HS-LS3 Inheritance and molecular genetics
Aligned to HS-LS3 — Next Generation Science Standards.
What this lesson teaches
DNA carries a code, written in four bases, that cells read to build proteins, and proteins determine an organism's traits. During meiosis, chromosomes shuffle and occasionally mutate, creating the genetic variation passed to offspring. Inheritance follows probability, which lets us predict trait ratios.
Worked examples
Example 1
The DNA sequence for a gene is transcribed into RNA and translated into a specific protein, like the pigment that colors skin. In a cross between two parents heterozygous for a trait (Aa x Aa), the predicted offspring ratio is 3 dominant to 1 recessive, a direct result of how alleles separate and recombine.
Example 2
Most traits are not single-gene. Skin colour involves many genes, which is why it comes in a continuous range rather than two options.
Example 3
A Punnett square gives probability per offspring, not a quota. Four children of Aa x Aa parents can all show the dominant trait.
Practice questions
- Trace the path from a DNA gene to a protein trait.
- Explain how meiosis and mutation generate genetic variation.
- For an Aa x Aa cross, calculate the expected ratio of dominant to recessive offspring and explain why.
Watch the lesson
Every lesson comes with a video taught in English and Spanish — the same video the QR code in the printed workbook opens.
▶ Watch this lessonEn español
HS-LS3 Herencia y genética molecular
El ADN lleva un código, escrito en cuatro bases, que las células leen para fabricar proteínas, y las proteínas determinan los rasgos de un organismo. Durante la meiosis, los cromosomas se mezclan y ocasionalmente mutan, creando la variación genética que se transmite a la descendencia. La herencia sigue reglas de probabilidad, lo que nos permite predecir las proporciones de rasgos.
Ejemplo 1: La secuencia de ADN de un gen se transcribe en ARN y se traduce en una proteína específica, como el pigmento que da color a la piel. En un cruzamiento entre dos progenitores heterocigotos para un rasgo (Aa x Aa), la proporción esperada de la descendencia es 3 dominantes por 1 recesivo, resultado directo de cómo los alelos se separan y recombinan.
Ejemplo 2: La mayoría de los rasgos no dependen de un solo gen. El color de piel involucra muchos genes, por eso se da en un rango continuo y no en dos opciones.
Ejemplo 3: Un cuadro de Punnett da probabilidad por descendiente, no una cuota. Cuatro hijos de padres Aa x Aa pueden mostrar todos el rasgo dominante.
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