaea peregra species, most snails are dextral, but a few sinistral snails occur in some populations.
the phenotype of a snail was determined not by its own genotype but by the genotype of its mother
polygenetic inheritance
many traits are produced by more than one gene
and the genes at different loci often interact in peculiar ways to produce that phenotype
In the last 100 years we have learned that polygenetic determination (or inheritance) should be thought of as the rule rather than the exception for many traits such as ___
height, IQ, athletic ability, etc.
Locating and understanding the loci in polygenetic inheritance is extremely difficult because
1) the vast majority of loci have very little impact
2) the loci can interact with each other in many ways
case where a single locus that highly impacts an otherwise highly polygenic trait
one form of dwarfism (there are at least 300 types) results from the action of a single locus (see Achondroplasia).
One possible molecular mechanism for how a dominant allele, given the designation of “+” (for wild type) masks (or covers) the recessive allele.
while both alleles are transcribed and translated, the mutant produces a non-functional protein.
WT allele produces enough of the enzyme to complete the cellular process when heterozygous - complete dominance.
haplosufficient
is when dominate allele (in complete dominance) produce the phenotype
incomplete dominance produces an ____ heterozygote
intermediate
one way incomplete dominance producing an intermediate heterozygote can ocour is
the amount (or dosage) of a gene product (e.g. red pigment in snapdragons).
Two copies of one allele produces enough pigment to produce red color, only one produces pink. The alternative allele lacks pigment and produces white when homozygous.
Codominance
Both Alleles are Displayed ex. ABO blood group
Human ABO* blood groups vs locus
The gene product is a sugar on the surface of red blood cells.
Three possible alleles (but only two in any individual): A and B alleles are different forms of the sugar, the i allele produces no sugar.
4 possible groups
Rh is extremely important for preventing immune reactions between
mother and baby in utero.
A second pregnancy with an opposing Rh factor can cause significant immune reaction from the mother to the baby.
Rh+ is ____ to Rh-
Most people are ___
dominant
Rh+
Will all babies from the same couple have the same Rh factor?
No, consider 2 alleles for one trait
sickle cell where the ____ produces the affliction and strongly sickled cells.
But it is not ___ because
homozygous recessive
not complete dominance because hetero are slightly sickled thus, incomplete dominance. Also, in hetero both alleles produce a different form of hemoglobin. so, is co-dominace
William Cardozo
was African American
provided health care to many poor people including African Americans
established Sickle Cell as genetic trait that was passed down in via Mendelian inheritance, and that the affliction was most common in those of African descent
(also researched other childhood pathologies such a Hodgkin’s disease)
Two models of a dominant mutants
haploinsufficiency & dominant negative
haploinsufficiency
where one does in insuffienct to produce the phenotype
dominate negative
where the product of mutant allele is “antagonistic” to the WT product
one way for this to occur is for the mutant to form a dimer with the WT, rendering it non-functional
appears to be the case in several forms of cancer
genetics of mice are of great interest because
they are much more similar to humans than many other models
The ____ yellow coat color mutant allele in mice, also acts as a lethal when _____.
When heterozygotes are crossed the resulting ratio is ___ of iving progeny.
dominant
homozygous
2:1 (not 1:2:1)
2 schools of thought on multi-locus genetic interactions/ multi-step pathways
1) because a mutation in a previous step will block all further steps this can be considered an interaction
2) others genes are not really interacting so much as depending on each other.
Understanding multi-locus genetic interactions is complicated by the fact that
different sources define an “interaction” in different ways.
for pepper color in the genus Capsicum
genes at two different loci interact to produce ____
phenotypes not seen in either homozygous parent or the F1.
note - the F1 consists of a single phenotype (Red)
when the F1’s are crossed we observe two additional phenotypes. this is inconsistent with our expectations of ___
Because we observe a very different result ___ we have confidence that ___
a single locus
(if single locus F2 would be 3:1 red: cream)
9:3:3:1 (Not like before where looking at 2 diff traits (same loci for one trait). Now, 1 trait but many loci), and two novel phenotypes
there is another loci involved (i.e. C locus) and polygenetic inheritance.
epistasis
One of the most common types of genetic interactions
a kind of multiple locus dominance where one locus covers or masks another locus.
In epistasis: the dominant loci (covering) is termed ______, and the masked loci is termed ________.
epistatic (on top of)
hypostatic (underneath)
Labrador retrievers occur in three color morphs___
Coat color is determined by ____
Black, Brown and Yellow
two interacting loci.
