allele

Alleles are variations of a gene; occurs at a specific place on a chromosome


dominant allele

dominant allele is expressed when two different alleles are present


Source
dominant alleles are represented with an UPPERCASE letter
http://www.shutterstock.com/pic-37507900/stock-photo-alphabetic-letter-in-gold-metal-on-white-background-uppercase-letter-t.html

genes

Specific region of DNA that codes for a particular protein


genome

All an organism's genetic material


genotype

refers to the genetic make up of a particular set of genes


Source
http://wmworia.wordpress.com/2007/11/19/personal-genetics-service-23andme/

Heterozygous

Heterozygous alleles are different from each other


Source
heterozygous/homozygous alleles
http://www.bbc.co.uk/schools/gcsebitesize/science/images/26_alleles.gif

homologous

Chromosomes that have the same length, appearance, and copies of genes, although the alleles may differ


Homozygous

Homozygous alleles are the same from each other


Source
homozygous/heterozygous alleles
http://www.bbc.co.uk/schools/gcsebitesize/science/images/26_alleles.gif

locus

the chromosomal position of a gene as determined by its linear order relative to the other genes on that chromosome.


phenotype

refers to the physical characteristics resulting from those genes


Source
attached/detached earlobes are examples of a phenotype
http://www.windows.ucar.edu/tour/link=/earth/Life/genetics_puzzle.html

polydactyly

a condition of having more than the typical number of fingers or toes. The allele for polydactyly is dominant.


recessive allele

A recessive allele is expressed only when two copies are present.


Source
recessive alleles are represented with lowercase letters
http://www.shutterstock.com/pic-20862046/stock-photo-green-stained-glass-with-silver-lowercase-letter-t.html