1. Intro to Genetics
Genetics is the branch of biology that deals with patterns of inheritance.
Heredity is the biological process by which parents pass on genetic information to their offspring through their gametes.
Gregor Mendel, an Austrian monk, started the science of genetics by experimenting with pea plants between 1856 and 1868.
Mendel came up with the Principles of Dominance, Segregation, and Independent Assortment.
Mrs. Degl

2. Each gene has a definite position on a chromosome, called a locus.
Based on his findings, Mendel proposed that certain traits were inherited as a result of the transmission of hereditary factors.
Mendel’s hereditary factors, called genes , are located on the chromosomes.
Each gene has a definite position on a chromosome, called a locus.
Mrs. Degl

3. A gene is a hereditary unit consisting of a sequence of DNA that occupies a specific location on a chromosome and determines a particular characteristic in an organism.
In the early 1900’s T.H. Morgan carried out breeding experiments (crosses) with the fruit fly (Drosophila), that supported Mendel’s findings. He received the Nobel Prize for Physiology or Medicine in 1933.
Mrs. Degl

4. Two alternate genes that control each trait are called alleles, and they are located on the same positions on homologous chromosomes. (Example: red or brown hair)
Homologous chromosomes are a pair of matching chromosomes in an organism, with one being inherited from each parent.
Mrs. Degl

5. GENOTYPE = the genes present in the DNA of an organism.
Pairs of letters are used to represent genotypes for one particular trait. 
There are always two letters in the genotype because (as a result of sexual reproduction) one code for the trait comes from the mother organism & the other comes from the father organism, so every offspring gets two codes (two letters).
PHENOTYPE = how the trait physically shows-up in the organism. 
Mrs. Degl

6. Tt = heterozygous = hybrid
When we have two capital or two lowercase letters in the GENOTYPE (ex: TT (tall) or tt (short)) it's called HOMOZYGOUS
("homo" means "the same").  Sometimes the term "PURE" is used instead of homozygous.
When the GENOTYPE is made up of one capital letter & one lowercase letter (ex: Tt) it's called HETEROZYGOUS
("hetero" means "other").  Just to confuse you, a heterozygous genotype can also be referred to as HYBRID.
Genotype = genes present in an organism (usually abbreviated as two letters)
TT = homozygous = pure
Tt = heterozygous = hybrid
tt = homozygous = pure
Mrs. Degl

7. Test Cross
In genetics, a test cross was first introduced by Mendel in order to determine if an individual exhibiting a dominant trait is a homozygous dominant or a heterozygous dominant. To do this, he crossed a homozygous recessive individual. If all offspring display the dominant phenotype, the individual in question is homozygous dominant; if the offspring are split equally between the dominant and recessive phenotype, the individual is heterozygous.
Mrs. Degl

8. The Principle of Dominance In a cross of parents that are pure for contrasting traits, only one form of the trait will appear in the next generation.  Offspring that are hybrid for a trait will have only the dominant trait in the phenotype.
Genotype Symbol
Genotype Vocab
Phenotype TT
homozygous DOMINANT or pure tall
tall Tt
heterozygous or hybrid tt
homozygous RECESSIVE  or pure short
short
Mrs. Degl

9. Punnett Squares help us illustrate the crosses that Mendel did. T T
T = the dominant allele for tall stems
t = recessive allele for short stems t
(Let’s fill it in)
A summary of this cross would be: 
Parent Pea Plants  (P Generation) = Parent
Offspring  (F1 Generation) = First Generation
Genotypes: TT x tt
Phenotypes: tall x short
Genotypes: 100% Tt
Phenotypes: 100% tall
Mrs. Degl

10. The Principle of Segregation During the formation of gametes (eggs or sperm), the two alleles responsible for a trait separate from each other.  Alleles for a trait are then "recombined" at fertilization, producing the genotype for the traits of the offspring. T t
Mendel now takes two of the "F1" generation (which are hybrid tall: Tt) & crosses them.  He thought he was going to get all tall again (since tall is dominant).  But no!  T t
(Let’s fill it in)
Mrs. Degl

11. Mendel’s F1 cross summary:
Parent Pea Plants  (Two Members of F1 Generation)
Offspring  (F2 Generation)
Genotypes:
Tt x Tt
Phenotypes:
tall x tall
Genotypes: 25% TT 50% Tt 25% tt
75% tall 25% short
A helpful thing to recognize:
Any time two parents have the same phenotype for a trait  but some of their offspring look different with respect to that trait,  the parents must be hybrid for that trait. 
Mrs. Degl

12. The Principle of Independent Assortment Alleles for different traits are distributed to sex cells (& offspring) independently of one another.
So far we've been dealing with one trait at a time.  For example,  height (tall or short), seed shape (round or wrinkled), pod color (green or yellow).
Mendel noticed during all his work that the height of the plant and the shape of the seeds and the color of the pods had no impact on one another.  In other words, being tall didn't automatically mean the plants had to have green pods, nor did green pods have to be filled only with wrinkled seeds, the different traits seem to be inherited INDEPENDENTLY. This is called Independent Assortment.
Mrs. Degl

13. Tt: medium height plant rr: white flower tt: dwarf plant
Four o'clock plants have a gene for color and a gene for height with the following phenotypes:
RR: red flower
TT: tall plant
Rr: pink flower
Tt: medium height plant
rr: white flower
tt: dwarf plant
If a dihybrid plant is self-fertilized, Let’s see proportions of genotypes and phenotypes produced on the next page.
Dihybrid Cross
Mrs. Degl

14. Number Genotype Phenotype 1 RRTT red-flower, tall plant
RrTT pink flower, tall plant
rrTT white flower, tall plant
RRTt red flower, medium height plant
RrTt pink flower, medium height plant
rrTt white flower, medium height plant
RRtt red flower, dwarf plant
Rrtt pink flower, dwarf plant
rrtt white flower. dwarf plant
Mrs. Degl

15. With incomplete dominance, a cross between organisms with two different phenotypes produces offspring with a third phenotype that is a blending of the parental traits. 
R = allele for red flowers W = allele for white flowers
red x white ---> pink RR x WW ---> 100% RW
It's like mixing paints, red + white will make pink.  Red doesn't totally block (dominate) the white, instead there is incomplete dominance, and we end up with something in-between.
We can still use the Punnett Square to solve problems involving incomplete dominance.  The only difference is that instead of using a capital letter for the dominant trait & a lowercase letter for the recessive trait, the letters we use are both going to be capital (because neither trait dominates the other). 
Mrs. Degl

16. With codominance, a cross between organisms with two different phenotypes produces offspring with a third phenotype in which both of the parental traits appear together. 
R = allele for red flowers W = allele for white flowers
red x white ---> red & white spotted RR x WW ---> 100% RW
The genetic gist to codominance is pretty much the same as incomplete dominance.  A hybrid organism shows a third phenotype --- not the usual "dominant" one & not the "recessive" one ... but a third, different phenotype.  With incomplete dominance we get a blending of the dominant & recessive traits so that the third phenotype is something in the middle (red x white = pink).
In COdominance, the "recessive" & "dominant" traits appear together in the phenotype of hybrid organisms.
Mrs. Degl