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Download A single characteristic may be influenced by many genes
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A single characteristic may be influenced by many genes •Polygenic inheritance •Additive effect of 2 or more •genes on a single trait •Example: •Skin color or height Gene with more than 2 alleles Human Inheritance can be controlled by a single gene that has more than 2 alleles Example blood type Codominance 2 alleles can combine because alleles are codominant they are written with superscripts Blood Type IA IB shows co-dominance in humans Blood Types Genotype IAIA or IAi IBIB or IBi A B I I ii Blood Type (phenotype) A B AB – universal recipient O – universal donor Traits Controlled by Many Genes Some human traits show a large number of phenotypes because the trait is controlled by many genes. The genes act together to produce a single trait Examples at least 4 genes control height at least 3 genes control skin color Effects of the Environment Effects of genes can be altered by the environment. Example – people’s diets can effect height Living conditions Medical care SEX CHROMOSOMES Chromosomes determine sex in many species A human male has one X chromosome and one Y chromosome A human female has two X chromosomes Whether a sperm cell has an X or Y chromosome determines the sex of the offspring X and Y Chromosome Sex-Linked Inheritance Single genes located on sex chromosomes Actually X-linked Males have no counterpart on Y chromosome Females have counterpart on second X chromosome Requires gene on both X’s for trait Hemophilia, Colorblindness, Duchene MS Sex-linked traits Females need two copies of a recessive allele to show a phenotype Males need only one Sex linked traits more common in males The individual who has one recessive allele for a trait is called a carrier Sex-linked disorders affect mostly males Most sex-linked human disorders are due to recessive alleles Examples: hemophilia, red-green color blindness These are mostly seen in males A male receives a single X-linked allele from his mother, and will have the disorder, while a female has to receive the allele from both parents to be affected Hemophilia is a sex-linked disease Pedigrees A tool used to trace the inheritance of traits in humans Symbols used on pedigree charts