Chromosomes, genes and proteins
Chromosomes, genes and alleles
- Chromosomes are made of DNA, which carries genetic information in genes.
- A gene is a length of DNA that codes for one protein.
- An allele is an alternative form of a gene (e.g. a brown allele and a blue allele for eye colour).
Practice
A gene is:
A gene is a length of DNA coding for one protein; an allele is an alternative form of a gene.
Practice
An allele is:
Alleles are different versions of the same gene (e.g. brown vs blue eye-colour alleles).
Sex chromosomes
- Sex is set by the sex chromosomes: females are XX, males are XY.
- An egg always carries an X; a sperm carries an X or a Y.
- So the sperm decides the sex, giving a 1:1 ratio of girls to boys.
Practice
Which cell decides the sex of a baby?
The egg always carries X; the sperm carries X or Y, so the sperm decides the sex, giving a 1:1 ratio.
You've got it
Key idea
- gene = a length of DNA coding for one protein; allele = one form of a gene
- chromosomes are made of DNA
- females XX, males XY; the sperm decides the sex → 1:1 boys to girls