6.1.6: Genetic inheritance
Not started yet — this one needs some love.
Gamete: a sex cell, such as a sperm cell, egg cell or pollen cell.
Chromosome: a structure containing DNA.
A gene is a small section of DNA on a chromosome.
Each gene may have different forms called alleles.
Some characteristics are controlled by a single gene, such as fur colour in mice and red-green colour blindness in humans.
The alleles present, or genotype, operate at a molecular level to develop characteristics that can be expressed as a phenotype.
A dominant allele is always expressed, even if only one copy is present.
A recessive allele is only expressed if two copies are present, so no dominant allele is present.
Homozygous means the two alleles present are the same for that trait.
Heterozygous means the two alleles present are different for that trait.
Most characteristics are a result of multiple genes interacting, rather than a single gene.
A single gene cross can be shown using a Punnett square to predict probability, ratios and proportions.
Family trees can be used to extract and interpret information about inherited alleles.
Common exam mistakes
Dominant means expressed when one copy is present; it does not mean stronger or more common.
Recessive means expressed only when two copies are present.
Genotype is the alleles present; phenotype is the expressed characteristic, such as having or not having a disorder.
In Punnett-square questions, label which gametes come from each parent and assign phenotypes to the offspring genotypes if asked.
Do not assign the same genotype to parents who have different phenotypes unless the evidence supports it.