Download Study Questions. 1) Explain how a continuously variable trait could

Study Questions.
1) Explain how a continuously variable trait could be governed by genes.
Study Questions.
1) Explain how a continuously variable trait could be governed by genes.
Because simple single-gene traits show discontinuous expression (Round or
wrinkled, green or yellow), many scientists assumed that continuous traits were
primarily governed by environmental effects (which can vary continuously, like
temperature…).
However, traits that are influenced by many genes – polygenic traits – can express
quantitative, continuous patterns of expression. For example, there are 16 genes
for melanin deposition in human skin. So, people can have all 16 genes ‘on’, and
be very dark, or 15 ‘on’ and 1 ‘off’ and be slightly lighter, through all the
combinations of ‘on’ and ‘off’ (14:2, 13:3, 12:4, 8:8, 6:10 etc…) forming a very
continuous expression of skin color from dark to light.
Study Questions.
1) Explain how a continuously variable trait could be governed by genes.
2) What is an epistatic interaction? Give an example.
Study Questions.
1) Explain how a continuously variable trait could be governed by genes.
2) What is an epistatic interaction? Give an example.
An epistatic interaction is where the expression of the genotype at one locus is
contingent upon, or depends upon, or is influenced by, the genotype at another
locus. They don’t just ‘add’ together’ like in quantitative inheritance – they
interact.
Albinism is an example. Albinism is caused by a different gene than the 16
melanin producing genes. Melanin is made from a precursor. An ‘aa’ individual
cannot make the precursor, so there will be no melanin produced REGARDLESS of
the genotypes at the 16 melanin-producing genes. Review the other forms of
epistasis.
Study Questions.
1) Explain how a continuously variable trait could be governed by genes.
2) What is an epistatic interaction? Give an example.
3) Describe how the position of a gene can affect its effect.
Study Questions.
1) Explain how a continuously variable trait could be governed by genes.
2) What is an epistatic interaction? Give an example.
3) Describe how the position of a gene can affect its effect.
A gene may be bound to histones and so be ‘off’. Or, a gene may be next to a
region of heterchromatin, and thus be ‘off’. Or it may be separated from a
promoter sequence and thus be ‘off’.
Study Questions.
1) Explain how a continuously variable trait could be governed by genes.
2) What is an epistatic interaction? Give an example.
3) Describe how the position of a gene can affect its effect.
4) How can the environment influence the expression of a trait?
Study Questions.
1) Explain how a continuously variable trait could be governed by genes.
2) What is an epistatic interaction? Give an example.
3) Describe how the position of a gene can affect its effect.
4) How can the environment influence the expression of a trait?
The environment may influence whether a gene is activated, or it may influence
the structure and function of the resulting protein.
Study Questions.
1) Explain how a continuously variable trait could be governed by genes.
2) What is an epistatic interaction? Give an example.
3) Describe how the position of a gene can affect its effect.
4) How can the environment influence the expression of a trait?
5) How can the environment influence the VALUE of a trait? Relate this to
Darwin's idea of the diverge of populations in different environments.
Study Questions.
1) Explain how a continuously variable trait could be governed by genes.
2) What is an epistatic interaction? Give an example.
3) Describe how the position of a gene can affect its effect.
4) How can the environment influence the expression of a trait?
5) How can the environment influence the VALUE of a trait? Relate this to
Darwin's idea of the diverge of populations in different environments.
Certain traits may be advantageous is certain environments but disadvantageous
in others. So, a gene that codes for an enzyme that functions best at warm
temperatures will be advantageous in warm environments but deleterious in cold
environments.
So, as Darwin surmised, different genes will be selected for in different
environments – genes that cause the expression of adaptive traits. Populations
of similar organisms (in the same species), placed in different environments, will
diverge from one another as they adapt to their particular environment.
Study Questions.
1) Explain how a continuously variable trait could be governed by genes.
2) What is an epistatic interaction? Give an example.
3) Describe how the position of a gene can affect its effect.
4) How can the environment influence the expression of a trait?
5) How can the environment influence the VALUE of a trait? Relate this to
Darwin's idea of the diverge of populations in different environments.
6) Why are most lethal alleles recessive? Answer with respect to the effects of
selection on a dominant, deleterious gene.
Study Questions.
1) Explain how a continuously variable trait could be governed by genes.
2) What is an epistatic interaction? Give an example.
3) Describe how the position of a gene can affect its effect.
4) How can the environment influence the expression of a trait?
5) How can the environment influence the VALUE of a trait? Relate this to
Darwin's idea of the diverge of populations in different environments.
6) Why are most lethal alleles recessive? Answer with respect to the effects of
selection on a dominant, deleterious gene.
A dominant lethal gene kills all the organisms that carry it, and it is quickly
weeded out of the population (unless it is only expressed after reproductive age).
The only other lethal alleles that can maintain themselves in a population are
those that are recessive; they can stay in the population at low frequency,
because they usually end up in the heterozygous condition and do not kill the
organism.
Study Questions.
7) 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?
Study Questions.
7) 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?
3 x 3 x 2 = 18
How many phenotypes are possible in the offspring?
3 x 2 x 2 = 12
Study Questions.
8. Conduct the following cross: Aa x Aa
Determine the genotypic and phenotypic ratios if there is:
- complete dominance
- Incomplete dominance
- Codominance
- Overdominance
Study Questions.
8. Conduct the following cross: Aa x Aa
Determine the genotypic and phenotypic ratios if there is:
- complete dominance
- Incomplete dominance
- Codominance
- Overdominance
Genotypic Ratio
Phenotypic Ratio
Complete Dom.
¼ AA : ½ Aa : ¼ aa
¾A:¼a
Incomplete Dom.
¼ AA : ½ Aa : ¼ aa
¼ A : ½ Aa : ¼ a
Codominance
¼ AA : ½ Aa : ¼ aa
¼ A : ½ Aa : ¼ a
Overdominance
¼ AA : ½ Aa : ¼ aa
¼ A : ½ Aa : ¼ a
Study Questions.
8. Conduct the following cross: Aa x Aa
Determine the genotypic and phenotypic ratios if there is:
- complete dominance
- Incomplete dominance
- Codominance
- Overdominance
9. Provide a cellular explanation for overdominance.
Study Questions.
8. Conduct the following cross: Aa x Aa
Determine the genotypic and phenotypic ratios if there is:
- complete dominance
- Incomplete dominance
- Codominance
- Overdominance
9. Provide a cellular explanation for overdominance.
If the heterozygote has two different functional alleles, they may be able to
function optimally under a wider range of conditions, expressing a more extreme
phenotype than either homozygote. So, a heterozygote for a growth enzyme,
where the alleles have different temperature optima, would be able to grow
throughout the growing season when it is warm AND cold, and would be taller
than either homozygote than would only grow best when it was warm OR cold.