Mendelian inheritance is the pattern of inheritance of traits in which each trait is determined by a gene located in a single locus. This pattern of inheritance was first discovered by Gregor Mendel in 1865 when he studied the pattern of single trait inheritance of garden peas. The human genome consists of two alleles for each locus. The possible patterns of transmission of a trait to the children following Mendelian inheritance rules can be found. The following is a summary of the inheritance patterns for genetic diseases: INHERITANCE PATTERNS Inheritance Type Explanation How it Happens Inheritance Rates autosomal dominant Offspring who carry the gene on one allele are affected Inherited from an affected parent; both sons and daughters have an equal chance of inheriting the gene 50% chance in each pregnancy. autosomal recessive Offspring must carry the gene on both alleles to be affected. Usually, the parents are either unaffected carriers or they have a mild version of the disorder. The condition is inherited from both parents who carry the gene; sons and daughters have an equal chance of inheriting the gene. 25% chance in each pregnancy. X-linked recessive Male offspring are affected; heterozygous females are either normal or only mildly affected. Homozygous females are fully affected by the genetic disorder. The gene is carried on the X chromosome and is passed on via the mother, who donates X chromosomes to all her children (the father will only donate an X to female children). A female carrier has a 50% chance of producing carrier daughters and a 50% chance of producing affected sons in each pregnancy. An affected male who has impregnanted a normal female will produce all carrier daughters and all normal sons. X-linked dominant All of the daughters but none of the sons of an affected male will have the disorder (because the sons do not get the X chromosome from the father). A heterozygous female has a 50% chance of passing the gene to her offspring regardless of their gender. A homozygous female passes the gene to all of her children. (A family that is inbred increases the chance for homozygous conditions) For information on determining the probable genetic ratios of offspring, see Punnet square.