Chapter 14 – Genetics and the Work of Mendel Modern genetics began in the mid-1800s in an abbey garden, where a monk named Gregor Mendel documented inheritance in peas used good experimental design used mathematical analysis collected data & kept detailed records excellent example of scientific method

Genes, Alleles and Dominance Trait – specific characteristic Mendel studied 7 different pea plant traits What are some examples? Plant Height, Seed Shape, Pod Color

Genes, Alleles and Dominance Genes – the chemical factors that determine traits (the segment of DNA) ex. pea plant: height Alleles – different forms of a gene from each parent ex. pea plants: tall and short (T or t)

Principle of Dominance Principle of dominance: some alleles are dominant and others are recessive Dominant allele – the trait is always shown Capital letter (T = tall) Recessive allele – the trait that will only show if there is no dominant allele Lower case letter (t = short)

Genes, Alleles and Dominance Heterozygous – organisms that have 2 different alleles for the same trait Hybrid for that trait Ex: Tt Homozygous – organisms that have 2 identical alleles for a particular trait True-breeding for a particular trait Ex: TT or tt

Genes, Alleles and Dominance Phenotype – physical characteristics Ex: tall, short, yellow, green Genotype – genetic make-up Ex: TT, Tt, tt Tall plants have the same phenotype (tall), but not the same genotype (TT or Tt)

Mendel crossed plants to study offspring P generation – (parents) – tall TT x short tt F1 generation – (offspring of P) tall Tt x tall Tt F2 generation – (offspring of F1) 3 tall TT Tt Tt, 1 short tt

Law of Segregation Segregation = separation The alleles for tall vs. short separate during the formation of gametes – sex cells Each gamete (sperm or egg) carries one allele for each gene

The Two-factor Cross: F1 4 alleles (2 genes) at the same time (cross true breeders – P generation) Provides the hybrid plants (F1 generation) Crossed a homozygous RRYY (round yellow peas) with a homozygous rryy (wrinkled, green peas)

The two factor cross: F2 F1 – yields all offspring heterozygous for both traits F2 – heterozygous parents crossed F2 – yields 9:3:3:1 ratio

Law of Independent Assortment Independent assortment – genes segregate independently during the formation of gametes Helps account for many genetic variations in organisms Seed shape & color gene do not influence each other

Law of Independent Assortment NOTE: Independent assortment only works if genes are on different chromosomes or are far apart on the same chromosome (unlinked)

Law of Independent Assortment Unlinked Genes – on different chromosomes, the two genes assort into gametes independently of each other. Follows expected ratio of 9:3:3:1 for heterozygous cross Linked Genes – on the same chromosomes, do not assort into gametes independently, unexpected phenotypic ratios. A/b and a/B are on the same chromosome, so they are linked

Genetics and Probability Probability – the likelihood a particular event will occur. Ex: probability of flipping a coin to heads = ½ or 50% Probability of heads 3 times in a row = ½ x ½ x ½ = 1/8 - can be multiplied together The greater the number or trials, the closer to the expected ratio Past outcomes do not affect future outcomes

Beyond Dominant and Recessive Alleles Genes can act in various ways Dominant vs. Recessive – one allele completely masks another allele. Incomplete Dominance Definition : one allele is not completely dominant over another Ex: flowers – white x red flowers = pink flowers

Beyond Dominant and Recessive 3. Codominance Definition: both alleles contribute to the phenotype of the organism Ex: chicken feather – black and white alleles = black and white feathers Colors don’t blend like incomplete dominance (roan horse and cow).

Multiple Alleles Definition: more than two alleles (more than 2 alleles exist in a population not an individual) Blood Types Ex: human’s blood type AB type is codominant

Polygenic Traits Definition: traits that are controlled by 2 or more genes Ex: fruit fly red eyes - 3 genes involved in making pigment Diff. combo of genes produce different eye colors Ex: Human skin color – more than 4 different genes Human height – more than 50 genes

Genetics and the Environment Genes provide a plan for development, but how the plan unfolds also depends on the environment: Ex. Butterflies have different wing colors depending on when they hatch Hydrangea flowers are different colors depending on soil pH.

Probability – Rules of Multiplication and Addition (pg Probability – Rules of Multiplication and Addition (pg. 270-271 in your text) Parents: PpYyRr x Ppyyrr What fraction of offspring from this cross would be expected to exhibit the recessive phenotypes for at least two of these traits? The genotypes: ppyyRr ppYyrr Ppyyrr PPyyrr ppyyrr