Modern Genetics Human Inheritance Patterns of Inheritance Some traits are controlled by a single gene with two alleles One dominant, one recessive with 2 distinct phenotypes Example: widows peak Some traits are controlled by single genes with multiple alleles Three or more forms of a gene that code for a single trait Example: blood type There are 4 main types of blood- A, B, AB and O. A and B are dominant, O is recessive. The allele for A is written I A, B is written I B and O is written i. Other traits are controlled by many genes At least 4 genes control height in humans. The genes act together as a group to produce a single trait. Skin color is another example of a human trait controlled by many genes Sex Chromosomes Sex chromosomes carry genes that determine whether a person is a male or a female. They also carry genes that determine other traits. Girl or boy? Sex chromosomes are the only chromosomes that do not always match. If you are a girl, they do match XX If you are a boy, they do not match XY Sex Linked Genes Genes on the X and Y chromosome are often called sex linked. Red-green color blindness Like other genes, sex linked genes carry dominant and recessive alleles. Any allele on an X chromosome, recessive or dominant, will produce that trait in a male. Because males only have one X chromosome, they are more likely than a female to have a sex linked trait that is controlled by a recessive allele. Colorblindness Colorblindness is a sex linked trait controlled by a recessive allele on the X chromosome. Many more males have the trait than females. Carrier- a person who has one recessive allele for a trait and one dominant. The carrier can pass on the trait, but does not have the trait. Environment The effects of your genes are often influenced by the environment- organisms surroundings. Height is a good example: A diet lacking protein, minerals or certain vitamins can prevent a person from growing as tall as might be possible Environmental factors can also effect human skills, such a playing an instrument or muscle coordination.