Mendelian genetics
52 flashcards covering Mendelian genetics for the MCAT Biology & Biochemistry section.
Mendelian genetics is the study of how traits are inherited from parents to offspring, based on the work of Gregor Mendel in the 19th century. Through experiments with pea plants, Mendel identified key principles, such as the idea that traits are determined by discrete units called genes, which come in different forms known as alleles. These alleles can be dominant or recessive, leading to patterns like simple dominant-recessive inheritance or more complex ones like codominance. Understanding these basics helps explain everyday phenomena, from eye color to genetic disorders, and forms the groundwork for more advanced genetics.
On the MCAT, Mendelian genetics frequently appears in the Biology and Biochemistry section through multiple-choice questions that test your ability to predict inheritance patterns using Punnett squares, analyze pedigrees, or calculate probabilities of trait transmission. Common traps include confusing dominant and recessive traits or overlooking exceptions like sex-linked inheritance, so focus on applying Mendel's laws accurately and practicing problem-solving. A solid grasp here can boost your score by connecting to broader topics like evolution and population genetics.
Practice Punnett squares with real-world examples to solidify your understanding.
Terms (52)
- 01
Gene
A gene is a segment of DNA that codes for a specific trait or function in an organism.
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Allele
An allele is a variant form of a gene that arises from mutations and determines different expressions of a trait.
- 03
Locus
A locus is the specific location of a gene on a chromosome where alleles are found.
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Genotype
Genotype refers to the genetic makeup of an organism, including the specific alleles it carries for a particular gene.
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Phenotype
Phenotype is the observable characteristics of an organism resulting from the interaction of its genotype with the environment.
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Dominant allele
A dominant allele is one that expresses its trait even if only one copy is present in the genotype.
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Recessive allele
A recessive allele expresses its trait only when two copies are present in the homozygous genotype.
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Homozygous
Homozygous describes an organism that has two identical alleles for a particular gene.
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Heterozygous
Heterozygous describes an organism that has two different alleles for a particular gene.
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Law of Segregation
The Law of Segregation states that during gamete formation, the two alleles for each gene separate so that each gamete carries only one allele.
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Law of Independent Assortment
The Law of Independent Assortment states that alleles of different genes assort independently of one another during gamete formation.
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Monohybrid cross
A monohybrid cross is a breeding experiment that examines the inheritance of one specific trait, typically between two individuals heterozygous for that trait.
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Dihybrid cross
A dihybrid cross is a breeding experiment that examines the inheritance of two different traits simultaneously in offspring.
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Punnett square
A Punnett square is a grid used to predict the probabilities of different genotypes and phenotypes in the offspring of a genetic cross.
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Test cross
A test cross is a breeding experiment where an individual of unknown genotype is crossed with a homozygous recessive individual to determine the unknown genotype.
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Parental generation
The parental generation, or P generation, consists of the original organisms used in a breeding experiment to produce offspring.
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First filial generation
The first filial generation, or F1 generation, is the offspring resulting from a cross between the parental generation.
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Second filial generation
The second filial generation, or F2 generation, is the offspring produced by crossing two individuals from the F1 generation.
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Incomplete dominance
Incomplete dominance is a pattern of inheritance where the heterozygous phenotype is intermediate between the two homozygous phenotypes.
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Codominance
Codominance is a pattern of inheritance where both alleles in a heterozygous genotype are fully expressed, resulting in a distinct phenotype.
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Multiple alleles
Multiple alleles refer to the existence of more than two alleles for a single gene in a population, though an individual carries only two.
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Polygenic traits
Polygenic traits are characteristics influenced by more than one gene, resulting in a range of phenotypes often seen in a bell curve distribution.
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Pleiotropy
Pleiotropy occurs when a single gene influences multiple, seemingly unrelated phenotypic traits.
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Epistasis
Epistasis is the interaction between different genes where one gene masks or modifies the expression of another gene.