1st loci determines the pigment color produced in the hair shaft. (two alleles B and b, where B is dominant and produces black, and b produces brown)
2d loci (e) is epistatic to the “B” loci and determines if any pigment is put into the shaft
phenotype 9:3:4
this example (labrador) is recessive epistasis
often a challenge to determine if a case is actually an interaction of two loci on a single trait (Capsicum), or two loci each impacting a separate trait such as Mendel’s peas.
How can we determine the genetic basis for these differences and if there is any interaction?
Crosses.
- o+/ ; b+/ _ is wild type orange and black (a)
- o/o; b+/ individual is black (b)
- o+/_; b/b individual is orange (c)
- o/o; b/b is albino (d)
Cross WT(a) to no color (d) get all F1 (o+/o; b+/b )
Cross F1, WT to F1, WT get 9:3:3:1 ratio of phenotypes (a,b,c,d)
indicates that black vs. orange color production are in two different (loci) and non-interfering (interacting) pathways.
two separate traits - border vs fill (that do not mix)
general method for detecting epistatic interactions is to
examine the results of a F1 x F1 cross
general expectation is: if there is no epistatic interaction then the ratio should be: a 9:3:3:1
multi-step pathway Recessive Epistasis
9:3:4 phenotype
Example: Collinsia flowers
two loci each (color) producing an enzyme
WT - both of the loci need at least one of the functional alleles (each act as a dominant).
either loci is homozygous for the mutant form a non-wild type flower is produced
If the homozygous mutant is at the m locus then the first step can proceed and pink results
If the homozygous mutant is at the m locus then the first step can proceed and pink results.
Thus the w loci is said to be _____
epistatic (covering) the second loci (m) that is termed hypostatic
This is “recessive” epistasis because the
double heterozygote remains the wild type.
Dominant Epistasis
12:3:1 phenotype
example: Foxglove
W/w D/d x W/w D/d
W allele at this locus (W/W or W/w) does not allow the deposition of color (red) by a second gene (D).
only when the alternative allele (w) is homozygous that the action of the second locus (red) is uncovered in the phenotype
double recessive epistasis
(or Interaction, Same Pathway)
example: blue & white flowers
ratio in the F2 of 9:7
discover two different color morphs of a flower: blue and white. Preliminary crosses indicate that the double wild type homozygote and the double heterozygote produce blue flowers but when two heterozygotes are crossed they produce a ratio of 9:7 blue to white.
The 3:3:1 produce only one phenotype.
How can double recessive epistasis (or Interaction, Same Pathway) have 3:3:1 produce only one phenotype?
a regulatory gene (r) that acts to control the transcription of a second loci (a).
the wild type alleles at both loci are dominant (thus the 9)
of haploid individuals showing a mutant non-lethal phenotype cross these to a mutagen and examine WT progeny.
What happened?
How tell what hapended?
1) a revertant - a mutation in the same gene that produced the initial mutant
2) suppression - a mutation in a second gene that suppresses the first mutant.
progeny x wild-type a+ (or a+/s+)
= all a+ phenotypes then - revertant
= 3 (WT):1 (OJ mutant) ratio in a haploid then - suppression
Diploid Suppressors
example: Drosophila Purple Eye Mutant
F2 produce a 13:3 ratio of red to purple
Locus 1:
pd+ = WT/red dominant.
pd = mutant/purple
Locus 2:
su+ =WT/no effect
su = mutant/red. if homozygous suppresses pd (but not pd+)
F2’s: 13:3
9 pd+/ ; su+/ (red)
3 pd+/; su/su (red)
1 pd/pd; su/su (red)
3 pd/pd; su+/ (purple)
suppression there are 2 phenotypes (like double recessive epistasis)
A modifier locus
changes the degree or amount of expression of second locus.
modifier
functional a+ allele, and a mutant allele (a)
A second loci (b) regulates how much of the product (a+) is produced.
type (b+) -full production,
mutant (b) - greatly reduces the product.
what are Genotype & Phenotype for haploid
a+ * b+
“wildtype” (functional protein, full transcription level)
a+ * b
“defective” (low transcription of a functional protein)
a * b+
“defective” (defective protein present, but at a full level)
a * b
“extremely defective” (low transcription of a defective protein)
Penetrance
yes or no (for having trait)
percent of individuals displaying the trait
Expressivity
varying level (intensity) of a phenotype
phenotype often deviates because of
the environment or interaction with other genes
other important non-Mendelian phenomena that produce changes from expectations
Chart plus:
Epigenetics - heritable traits that cannot be explained by changes in DNA sequence.
Environmental effects
