Study Questions:
1) Explain how a continuously variable trait could be governed by genes.
2) What is an epistatic interaction? Give an example.
3) How can genes interact to affect a single phenotypic trait? Give an example.
4) Describe how the position of a gene can affect its effect.
5) How can the environment influence the expression of a trait?
6) How can the environment influence the VALUE of a trait? Relate this to Darwin's idea of the diverge of populations in different environments.
7) Why are most lethal alleles recessive? Answer with respect to the effects of selection on a dominant, deleterious gene.
8) As such, how can a dominant lethal allele be maintained in a population?
9) A typical problem on the next exam:
10) Consider this cross:
AaBbCc x AaBbCC
- assume independent assortment of the three genes
- There is incomplete dominance at the A locus (meaning A is incompletely dominant to a).
- There is complete dominance at the B locus.
- There is overdominance at the C locus.
How many genotypes are possible in the offspring?
How many phenotypes are possible in the offspring?
11. Conduct the following cross: Aa x Aa
Determine the genotypic and phenotypic ratios if there is:
- complete dominance
- Incomplete dominance
- Codominance
- Overdominance
12. Provide a cellular explanation for overdominance.
13. A typical problem on the next exam:
14. Consider this cross:
AaBbCc x AaBbCC
- assume independent assortment of the three genes
- There is incomplete dominance at the A locus (meaning A is incompletely dominant to a).
- There is complete dominance at the B locus.
- There is overdominance at the C locus.
How many genotypes are possible in the offspring?
How many phenotypes are possible in the offspring?
- the effect of an allele, and the "value" of the allele to the organism (is it 'good' or 'bad'?) is not an intrinsic property of the allele. Rather, the effect of an allele and it's value is also determined by these three classes of factors. Keep this in mind; it is the overriding message of transmission genetics. Unfortunately, it has not always been our unbderstanding.
- AA = Aa > aa (Mendel's peas provide examples)
- Cellular explanation? one gene produces enough product for complete cell function and phenotypic expression. Surplus has no effect. Or the product of the dominat allele has a higher reactivity with the substrate and always reacts with it, it's way, even in the heterozygous condition.
2. Incomplete Dominance/Intermediate Inheritance
- AA > Aa > aa
- Flower color in four o'clocks
- Cellular explanation? There is a quantitative, dosage effect at the cellular level. Two 'on' genes produce more functional product than one, and this surplus influences cell function and phenotypic expression.
3. Codominance
- heterozygote expresses both traits completely; not an "in between" phenotype but both phenotypes. (AB blood)
- Cellular explanation? The allelic products are both functional and work on slightly different substrates. Having both gives the phenotype a qualitative diffence, not a quantitative difference.
4. Overdominance (Heterosis)
- The heterozygote expresses a phenotype "more extreme" than either homozygote. Aa = tallest, AA = tall, aa = short
- Cellular explanation? the two alleles work synergistically or competitively (interfere with one another) and expression is either augmented or supressed relative to the homozygote phenotypes. For instance, consider a gene that codes for an enzyme that influences growth. Maybe the two alleles for this enzyme work at different temperatures (H = warm and h = cold). Id the organism lives in an environment that is warm most of the time, then the homozygote for HH will grow most of the time, and will be taller than the hh homozygotes that only grow during the rare colder periods. However, the heterozygote Hh grows ALL the time, when it is cold and warm, and so is taller than either homozygote.
5. Multiple Alleles at a locus (A, B, O)
- dramatically increases the number of genotypes possible
- 1 allele = 1 diploid genotype
2 alleles = 3 diploid genotypes
3 alleles = 6 diploid genotypes
4 alleles = 10 diploid genotypes
5 alleles = 15 ....etc.
So, when a new gene is produced by mutation, it does not just make ONE new genotype. It has the potential, through independent assortment, to be joined with all the other alleles already present. So, in the example above, when a new FOURTH allele is added, you don't just get one new genotype (over the 6 that existed in the three-allele population), you get an extra 4 genotypes. There is a multiplicative effect on variation.
- As a consequence of many loci acting on a trait, there are many more combinations that are possible - resulting in a wide variety of phenotypic expression that forms a nearly 'continuous' range of variation.
- For example, human skin pigmentation may be governed by as many as 16 gens for melanin production. Although there may be dominance at each locus, this multiplication of genes allows for an extraordinary and continuous amount of phenotypic variation, from all 16 genes 'on', to 15 on and 1 off, to 14:2, 13:3, etc.
- In sweet peas, there are two genes that influence flower color. Both exhibit complete dominance, and both proteins must be produced in order for purple flower color to be expressed. So:
aaBB (white) x AAbb (white)
|
100% AaBb (purple)
|
9/16 A_B_ (purple)
7/16 aaB_ or A_bb or aabb (white)
- In Summer Squash, fruit shape is influenced by interacting loci. Disc-shaped fruits form when both loci have dominant alleles present (A_B_). Long fruits are produced when dominant alleles are absent (aabb). SO:
AABB (disk) x aabb (long)
|
100% AaBb (disk)
|
9/16 A_B_ (disk)
6/16 A_bb or aaB_ (ROUND)
1/16 aabb (long)
- Sickle cell anemia is caused by an altered beta globin allele, which causes a single amino acid change in the beta proteins in hemoglobin. The trait exhibits incomplete dominance - one "a" allele will result in some "sickling" of red blood cells at low oxygen concentration, but the condition is not nearly as severe as it is in the homozygous condition. The affect of this gene depends on the environment. In particular, it depends on the presence of malaria.
In the tropics, a primary source of mortality is malaria. Malaria is caused by a single-celled protist, Plasmodium. This single celled parasite infects red blood cells and divides mitotically, producing hundreds of offspring and eventually rupturing the cell. Thus, infection causes extreme loss of RBC's - "anemia" (don't get this confused with sicle cell anemia). Curiously, the Plasmodium parasite can not reproduce in cells with the altered form of "sickle-cell" hemoglobin - even in the heterozygous condition. So, the heterozygote suffers some sickling on occasion, but is protected from malaria. AA homozygotes have lower survivorship than the Aa heterozygotes because the AA individuals are exposed to malaria. The aa homozygotes have lower survivorship than the Aa heterozygotes because of teh more prounced debilitating effects of the sickle cell.
So, is an 'a' allele "good" or "bad"? Well, that depends on other alleles at that locus (if it's with another 'a' allele it is always bad), but it also depends on the environment. If the 'a' is with an 'A' in the temperate zone, it is bad (realtive to the AA homozygote).... but if the 'a' allele is paired with an 'A' allele in the tropics, then it is "good" - better than having two A's (AA).
TEMPERATE ZONE: survivorship: AA > Aa > aa
TROPICS: survivorship: AA < Aa > aa
You should relate this to the corollary of Darwin's Theory of Natural
Selection. Populations in different environmens will dvierge from one another genetically, as the environment selects for different traits (genotypes).