Mendel and the Gene Idea

Gregor Mendel Studied pea plants for specific traits and how those traits passed from one generation to the next He noticed a 3:1 inheritance pattern which led to the Law of Segregation Alternative versions of genes account for variations in inherited characters For each character, an organism inherits 2 alleles, one from each parent If the 2 alleles at a locus differ, then one, the dominant allele, determines the organisms appearance; the other, the recessive allele, has no noticeable effect on the organism’s appearance The two alleles for a heritable character segregate during gamete formation and end up in different gametes

Law of Segregation

Law of Independent Assortment Each pair of alleles segregates independently of each other pair of alleles during gamete formation

Rules of Probability The outcome of any event is unaffected by what has happened previously The multiplication rule To determine the probability of 2 events that do not depend on each other, multiply the two probabilities The addition rule The probability that any one of 2 or more mutually exclusive events will occur is calculated by adding their individual probabilities

Beyond Mendel Mendel looked at traits that had 2 possible characteristics and 2 possible alleles, but some traits are more complex In single genes this can be seen in: Codominance Multiple Alleles Pleiotropy

Incomplete Dominance For some genes, neither allele is completely dominant over the other Example: When red snapdragons are crossed with white snapdragons, the offspring are pink Should not be confused with codominance where both traits are seen, but it is not a mix or intermediate (ie AB blood)

Multiple Alleles Many genes exist with more than two possible allelic forms Example: ABO blood groups can take the form of type A, type B, Type AB or type O, where A, B and AB are all dominant forms and O is recessive

Pleiotropy A single gene can have multiple affects on a number of characteristics in an organism Example: In pea plants, the gene that controls flower color also affects the seed coat color

Beyond Mendel At times, multiple genes are involved in determining a particular phenotype Epistasis Polygenic Inheritance

Epistasis A gene at one locus alters the phenotypic expression of a gene at a second locus Example: In mice, black coat color is dominant to brown. A second gene determines if pigment is deposited in the fur. If the second gene is homozygous recessive it causes white fur regardless of the first genotype

Polygenic Inheritance When multiple genes work together, the number of alleles involved cause a variation like that on a continuum; quantitative characters Example: Skin pigmentation in humans is thought to be controlled by at least 3 separate genes that together give us the variation in skin colors. The greater number of dominant alleles, the darker the skin

Nature vs. Nurture Genotype will influence what the phenotype will be, but the environment and outside influences also play roles Available nutrients Weather

Pedigrees Show traits in a family tree format

Recessive Disorders These are disorders caused by recessive alleles Those individuals that are heterozygous or carry one allele for a recessive disorder are considered carriers They can pass on the disease, but they do not have the disease Examples: Albinism, cystic fibrosis, sickle cell disease

Dominant Disorders A disorder or disease that is seen with the presence of one dominant allele Dominant alleles that cause lethal diseases are much less common than recessive All LETHAL alleles arise by mutations and dominant alleles are more lethal. You can not ‘carry’ and pass on the disorder Examples: Achondroplastic dwarfism, Huntington’s Disease

Genetic Testing and Counseling Today couples can go through genetic testing to determine risk of genetic disorders prior to having a family, during pregnancy or after birth Tests for identifying carriers Often done with Tay-Sachs, cystic fibrosis and sickle cell Fetal testing can be accomplished using amniocentesis or chorionic villus sampling Newborns are also screened for many diseases at birth