An organism's traits are controlled by the alleles it inherits from its parents. In a genetic cross, the allele that each parent will pass on to its offspring is inherited based on prbability. An organisms's phenotype is its physical appearance, or visible traits. An organism's genotype is its genetic makeup, or allele combinations.
Chromosomes are made up of many genes joined together like beads on a string. According to the chromosome theory of inheritance, genes are carried from parents to their offspring on chromosomes. During meiosis, the chromosome pairs separate and are distributed to two diffent cells. The resulting sex cells have only half as many chromosomes as the other cells in the organism.
The order of the nitrogen bases along a gene fomrs a genetic code that specifies what type of protein will be formed. During protein synthesis, the cell uses information from a gen on a chromosome to produce a specific protein. Mutations can cause a cell to produce an incorrect protein during protein synthesis. As a result, the organism's traits or phenotype, may be different from what it normally would have been.
Some human traits are controlled by single genes with two alleles, and others by single genes with multiple alleles. Still other traits are controlled by many genes that act together. The sex chromosomes carry genes that determine whether a person is male or female. They also carry genes that determine other traits.