Biology B1 GCSE Revision Notes OCR (higher)
- Created by: Anouschka
- Created on: 09-05-11 17:50
SIZE, LARGEST TO SMALLEST
Organism. Organ. Cell. Nucleus. DNA. Chromosome. Gene. Molecule. Atom.
CHROMOSOMES
A "normal" human cell nucleuscontains 46 chromosomes (23 pairs)
Females - XX
Males - XY
All normal egg cells produced by a human ovary carry the X chromosome.
Half of the sperm carries an X chromosome, and half carries Y chromosome.
The baby's gender is determined by the sperm that fertilises the egg. If the sperm carries the X chromosome, it will be a girl, and if a Y, a boy.
50/50 chance.
*Sex Cells are known as gametes.
ALLELES
- Different versions of the same gene
- Eg. the gene for eye colour has an allele for blue eyes and an allele for brown.
- For any gene, a person may have the same two alleles or two different ones
- Can be recessive or dominant
Heterozygous
of an organism which has two different alleles of a given gene
Recessive
only shows if the individual has two copies of it. Eg. Blue/green eyes.
Dominant
Always shows, even if the individual only has one copy of it. Eg. Brown eyes.
FOR EXAMPLE:
Even if both the parents have brown eyes, if they are both carriers of the blue-eyed allele, they would have a 1 in 4 chance of having a blue eyed child.
Even if…
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