Chromosomal Basis of Inheritance (1C) - MCAT Biological and Biochemical Foundations of Living Systems

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.

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

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

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

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.

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.

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.

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.

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.

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

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.

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

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

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.

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

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

$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.

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.

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.

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.

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.

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.

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.