2. Gregor Mendel Humans have noticed family resemblances for thousands of years. Heredity- the passing of traits from parents to offspring, was used for breeding plants and animals but was poorly understood. Mendel was the first person to explore genetics, the study of heredity, in a scientifically controlled manner. Mendel studied pea plants because it is relatively easy to control their reproduction. Mendel’s experiment were carefully controlled- at first, he studied only one trait at a time from lines of true breeding plants

3. Trait vs. Characteristic A characteristic or character is an inherited characteristic. A trait is a single type of a character

4. Mendel’s Hypotheses Gene- a segment of DNA that codes for a particular trait An individual has two copies of a single gene- one from each parent. There are alternate forms of genes- alleles. When two different alleles occur together one will be expressed and one will not- (dominant/recessive) Gametes carry only one allele for each inherited character.

5. Heredity Zygote- a fertilized egg, a combination of a sperm and an egg. Homozygous- Two alleles of one gene that are the same (ex: TT) Heterozygous- Two alleles of one gene that are different (ex: Tt) Dominant- the most common expressed form of a gene. Recessive- the least commonly expressed form of a gene, will only be expressed with another recessive form.

6. Monohybrid Crosses Parent plants differing in only one trait were crossed and the traits of the offspring were observed The offspring of this cross are called the first filial generation (F 1 ) The F 1 were then crossed with eachother to produce the second filial generation (F 2 ) and traits were observed

7. Results of Monohybrid Crosses Entire F 1 generation carried on only the traits of one of the parents Example: Parent were tall and short, all F 1 generation was tall Two F 1 crossed and 75% of offspring carry on the F 1 traits, 25% of the offspring however resemble the original parental trait that was not seen in the F 1 generation (3:1 ratio) Example: 2 F 1 (tall) offspring crossed and 3/4 offspring were tall, 1/4 offspring were short

8. Mendel’s Conclusions: Each organism has two factors (genes) for each trait Genes can have alternate forms called alleles. Example: The height gene has two alleles, tall (T) and short (t). The allele expressed in the F 1 generation is dominant to the recessive allele that is hidden. Plants with the tall phenotype (outward appearance) can be homozygous dominant (TT) or heterozygous (Tt). Plants showing the short phenotype must be homozygous recessive (tt).

9. Law of Segregation The two alleles possessed by a parent separate when forming gametes; only one is randomly passed on to offspring. Ensures that offspring have same number of chromosomes as their parents (2 parents each with 2 alleles, if each passed on both would result in an offspring with 4 alleles)

10. Rule of Dominance When 2 alleles are present for the same trait, one will dominate over the other Dominant = the trait that is observed Recessive = the trait that is not seen or “disappears”

11. Homozygous vs Heterozygous Each trait is coded for by two alleles (1 allele passed on from each parent) Allele can be dominant (T) or recessive (t) If alleles are the same, the organism is said to be homozygous for that trait Homozygous dominant (TT) Homozygous recessive (tt) If alleles are different, the organisms is said to be heterozygous for that trait (Tt)

12. Genotype vs. Phenotype Genotype- the arrangement of alleles that are present (ex: TT, Tt, tt, etc) TT or Tt will show dominant trait tt will show recessive trait Phenotype- the physical expression of a trait Cross- a method to determine the possible offspring of the cross between two parents

13. Mendel’s Laws Rule of Dominance- with different alleles one may be fully expressed while one may have no observable effect. Law of Segregation- two alleles of a gene separate while gametes are formed. Law of Independent Assortment- the alleles of different genes separate independently of one another during gamete formation.

14. Incomplete Dominance Incomplete Dominance is when there are three phenotypes for three genotypes Heterozygous phenotype will be its own genotype The heterozygous phenotype will usually be a mixture of both homozygous phenotypes

15. Multiple Alleles Genes with three or more possible alleles. Example: Blood Type Blood Type is determine by the I A, I B, i Even for genes with multiple alleles, an individual can only have two alleles.

16. Codominance Two dominant alleles are expressed at the same time. Codominance is different from incomplete dominance, both dominant traits are displayed. Example: Blood type

17. Polygenic Inheritance Some traits can not be determined by one gene. Some traits are determined by several genes- polygenic inheritance. Examples of polygenic inheritance is hair, color, skin color, eye color, etc.

18. Dihybrid Crosses Dihybrid Crosses are Punnett Squares that look at the cross two genes of two individuals.