MCAT Biological and Biochemical Foundations of Living Systems Flashcards: 1c Chromosomal Basis Inheritance

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This deck focuses on 1c Chromosomal Basis 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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Flashcard 1: Which inheritance pattern is most consistent with a gene located on the Y chromosome?

Answer: Father-to-son transmission only. Y-linked genes pass solely from fathers to sons, as the Y chromosome is male-specific and not in females.

Flashcard 2: What is the recombination frequency formula between two loci?

Answer: rac{ ext{recombinants}}{ ext{total offspring}} imes 100 ext{%}. Recombination frequency measures linkage by calculating the percentage of offspring with new allele combinations from crossing over.

Flashcard 3: Which meiotic error produces two normal gametes plus one $n+1$ and one $n-1$ gamete?

Answer: Nondisjunction in meiosis II (sister chromatids fail to separate). Meiosis II nondisjunction affects one cell, producing half normal gametes and half with chromosome gain or loss.

Flashcard 4: What is X-inactivation (lyonization) in human females?

Answer: Random silencing of one X chromosome to form a Barr body. X-inactivation equalizes X-linked gene expression between sexes by condensing one X into a transcriptionally inactive Barr body.

Flashcard 5: What is the chromosomal basis of inheritance as demonstrated by Mendel's laws?

Answer: Genes are located on chromosomes that segregate and assort in meiosis. Mendel's laws of segregation and independent assortment are explained by genes residing on chromosomes that separate and randomly align during meiotic divisions.

Flashcard 6: What is the difference between homologous chromosomes and sister chromatids?

Answer: Homologs are maternal vs paternal; sisters are replicated copies of one chromosome. Homologous chromosomes are pairs inherited from each parent, while sister chromatids form after DNA replication of a single chromosome.

Flashcard 7: What is meant by a gene locus on a chromosome?

Answer: The specific physical position of a gene on a chromosome. A gene locus denotes the precise chromosomal location where alleles of a gene are found, determining inheritance patterns.

Flashcard 8: Identify the maximum recombination frequency expected for two loci in a diploid organism.

Answer: 50%. For unlinked loci or those far apart, recombination mimics independent assortment, yielding a maximum of 50% recombinant gametes.

Flashcard 9: What is polyploidy?

Answer: More than two complete sets of chromosomes (e.g., $3n$, $4n$). Polyploidy results from whole genome duplication, common in plants and leading to viable organisms with multiple chromosome sets.

Flashcard 10: What karyotype notation describes Turner syndrome?

Answer: 45,X. Turner syndrome involves monosomy X, with 45 chromosomes lacking one sex chromosome, leading to female developmental issues.

Flashcard 11: What karyotype notation describes a female with trisomy 21?

Answer: 47,XX,+21. Trisomy 21 notation indicates 47 chromosomes with two X and an extra chromosome 21, causing Down syndrome in females.

Flashcard 12: What is aneuploidy?

Answer: An abnormal number of individual chromosomes (not whole sets). Aneuploidy arises from errors like nondisjunction, altering chromosome count by gains or losses of specific chromosomes.

Flashcard 13: What karyotype notation describes Klinefelter syndrome?

Answer: 47,XXY. Klinefelter syndrome features an extra X in males, totaling 47 chromosomes, often causing infertility and other traits.

Flashcard 14: What is genetic linkage?

Answer: Tendency of nearby genes on one chromosome to be inherited together. Genes close on the same chromosome are linked, reducing recombination and causing them to co-segregate more often than by chance.

Flashcard 15: What is nondisjunction, in one sentence?

Answer: Failure of homologs or sister chromatids to separate during meiosis. Nondisjunction disrupts proper chromosome distribution, leading to gametes with abnormal numbers and potential aneuploidy in offspring.

Flashcard 16: What map distance in centimorgans corresponds to a recombination frequency of 1 ext{%}?

Answer: 1 cM. One centimorgan equals 1% recombination frequency, serving as a unit for mapping genetic distances on chromosomes.

Flashcard 17: Which meiotic event directly explains Mendel's law of segregation?

Answer: Separation of homologous chromosomes in anaphase I. During anaphase I, homologous chromosomes separate, ensuring each gamete receives one allele of a gene, aligning with segregation.

Flashcard 18: Which option best defines a testcross used in linkage analysis?

Answer: Cross an unknown (often heterozygote) with a homozygous recessive. A testcross reveals an individual's genotype and linkage by mating with a recessive homozygote to observe offspring ratios.

Flashcard 19: Which meiotic event best explains Mendel's law of independent assortment?

Answer: Random orientation of homologous pairs at metaphase I. At metaphase I, independent alignment of homologous pairs allows genes on different chromosomes to assort randomly into gametes.

Flashcard 20: What offspring phenotypic ratio is expected from a dihybrid testcross with unlinked genes?

Answer: $1:1:1:1$. In a dihybrid testcross of unlinked genes, independent assortment produces equal proportions of all four phenotypic classes.

Flashcard 21: Which meiotic error produces $n+1$ and $n-1$ gametes after meiosis I?

Answer: Nondisjunction in meiosis I (homologs fail to separate). Meiosis I nondisjunction causes both homologs to go to one pole, yielding cells with extra or missing chromosomes post-division.

Flashcard 22: Identify the outcome pattern in a testcross that suggests two genes are linked.

Answer: Parental phenotypes exceed recombinant phenotypes. Linkage in a testcross reduces recombinants, resulting in more parental types due to genes inheriting together.

Flashcard 23: What is crossing over, and during which meiotic stage does it occur?

Answer: Exchange between homologs during prophase I (pachytene). Crossing over involves genetic material exchange between non-sister chromatids of homologs, increasing diversity in pachytene of prophase I.

Flashcard 24: Identify the recombination frequency when $18$ recombinants occur among $200$ total offspring.

Answer: rac{18}{200} imes 100 ext{%} = 9 ext{%}. Recombination frequency is calculated as the percentage of recombinant offspring, indicating linkage strength between loci.