So far, the hypothesis that one gene is responsible for the paralyzed trait is the simplest explanation that fits the data The way to distinguish most easily between a heterozygote and a homozygote expressing a dominant trait is to cross to a homozygous recessive test strain Test cross: cross to homozygote recessive A/A a/a gives all A/a. i.e. all offspring will express the dominant trait Ala x a/a gives 1/2 A/a and1/2a/a. i.e. one half of the offspring will express the dominant trait Mendelian inheritance in humans For humans we can't do test crosses, of course, but by following inheritance of a trait for several generations the modes of inheritance can usually be identified by applying basic principles of Mendel. The following are guidelines for identifying different modes of inheritance in pedigrees Autosomal dominant D) Affected individuals must have at least one affected parent Exceptions to this rule will occur if a new mutation arises in one of the parents( in real life a more likely explanation is extramarital paternity). Another possibility is incomplete penetrance, where other genetic or environmental factors prevent the trait from being expressed in one of the parents Autosomal recessive iWhen both parents are carriers, on average 1/4 of the children will be affected ii)When both parents are affected, then all of the children will be affected ii If the trait is very rare then consanguinity is likely. That is, it is likely that parents of affected children are themselves related (e.g. cousins) X-Iinked inheritance Q XCX+ x OX+y (carrier) ♀XcX+,♀Xx+,dXcY.可xy (carrier) (color blind)So far, the hypothesis that one gene is responsible for the paralyzed trait is the simplest explanation that fits the data. The way to distinguish most easily between a heterozygote and a homozygote expressing a dominant trait is to cross to a homozygous recessive test strain. Test cross: cross to homozygote recessive: A/Ax a/agives all A/a. i.e. all offspring will express the dominant trait. A/ax a/agives 1/2 A/aand 1/2 a/a. i.e. one half of the offspring will express the dominant trait. Mendelian inheritance in humans For humans we can’t do test crosses, of course, but by following inheritance of a trait for several generations the modes of inheritance can usually be identified by applying basic principles of Mendel. The following are guidelines for identifying different modes of inheritance in pedigrees. Autosomal dominant i) Affected individuals must have at least one affected parent Exceptions to this rule will occur if a new mutation arises in one of the parents (in real life a more likely explanation is extramarital paternity). Another possibility is incomplete penetrance, where other genetic or environmental factors prevent the trait from being expressed in one of the parents. Autosomal recessive i) When both parents are carriers, on average 1/4 of the children will be affected. ii) When both parents are affected, then all of the children will be affected. iii) If the trait is very rare then consanguinity is likely. That is, it is likely that parents of affected children are themselves related (e.g. cousins). X-linked inheritance O XcX+ x O X+Y (carrier) ↓ X+Y (carrier) (color blind) O XcX+, O X+X+, O XcY, O