1. Mendel’s Principles of Inheritance
Chapter 1
Mendel’s Principles of Inheritance
I. History of genetics before Mendel:
1. First genetic practice was artificial selection. This applies to cultivated plants (corn, wheat, potatoes, etc..) and domesticated animals (dogs, cattle, sheep, etc…).
2. Two myths led to misconceptions about heredity and transmission of traits to offsprings
a. One parent contributes most to the offspring’s inherited features Aristotle believed it was the male parent.
b. The concept of blended inheritance i.e., the parental traits are mixed and and forever changed in the offspring

2. II. Genetic analysis according to Mendel
A. Mendel’s success was based on making good choices:
1. Used a self-fertilizing plant (the garden pea)
2. Chose traits that had clear cut alternative forms (7 traits)
3. Started his experiments with pure-breeding lines
4. Did reciprocal crosses to eliminate maternal effects
5. Analyzed a large number of plants (quantitative analysis)
6. All plants were planted under same conditions

5. B. Monohybrid crosses
1. Revealed units of inheritance and the law of segregation
2. The discrete units of inheritance are genes which control the appearance of inherited traits
3. Genes come in alternative forms called alleles that are responsible for the expression of different forms of a trait. E.g. height of plant is trait encoded by a gene which has two forms, short (one allele for the gene) and short (a second allele for the gene)

8. Important terms:
a. Body cells of sexually reproducing organisms carry two copies of each gene. When the two copies of a gene are the same allele, the individual is homozygous for that gene. When the two copies of a gene are different alleles, the individual is heterozygous for that gene.
b.The genotype is a description of the allelic combination of the two copies of a gene present in an individual. The phenotype is the observable form of the trait that the individual expresses.
c. A cross between two parental lines (P) that are pure bred for alternative alleles of a gene will produce a first filial (F1) generation of hybrids that are heterozygous. The phenotype expressed by these hybrids is determined by the dominant allele of the pair, and is the same as that expressed by individuals homozygous for the dominant allele. The phenotype associated with the recessive allele will only appear in the F2 generation in individuals homozygous for this allele. In crosses between F1 heterozygotes, the dominant and recessive phenotypes will appear in the F2 generation in a ratio of 3:1.

10. Law of segregation: The two alleles of each gene (trait) segregate independently from one another during the formation of gametes. As a result, each egg and sperm (or pollen grain) contain only one allele of each gene. At fertilization, male and female gametes unite at random during fertilization so that the formed zygote will contain the two alleles (one parental and one maternal) of a gene.

14. Mendel’s results reflect basic rules of probability
a. Further crosses confirmed ratios predicted by the law of segregation
b. Test crosses establish genotype

16. C. Dihybrid crosses 1. reveal the law of independent assortment
2. Law of independent assortment “During the formation of gametes, the segregation of alleles of any one gene is independent of the segregation of alleles of other genes.”
3. According to this law, crosses between AaBb F1 dihybrids will generate F2 progeny with a phenotypic ratio of 9(A-B-):3(A-bb):3(aaB-):1(aabb).