1. Genetics Since Mendel In science, there is an exception to every rule, particularly in Genetics
2. Incomplete Dominance Neither allele for a trait is dominant The phenotype produced is intermediate between the two homozygous parents Ex: The four-o’ clock plants showed pink flowers when a homozygous white and homozygous red plant were crossed
3. Multiple Alleles More than two alleles that control a trait are called multiple alleles Traits controlled by multiple alleles produce more than three phenotypes Ex: Bloodtype Alleles for bloodtype are A, B, and O Possible genotypes are: AB, AA, AO, B, BO, OO
5. Polygenic Inheritance A group of gene pairs acts together to produce a trait, which creates more variety in phenotypes Many human traits are controlled by polygenic inheritance, such as hair and eye color and the effects of so many alleles acting together produces a wide variety of phenotypes
6. An example of this is human stature. NOTE: whether an individual achieves his or her genetically programmed height is significantly affected by human growth hormones (HGH) produced in the pituitary gland. A deficiency in the amount of these hormones during childhood and puberty can result in stunted growth. Too much of them can cause excessive growth resulting in exceptional height. Differences in diet and other environmental factors during the crucial growth years can also be important in determining stature and other complex traits. Usually, about 10% of an individual's height is due to the environment.
7. Genetics and the Environment Your environment plays a role in how some of your genes are expressed or whether they are expressed at all. Ex: skin cancer: if you are genetically predisposed to this cancer, using sunscreen can prevent it and overexposure can cause it Environmental factors can affect how large organisms grow
8. Mutations – genes that are altered or copied incorrectly A mutation can be harmful, beneficial, or have no effect Chromosome disorders – caused by more or fewer chromosomes than normal Down’s syndrome – caused by an extra copy of chromosome 21
10. Recessive Genetic Disorders Both parents have a recessive allele responsible for the disorder and pass it to their child Because the parents are heterozygous, they don’t show any symptoms Cystic fibrosis is a recessive disorder
11. Sex Determination Chromosomes that determine the sex of an organism are XX in females and XY in males Females produce eggs with an X chromosome only. Males produce sperm with either an X or a Y chromosome
12. Sex-linked disorders An allele inherited on an X or Y chromosome is a sex-linked gene Color blindness is a sex-linked disorder caused by a recessive allele on the X chromosome A pedigree follows a trait through generations of a family
