Biology: DNA, genetics, and inheritance (grade 9) · lesson

DNA and base pairs

💡 Genes are sections of DNA, and DNA's bases pair A with T and G with C.

DNA is a long molecule packed into chromosomes in the nucleus. A gene is one section of DNA that codes for a protein.

DNA has two strands held together by base pairs. Adenine (A) always pairs with thymine (T), and guanine (G) always pairs with cytosine (C). RNA uses uracil (U) in place of thymine.

🧠 Let's work it out

To write a matching strand, go one base at a time and swap each for its partner.
A → T, T → A, G → C, C → G.

What do you think comes next?

chromosome: a tightly packed strand of DNA
gene: a section of DNA that codes for a protein

🔗 Khan Academy: Inheritance and variation of traits · Khan Academy (free) ↗

From gene to trait

💡 Genes make proteins, and proteins make traits, so a change in a gene can change a trait.

A gene is copied into mRNA. A ribosome reads the mRNA and builds a protein. The protein does a job, such as making eye pigment, and that job shows up as a trait.

A mutation changes the DNA. If one base changes, one group of three bases (a codon) changes. It may now code for a different amino acid. But some amino acids have more than one code, so the change can also be silent, leaving the protein the same.

🧠 Let's work it out

Ask how far the change reaches.
One base sits in one gene, so it affects only that gene's protein.
It can't change every gene or the number of chromosomes.

What do you think comes next?

mutation: a change in the DNA sequence
codon: a group of three bases that codes for one amino acid
trait: a feature, like eye color or height

Predicting offspring

💡 A Punnett square lists every way parents' alleles can combine, so you can predict ratios.

Each parent gives one allele (version of a gene) to each offspring. A dominant allele (T) shows up even with one copy. A recessive allele (t) shows up only with two copies (tt).

Write one parent's alleles across the top and the other's down the side. Fill each box with one from each. Tt × tt gives Tt, Tt, tt, tt: 1 tall to 1 short.

Some traits blend: red × white snapdragons give pink (one of each allele). Some genes sit on the X chromosome. A son gets his only X from his mother, so a carrier mother passes the recessive X to about half her sons.

🧠 Let's work it out

Pp × Pp: boxes are PP, Pp, Pp, pp.
Only pp shows the recessive trait, so 1 of 4, or 25%.
For sons of a carrier mother (XᴮXᵇ), the X comes from mom: half Xᴮ, half Xᵇ, so 50%.

What do you think comes next?

allele: one version of a gene
dominant: shows up with just one copy
recessive: shows up only with two copies

Meiosis mixes and halves

💡 Meiosis makes sex cells with half the chromosomes and a new mix of alleles.

Body cells are copied by mitosis, which makes identical cells. Sex cells (eggs and sperm) are made by meiosis, which halves the chromosome number. When an egg and sperm join, the full number returns.

Meiosis also shuffles alleles in two ways. In crossing over, paired chromosomes swap pieces. In independent assortment, each pair splits on its own, so many mixes are possible.

🧠 How to think about it

If body cells have 46 chromosomes, egg and sperm must each have 23, because 23 + 23 = 46. Halve the body-cell number to find the sex-cell number.

meiosis: cell division that makes sex cells with half the chromosomes
mitosis: cell division that makes identical copies

Genes and environment

💡 Traits come from genes and the environment working together.

Genes set a range of possible results. The environment, such as food, sunlight, or exercise, affects where in that range a living thing ends up.

Identical twins share the same DNA. So when they differ, the difference must come from their environments, not their genes. Diet does not rewrite DNA in every cell.

🧠 How to think about it

Find what is the same and what is different. Same genes plus different results points to the environment as the cause.