Biology: Cells to AP Biology · Module 5 of 15
Genetics & Heredity
Mendelian inheritance, more complex inheritance patterns, sex-linked traits, and DNA itself.
Mendelian Genetics & Punnett Squares
- Genes come in alternate forms called alleles; an organism inherits one allele from each parent for each gene.
- A Punnett square predicts the probability of offspring genotypes and phenotypes from a genetic cross.
- Dominant alleles are expressed even with one copy; recessive alleles only show up with two copies (homozygous recessive).
In pea plants, purple flowers (P) are dominant over white (p). Cross two heterozygous (Pp) plants. What fraction of offspring is expected to have white flowers?
Solution. The Punnett square of Pp × Pp gives genotypes PP, Pp, Pp, pp. Only pp (1 out of 4) is white, since p is recessive.
Incomplete Dominance, Codominance & Multiple Alleles
- Incomplete dominance: heterozygotes show a blended intermediate phenotype (e.g. red × white flowers → pink).
- Codominance: both alleles are fully expressed at once (e.g. AB blood type shows both A and B markers).
- Multiple alleles: a gene can have more than two possible alleles in a population (e.g. human ABO blood type has three: A, B, O).
A red-flowered plant is crossed with a white-flowered plant, and all offspring are pink. What pattern of inheritance is this?
Solution. Incomplete dominance — the heterozygous offspring show a blended phenotype rather than either parent color.
Sex-Linked Traits & Pedigrees
- Sex-linked traits are carried on the X or Y chromosome; X-linked recessive traits (like red-green colorblindness) appear more often in males, since males have only one X chromosome.
- Pedigrees track the inheritance of a trait through a family tree across generations, helping determine dominant/recessive and sex-linked patterns.
Why is colorblindness (X-linked recessive) much more common in men than women?
Solution. Men have only one X chromosome, so a single recessive allele causes colorblindness; women have two X chromosomes and need both to carry the recessive allele to be colorblind.
DNA Structure & Replication
- DNA is a double helix made of two strands of nucleotides, held together by hydrogen bonds between complementary base pairs: A–T and C–G.
- DNA replication is semi-conservative: each new DNA molecule has one original strand and one newly made strand.
- DNA polymerase is the main enzyme that builds the new strand by matching complementary bases.
If one strand of DNA reads A-T-G-C, what is the complementary strand?
Solution. T-A-C-G — A pairs with T, and G pairs with C.
Check yourself
2 of this module's 5 quiz questions. Pick your answers, then check them in the app, with an explanation for every question.
1. Crossing two heterozygous (Pp) parents, what fraction of offspring is expected to show the recessive phenotype?
2. Which inheritance pattern produces a blended phenotype in heterozygotes?