History of Evolutionary Theory

Historical Development of Genetics

• Preformation theory (1665): Development is simply a matter of growth of a tiny homunculus, with two opposing views - "spermatist" and "ovist".
• Epigenesis theory (1759): Development occurs through a series of complex interactions, with sex cells being homogenous bits of organic matter containing nothing resembling the body that will develop from them.

Key Figures in Genetics

• Jan Swammerdam
• Charles Bonnet
• Karl Ernst von Baer
• Charles Lamarck
• August Weismann
• Gregor Mendel

Principles of Segregation

• Useful in genetics studies for determining the genotypes of individual organisms (homozygous or heterozygous)
• Helps isolate (separate out) certain characteristics in a related group of animals or plants

First Experiment: Testing the Principle of Segregation

• F1 Generation: Cross between two individuals with different genotypes (e.g., Dd x dd)
• Backcross: F1 (progeny) is mated or crossed back to one of their parents or with an individual that has a parental genotype

Difference between Testcross and Backcross

• Testcross: A recessive homozygote is always used as one of the parents, with a specific goal of identifying the genotype of the other parent
• Backcross: Not necessarily true, with the goal of identifying the genotype of the F1 progeny### First Experiment: Testcross and Backcross
• In testcross, a recessive homozygote is always used as one of the parents.
• The testcross parent is homozygous recessive for the gene of interest.
• This is not necessarily true in a backcross.

Principles of Segregation

• The results of a testcross can be mathematically expressed as two monohybrid crosses, each expected to result in a 3:1 ratio.
• The combined result is a 9:3:3:1 ratio, which conforms to the law of probability called the "Product Rule".

Second Experiment: Dihybrid Backcross

• When a 9:3:3:1 ratio results in an experiment, geneticists may postulate that two independent pairs of alleles are involved.
• One member of each pair behaves like a dominant over its allele.
• The F1 generation is heterozygous for both genes (RrYy).
• The gametes produced are RY, Ry, rY, and ry.
• The resulting genotype and phenotype ratios are 1:1:1:1.

Trihybrid Ratios

• A cross between homozygous parents that differ in three gene pairs is a combination of three-pair crosses operating together.
• The genotype ratio is a combination of three independent monohybrid crosses.

Probability and Genetic Events

• Probability is used to calculate the likelihood of genetic events.
• The independence of genetic events is a key concept in probability calculations.
• The binomial expansion/distribution is a mathematical formula used to calculate the probability of multiple events.
• The formula is P = (n!/s!t!)(psqt), where n is the total number of events, s is the number of successes, t is the number of failures, p is the probability of success, and q is the probability of failure.

Examples of Probability Calculations

• If four babies are born at a given hospital on the same day, the probability that two will be boys and two will be girls is 10/32.
• If a heterozygous man and woman plan to have four children, the probability that any given child will be normal is 1/2, and the probability that all four will be normal is 1/16.