Chapter 14: Mendel & the Gene Idea AP Biology

14.1 Mendel used the scientific approach to identify two laws of inheritance

Gregor Mendel Known as the “Father of Genetics” Experimented with pea plants to develop principles of genetics In the 1800s… before anyone even knew: what DNA was, let alone how it worked (1940s-1950s) What meiosis was or how it worked (1870s-1880s)

Gregor Mendel Knew some “factor” caused organisms to have “characters” like their parents Now we call them genes and traits

Mendel’s Experiments P generation – true breeding F1 generation – heterozygous F2 generation – 3:1 ratio; recessive trait reappears

New Vocabulary, courtesy of Mendel Dominant: gene that is always expressed if present Recessive: gene that only expressed when 2 copies are inherited Homozygous/True-breeding/Pure: two of the same allele (ex: BB, bb) Heterozygous/Hybrid: two different alleles (ex: Bb) Phenotype: physical appearance of an organism (based on its genes) Genotype: genetic makeup of an organism (letters) P generation: parent generation (Mendel – true-breeding plants) F1 generation: first generation made from crossing P generation F2 generation: second generation made from crossing F1generation

Mendel’s Laws Law of Segregation Two alleles of a gene separate during meiosis Each parent can give one of each homologous chromosome (one copy of each gene) to offspring

Mendel’s Laws Law of Independent Assortment Each allele pair segregates into gametes independently of other pairs (random) How homologous chromosomes align in metaphase I

Genetic Crosses Monohybrid crosses cross 1 trait Ex: pea color F2 generation shows 3:1 ratio

Genetic Crosses Dihybrid crosses cross 2 traits at once Ex: pea color and shape F2 generation shows 9:3:3:1 ratio

14.2 The laws of probability govern Mendelian inheritance

Laws of Probability Rule of Multiplication Probability that independent events will occur in sequence Look for “and”

Laws of Probability, cont. Rule of Addition Probability that events will occur mutually exclusive of each other Look for “or”

Bozeman Probability in Genetics: Multiplication & Addition Rules

14.3 Inheritance patterns are often more complex than predicted by simple Mendelian genetics

Inheritance Patterns Complete dominance (as shown in Mendel’s pea plants) Dominant is always expressed over recessive Recessive only shows if both alleles are recessive RR = purple; Rr = purple; rr = white

Mendel was lucky – most inheritance follows other patterns… Let’s look at the exceptions to the norms

Incomplete Dominance One allele is not completely dominant over another Heterozygous organisms display a blend between both phenotypes Example: snapdragons

Codominance Alleles do NOT blend Both traits show in heterozygous organism Both alleles expressed dominantly Neither allele is recessive Ex: roan cattle

Multiple Alleles Many genes have more than 2 alleles Examples: human blood type fur color in rabbits

Polygenics Multiple genes affect single phenotype Ex: skin color, height The more dominant alleles inherited, the darker the skin, taller the offspring

Environmental Influence on Genes DNA isn’t the only factor that influences a phenotype Environment can influence genes too In reality, phenotype is a combination of an organism’s genes, environment, as well as other factors Examples: Temperature Light Moisture Minerals Nutrients

Environmental Influence on Genes – Temperature Snowshoe hare Genes code for pigments that give its hair a brownish-grey color called agouti. However, during cold winter months the alleles for pigment production are turned-off and the hare appears white in color. In this case temperature regulates the expression of the coat color alleles.

Environmental Influence on Genes – Temperature Himalayan rabbits Genotype ch/ch should produce black pigments in their hair. when the rabbit’s body temperature is above 37 o C, the ch alleles are turned-off and the rabbit’s hair appears white if the rabbit’s body temperature falls below 37 o C, black pigments will appear in the rabbit’s hair

Environmental Influence on Genes – Soil pH Hydrangeas flower coloration is subject to the pH of the soil in which the hydrangea bush is growing. In acidic soil, the flowers pink pigments are produced. In basic soil, blue pigmentation is produced in flowers.

14.4 Many human traits follow Mendelian patterns of inheritance

Pedigrees Analyze inheritance of traits through family relationships

Cystic Fibrosis Autosomal recessive Defective chloride channels in cells leads to build up of mucus in pancreas, lungs, digestive tract, etc.

Tay Sachs Autosomal recessive Fatal neurological/brain degeneration beginning at around 6 months of age

Sickle Cell Disease Autosomal recessive Causes abnormally shaped red blood cells, leads to anemia, clots

Achondroplasia Autosomal dominant Dwarfism

Huntington’s Disease Autosomal dominant Progressive degeneration of brain/nervous system Late onset (age 35-45)

Fetal Testing & Newborn Screening Amniocentesis Chorionic villus sampling Blood testing Newborn screening Blood analysis for genetic disorders