1. Genetics: The Science of Heredity
Chapter 3

2. Section 1: Gregor Mendel
A young priest from Europe, became a teacher and cared for hundreds of pea plants
Became curious about why some of the plants had different physical characteristics, or traits
He noticed that some plants were short and some were tall
He also noticed some had yellow seeds while some had green seeds

3. Mendel continued
Mendel observed that some pea plants had different traits that their parents
The passing of traits from parents to offspring is called heredity
The study of heredity is genetics

4. Mendel’s Peas
Mendel chose pea plants to study because many of their traits exist in only two forms: for example, height is either tall or short, not medium
Mendel developed a way to cross-pollinate, or “cross” pea plants
He removed pollen from a flower on one plant and brushed it on a flower of a second plant

5. Mendel’s Experiments
To study inheritance of traits, Mendel decided to cross plants with opposite forms of a trait…
For example, he crossed tall plants with short plants to see what the offspring would be
He always started his experiment with purebred plants
A purebred plant is one that always produces offspring with the same form of a trait as the parent (purebred short peas always produce short offspring)

6. First Experiment
In his first experiment, Mendel crossed purebred tall plants with purebred short plants (“P generation”)
He called the offspring of the “P generation” the first filial, or the “F1 generation” (Filial means son in Latin)
To his surprise, all of the F1 generation plants were tall even though one of the parents were short

7. Second Experiment Mendel let the F1 generation plants self-pollinate
These offspring were known as the F2 generation
Mendel was surprised to see the F2 generation were both tall and short!
This occurred even though all of the F1 parents were tall!
Mendel discovered about ¾ of the plants were tall and ¼ were short

8. Other Traits Mendel also did experiments to check for:
Seed shape, seed color, seed coat color, pod shaped, pod color, and flower position

9. Dominant and Recessive Alleles
Factors that control each trait exist in pairs…one from the female parent and one from the male
One factor in a pair can hide the other factor
In the F1 generation, the tall factor hid the short factor

10. Today, factors that control traits are called genes
Alleles continued…
Today, factors that control traits are called genes
Different forms of genes are called alleles
In Mendel’s work, each pea plant inherits a combination of two alleles from its parents
Either:
Two alleles for tall stems
Two alleles for short stems
One of each

11. Alleles continued…
Individual alleles control the inheritance of traits
Some alleles are dominant
A trait that always shows up in the organism when the allele is present
Some alleles are recessive
These are masked, or covered up, when the dominant allele is present
A trait controlled by a recessive allele will only show up if the organism does not have the dominant trait

12. Understanding Mendel’s Crosses
Here is how Mendel’s results work:
The P generation tall plants (purebred) had two alleles for tall stems
The purebred short plants had two alleles for short stems
The F1 generation received one tall and one short allele
The F1 plants are called hybrids (they have different alleles for the trait)
All the F1 plants are tall because the dominant allele (tall) covers the recessive allele (short)

13. Understanding Mendel’s Crosses continued…
When the F1 plants (hybrids) were crossed with each other, some inherited two dominant alleles (these became tall plants)
Some inherited one dominant and one recessive allele (these were tall)
Some inherited two recessive alleles (these were short!)

14. Symbols in Genetics
Letters are used to represent traits in today’s world
A dominant allele is represented by a capital letter (T)
A recessive allele is represented by a lowercase letter (t)
TT would mean two dominant alleles
tt would mean two recessive alleles
Tt would mean one dominant and one recessive allele

15. Mendel’s Contribution
In 1900, three scientists rediscovered Mendel’s work
Many of Mendel’s discoveries are still used today
Because of his important work, Gregor Mendel is known as the “Father of Genetics”

16. Section 2: Probability
Probability is the likelihood that a particular event will occur
If you flip a coin… the probability of landing on heads is 50%, and the probability of landing on tails is 50%

17. Principles of Probability
There are three ways to express probability
As a ratio: 1 to 2
As a percent: 50%
As a fraction ½
The laws of probability predict what MIGHT occur.
If you flip a coin 10 times you might get 5 heads and 5 tails, or you might get 1 head and 9 tails…

18. Mendel and Probability
Remember…when Mendel crossed two hybrid plants (Tt), ¾ were tall and ¼ were short
Mendel stated that the probability of producing a tall plant was 3 in 4, and the probability of producing a short plant was 1 in 4
Mendel was the first scientist to recognize that the principle of probability can be used to predict the results of genetic crosses

19. Punnett Squares
A Punnett square is a chart that shows possible combinations of alleles that can result from a genetic cross
They are also used to determine the probability of a particular outcome
TT = 25%
Tt = 50%
tt = 25%

20. Phenotypes An organisms phenotype is its physical appearance
Phenotypes are visible traits
An example:
Pea plants can have one of two different phenotypes for stem height: tall or short
These are physical characteristics of something!

