Chromosomal Inheritance — AP Biology
1. What Is Chromosomal Inheritance? ★★☆☆☆ ⏱ 3 min
Chromosomal inheritance describes how genes located at specific loci on chromosomes are transmitted from parents to offspring, forming the foundation of the chromosome theory of inheritance. This unifying principle extends Mendelian genetics to explain deviations from classic 9:3:3:1 dihybrid ratios, accounting for non-independent assortment, sex-linked traits, and errors in chromosome transmission. Unit 5 (Heredity) makes up 8-11% of your total AP exam score, with chromosomal inheritance contributing roughly 2-4% of overall points, tested in both MCQ and FRQ.
2. Sex-Linked Inheritance ★★☆☆☆ ⏱ 3 min
Sex-linked traits are encoded by genes located on the X or Y sex chromosomes. Y-linked traits are rare, passed exclusively from father to all sons, as daughters do not inherit a Y chromosome. For AP Biology, the most commonly tested traits are X-linked, carried on the X chromosome.
Males are hemizygous for all X-linked genes: they only have one X chromosome, so any allele (even recessive ones) is directly expressed with no masking. This explains why X-linked recessive disorders like red-green color blindness and hemophilia are far more common in males than females.
Key patterns for X-linked recessive traits: affected sons always inherit the disease allele from their (usually phenotypically normal carrier) mother; affected fathers cannot pass the disease to sons; all daughters of an affected father are carriers. For X-linked dominant traits: affected fathers pass the trait to all daughters and no sons, while heterozygous affected mothers pass the trait to 50% of sons and 50% of daughters.
Exam tip: Always circle the condition in the question: if it asks for the probability that a son is affected, the answer is $\frac{1}{2}$; if it asks for the probability of an affected son out of all offspring, the answer is $\frac{1}{4}$.
3. Linked Genes and Recombination Frequency ★★★☆☆ ⏱ 4 min
Linked genes are genes located close together on the same chromosome that tend to be inherited together, because they do not assort independently during meiosis. Crossing over (homologous recombination) during prophase I of meiosis can separate linked genes, producing recombinant gametes with new allele combinations not present in the parental generation. The farther apart two genes are on a chromosome, the more likely a crossing over event will occur between them, resulting in a higher proportion of recombinant offspring.
RF = \frac{\text{Number of recombinant offspring}}{\text{Total number of offspring}} \times 100\%
Linkage maps (genetic maps of chromosomes) are built by adding recombination frequencies between adjacent genes to get the relative order of genes on a chromosome. If RF equals 50%, genes are considered unlinked: this occurs either when genes are on different chromosomes, or when they are very far apart on the same chromosome, so crossing over always separates them.
Exam tip: If your calculated recombination frequency is greater than 50%, you almost certainly used parental offspring instead of recombinant offspring. Double-check which class is less abundant before proceeding.
4. Nondisjunction and Chromosomal Alterations ★★★☆☆ ⏱ 3 min
Nondisjunction is the failure of chromosomes to separate properly during meiosis, resulting in gametes with an abnormal number of chromosomes. Nondisjunction can occur in meiosis I (when homologous chromosome pairs fail to separate) or meiosis II (when sister chromatids fail to separate). The resulting condition of having an abnormal chromosome number is called aneuploidy, which has two common forms: monosomy (one copy of a chromosome, $2n-1$) and trisomy (three copies of a chromosome, $2n+1$).
Most full autosomal aneuploidies are lethal in humans, but trisomy 21 (Down syndrome) is viable. Sex chromosome aneuploidies are often viable due to X-inactivation (the process that silences extra X chromosomes in female mammals); common examples include Turner syndrome (XO, monosomy X) and Klinefelter syndrome (XXY). In addition to number changes, chromosomes can have structural alterations: deletions (loss of a chromosome segment), duplications (extra copy of a segment), inversions (reversal of a segment's orientation), and translocations (movement of a segment between non-homologous chromosomes). All structural alterations can change phenotype by altering gene number or regulation.
Exam tip: Remember that nondisjunction in meiosis II only produces half abnormal gametes, while nondisjunction in meiosis I produces all abnormal gametes. Don't assume every gamete from a meiosis with nondisjunction is abnormal.
5. AP-Style Concept Check ★★★☆☆ ⏱ 4 min
Common Pitfalls
Why: Students mix up phenotypic classes, forgetting linked genes produce more parental offspring than recombinants
Why: Students confuse autosomal and sex-linked transmission patterns
Why: Students automatically recall 1/2 of males are affected, and forget to account for the 1/2 chance of the child being male
Why: Students forget very distant genes on the same chromosome have a 100% chance of crossing over, leading to 50% RF
Why: Students memorize most autosomal aneuploidies are lethal and generalize to all aneuploidies
Why: Students confuse recombination frequency probability rules with linkage map construction