Genetics Exam 1 Study Notes Flashcards

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Alleles

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Alternate forms of a gene at the same locus that can produce different phenotypes. Alleles may be dominant, recessive, or show other interaction patterns and determine an organism's genotype for a trait. In diploids an individual carries two alleles per gene, one from each parent.

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Segregation

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Mendel’s law stating that the two alleles for a heritable character separate during gamete formation so each gamete carries only one allele. This explains why a monohybrid F2 generation often shows a 3:1 dominant-to-recessive phenotypic ratio. Segregation is a consequence of homologous chromosome separation in meiosis I.

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Independent assortment

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Mendel’s law that alleles of different genes assort independently of one another during gamete formation, provided the genes are unlinked. It predicts the 9:3:3:1 phenotypic ratio in the F2 of a dihybrid cross. The physical basis is the random alignment of different homolog pairs at metaphase I of meiosis.

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Testcross

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A cross between an individual with an unknown genotype and a homozygous recessive tester to reveal the unknown genotype by the progeny phenotypes. It is commonly used to determine whether an individual showing the dominant phenotype is homozygous or heterozygous. Results are interpreted using expected Mendelian ratios.

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Monohybrid

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A cross that follows the inheritance of a single trait controlled by a single gene with two alleles. The F1 generation of true-breeding parents is typically uniform, and the F2 often shows a 3:1 phenotypic ratio for dominant versus recessive traits. Monohybrid crosses illustrate the law of segregation.

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Dihybrid

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A cross that follows two different traits controlled by two genes, each with two alleles. If genes assort independently, the F2 phenotypic ratio from two heterozygous parents is typically 9:3:3:1. Dihybrid crosses provide evidence for independent assortment.

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Codominance

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An allele interaction in which both alleles in a heterozygote are fully expressed, producing a phenotype that shows both parental traits simultaneously. A classic example is ABO blood group antigens where $I^A$ and $I^B$ are codominant. Codominance differs from incomplete dominance because both traits appear rather than blending.

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Incomplete dominance

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An allele interaction where the heterozygote shows an intermediate phenotype between the two homozygotes, often due to dosage effects of gene product. For example, a red and white flower producing pink offspring indicates incomplete dominance. It reflects molecular mechanisms like reduced enzyme activity from one allele.

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Epistasis

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An interaction between genes where alleles at one locus mask or modify the expression of alleles at another locus. Epistasis can produce altered phenotypic ratios (e.g., 9:7 or 12:3:1) that deviate from simple Mendelian expectations. Recognizing epistasis requires analyzing progeny ratios from two-gene crosses.

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Recessive lethal

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A mutant allele that is viable in heterozygotes but causes death when homozygous, leading to reduced numbers of homozygous offspring. Crosses involving recessive lethals often show altered ratios (e.g., 2:1 instead of 3:1). Identification requires observing missing genotypic classes and understanding embryonic or early-life lethality.

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Penetrance

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The proportion of individuals with a particular genotype who actually express the expected phenotype. Penetrance can be complete or incomplete and is influenced by genetic background and environment. It differs from expressivity, which describes the degree of phenotypic expression among individuals with the genotype.

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Complementation test

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A genetic test to determine whether two mutations that produce similar phenotypes are in the same gene or in different genes. When two mutants are crossed, restoration of wild-type phenotype indicates mutations are in different genes (complementation). Failure to complement implies the mutations affect the same gene.

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Meiosis

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A specialized cell division that reduces chromosome number from diploid to haploid, producing gametes with one set of chromosomes. It involves two sequential divisions (meiosis I and II) and features events like synapsis and crossing-over during prophase I. Meiosis underlies Mendel’s laws by segregating homologous chromosomes and assorting them independently.

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Crossing-over

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The physical exchange of DNA between non-sister chromatids of homologous chromosomes during prophase I of meiosis. Crossing-over creates recombinant chromosomes and increases genetic diversity among gametes. The cytological manifestation of crossing-over is the chiasma and it is the basis for recombination-based gene mapping.

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Homologs

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Paired chromosomes in a diploid cell that carry the same set of genes, one inherited from each parent. Homologous chromosomes align and undergo recombination during meiosis I. They are distinct from sister chromatids, which are identical copies produced by DNA replication.

