Heredity — AP Biology
1. What Is Heredity? ★☆☆☆☆ ⏱ 5 min
Heredity is the transmission of genetic information from parent organisms to their offspring, forming the basis of shared family traits and genetic variation across generations. Questions on this unit make up 8-11% of your total AP Biology exam score, per the official College Board CED. It covers both simple Mendelian inheritance patterns and complex non-Mendelian patterns driven by gene linkage, chromosomal abnormalities, and sex-linked gene expression.
2. Mendelian Genetics ★★☆☆☆ ⏱ 20 min
Mendelian genetics describes the foundational inheritance patterns first identified by Gregor Mendel in his 19th-century pea plant breeding experiments.
- **Law of Segregation**: The two alleles for a single gene separate during gamete formation in meiosis, so each gamete receives only one allele for each gene.
- **Law of Independent Assortment**: Alleles of two or more different genes assort independently of one another during gamete formation, as long as the genes are located on different chromosomes or very far apart on the same chromosome.
3. Linkage and Recombination ★★★☆☆ ⏱ 20 min
Linked genes are genes located close together on the same chromosome, which tend to be inherited together and violate the Law of Independent Assortment. The only way linked genes are separated is via crossing over, the exchange of homologous chromosome segments during prophase I of meiosis, which produces recombinant gametes with new allele combinations not present in either parent.
Recombination frequency (RF) is the percentage of recombinant offspring produced in a cross, calculated as:
\text{Recombination Frequency (RF)} = \frac{\text{Number of Recombinant Offspring}}{\text{Total Number of Offspring}} \times 100\%
A 1% RF equals 1 map unit (centimorgan, cM), the standard unit for relative distance between genes. An RF of 50% indicates genes assort independently, either because they are on separate chromosomes or very far apart on the same chromosome.
4. Sex Linkage ★★☆☆☆ ⏱ 15 min
Sex-linked genes are located on the sex chromosomes (X and Y in humans). Most sex-linked traits are X-linked, as the Y chromosome is very small and carries only ~50 functional genes compared to ~1000 on the X chromosome. Males are hemizygous for all X-linked genes, meaning they only have one copy, so they will express any recessive X-linked allele even with one copy. Females need two copies of a recessive allele to express the trait, so X-linked recessive traits are far more common in males.
5. Pedigree Analysis ★★★☆☆ ⏱ 18 min
A pedigree is a standardized family tree diagram that tracks the occurrence of a genetic trait across multiple generations, used to infer the trait's inheritance pattern. Standard symbols: circles = female, squares = male, filled symbols = affected individual, horizontal line = mating, vertical lines = offspring.
- **Autosomal dominant**: Affected individuals have at least one affected parent, trait appears in every generation, males and females equally likely to be affected.
- **Autosomal recessive**: Affected individuals often have unaffected carrier parents, trait skips generations, males and females equally likely to be affected.
- **X-linked dominant**: Affected males pass the trait to 100% of their daughters and 0% of their sons, trait does not skip generations.
- **X-linked recessive**: More males are affected, trait skips generations, no father-to-son transmission (fathers pass Y chromosome to sons, not X).
6. Chromosomal Inheritance ★★★☆☆ ⏱ 15 min
The chromosomal theory of inheritance states that genes are located at specific fixed positions (loci) on chromosomes, and the segregation and independent assortment of chromosomes during meiosis directly explain Mendel's laws of inheritance.
Most large-scale genetic disorders are caused by chromosomal abnormalities, which fall into two main categories:
- **Aneuploidy**: An abnormal number of a single chromosome, caused by nondisjunction (failure of chromosomes to separate during meiosis I or II). Nondisjunction in meiosis I produces 4 abnormal gametes, while nondisjunction in meiosis II produces 2 abnormal and 2 normal gametes. Common examples: Down syndrome (trisomy 21), Turner syndrome (XO), Klinefelter syndrome (XXY).
- **Structural abnormalities**: Changes to chromosome structure, including deletion (e.g. Cri-du-chat syndrome, deletion of part of chromosome 5 short arm), duplication, inversion, and translocation (e.g. chronic myelogenous leukemia, translocation between chromosomes 9 and 22).
Common Pitfalls
Why: Students memorize Mendel's results without understanding that this ratio only applies to unlinked genes.
Why: Students treat X-linked traits the same as autosomal traits, ignoring hemizygosity in males.
Why: Students confuse autosomal recessive and X-linked recessive pedigree patterns, since both can skip generations.
Why: Students forget that crossing over occurs regularly between genes that are far apart on the same chromosome.
Why: Students assume 1 cM equals a fixed number of DNA base pairs across the entire genome.