Genetics & Heredity: From Mendel to CRISPR

Part 1: Mendelian Inheritance

Gregor Mendel discovered the basic rules of inheritance in the 1860s by crossing pea plants and carefully counting the results. His two laws still explain most inheritance patterns you will be tested on.

Mendel's Two Laws

• Law of Segregation: each organism has two alleles for each trait; they separate during gamete formation so each gamete carries only one allele
• Law of Independent Assortment: genes on different chromosomes are inherited independently of each other (does not apply to linked genes)

Punnett Square Basics

For a monohybrid cross (one trait), a heterozygote × heterozygote (Aa × Aa) gives a 3:1 phenotypic ratio and a 1:2:1 genotypic ratio. For a dihybrid cross (two traits, unlinked), a double heterozygote cross (AaBb × AaBb) gives a 9:3:3:1 phenotypic ratio.

Part 2: Non-Mendelian Inheritance Patterns

Part 3: Sex-Linked Traits

Genes located on the X chromosome follow X-linked inheritance patterns. Because males have only one X chromosome (XY), they express X-linked recessive traits whenever they carry the allele — there is no second X to mask it.

• X-linked recessive (e.g., hemophilia, red-green colorblindness): more common in males; females can be carriers (XˢX)
• Carrier female (XˢX): normal phenotype but 50% of sons will be affected
• X-linked dominant: affected fathers pass the trait to ALL daughters, NO sons
• Y-linked traits: passed from father to all sons only

Part 4: Gene Linkage and Recombination

Genes on the same chromosome are linked and do not assort independently. However, crossing over during Prophase I can separate them. The frequency of recombination (crossover frequency) is used to map gene distances.

• Recombination frequency = (# recombinant offspring / total offspring) × 100%
• 1% recombination frequency = 1 centimorgan (cM) = 1 map unit (mu)
• Genes more than 50 cM apart behave as if independently assorting even though they are on the same chromosome

Part 5: Chromosomal Abnormalities

Errors in meiosis can produce gametes with the wrong number of chromosomes, leading to aneuploidy in offspring.

Part 6: From PCR to CRISPR

Modern molecular genetics gives scientists tools to read, copy, edit, and insert specific DNA sequences with extraordinary precision.

• PCR (Polymerase Chain Reaction): amplifies a specific DNA sequence exponentially using primers and Taq polymerase. 3 steps: denaturation (94°C), annealing (50–65°C), extension (72°C).
• Restriction enzymes (endonucleases): cut DNA at specific palindromic sequences, creating sticky ends for recombinant DNA work.
• Gel electrophoresis: separates DNA fragments by size through a porous agarose gel. Smaller fragments migrate further from the well.
• CRISPR-Cas9: a bacterial immune system repurposed as a gene-editing tool. A guide RNA directs the Cas9 nuclease to a specific genomic sequence, which it cuts. The cell's own repair machinery then introduces the desired edit.