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Sister chromatids

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Two identical copies of a single replicated chromosome joined at the centromere after DNA replication. They separate during mitosis (anaphase) and during meiosis II to produce individual chromosomes in daughter cells. Sister chromatids are not the same as homologs, which are similar but nonidentical.

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Nondisjunction

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The failure of homologous chromosomes or sister chromatids to separate properly during meiosis or mitosis. Nondisjunction produces aneuploid gametes and can lead to conditions such as Down syndrome (trisomy 21) or sex chromosome abnormalities. Its occurrence explains many chromosomal disorders and altered gene dosage.

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Karyotype

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A photographic display of an individual's chromosomes arranged by size, shape, and number to detect chromosomal abnormalities. Karyotyping is used clinically to identify aneuploidies and large structural rearrangements. Generating a karyotype typically involves arresting cells in metaphase and staining chromosomes.

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Sex chromosomes

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Chromosomes that determine the sex of an organism, such as X and Y in humans, which differ from autosomes in number and content. Different species use various systems (XY, ZW, haplodiploidy) for sex determination. Sex chromosomes often carry genes that produce sex-linked patterns of inheritance.

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Dosage compensation

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Mechanisms that equalize expression of X-linked genes between sexes despite differences in X chromosome number, such as X inactivation in mammals. Dosage compensation prevents harmful imbalances in gene products that would arise from unequal sex chromosome dosage. Different organisms achieve compensation by different molecular strategies.

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X inactivation

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The process in female mammals where one X chromosome is transcriptionally silenced in each cell, producing a Barr body and equalizing X-linked gene expression with males. Inactivation is typically random in early embryogenesis, leading to mosaic expression of X-linked traits. It is a key form of dosage compensation.

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Hemizygous

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Having only one allele for a gene in a diploid organism, as occurs for most X-linked genes in XY males. Hemizygosity means a single recessive allele on the X chromosome will be expressed in males because there is no second allele to mask it. This concept explains many sex-linked inheritance patterns.

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SRY

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The sex-determining region Y gene on the human Y chromosome that triggers male sex determination by promoting testis development. Mutations or translocations involving SRY can cause sex-reversal or intersexual phenotypes. SRY is a key example of how a single gene can direct a major developmental pathway.

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Turner syndrome

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A human sex chromosome disorder in which an individual has a single X chromosome (45,X) and typically exhibits short stature, ovarian failure, and other features. Turner syndrome results from nondisjunction leading to monosomy X. Clinical recognition and karyotyping allow diagnosis and management.

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Linkage

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The tendency of genes located close together on the same chromosome to be inherited together because they are less likely to be separated by crossing-over. Linked genes violate independent assortment and produce excess parental-type progeny in crosses. Mapping experiments exploit deviations from expected ratios to infer linkage.

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Recombination frequency

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A measure of the proportion of recombinant offspring produced by a cross and used to estimate genetic distance between loci. It is calculated as $\frac{\#\; recombinants}{\text{total}\; \#\; \text{offspring}} \times 100$, and values range from 0% (complete linkage) up to 50% (independent assortment).

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Map unit

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A genetic distance measure equivalent to 1% recombination frequency, also called a centimorgan (cM). Map units estimate relative positions of genes on a chromosome based on recombination data but do not always correspond linearly to physical distance because of interference and variation in recombination rates. Genetic maps are constructed from two- and three-point testcrosses.

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Chi-square test

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A statistical test used to compare observed and expected progeny counts under a null hypothesis, such as Mendelian segregation ratios. It yields a $p$ value that helps determine whether deviations from expectation are statistically significant. Proper interpretation requires knowing degrees of freedom and chosen significance level.

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Interference

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The phenomenon where one crossover event reduces the probability of another occurring nearby, affecting expected double-crossover frequencies. Interference is quantified using the coefficient of coincidence and alters the relationship between recombination frequency and physical distance. Accounting for interference improves accuracy of genetic maps.

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Tetrad analysis

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A genetic technique in fungi that analyzes the four products of a single meiosis (an ascus) to determine linkage and gene order. Classifying tetrads as parental ditype (PD), nonparental ditype (NPD), or tetratype (T) reveals recombination patterns. Ordered tetrad analysis can map gene positions relative to centromeres.