Chapter 2- Mendelian Genetics

Mendel's Laws of Inheritance- Ch 2.1

Mendel and his Pea Plants (pgs. 18-20) When 2 distinct individuals with different characteristics are mated or crossed, this is called a hybridization. The offspring are referred to as hybrids (18). In plants, fertilization occurs when sperm enters the micropyle in the ovule wall and fuses with an egg cell (20). Gamete is used to describe the haploid reproductive cells that can unite to form a zygote (20). The process of crossing the genetics of 2 plants rather than allowing them to self-fertilize is called cross-fertilization. This requires that the pollen from one plant be placed on the sigma of another plant (20). 7 Traits that Bred True (pgs. 20-21) Unique characteristics of an organism are called traits (20). "Bred true" means that a trait did not vary in appearance from generation to generation. A variety that continues to produce the same characteristic after several generations of self-fertilization is called a true-breeding line, or strain (20). A variant refers to a particular trait that may be found in 2 or more variations within a single species (20). A cross in which an experimenter is observing only 1 trait is called a single-factor cross, or a monohybrid cross (20). When the parents are different variants for a given trait, this type of cross produces single-trait hybrids, also known as monohybrids. Following the Outcome of a Single Trait for 2 Generations (21-23) Mendel began with true-breeding plants that differed with regard to a single trait. This is the parental, or P, generation (21). The true-breeding parents were crossed with each other (P cross) and the offspring was called the F1 generation. All of these plants showed the phenotype of one parent, but not the other (21). The F1 generation was allowed to self-fertilize, and this made the F2 generation (21). Mendel proposed 3 different ideas: The variant for 1 trait is dominant over another variant. Recessive is used to describe the variant that is masked by the presence of a dominant trait. When a true-breeding parent with a dominant trait was crossed with a true-breeding recessive parent, the dominant trait is was always observed in F1, but was variable in F2. Because he observed recessive traits only in the 2nd generation, he proposed that the genetic determinants of traits are passed along as "unit factors", which are now called genes. This was consistent with the particulate theory of inheritance (22-23). There is always a 3:1 ratio between the dominant and recessive in the F2 generation (23).

Mendel's Laws of Inheritance- Ch 2.1

3:1 Phenotypic Ratio is Consistent with Law of Segregation (23) A gene is defined as a "unit of heredity" that may influence the outcome of an organism's traits (23). An allele refers to different variations of the same gene. Eukaryotes have 2 copies of most genes because genetic material is organized into pairs of chromosomes (23). Each parent transmits only 1 copy of each gene (23). Mendel's law of segregation states that 2 copies of a gene separate from each other during transmission from parent to offspring (23). Therefore, only 1 copy of each gene is found in a gamete. At fertilization the gametes combine randomly and potentially produce different allelic combos. Genotypes refer to genetic composition. Phenotypes refer to the observable characteristics (23). Crosses Involving 2 Different Traits (25-28) Two factor, or dihybrid crosses, are used to investigate the pattern of inheritance for 2 different traits (25). According to Mendel's law of independent assortment, 2 different genes will randomly assort their alleles during the formation of haploid cells (25-27). A single individuals can produce a vast array of genetically different gametes. This law is rooted in the random pattern by which the homologues assort themselves during meiosis. If a species contains a large number of homologues, this creates potential for an enormous amount of genetic diversity (27-28). Mendel began with 2 different strains of true-breeding pea plants that were different in regard to 2 seed traits (color and consistency of seeds) (25). The F2 generation had round and green and wrinkled and yellow seeds (whereas F1 only had round and yellow seeds). Round/green and wrinkled/yellow seeds are called nonparentals because their phenotypes were not found in the true-breeding plants of the P generations (25-26). The phenotypic ratio is generally 9:3:3:1when a dihybrid is allowed to self-fertilize (27). When an offspring recieves a combo of alleles that differs from those in the parental generation, the phenomenon is called genetic recombination (27). One mechanism that accounts for this is independent assortment.

Mendel's Laws of Inheritance- Ch 2.1

Molecular Expression of Genes and Outcomes of Traits (29) A genetic approach can tell us relationship btwn protein's function and its effect on genotype.  Loss-of-function alleles are defective genes. Geneticists will try to identify an individual  that has a defective copy of a gene to see how that will affect the phenotype of the organism (29) Such alleles are often recessive, though that isn't always the case. Pedigree Analysis and Mendelian Inheritance (29- A pedigree analysis is aimed at determining the type of inheritance pattern a gene will follow (29).  Pedigrees often provide important clues concerning the pattern of inheritance of traits within human families (29).   Pedigrees are commonly use to determine inheritance pattern of human genetic diseases. One way to discern dominant/recessive relationship between 2 alleles is by a pedigree analysis (30).  A recessive pattern makes 2 important predictions: (30) 2 heterozygous normal individuals will, on average, have 1/4 of offspring affected. All offspring of 2 affected individuals will be affected.  Dominant pattern makes the prediction that affected individuals will have inherited the gene from at least 1 parent unless a mutation has occurred (30).

Probability and Statistics- Ch 2.2

Probability is the Likelihood That an Event Will Occur (30-31) The chance that an event will occur in the future is called the probability (30). It depends on the number of possible outcomes. The general formula for probability: Number of times an event occurs/total number of events (31). Probability calculations are great and all, but the accuracy depends to a great extent on the size of the sample (31).  The deviation between the observed and expected outcomes is called the random sampling error (31).  In a larger sample, we expect the random sampling error to be a much smaller percentage.  The Sum Rule Can Predict the Occurence of Mutually Exclusive Events (31) The sum rule states that the probability that one of 2 or more mutually exclusive events will occur is equal  to the sum of the individual probabilities of the events (31).  This allows us to determine the probability that we will obtain any 1 of 2 or more different types of offspring.  The Product Rule Can Be Used to Predict Independent Events (31-32) The product rule states that the probability of 2 or more independent events will occur is equal to the product of their individual probabilities (32). We can utilize the product rule to predict probability of a sequence of events that involves combinations of different offspring (32).  The product rule can also be used to predict the outcome of a cross involving 2 or more genes (32). The Binomial Expansion Equation Used to Predict Probability of an Unordered Combo of Events (32-33) In this case, we are not concerned with the order in which the offspring are born. We are instead concerned with the final numbers of each phenotype (32). The binomial expansion equation can be used to represent all of the possibilities for a given set of unordered events (32). A multinomial expansion equation can be used to solve unordered genetic problems that involve 3 or more phenotypic categories (33).  Chi Square Test For Validity Reasons Hypothesis testing is used to determine if the data from genetic crosses are consistent with a particular pattern of inheritance (33).