11.1 Reproduction and the Mechanisms of Heredity

Sexual vs Asexual Reproduction

Organisms reproduce by one of two broad strategies. Both produce offspring, but they differ profoundly in genetic outcome.

Mitosis produces two diploid daughter cells with identical chromosomes; meiosis produces four haploid gametes with half the chromosome number and recombined alleles. The genetic variation generated by sexual reproduction — through crossing over during prophase I, independent assortment of homologous chromosomes, and random fertilization — is the evolutionary advantage: it gives populations allele diversity on which natural selection can act.

DNA, Genes, Chromosomes, and Alleles

DNA is a double helix of two antiparallel nucleotide strands. Each nucleotide has three parts: a phosphate group, a deoxyribose sugar, and one of four nitrogen bases — adenine (A), thymine (T), guanine (G), and cytosine (C). Base pairing follows Chargaff's rule: A pairs with T through two hydrogen bonds, and G pairs with C through three hydrogen bonds. The sugar-phosphate backbone is constant; the sequence of bases carries the genetic code.

A gene is a segment of DNA that codes for a specific protein or functional RNA. Chromosomes are tightly coiled DNA-protein complexes (DNA wrapped around histone proteins) that compact DNA for cell division; humans have 23 pairs (46 total) — 22 autosomes plus one pair of sex chromosomes (XX or XY). An allele is one variant form of a gene; a person with two identical alleles is homozygous, and a person with two different alleles is heterozygous.

Genotype vs Phenotype

The genotype is the allele combination (for example, Aa), while the phenotype is the observable trait (for example, brown eyes). Not every allele in the genotype shows in the phenotype — recessive alleles can be masked by dominant alleles in heterozygotes.

Mendelian Genetics

Gregor Mendel's pea-plant experiments (1866) established two laws that anchor classical genetics:

1. Law of segregation — the two alleles for a gene separate during gamete formation, so each gamete carries only one allele.
2. Law of independent assortment — alleles of different genes are distributed to gametes independently when genes are on different chromosomes or far apart on the same chromosome.

Dominant alleles mask recessive alleles in heterozygotes. By convention, the dominant allele is capitalized (T for tall) and the recessive allele is lowercased (t for short).

Punnett Square — Worked Monohybrid

Cross a heterozygous tall plant (Tt) with another heterozygous tall plant (Tt):

Genotype ratio: 1 TT : 2 Tt : 1 tt (genotypic 1:2:1). Phenotype ratio: 3 tall : 1 short (3:1). Probability of a short offspring: 1/4 = 25%.

Punnett Square — Worked Dihybrid

Cross plants heterozygous for both seed color (Yy) and seed shape (Rr): YyRr × YyRr. Independent assortment gives four gamete types from each parent — YR, Yr, yR, yr. The 16-cell Punnett square yields a 9:3:3:1 phenotypic ratio: 9 yellow round : 3 yellow wrinkled : 3 green round : 1 green wrinkled. This classic ratio is the signature of independent assortment for two heterozygous genes with complete dominance.

Single-Gene, Polygenic, and Pleiotropic Traits

Not every trait follows the simple Mendelian pattern. Three patterns a TExES teacher must distinguish:

• Single-gene traits — one gene controls one phenotype. Mendel's pea traits and human ABO blood type are classic examples; the phenotype ratios follow Mendel's laws.
• Polygenic traits — many genes contribute to one trait, producing a continuous distribution. Human height, skin color, and grain yield in corn are polygenic; the distribution often forms a bell curve.
• Pleiotropy — one gene influences many traits. The classic example is sickle-cell anemia: a single base substitution in the beta-globin gene changes hemoglobin structure, which alters red blood cell shape, oxygen transport, resistance to malaria, and vulnerability to organ damage — multiple phenotypes from one gene.

Environmental Influence on Phenotype

The phenotype is not the genotype alone — the environment modulates expression. Hydrangea flowers are blue in acidic soil (aluminum is available for uptake) and pink in alkaline soil. Identical twins with the same genotype can differ in height, weight, and disease risk because of nutrition, stress, and exposure. Temperature-sensitive enzymes in Siamese cats produce darker fur on cooler body parts. For TExES, the key teaching point is that genes set a range of reaction — a range of possible outcomes — and the environment determines where within that range the phenotype actually lands.

Genetic Research Applications

Modern genetics touches every part of life, and a 4-8 teacher frames these applications as extensions of heredity knowledge:

• Selective breeding — choosing parents with desired traits to shape offspring over generations (disease-resistant crops, high-yield dairy cattle, dog breeds). Used for millennia and now accelerated by marker-assisted selection.
• Genetic engineering and GMOs — transferring genes between organisms. Bt corn carries a bacterial insect-toxin gene; insulin-producing bacteria carry the human insulin gene; Golden Rice adds beta-carotene to address vitamin A deficiency.
• Cloning — producing genetically identical individuals. Plant cuttings are ancient clones; somatic cell nuclear transfer (SCNT) produced Dolly the sheep in 1996 by replacing an egg cell nucleus with an adult mammary cell nucleus.
• Gene therapy — introducing functional genes to treat genetic disorders. CAR-T cell therapy engineers a patient's immune cells to target cancer, and approved therapies now treat sickle-cell disease and some forms of inherited blindness.
• Personalized medicine — genomic profiling guides drug choice, and pharmacogenomics predicts adverse drug reactions.

A teacher who frames these applications helps students evaluate benefits (food security, disease treatment) alongside ethical questions (privacy, ecological impact of GMOs, animal welfare).