Genes

• Each gene occupies a specific position on the type of chromosome where it is located. This position is called the locus of the gene.
• Alleles can be dominant or recessive, with only one copy needed to express the trait.
• The alleles are alternative forms of genes that code for different versions of the same protein or trait.
• Dominant alleles mask the expression of recessive alleles.
• New alleles are formed from other alleles by gene mutation.
• The genome is the whole of the genetic information of an organism.
• In humans the genome consists of the 46 molecules that form the chromosomes in the nucleus plus the DNA molecule in the mitochondrion.
• The genome of prokaryotes is much smaller and consists of the DNA in the circular chromosome, plus any plasmids that are present.
• A gene is a sequence of bases on a DNA molecule that codes for a protein.
• Genotype is the genetic constitution of an individual (i.e. all the alleles that an individual has).
• Phenotype is the set of observable characteristics of an individual, resulting from the interaction of its genotype with the environment.
• Diploid is a term that refers to the presence of two complete sets of chromosomes in an organism's cells, with each parent contributing a chromosome to each pair. 
• A karyotype is an individual's complete set of chromosomes.
• An autosome is one of the numbered chromosomes, as opposed to the sex chromosomes.
• Humans have 22 pairs of autosomes and one pair of sex chromosomes (XX or XY).
• Homozygous - the presence of two identical alleles at a particular gene locus.
• Heterozygous - having inherited different versions (alleles) of a genomic marker from each biological parent.
• The characteristic of a dominant allele is present even if only one allele is present.
• The characteristic of a recessive allele is present only if both alleles are present.
• Mendelian inheritance refers to an inheritance pattern that follows the laws of segregation and independent assortment in which a gene inherited from either parent segregates into gametes at an equal frequency.
• Incomplete dominance is a condition in which a dominant allele does not completely mask the effects of a recessive allele.
• In an X-linked disease the defective gene is present on the X chromosome and therefore, males are more likely to inherit the defective gene.
• Codominant inheritance occurs when two alleles contribute equally to the expression of a particular trait.