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Definition
occurs when the phenotypic expression of one gene is affected by another gene.
For example, two genes (B and E) determine coat color in Labrador retrievers:
- Allele B (black pigment) is dominant to b (brown)
- Allele E (pigment deposition in hair) is dominant to e (no deposition, so hair is yellow)
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| Farmers have known for centuries that matings among close relatives (known as inbreeding) can result in offspring of lower quality than matings between unrelated individuals. Agricultural scientists call this inbreeding depression. The problems with inbreeding arise because close relatives tend to have the same recessive alleles,some of which may be harmful. |
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after crossing inbred lines is called heterosis (short for heterozygosis). The cultivation of hybrid corn spread rapidly in the United States and all over the world, quadrupling grain production.
The practice of hybridization has spread to many other crops and animals used in agriculture.
EXAMPLE:
beef cattle that are crossbred are larger and live longer than cattle bred within their own genetic strain. |
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What determines the phenotype of an organism?
(12.3.1) |
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| Genotype and environment interact to determine the phenotype of anorganism. This is especially important to remember in the era of genome sequencing |
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Figure 12.16 (from 12.1.3.2)
Rabbits:The Environment Influences Gene Expression |
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| This rabbit expresses a coat pattern known as “chocolate point.” Its genotype specifies dark fur, but the enzyme for dark fur is inactive at normal body temperature, so only the rabbit’s extremities—the coolest regions of the body—express this phenotype. |
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| Two parameters that describe the effects of genes and environment on the phenotype: |
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Definition
| Penetrance and Expressivity |
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Definition
is the proportion of individuals in a group with a given genotype that actually show the expected phenotype.
EXAMPLE: Huntington’s disease in humans. The disease results from the presence of a dominant allele, but 5% of people with the allele do not express the disease. So this allele is said to be 95 percent penetrant. |
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is the degree to which a genotype is expressed in an individual.
EXAMPLE: environmental effects: consider how Siamese cats kept indoors or outdoors in different climates might look. |
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| Qualitative vs. Quantitative |
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Definition
Qualitative:The differences between individual organisms in simple characters, such as those that Mendel studied in pea plants.
EX: the individuals in a population of pea plants are either short or tall
Quantitative:Such variation within a population, or continuous, variation (due to both genes and environment)
EX:Unlike humans where variation is different, some people are short, others are tall, and many are in between the two extremes. |
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A set of genes that determines a complex character that exhibits quantitative variation.
human characteristics such as disease susceptibility and behavior are caused in part by quantitative trait loci.
FUN FACT:Recently, one of the many genes involved with human height was identified. The gene, HMGA2, has an allele that apparently has the potential to add 4 mm to human height. |
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Definition
| In epistasis, one gene affects the expression of another. Perhaps the most challenging problem for genetics is the explanation of complex phenotypes that are caused by many interacting genesand the environment. |
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