Everything from the 5 lessons of this topic on a few pages: the key idea of each lesson, the diagram to remember, the points to learn and the traps examiners set. Read it, cover it, test yourself.
Chromosomes are made of DNA, which contains genetic information in the form of genes. A gene is a length of DNA that codes for a protein, and an allele is an alternative form of a gene. Females are XX and males are XY. Extended: the sequence of bases in a gene sets the sequence of amino acids in a protein, and that sequence sets the protein's shape.
Do not mix up gene and allele. The gene is the instruction for a feature such as flower colour; the alleles are the different versions of that instruction, such as purple and white.
The mother cannot decide the sex of a child: all her eggs carry an X. Whether the child is XX or XY depends on which sperm fertilises the egg.
The DNA itself never leaves the nucleus. It is the mRNA copy that travels to the ribosome.
Mitosis is nuclear division giving rise to genetically identical cells. It is used for growth, repair of damaged tissues, replacement of cells and asexual reproduction. Extended: the chromosomes are replicated exactly before mitosis, and during mitosis the copies separate, so each daughter cell keeps the same chromosome number.
Mitosis does not halve the chromosome number. A human body cell with 46 chromosomes gives two cells that each have 46, with exactly the same alleles.
Stem cells are not already specialised. They become specialised only after dividing, when a daughter cell develops a particular structure and job.
Meiosis is involved in the production of gametes. Extended: it is a reduction division in which the chromosome number is halved from diploid to haploid, resulting in genetically different cells.
Haploid does not mean half a chromosome or half of each chromosome. It means a single set: one whole chromosome of each type.
The zygote divides by mitosis, not meiosis. Meiosis happens only when gametes are being made.
Genotype is the genetic make-up of an organism in terms of its alleles; phenotype is its observable features. A dominant allele is expressed if it is present; a recessive allele only when no dominant allele is present. Crossing two heterozygotes gives a 3 : 1 ratio; crossing a heterozygote with a homozygous recessive gives 1 : 1.
Each gamete carries only ONE allele of the gene. Write the gametes as T and t, never as Tt, and circle each one.
All-black offspring do not prove the parent is BB: a Bb parent could, by chance, give only black pups. A large number of offspring makes the conclusion much more reliable.
In a pedigree diagram, two unaffected parents with an affected child show that the allele is recessive and both parents are carriers. Extended: in codominance both alleles in a heterozygote contribute to the phenotype (IAIB is blood group AB). A sex-linked gene is on a sex chromosome, so the feature is more common in one sex, as with red-green colour blindness.
An unaffected person is not automatically AA. Look at their parents and children: an aa child or an aa parent proves they carry a.
A person can only have TWO of the three alleles. Group A is not always IAIA: it can be IAIo, which is how two group A parents can have a group O child.
A son can never inherit colour blindness from his father: a father passes his Y, not his X, to every son.