Non-Mendelian Genetics

 Some traits don’t follow the simple dominant/recessive rules that Mendel first applied to genetics.  Traits can be controlled by more than one gene.  Some alleles are neither dominant nor recessive.

Incomplete Dominance  One allele is not completely dominant over another.  The heterozygous phenotype is a blending of the two homozygous phenotypes. Example: four o’clock flowers  rr=red  ww=white  rw=pink (blending of the two alleles)

Codominance  Two alleles are both expressed as a dominant phenotype  Coat color in cows  RR: Red  WW: White  RW: Roan, white with red spots (NOT pink!)

Sex-Linked Inheritance

Review  Males have an X and a Y chromosome  Females have two X chromosomes  These chromosomes determine sex, so genes located on these chromosomes are known as sex-linked genes.

 The X chromosome is much larger than the Y, so it carries more genes than the Y chromosome. Disorders that are sex-linked are much more common in males, because they would only need 1 recessive allele to have the trait; rather than the two recessive alleles the females need.

Hemophilia  Recessive trait  Disorder where individuals are missing the normal blood clotting protein.  Uncontrolled bleeds from minor cuts or bruises.  Female hemophiliac: X h X h  Female carrier: X h X  Normal: XX  Male hemophiliac: X h Y  Normal: XY

Colorblindness  Recessive  Inability to see certain colors

Duchenne Muscular Dystrophy  Recessive  Progressive weakening and loss of skeletal muscle.  Defective version of gene that codes for muscle protein

 EXAMPLES!!  A woman who is heterozygous for normal vision marries a man who is colorblind. What are the chances of them having a son or daughter who is colorblind? **NOTE: You have to use X’s and Y’s, and read the punnett square separately for boys and girls!**

 A woman who is homozygous for normal blood clotting marries a man who has hemophilia. What are the chances of them having a son or daughter with hemophilia?

-A pedigree is a chart for tracing genes in a family. - When more than one individual in a family is afflicted with a disease, it suggests that the disease might be inherited. Pedigrees

Introduction to Pedigree Symbols -Males are represented by squares. -Females are represented by circles. -Filled symbols show individuals who exhibit the trait in question. -A horizontal line between two symbols represents a mating. -A vertical line descends from parents to a horizontal line shared by all their offspring. -A diagonal line through a symbol indicates that person is deceased. -Roman numerals (I, II, III,…) represent generations. -Arabic numerals (1, 2, 3,…) represent the birth order of siblings. -Specific combinations of numerals (like II-3) uniquely identify each individual in the pedigree.

Phenotypes are used to infer genotypes on a pedigree. Autosomal genes show different patterns on a pedigree than sex-linked genes.

If the phenotype is more common in males, the gene is likely sex-linked.

THE STEPS WHEN INTERPRETING A PEDIGREE CHART -Determine if the pedigree chart shows an autosomal or X- linked disease. -If most of the males in the pedigree are affected, then the disorder is X-linked -If it is a 50/50 ratio between men and women the disorder is autosomal.

-Determine whether the disorder is dominant or recessive. -If the disorder is dominant, one of the parents must have the disorder. -If the disorder is recessive, neither parent has to have the disorder because they can be heterozygous.