DNA carries the genetic information in a cell. A gene and an allele are both made of DNA. A gene is a length of DNA that codes for a particular characteristic, whereas an allele is the alternative form of a gene containing a different base sequence. A gene for a characteristic consists of two alleles.
Remember adenine pairs with thymine as 'Apples grow on Trees'.
Remember guanine pairs with cytosine as 'Carrots grow in the Ground'.
In pigs, curly tails are dominant. To work out if a curly-tailed pig is homozygous dominant or heterozygous for this trait, mate the curly-tailed pig with a straight-tailed pig. We know the straight-tailed pig's genotype is tt. If any offspring have straight tails we know the curly-tailed pig is heterozygous because they must be carrying the recessive allele for a recessive offspring to be produced. If no offspring have straight tails it is likely that the curly-tailed pig is TT.
Genetics Cheat Sheet by liviabrookes
Level 1 Genetics, basic meiosis, and mitosis, definitions, etc.