Genetics Study Notes: Alleles, Inheritance, Natural Selection, and Pedigrees Flashcards

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Alleles

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Alternative versions of a gene found at the same locus on homologous chromosomes. Alleles determine the variation in a specific trait and are represented in genotypes such as $AA$, $Aa$, or $aa$.

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Genotype

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The genetic makeup of an organism for a particular trait, shown by allele combinations like $AA$, $Aa$, or $aa$. Genotype predicts potential phenotypes but does not always determine the observable trait when interactions like dominance occur.

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Phenotype

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The observable characteristics or traits of an organism produced by the interaction of its genotype and the environment. Phenotypes are what natural selection acts upon, even though genotype frequencies change over time.

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Dominant allele

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An allele that expresses its phenotype when present in either heterozygous or homozygous states, typically written as a capital letter such as $A$. A dominant allele masks the effect of a recessive allele in a heterozygote, so $Aa$ shows the dominant trait.

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Recessive allele

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An allele that is phenotypically expressed only when homozygous, written in lowercase such as $a$. Recessive traits often skip generations and appear when an individual has genotype $aa$.

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Heterozygous

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Having two different alleles for a given gene, for example $Aa$. Heterozygotes can show dominant phenotypes or intermediate phenotypes in cases like incomplete dominance.

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Natural selection

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The process by which organisms with traits that increase survival or reproduction become more common in a population over generations. It requires variation, inheritance, differential survival or reproduction, and time to change allele frequencies.

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Fitness

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The relative reproductive success of a genotype compared to others in the population. Fitness determines which alleles increase in frequency under selection because higher-fitness genotypes contribute more offspring.

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Directional selection

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A form of natural selection that favors one extreme phenotype, shifting the population mean toward that extreme. An example is antibiotic resistance increasing in bacteria when drugs select for resistant variants.

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Stabilizing selection

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A type of selection that favors the average phenotype and reduces variation by selecting against extremes. This mode maintains traits near the population mean and can reduce the frequency of extreme alleles.

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Disruptive selection

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Selection that favors extreme phenotypes at both ends of the distribution, which can increase variation and potentially lead to speciation. It reduces the frequency of intermediate phenotypes and favors divergent alleles.

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Genetic drift

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Random changes in allele frequencies due to chance events, which are especially important in small populations. Drift can lead to loss or fixation of alleles independent of fitness and reduces genetic variation over time.

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Punnett square

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A diagram used to predict the genotype and phenotype ratios of offspring from a genetic cross. It applies Mendelian logic by combining parental alleles and helps test if observed offspring fit an inheritance model.

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Codominance

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A pattern where both alleles in a heterozygote are fully expressed, producing a phenotype that displays both traits simultaneously. A classic example is blood type genotype $I^A I^B$, where both A and B antigens are present.

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Incomplete dominance

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A pattern in which heterozygotes show an intermediate phenotype between the two homozygotes rather than expressing one allele fully. For example, $RR$ red crossed with $WW$ white producing $RW$ pink offspring demonstrates blending.

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Pedigree

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A family tree that tracks the inheritance of a trait across generations using standard symbols (square = male, circle = female, shaded = affected). Pedigrees help determine modes of inheritance by noting patterns like skipped generations or sex bias.

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X-linked recessive

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A recessive mutation on the X chromosome that causes disease when males have genotype $X^cY$ because they are hemizygous for X. Females must be $X^cX^c$ to be affected and $X^cX$ females are typically carriers; many more affected males and no father-to-son transmission are typical patterns.

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Y-linked

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Traits determined by genes on the Y chromosome that affect only males and show consistent father-to-son transmission. Y-linked traits are rare because the Y chromosome carries relatively few genes.