Genes and Continuously Variable Traits

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 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.

Genes and Continuously Variable Traits