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Sex-linked inheritance
Sex-linked inheritance involves traits controlled by genes on the sex chromosomes, primarily the X chromosome, leading to different patterns in males and females.
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X-linked trait
An X-linked trait is one determined by a gene on the X chromosome, often showing different inheritance patterns based on the sex of the individual.
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Autosomal dominant inheritance
Autosomal dominant inheritance means a trait is expressed if at least one copy of the dominant allele is present on an autosome.
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Autosomal recessive inheritance
Autosomal recessive inheritance means a trait is expressed only if two copies of the recessive allele are present on an autosome.
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Pedigree analysis
Pedigree analysis is the study of inheritance patterns in families through a diagram that shows the relationships and traits across generations.
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Carrier
A carrier is an individual who has one copy of a recessive allele and does not exhibit the trait but can pass the allele to offspring.
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Chi-square test
The chi-square test is a statistical method used to determine if observed genetic ratios deviate significantly from expected ratios due to chance.
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Linkage
Linkage occurs when genes on the same chromosome are inherited together because they do not assort independently.
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Crossing over
Crossing over is the exchange of genetic material between homologous chromosomes during meiosis, which can create new allele combinations.
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Genetic recombination
Genetic recombination is the process that produces new gene combinations through crossing over and independent assortment.
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True-breeding
True-breeding organisms are those that, when self-pollinated or crossed with another of the same type, produce offspring with the same traits.
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Backcross
A backcross is a breeding experiment where an offspring is crossed with one of its parents to analyze inheritance patterns.
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Reciprocal cross
A reciprocal cross involves swapping the parental phenotypes in two separate breeding experiments to check for sex-linked inheritance.
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Phenotypic ratio
The phenotypic ratio is the proportion of different observable traits in the offspring of a genetic cross.
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Genotypic ratio
The genotypic ratio is the proportion of different genotypes in the offspring of a genetic cross.
- 40
Segregation ratio
The segregation ratio is the expected ratio of offspring phenotypes or genotypes based on Mendel's laws, such as 3:1 in a monohybrid cross.
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Strategy for Punnett squares
To use Punnett squares effectively, identify the genotypes of parents, list all possible gametes, and fill the grid to calculate offspring probabilities.
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Common trap: Confusing dominance
A common error is assuming an allele is dominant without evidence, as dominance is determined by expression patterns, not by the allele itself.
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Worked example: Monohybrid cross
In a monohybrid cross between two heterozygous tall pea plants (Tt x Tt), the offspring genotypes are 1 TT, 2 Tt, and 1 tt, resulting in a 3:1 phenotypic ratio of tall to short.
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Worked example: Dihybrid cross
In a dihybrid cross between two heterozygous individuals for seed shape and color (RrYy x RrYy), the expected phenotypic ratio is 9:3:3:1 for round yellow, round green, wrinkled yellow, and wrinkled green seeds.
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Probability of inheritance
The probability of inheriting a specific genotype is calculated by multiplying the independent probabilities of each allele, such as 0.5 for each allele in a heterozygous parent.
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Mendel's experiments
Mendel's experiments involved crossing pea plants with different traits and observing patterns in offspring to establish the foundational laws of inheritance.
- 47
Hybrid
A hybrid is an offspring resulting from the cross between two individuals that differ in one or more genes.
- 48
Expected ratios in genetics
Expected ratios, like 3:1 or 9:3:3:1, are theoretical outcomes based on Mendelian principles and are used to predict inheritance patterns.
- 49
Autosomal traits
Autosomal traits are those controlled by genes on autosomes, not sex chromosomes, and follow Mendelian inheritance patterns equally in both sexes.
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Gamete formation
Gamete formation, or meiosis, ensures that each gamete receives one allele per gene, maintaining the diploid number in offspring.
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Phenotypic variation
Phenotypic variation in Mendelian genetics arises from different allele combinations and can be predicted using tools like Punnett squares.
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Genetic cross types
Genetic cross types include monohybrid, dihybrid, and test crosses, each designed to reveal specific aspects of inheritance.