Cell Biology
The concept of phenotype refers to an organism's observable characteristics determined by a combination of the organism's genotype and environment. One specific type of phenotypic variation is genetic polymorphism, which occurs when there are multiple alleles of one or more genes causing variation in a trait. Traits can be categorized as either qualitative (traits that can be grouped into discrete categories) or quantitative (traits that have a continuous distribution and can be measured in precise units).
The lesson delves into patterns of dominance and the concept of Mendelian traits, where one dominant and one recessive allele control the phenotype. It highlights three dominance patterns: complete dominance (dominant allele fully masks a recessive allele), codominance (two different alleles are fully expressed in heterozygotes), and incomplete dominance (the phenotype of heterozygotes is a blend of the two alleles). The lesson also discusses the measures of penetrance (the percentage of individuals with a given genotype expressing the phenotype) and expressivity (the degree to which individuals with a given genotype express the associated phenotype).
Lesson Outline
<ul> <li>Introduction to Phenotype <ul> <li>Definition of phenotype: the oservable characteristics of organisms</li> <li>Influence of genotype and environment</li> <li>Phenotypic variation</li> </ul> </li> <li>Genetic Polymorphism <ul> <li>Qualitative traits <ul> <li>Easily categorized</li> <li>Example: Eye color, hair color, attached or detached earlobes</li> </ul> </li> <li>Quantitative traits <ul> <li>Continuous distribution</li> <li>Measurable in precise units</li> <li>Example: height, weight</li> </ul> </li> </ul> </li> <li>Mendelian Traits <ul> <li>Definition: qualitative traits controlled by a single gene with clear dominant and recessive alleles</li> <li>Heterozygous and homozygous individuals</li> <li>Disease carriers (heterozygous individuals who carry a recessive disease gene)</li> </ul> </li> <li>Patterns of Dominance <ul> <li>Complete dominance <ul> <li>Dominant allele masks recessive allele</li> </ul> </li> <li>Codominance <ul> <li>Two different alleles are fully expressed</li> </ul> </li> <li>Incomplete dominance <ul> <li>Blend of both alleles</li> </ul> </li> </ul> </li> <li>Evaluating Genotype Expression <ul> <li>Penetrance <ul> <li>Percentage of individuals with given genotype that express associated phenotype</li> </ul> </li> <li>Expressivity <ul> <li>Degree to which individuals with given genotype express associated phenotype</li> </ul> </li> </ul> </li> </ul>
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FAQs
Qualitative traits are traits that are controlled by a single gene and exhibit distinct phenotypes, like Mendelian traits, with clear dominant and recessive alleles. Examples include eye color or the ability to roll one's tongue. Quantitative traits, on the other hand, are traits influenced by multiple genes and environmental factors, leading to continuous variation in phenotypes. Examples include height and body weight.
Incomplete dominance occurs when a heterozygous individual shows a phenotype that is intermediate between the two homozygous phenotypes. In this case, neither allele is fully dominant, so the resulting phenotype is a blend of both alleles' expressions. Codominance, on the other hand, is when both alleles are fully expressed in the heterozygous individual, making both phenotypes visible simultaneously. This results in a phenotype not seen in either homozygous parent and is a form of genetic polymorphism where multiple phenotypes exist in a population.
In Mendelian traits, inheritance patterns follow Mendel's laws of segregation and independent assortment. Patterns of dominance can be observed through the phenotypes of offspring resulting from a specific genetic cross. Mendelian traits are controlled by single genes with distinct dominant and recessive alleles. The dominant allele's phenotype will be observed when at least one copy is present in a heterozygous individual. In contrast, the recessive allele's phenotype will only be expressed when two copies are present in a homozygous individual.
Dominant alleles are genetic variations that can fully express their associated phenotypes even if an organism has only one copy of the allele. When a dominant allele is present, its phenotype will mask the expression of the recessive allele. On the other hand, recessive alleles only express their associated phenotype when two copies are present (i.e., the organism is homozygous for the recessive allele). The combination of dominant and recessive alleles inherited from the parents determines the organism's phenotype for a specific trait.
