MCAT Biological and Biochemical Foundations of Living Systems Flashcards: 1c Mendelian Genetics Inheritance

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This deck focuses on 1c Mendelian Genetics Inheritance, giving you a quick way to review the definitions, rules, and examples that matter most for MCAT Biological and Biochemical Foundations of Living Systems.

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Work through these flashcards in short sessions. Try to answer each prompt before flipping the card, then revisit any cards you miss until the explanation feels automatic.

Flashcard 1: What is the expected phenotypic ratio for a dihybrid cross $AaBb \times AaBb$ with complete dominance and unlinked genes?

Answer: $9:3:3:1$. The dihybrid ratio results from independent assortment of two genes, combining monohybrid ratios for four phenotypic classes.

Flashcard 2: What is the definition of homozygous at a gene locus?

Answer: Having two identical alleles (e.g., $AA$ or $aa$). Homozygosity occurs when both alleles at a locus are the same, resulting in consistent expression regardless of dominance.

Flashcard 3: What is the probability that $Aa \times Aa$ produces a heterozygous $Aa$ offspring?

Answer: $\frac{1}{2}$. Heterozygous offspring require one dominant and one recessive allele, occurring in two of four possible combinations.

Flashcard 4: Which cross is a testcross for a dominant-phenotype individual of genotype A_?

Answer: Cross with homozygous recessive: $A_ \times aa$. A testcross reveals unknown genotypes by crossing with homozygous recessive, exposing hidden recessive alleles in offspring.

Flashcard 5: What does variable penetrance mean for a genotype in a population?

Answer: Not all individuals with the genotype show the phenotype. Incomplete penetrance means environmental or genetic factors prevent phenotype manifestation in some genotype carriers.

Flashcard 6: What inheritance pattern is suggested by a trait that appears more often in males and has no father-to-son transmission?

Answer: X-linked recessive inheritance. X-linked recessive traits skip generations and affect males more due to their single X chromosome lacking a compensating allele.

Flashcard 7: What is the definition of an allele?

Answer: An alternative version of a gene at a locus. Alleles represent variations of a gene occupying the same chromosomal locus, leading to potential differences in trait expression.

Flashcard 8: What is the difference between genotype and phenotype?

Answer: Genotype = alleles; phenotype = expressed traits. Genotype refers to the genetic makeup, while phenotype is the observable outcome influenced by both genetics and environment.

Flashcard 9: What is the probability that $AaBb$ produces a gamete $ab$ assuming independent assortment?

Answer: $\frac{1}{4}$. With independent assortment, each allele pair segregates separately, so the chance of both recessive alleles in a gamete is $\frac{1}{2} \times \frac{1}{2}$.

Flashcard 10: What is the definition of heterozygous at a gene locus?

Answer: Having two different alleles (e.g., $Aa$). Heterozygosity arises from differing alleles at a locus, allowing for interactions like dominance or codominance in phenotype.

Flashcard 11: What is the definition of a dominant allele in a heterozygote?

Answer: An allele expressed in the phenotype of $Aa$. Dominant alleles override recessive ones in heterozygotes, determining the observed trait in Mendelian inheritance.

Flashcard 12: What is the definition of a recessive allele in a heterozygote?

Answer: An allele masked in $Aa$; expressed only in $aa$. Recessive alleles require homozygosity for expression, as they are suppressed by dominant alleles in heterozygotes.

Flashcard 13: What does Mendel's law of segregation state about allele separation?

Answer: Alleles separate into different gametes during meiosis. Mendel's law of segregation explains how paired alleles divide during gamete formation, ensuring each gamete receives one allele.

Flashcard 14: What does Mendel's law of independent assortment state (with unlinked genes)?

Answer: Alleles of different genes assort independently into gametes. Independent assortment occurs when genes on different chromosomes segregate randomly, producing varied gamete combinations.

Flashcard 15: What is the expected phenotypic ratio for a monohybrid cross $Aa \times Aa$ with complete dominance?

Answer: $3:1$ (dominant:recessive). In complete dominance, the monohybrid cross yields three dominant phenotypes to one recessive due to heterozygote masking.

Flashcard 16: What is the expected genotypic ratio for a monohybrid cross $Aa \times Aa$?

Answer: $1:2:1$ ($AA:Aa:aa$). The genotypic ratio reflects the probabilities of homozygous dominant, heterozygous, and homozygous recessive outcomes from segregation.

Flashcard 17: What is the probability that $Aa \times Aa$ produces an $aa$ offspring?

Answer: $\frac{1}{4}$. The probability of homozygous recessive offspring is $\frac{1}{2} \times \frac{1}{2}$ from each parent's contribution of the recessive allele.

Flashcard 18: What offspring phenotypic ratio indicates that a dominant-phenotype parent is $Aa$ in a testcross $A_ \times aa$?

Answer: $1:1$ (dominant:recessive). The 1:1 ratio in a testcross confirms heterozygosity, as half the offspring inherit the recessive allele from the tested parent.

Flashcard 19: What is epistasis in the context of two-gene inheritance?

Answer: An allele at one gene masks/modifies another gene’s phenotype. Epistasis involves gene interactions where one gene's product affects another's expression, altering expected dihybrid ratios.

Flashcard 20: What is the definition of a gene in Mendelian genetics?

Answer: A heritable DNA unit that affects a trait. Genes serve as the fundamental units of heredity, encoding information in DNA that determines specific traits passed from parents to offspring.

Flashcard 21: What does variable expressivity mean for individuals with the same genotype?

Answer: Phenotype severity varies among individuals with the genotype. Variable expressivity results from modifiers causing differing degrees of trait severity among individuals with identical genotypes.

Flashcard 22: What does pleiotropy mean for the effect of a single gene?

Answer: One gene influences multiple phenotypic traits. Pleiotropy occurs when a single gene mutation affects multiple systems, as in sickle cell anemia impacting blood and organs.

Flashcard 23: What is incomplete dominance (as judged by heterozygote phenotype)?

Answer: Heterozygote shows an intermediate phenotype. Incomplete dominance results from neither allele fully dominating, blending traits in heterozygotes like in snapdragon flower color.

Flashcard 24: What is codominance (as judged by heterozygote phenotype)?

Answer: Heterozygote fully expresses both alleles. Codominance allows both alleles to contribute equally to the phenotype, as seen in ABO blood types with distinct expressions.

Flashcard 25: What is the phenotypic ratio for $Aa \times Aa$ under incomplete dominance?

Answer: $1:2:1$ (three distinct phenotypes). Incomplete dominance produces three phenotypes in the ratio due to the intermediate heterozygote differing from both homozygotes.