DNA Replication and Genetics
DNA stores genetic information using four bases that pair specifically: adenine with thymine (A-T) and guanine with cytosine (G-C).
Semiconservative replication
When a cell divides, DNA copies itself semiconservatively: the double helix unwinds and each old strand serves as a template for a new complementary strand. Each daughter molecule has one old and one new strand. The enzyme DNA polymerase adds nucleotides, and helicase unwinds the helix.
Mendelian inheritance
Genes come in versions called alleles. An organism's genotype (allele combination) determines its phenotype (observable trait). A dominant allele masks a recessive one.
Worked example
Cross two heterozygous pea plants for seed color, Yy x Yy, where Y (yellow) is dominant over y (green). Use a Punnett square:
| Y | y | |
|---|---|---|
| Y | YY | Yy |
| y | Yy | yy |
Genotypes: 1 YY : 2 Yy : 1 yy. Since YY and Yy both show yellow, the phenotype ratio is 3 yellow : 1 green, a probability of 3/4 yellow.
Key takeaways
- •Bases pair A-T and G-C; replication is semiconservative.
- •Genotype determines phenotype; dominant masks recessive.
- •A monohybrid Yy x Yy cross yields a 3:1 phenotype ratio.
