North Carolina Biology EOC: Inheritance and Variation - a complete overview of meiosis, Mendelian genetics, inheritance patterns, sex-linked traits, and gene-environment interaction

What the inheritance and variation content demands

The inheritance content sits in the Heredity strand and is the part of the course where genetics becomes predictive. This guide runs from meiosis (how gametes form and variation arises), through Mendelian genetics and Punnett squares, the non-Mendelian patterns, sex-linked inheritance and pedigrees, to how the environment shapes traits. The recurring crosscutting concepts are cause and effect and patterns, and the recurring skill is predicting and reasoning about offspring.

This guide ties together the matching topic pages, each with its own practice questions: meiosis and genetic variation, Mendelian genetics and Punnett squares, patterns of inheritance, sex-linked traits and pedigree analysis, and the environment and gene expression.

Meiosis and variation

Meiosis makes gametes with half the chromosome number (haploid), so that fertilization restores the full number. It produces four genetically different cells (unlike mitosis's two identical ones). Variation comes from crossing over (homologous chromosomes exchange segments), independent assortment (random chromosome arrangement), and random fertilization. This variation is the raw material for natural selection.

Mendelian genetics

An organism has two alleles per gene. A dominant allele shows whenever present; a recessive allele only when both are recessive. Genotype is the alleles; phenotype is the trait. A Punnett square predicts a cross: $Tt \times Tt$ gives a $1:2:1$ genotype ratio and a 3:1 phenotype ratio (a $\frac{3}{4}$ chance of the dominant trait); $Tt \times tt$ gives 1:1.

Inheritance beyond simple dominance

Incomplete dominance gives a blended heterozygote (pink snapdragons). Codominance shows both alleles at once (roan coat, type AB blood). Multiple alleles mean more than two versions exist in the population (ABO blood type, with three). Polygenic traits (height, skin color) are set by many genes, giving a continuous range.

Sex-linked traits and pedigrees

Females are XX, males XY. X-linked recessive traits appear more in males, who have only one X, so a single recessive allele is expressed. A carrier has one recessive allele but does not show the trait. A pedigree traces a trait through a family: two unaffected parents with an affected child indicates a recessive trait (the parents are carriers).

The environment and gene expression

Phenotype = genotype + environment. Genes set the potential, but the environment influences how traits are expressed: identical plants differ in height with different light, and temperature affects coat color or sex in some animals. Inherited traits come from genes and pass on; acquired traits (a suntan, a scar) come from the environment and do not, because they do not change gamete DNA.

Check your knowledge

A mix of recall and reasoning questions covering the inheritance and variation content. Attempt them under timed conditions, then check against the solutions.

1. A body cell has 46 chromosomes. State the number in its gametes and why. (2 marks)
2. Name two processes during meiosis that create genetic variation. (2 marks)
3. In a $Tt \times Tt$ cross, state the phenotype ratio and the probability of the dominant trait. (2 marks)
4. An organism shows a recessive trait. State its genotype and why. (2 marks)
5. Snapdragons: red x white gives all pink. Name this inheritance pattern. (1 mark)
6. Explain why type AB blood is an example of codominance. (2 marks)
7. Explain why X-linked recessive traits appear more often in males. (2 marks)
8. In a pedigree, two unaffected parents have an affected child. State whether the trait is dominant or recessive and why. (2 marks)
9. State whether a suntan is inherited or acquired, and why it is not passed on. (2 marks)