Chapter 14: Mendel & The Gene Idea Quantitative approach to science Pea plants Austrian Monk

Mendel vs. Conventional Inheritance “Blending”hypothesis Traits of offspring are a blending of parents genetic material “Particulate” hypothesis Parents pass on discrete heritable units Gene Mendel’s hypothesis

Why Peas? They are available in many varieties They have heritable features or character (flower color). Traits, or variations of heritable features are obvious(flower color- purple vs white) Mendel could easily control which traits he wanted to cross

Mendelian genetics True-bred Results of true breeding???

Mendelian genetics Hybridization Hybridization The crossing of two true breeding varieties P generation (parents) Purple flowers X White flowers F 1 generation (first filial generation) All purple

Mendelian Genetics: Self pollinate the F1 generation produces the F2 generation. Second filial generation 3: 1 ratio 3 purple: 1 white

Leading to the Law of Segregation 1. Alternative versions of genes (alleles) account for variations in inherited characteristics 2. For each character, an organism inherits 2 alleles, one from each parent 3. If the two alleles differ, then one, the dominant allele, is fully expressed in the organism’s appearance; the other, the recessive allele, has no noticeable effect on the organism’s appearance 4. The alleles for each character segregate (separate) during gamete production (meiosis). Mendel’s Law of Segregation

Genetic vocabulary……. Punnett square: predicts the results of a genetic cross between individuals of known genotype Homozygous: pair of identical alleles for a character Heterozygous: two different alleles for a gene Phenotype: an organism’s traits Genotype: an organism’s genetic makeup

Problem: How could you determine the genotype of a dominant trait? Example: purple flower are dominant to white flowers We know the genotype of the white flower to be homozygous recessive (pp). What is the genotype of the purple flower? Pp or PP?

Test Cross: breeding of a recessive homozygote X dominate phenotype (but unknown genotype)

The Law of Independent Assortment Law of Segregation involves 1 character. What about 2 (or more) characters? Monohybrid cross vs. dihybrid cross The two pairs of alleles segregate independently of each other. Mendel’s Law of Independent Assortment

Dihybrids: Are two characters, i.e. flower and seed color, transmitted from parents to offspring as a package?

Trihybrid: What would the F1 generation look like if you were to cross three true breeding traits? such as: Flower color (PP, pp) Seed color (YY, yy) Seed shape (RR, rr) What would the F1 gametes look like?

Mendel’s inheritance is governed by the laws of probability: Multiplication rule : Multiplying the probability of one event by the probability of the other event

Mendel’s inheritance is governed by the laws of probability: Addition rule : the probability that any one of two or more mutually exclusive events will occur is calculated by adding together their individual probabilities

Complex inheritance patterns: Complete dominance: One allele dominant over the other Homozygous dominant Heterozygous dominant

Complex inheritance patterns: Codominance: The two alleles affect the phenotype in separate and distinguishable way. Human blood: AB

Complex inheritance patterns: Incomplete dominance: The phenotypic expression falls between the two alleles Flower color of snapdragons Red x white = pink

Complex inheritance patterns: Multiple alleles: more than 2 possible alleles for a gene. Ex: human blood types

Complex inheritance patterns:

Pleiotropy: genes with multiple phenotypic effect. sickle-cell anemia

Complex inheritance patterns: Epistasis: a gene at one locus (chromosomal location) affects the phenotypic expression of a gene at a second locus. Epistatic vs. hypostatic hypostatic mice coat color Combs in Chickens

Complex inheritance patterns: Polygenic Inheritance: an additive effect of two or more genes on a single phenotypic character human skin pigmentation and height

The relationship between dominance and phenotype: No interactions between dominant and recessive alleles: Pathway to expression Example: round peas vs. wrinkled Example: Tay Sachs (recessive)- depends on the level at which the phenotype is examined

Frequency: Dominance does not necessarily mean more frequent. Example: polydactly- dominant trait affects 1 out of 400 born in US

Polydactly is also an example of variable expressivity - the degree to which a gene is expressed

Recessively inherited disorders: Always homozygous One recessive allele inherited from each parent Parents are usually carriers Can be life threatening or harmless

Recessively inherited disorders: Albinism: Lack of pigment- relatively mild- may lead to blindness

Recessively inherited disorders: Cystic Fibrosis: Lethal Sickle Cell Anemia Can also be lethal

Dominantly Inherited Disorders: Homozygous or Heterozygous individuals express the disorder Much less common than recessive disorders Less chance of being passed from one generation to the next if lethal No heterozygous carrier

Dominantly Inherited Disorders: Achondroplasia: dwarfism One out of 25, % of population is homozygous recessive

Dominantly Inherited Disorders: Huntington’s disease: Degenerative disease of the nervous system No obvious phenotype until 35 – 40 yrs. 1 out of 10,000 Children with a parent who has this disease will have a 50% chance of acquiring the disease

Testing: Amniocentesis: Chorionic Villus Sampling: CVS Genetic Counseling

Pedigree: A family tree describing the interrelationships of parents and children across generations for a particular trait.

Human disorders