Chapter 4genetics

Explain why Tay-Sachs is considered recessive at the organismal level and co-dominant at the molecular level.

Children with two Tay-Sachs alleles (homozygotes) have the disease. Both heterozygotes with one working allele and homozygotes with two working alleles are healthy and normal at the organismal level. The activity level of the lipid-metabolizing enzyme is reduced in heterozygotes. At the biochemical level, the alleles show incomplete dominance. Heterozygous individuals produce equal numbers of normal and dysfunctional enzyme molecules. At the molecular level, the Tay-Sachs and functional alleles are codominant.

Explain how phenotypic expression in the heterozygote differs with co-dominance

Codominance: A similar mixing, but both traits are clearly evident. This is also usually expressed in color, but would be seen as patches of both colors as in cows. If you can have white or black cows, then a black and white splotched cow would be codominance. It would NOT be White + Black = Gray.

Explain how phenotypic expression in the heterozygote differs with complete dominance.

Complete dominance is simple dominance: whichever trait is dominant is the only one which will be expressed. There will be no sign of the recessive

4. Explain why genetic dominance does not mean that the dominant allele subdues a recessive allele. Illustrate your explanation with the example of round versus wrinkled pea seed shape.

Dominant alleles do not subdue or suppress recessive alleles in any way because the two do not physically interact. Dominance and recessiveness refer to the mechanisms by which genotype is translated into phenotype. In the case of round vs. wrinkled seeds... Round seeds (homozygous dominant and heterozygous) make enough of a certain enzyme that allows them to convert accumulating sugars and water into starches. This gives the peas more volume, smoothness, and roundness. Wrinkled seeds (homozygous recessive), on the other hand, lack the ability to make that enzyme- so, no starch is formed, and the seeds wrinkle when they dry

What is Epistasis and give an example

Epistasis - the interaction between two or more genes to control a single phenotype, For example, the gene causing albinism would hide the gene controlling color of a person's hair

Explain how phenotypic expression in the heterozygote differs with incomplete dominance

Incomplete Dominance: There is a mixing between the dominant and recessive traits, usually in the case of color where it would be similar to two paint colors being mixed. I.E. Red Snapdragons and White Snapdragons produce Pink Snapdragons. Neither dominant nor recessive traits are fully expressed.

Give an example of incomplete dominance and explain why it does not support the blending theory of inheritance.

Incomplete dominance can happen in flowers such as snap dragons where a red flower plant and a white flower plant have an offspring that is neither red nor white but is a mix so in this case it would be pink. It does not support the blending theory as it does not get its colour from the dominant plant in this case but from both.

Explain why dominant alleles are not necessarily more common in a population. Illustrate your explanation with an example.

It depends on the trait. If the dominant genotype is selected for more often than the recessive genotype, then the dominant allele will become more common in the gene pool. If the recessive genotype is selected for more often than the dominant genotype, the recessive allele will become more common than the dominant allele in the gene pool. For example, in humans, having five digits on hands and feet is recessive, and six digits is dominant. Most people have five digits because the recessive allele for numbers of digits is much more common in the human gene pool than the dominant allele.

What is Pleiotropy and give an example

Pleiotropy refers to the phenomenon in which a single gene controls several distinct, and seemingly unrelated, phenotypic effects. A classic example of pleiotropy is the human disease PKU (phenylketonuria).

Describe the inheritance of the ABO blood system and explain why the IA and IB alleles are said to be co-dominant.

The ABO blood system has to do with the glycoproteins that are absent or present on the cell surfaces of the red blood cells. There are three alleles. O, is the first, and does not cause the production of a glycoprotein, A codes for the production and insertion into the membrane of the "A" type and B codes for the production and insertion into the membrane of the "B" type. Genetically, people can be homozygous for O. If they have the genotype AO or BO, the glycoprotein indicated is expressed. Thus, a person with type A blood could be AA or AO, and a person with type B blood could be BB or BO. If a person's genotype is AB, s/he expresses both the A and the B type glycoprotein (so neither is dominant).