Grade 9 · Genetics and Heredity · Inheritance of Traits
DNA, Genes and Chromosomes
The instruction manual inside every one of your cells.
Learning objectives
- Describe the relationship between DNA, genes and chromosomes.
- Explain what a gene codes for.
- Distinguish between genotype and phenotype.
NGSS alignment
HS-LS3-1
Ask questions to clarify relationships about the role of DNA and chromosomes in coding the instructions for characteristic traits passed from parents to offspring.
MS-LS3-1
Develop and use a model to describe why structural changes to genes located on chromosomes may affect proteins and may result in harmful, beneficial, or neutral effects to the structure and function of the organism.
MYP criteria
- Criterion A — Knowing and Understanding
ENGAGE
Two metres of instructions
Stretched end to end, the DNA in one of your cells would be about two metres long, yet it fits inside a nucleus far too small to see.
Think about it
- How could such a long molecule be packed so small?
- Why does every cell need its own copy?
EXPLAIN
From DNA to trait
- DNA is a long molecule made of paired bases
- A gene is a section of DNA that codes for one protein
- A chromosome is a tightly coiled DNA molecule
- Humans have 23 pairs of chromosomes in most cells
| Term | Meaning | Example |
|---|---|---|
| Allele | A version of a gene | Allele for brown eyes |
| Genotype | The alleles an organism carries | Bb |
| Phenotype | The observable characteristic | Brown eyes |
Proteins built from gene instructions become enzymes, pigments and structures, and these produce the characteristics we can observe.
Remember
Order of size: DNA base, then gene, then chromosome, then nucleus, then cell.
Watch
DNA, Chromosomes, Genes and Traits
Amoeba Sisters · 9 min
Builds the size order from base to chromosome.
Before you watch: Which is bigger: a gene or a chromosome?
- Define allele in one sentence.
- Explain the difference between genotype and phenotype.
Interactive simulation · PhET
Gene Expression Essentials
Build a protein from a gene, then change the gene and see what happens.
While you explore
- What does the gene actually produce?
- How could a change in DNA change a phenotype?
Key vocabulary
- Chromosome
- A tightly coiled thread of DNA in the nucleus.
- Gene
- A section of DNA with instructions for one feature.
Practice questions
0/2 correct
Level 1 · Criterion A
Which sequence is in order of increasing size?
Level 2 · Criterion A
An organism with genotype Bb has brown eyes. Its phenotype is:
MYP criterion tasks
Level 3 · Criterion A
Explain how a change in a single gene can change an organism's observable characteristics.
Reflect & track
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