Exceptions to Mendel's Laws/ "Sex Traits" Flashcards
Continuous Variation in Quantitative Traits
Polygenic Inheritance: the greater number of genes influencing a phenotype, the more continuous the distribution of phenotype
* Often formed in a bell curve
Important to note continuous can also show the continuum changing over time
Example:
The average height today being slightly greater than in 1910 possibly because of access to nutrition; (note): could also apply to Mendel not believing on outside influence
Incomplete Dominance
None of the alternate alleles are fully dominant or recessive; dominant allele is not completely masking the recessive allele(results in blending)
Example: flowers
Pigment alleles not evenly distributed throughout flower, so cannot be “either/or” under Mendel’s principles; Range of pigments
Codominance
no single alleles is dominant, and each phenotype is distinctly present
Example: A-B-O blood groups, system exhibits codominance, where individuals with the IAIA (also IAi)and IBIB(and also IBi) genotypes express the A and B antigens, respectively, while individuals with the IAIB genotype express both A and B antigens.
(Blood type O does not have antigens connected)
MANY subtypes(6 genotypes and 4 phenotypes)
Consists of a glycoprotein in plasma membrane of red blood cells(core polysaccharide and allele attached to membrane)
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Epistasis(related to pleiotropic effects)
In this case, the expression of one gene affects the expression of another gene. For example, in mice, the gene for coat color is affected by the gene for pigment production. If the pigment production gene is defective, then the coat color gene will not be expressed.
Pleiotropic effects and examples
When an allele has more than one effect on the phenotype
Example: Marfan’s syndrome: Dominant mutation to TGF-beta; (Overexpression of gene, Involved in forming many connective tissues, Leads to huge increase of this protein in those that have it, Genetic disorder of connective tissues)
Example: Coat color
* Calico cats only occur for females
Environmental Effects
Allele expression may depend on environment
Example: Tyrosinase=pigment
Can be affected by temperature; Tyrosinase inactive at higher temperature, no pigment; Tyrosinase active at lower temperature, pigment
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Epistasis
One gene affects the expression of another gene
Example: Labrador retrievers
Gene determines color of pigment(brown vs. black)
We have ee and E mixing with bb and B_
When ee is present: no dark pigment in fur(yellow lab), no matter if bb or B_ is present
When E_ is present: dark pigment in fur, no matter if bb or B_ is present
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Sex determination: what defines a sex?
Gametic sex
Chromosomal sex
Genetic sex
Phenotypic or biological (Expression of genitalia)
What DETERMINES SEX?
- Chromosomes: Species do it differently
Example:
Humans: XX(female), XY
Reptiles, birds: ZZ(male), ZW(female)
- Genes on chromosomes: Testes determination factor, Determines a male
- Could be affected by expressed XX gene but have male genetalia, vice versa
- Environmental factors: Pheromones used to cause reproduction; Can change process at points in life to reproduce and develop
Why is Sex Determination not Clear Cut?
- Chromosomal Abnormalities can occur
(XO, XXY, XXXY, etc.) - intersex/intermediate sex conditions: can have features of both testes and ovaries(ex.)
Sex-Linked Traits: X-linked recessive
Males exhibit trait if they inherit it from mothers; Females only exhibit traits if they are homozygous for it
Often “skips a generation”
Male passes to half of offspring
Example: red-green colorblindness
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Sex-linked traits: X-linked dominant
Male passes trait to all female offspring, none to males; Females pass trait to half of offspring
Example: hypophosphatemia
* A disease that affects bone development
