Georgia Milestones Biology EOC, Heredity (SB3): a complete overview of meiosis, Mendelian genetics, complex inheritance, pedigrees, sex linkage, and reproduction strategies

What the Heredity domain demands

Heredity (SB3) is the half of the genetics content that deals with inheritance: how traits pass from parents to offspring, and how to predict them. Together with the molecular genetics standard SB2, the genetics standards make up near 23 percent of the Georgia Milestones Biology EOC. The recurring skill is the Punnett square (and reading a pedigree), so practice setting up and reading crosses until it is automatic.

This guide ties together the matching topic pages, each with its own practice questions: meiosis and genetic variation, Mendelian genetics and Punnett squares, complex inheritance patterns, pedigrees and sex-linked traits, and sexual and asexual reproduction.

Meiosis and variation

Meiosis makes gametes by halving the chromosome number (diploid to haploid), so fertilization restores the full number. It differs from mitosis: two divisions, four genetically different haploid cells, versus one division and two identical diploid cells. Meiosis creates variation through crossing over and independent assortment, and random fertilization adds more. This variation is the raw material for evolution and the reason siblings differ.

Mendelian genetics

A gene has versions called alleles; an organism has two, one from each parent. A dominant allele shows whenever present; a recessive one shows only when both alleles are recessive. The genotype is the alleles; the phenotype is the trait. Mendel's laws (segregation and independent assortment) underlie the Punnett square, which predicts ratios: $Tt \times Tt$ gives a 3:1 phenotype ratio (and $1:2:1$ genotype ratio); $Tt \times tt$ gives 1:1. Ratios are probabilities, not guaranteed counts.

Complex inheritance

Some traits go beyond simple dominance. Incomplete dominance gives a blended heterozygote (red plus white gives pink). Codominance shows both alleles fully (red and white hairs together). Multiple alleles means a gene has more than two versions in the population; ABO blood type uses $I^A$, $I^B$ (codominant with each other), and $i$ (recessive), giving types A, B, AB, and O. You still use a Punnett square; only the phenotype rule changes.

Pedigrees and sex linkage

A pedigree is a family tree (squares are males, circles females, shaded means affected). The key rule: two unaffected parents with an affected child means the trait is recessive, and the parents are carriers (heterozygous, unaffected). Sex-linked traits sit on the X chromosome. Because males are XY (one X), a single X-linked recessive allele is expressed in them, while females (XX) need it on both Xs, so traits like color blindness appear more often in males.

Reproduction strategies

Sexual reproduction (two parents) produces varied offspring; asexual reproduction (one parent) produces identical clones. Sexual reproduction's advantage is variation, favored in a changing environment; asexual reproduction's advantages are speed and no mate, favored in a stable environment where a well-adapted parent's traits should pass on unchanged. The lack of variation makes an asexual population vulnerable to a new threat, since all individuals are alike.

Check your knowledge

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

1. A body cell has 46 chromosomes. How many are in a gamete? (1 mark)
2. Name two processes in meiosis that create genetic variation. (2 marks)
3. Define genotype and phenotype. (2 marks)
4. State the phenotype ratio expected from a $Tt \times Tt$ cross. (1 mark)
5. State the difference between incomplete dominance and codominance. (2 marks)
6. Give the genotype of a person with blood type O. (1 mark)
7. In a pedigree, what does a shaded circle represent? (1 mark)
8. Explain why X-linked recessive traits are more common in males. (2 marks)
9. State the main advantage of sexual reproduction. (1 mark)
10. State the environment in which asexual reproduction is most advantageous. (1 mark)