21. Genotypes A genotype is the genetic makeup or the allele combinations
TT, Tt, tt
An organism that has two identical alleles for a trait is homozygous (TT, tt)
An organism with different alleles for a trait is heterozygous (Tt)

22. Codominance Sometimes, there is no dominant or recessive allele
When this happens, the offspring inherits both alleles
For example, if a purebred black cow is crossed with a purebred white cow, the cow will be both black and white.

23. Section 3: Chromosomes and Inheritance
In humans, each body cell (skin, blood, liver, etc.) has 46 chromosomes (or 23 pairs)
Sex cells (sperm and egg) contain 12 chromosomes
Walter Sutton, an American geneticist discovered this
He concluded that genes are carried from parents to their offspring on chromosomes.
This is known as the Chromosome Theory of Inheritance

24. Meiosis
Meiosis is the process by which the number of chromosomes is reduced by half to form sex cells (sperm and egg)
During meiosis the chromosome pairs separate and are distributed to two different cells.
The sex cells that are created have only half as many chromosomes as the other cells in the organism.

25. Chromosomes
In body cells of humans, there are 23 pairs (46 total) chromosomes
Chromosomes are made of many genes joined together like beads on a string
Each gene controls a particular trait

26. Section 4: DNA Connection
The main function of genes is to control the production of proteins in cells
Proteins help to determine the size, shape, and other traits
DNA is a major component in chromosomes
A = Adenine
T = Thymine
G = Guanine
C = Cytosine
Remember, these four bases form the “steps” of the DNA ladder

27. Making Proteins
The order of the four nitrogen bases (A, T, G, C) along each gene forms a genetic code that specifies what type of protein will be produced
The production of proteins is called Protein Synthesis
During this process, the cell uses information from a gene on a chromosome to produce a specific protein
Protein Synthesis takes place in the ribosomes

28. The Role of RNA The ribosomes are found in the cytoplasm
The chromosomes are in the nucleus
How does the information needed to make proteins get from the nucleus to the ribosomes?
A “messenger” must first carry the genetic code from the DNA (in the nucleus) into the cytoplasm (to the ribosomes)
The “messenger” is RNA
RNA looks like one side of the “DNA ladder”
RNA also has four nitrogen bases: adenine (A), guanine (G), cytosine (C), and uracil (U)

29. Types of RNA There are two main types of RNA Messenger RNA:
Copies the coded message from the DNA in the nucleus, and carries the message to the cytoplasm for the ribosomes
Transfer RNA:
Carries amino acids and adds them to growing proteins

30. Protein Synthesis Step 1: mRNA Production
In the nucleus, DNA “unzips” between base pairs, and RNA bases match up along the DNA strands…the genetic info from DNA is transferred to the RNA
Step 2: mRNA attaches to a Ribosome
mRNA enters cytoplasm, attaches to a ribosome, production of protein begins
Step 3: tRNA attaches to mRNA
tRNA carries amino acids to the ribosome, they match up with bases on the mRNA, protein chain grows
Step 4:Protein Production Complete
Chain grows until the ribosome tells it to stop…the chain is released and the protein is complete

31. Mutations
Any change that occurs in a gene or chromosome is called a mutation
Mutations can cause a cell to produce an incorrect protein during protein synthesis
If this happens, an organism’s phenotype (traits) will be different from what it should have been
If a mutation happens in a body cell, only that cell is affected
If a mutation happens in a sex cell, offspring can be affected

32. Effects of Mutations Some mutations can be harmful
These mutations can reduce an organism’s chance of survival
Some mutations can be helpful
These mutations can improve an organism’s chance of survival
Some can be neither