Extensions of Mendelian Genetics Flashcards
Limitations of Mendel’s Studies
1) Used a model that had a strict “either/or” trait type (only 2 alleles)
2) Characters that were studied were controlled by only ONE gene
–> His model was too simple to account for all the complexities of genetics
Inheritance of characters determined by a single gene can differ from Mendelian genetics patterns when: (3)
1) Alleles are not completely dominant or recessive
–> Degrees of dominance
2) A particular gene has MORE than 2 alleles
3) A single gene affects more than one phenotype
Dominance
The relationship between the alleles of one gene in which the phenotype of one allele “masks” the other
Complete Dominance
When the phenotype of a heterozygote and homozygous dominant are indistinguishable
–> In a heterozygote, the dominant phenotype is expressed
(Essentially what Mendel studied)
Incomplete Dominance
The phenotype of the heterozygote is an INTERMEDIATE between those of the 2 homozygotes –> A mix of both allele phenotypes
Tay Sachs Disease is an example of…
Incomplete Dominance
Tay Sachs Disease
Neurodegenerative autosomal recessive disease
–> Due to a recessive lethal mutation
Tay Sachs Disease: what is mutated?
An enzyme: Hexoseaminidase –> An enzyme that breaks down lipids in nerve cells
TS Homozygous Recessive
= deficiency in hexoseaminidase = Disease Phenotype
TS Homozygous Dominant
= Normal # hexoseaminidase = Normal Phenotype
TS Heterozygous
= 1/2 # hexoseaminidase = Normal Phenotype
–> Incomplete dominance at biochemical level (intermediate amount of the enzyme is produced)
–> Complete dominance at physical level
Incomplete Dominance: Carnation Flower Color Example
Homo Dominant = Red Flower
Hetero Recessive = White Flower
Heterozygous = PINK flower –> Intermediate color
Co-Dominance
Two different phenotypes BOTH get expressed in the heterozygote
–> The 2 alleles both affect the phenotype in separate, distinguishable manners
What part does the I-Gene play in blood typing?
I-Gene = Encodes for the enzyme glycosyltransferase
== Enzyme that adds sugar onto its substrate (for blood it’s protein that gets secreted by rbcs and then resides on their surfaces)
Blood Type Alleles
3 of them: Ia, Ib, i
Type A Blood Genotype
Ia + Ia OR Ia + i
Type B Blood Genotype
Ib + Ib OR Ib + i
Type AB Blood Genotype
Ia + Ib
Type O Blood Genotype
i + i
Type AB blood is a form of….
Co-dominance
–> Both the A and B sugars are attached to the RBCs
–> Both the Ia and Ib alleles are being expressed at the same time
Multiple Alleles
Within a population there may be many different alleles of a particular gene –> Most genes have > 2 alleles
No matter how many alleles exist within a population…
a single individual will only have TWO alleles per gene
Common Misconceptions of Dominance (2)
1) The dominant allele is the “normal/wild type” and the recessive is the mutant (not normal)
2) The dominant allele is the one found most frequently in a population
–> These may be true in many cases but NOT all
Polydactyl
The dominant allele (P) leads to the growth of an extra digit (in humans and cats)
–> HOWEVER, most of the population is recessive = no extra digit
–> Although the trait is dominant, it is not the trait most frequently found in the population: It’s found in every 1 in 1000 people
Extensions that Impact Phenotypic Expression (4)
1) Epistasis
2) Pleiotropy
3) Environment
4) Lethal Dominant Allele
Lethal Dominant Alleles
Homozygous lethal dominant alleles can disrupt the phenotypic ratios: Changes the probability of the offspring
Manx Cat
Homozygous Dominant (MM) = Early Embryonic Death
–> Insufficient spinal development
Heterozygous (Mm) = Short/No tail (manx)
Homozygous Recessive (mm) = Normal tail (non-manx)
Epistasis
A gene at one locus alters the phenotypic expression of another gene at a second locus
–> Two genes contribute to an overall single phenotype
Hair Color and Baldness Example
Baldness gene is epistatic to hair color gene
–> If you have the baldness allele, the hair color won’t even matter because it won’t get the chance to be phenotypically expressed
–> ***Baldness affects the expression of the hair color phenotype
Albino Mice Example
2 genes control hair coat color:
1) Pigment color gene (brown or black)
2) Pigment deposition gene –> Homo. Rec. = No pigment deposited
Any mouse with “cc” for the 2nd gene = ALBINO
(because no pigment deposition occurs)
–> Prevents hair color phenotype from being expressed
Pleiotropy
The ability of a gene to affect an organism’s phenotype in multiple ways (affecting more than one trait)
Deaf-White Cats Example
Most cats with albino (white) fur and blue eyes are also DEAF
–> ***The gene for pigmentation is also involved in ear development
–> White cats with only one blue eye are deaf only in one ear
Environmental Impacts
NATURE vs NURTURE
–> We are not just a product of our genes, but a product of our environment as well
Sickle Cell Anemia Example
A single point mutation affects all the following:
1) Hemoglobin
2) Organ Function
3) Respiration
ETC……
(Affects many aspects of human physiology = pleiotropy)
A genotype is not generally associated with…
a rigidly defined phenotype, but rather with a RANGE of phenotypic possibilities due to environmental impacts
Siamese Cats Example
Phenotype depends on climate!
Temp. sensitive (TS) tyrosinase = enzyme that catalyzes production of dark pigment
–> Only works at temp < 33C
== Extremities tend to be colder, thus the cats have dark pigment in their extremities
Siamese Cats in Warm Climate
Tend to have lighter coats than those in colder climates
If a patch of white fur is removed from a siamese cat and then an icepack is placed over the patch…
The fur will grow back in dark
An organism’s overall phenotype is composed of/is a
1) Physical appearance, internal anatomy, physiology, and behavior
2) Reflection of its overall genotype and unique environmental history
