1. Table of Contents Mendel’s Work Probability and Heredity
The Cell and Inheritance
The DNA Connection

2. Heredity, Traits and Genetics
*Heredity is the passing
of traits from parent to
offspring
*A trait is a characteristic
of an organism, such as
hair color, eye color, etc.
*Genetics is the scientific
study of heredity.
Trait Quiz

3. The Father of Modern Genetics
*Gregor Mendel outlined the
basic laws of heredity.
*Mendel was an Austrian monk
born in 1822.
*Experimented with garden
pea plants
*Presented the first paper on
genetic research in 1866 titled,
“Experiments with Plant Hybrids”

4. Mendel’s Experimental Garden
Mendel chose garden pea plants for his experiments. Why?

5. *Pea plants usually self-pollinate,
producing purebred plants.
*Mendel found a way to cross-pollinate
them so he could mix traits.
*Easy to grow large numbers of plants.
*Many different traits such as height,
seed color, seed shape, pod color, pod
shape, and flower color can be easily
studied.

6. wrinkled seeds
Side or end flowers

7. How did he do it? - Mendel’s Work
Gregor Mendel crossed pea plants that had different traits. The illustrations show how he did this.

8. Details of Mendel’s First Experiment
Mendel’s first step was to cross pure tall plants with pure short plants.
What were his results? X

9. The first generation of offspring were all tall.X
P generation
How can this be? F1
generation

10. Mendel’s Explaination
In pea plants, tall traits overpower short traits. Each plant, although it looked tall, was hiding a factor for short height.
Will the short trait reappear in the next generation?

11. To find out, Mendel now crossed tall plants of the first generation with each other.x
What were his results?

12. In the second generation, the offspring were both tall and short
In the second generation, the offspring were both tall and short. Tall outnumbered short 3 : 1 X
F1 generation F2
generation

13. - Mendel’s Work
In all of Mendel’s crosses, only one form of the trait appeared in the F1 generation. However, in the F2 generation, the “lost” form of the trait always reappeared in about one fourth of the plants.

14. Mendel’s Conclusion
Tall is a dominant factor for height since it overpowers or hides the short factor.
Short is a recessive factor for height since it appears to disappear if the tall factor is present.
We now call these “factors” alleles.

15. Dominant and Recessive Alleles
- Mendel’s Work
Dominant and Recessive Alleles
Mendel studied several traits in pea plants.

16. Traits, Genes and Alleles
Sometimes the words "gene", "allele" and "trait" seem to be used to mean the exact same thing.  Although they all are closely related, there is a difference between genes, alleles, and traits. 

17. A trait is a characteristic of an organism, such as tall height.
A gene is a small section of DNA on a chromosome that controls a trait. (Mendel didn’t know about chromosomes).
The different forms a gene may come in are its alleles (example—short or tall are the alleles for the gene that controls the height of a plant).
Gene
Allele
Trait
Homologous Chromosomes
Tall

18. Mendel showed that organisms have two alleles for every trait
Mendel showed that organisms have two alleles for every trait. (One that we get from mom and one that we get from dad).
He also showed that these alleles can be either dominant or recessive.
Capital letters are used to represent dominant alleles and lower case letters are used to represent recessive alleles.

19. Dominant traits will be seen even if there is only one dominant allele in the pair.
TT = tall plants Tt = tall plants
Recessive traits will only be seen when both recessive alleles are present in the pair tt = short plants
This explains the 3:1 ration of tall to short plants observed by Mendel.

20. Heterozygous and Homozygous
An individual with two alleles for a trait that are exactly the same (TT or tt) is said to be homozygous or “pure” for that trait.
An individual with two different alleles for a trait (Tt) is said to be heterozygous or “hybrid” for that trait.

21. Genotypes and Phenotypes
Genotype is an organism’s genetic make-up: it includes hidden traits and is represented by letters:
Examples: TT Tt or tt
Phenotype is the physical trait that you see in an organism: it is a description
Examples: tall plants or short plants

22. Phenotypes and Genotypes
- Probability and Heredity
Phenotypes and Genotypes

23. Remember, Mendel did not know how factors were passed from parent to offspring.
In 1903, Walter Sutton discovered that Mendel’s “factors” were actually genes carried on chromosomes.
These genes are passed to the offspring in the sperm and egg cells produced during meiosis.

24. Sutton discovered this while studying the development of sperm and egg cells in grasshoppers.
This is now known as the chromosome theory of inheritance.

25. Click the Video button to watch a movie about chromosomes.
- The Cell and Inheritance
Chromosomes
Click the Video button to watch a movie about chromosomes.

26. - The Cell and Inheritance
A Lineup of Genes
Chromosomes are made up of many genes joined together like beads on a string.
The chromosomes in a pair may have different alleles for some genes and the same allele for others.

27. When a pair of chromosomes separate during meiosis, the different alleles for a trait move into separate sex cells.
Ee = parent genotype
ee = parent genotype
EE = parent genotype

28. The genotype of the offspring will be determined by which sperm cell fertilizes which egg cell.
Ee = genotype of offspring Ee

29. Punnett Squares
Punnett squares can be used to show the possible ways that genes can combine at fertilization

30. - The Cell and Inheritance
A Punnett square is actually a way to show the events that occur at meiosis.

31. Monohybrid Cross
A Punnett Square that shows only one trait is known as a Monohybrid Cross.

32. T T t T t T t t T t T t Example: TT x tt
The only possible combination for the offspring of these two parents is one tall allele (T) and one short allele (t). All offspring will be hybrid (heterozygous) tall.
Genotype = Tt
Phenotype = tall T T t
T t
T t t
T t
T t

33. Let’s look at another example: T t x T t
There are 3 combinations for the offspring of these two parents. They occur in the following ratios:
Pure (homozygous) tall
TT = 25%
Hybrid (heterozygous) tall
Tt = 50%
Pure (homozygous) short
tt = 25% T t T
T T
T t t
T t
t t

34. What do you actually see?
75% tall plants and 25 % short plants.

35. Mendel’s Law of Independent Assortment
Mendel discovered that it was possible to study more than one trait at a time. Example:
There are 4 possible outcomes for the gametes. One letter does not affect the selection of the other.

36. RY Ry rY ry RY Ry rY ry Dihybrid Crosses show two traits:
Example: Rr Yy x Rr Yy
R = round seeds Y = yellow seeds
r = wrinkled seeds y = green seeds RY Ry rY ry RY
RRYY
RRYy
RrYY
RrYy
9 = round/ yellow
3 = round/ green
3 =wrinkled/ yellow
1 = wrinkled/ green Ry
RRYy
RRyy
RrYy
Rryy rY
RrYY
RrYy
rrYY
rrYy ry
RrYy
Rryy
rrYy
rryy

37. Pure Red Flowers X Pure White Flowers Result: All Pink Flowers
Codominance
Codominance occurs when neither allele in the pair is dominant over the other.
Example:
Pure Red Flowers X Pure White Flowers
Result:
All Pink Flowers

38. X RR WW RW RW RW RW

39. Punnett Squares for codominance
do not use lower case letters, since neither trait is dominant.
Example: RR X WW R R W RW RW
All offspring are
PINK W RW RW

40. - Probability and Heredity
Codominance
Another example of codominance shows each trait appearing together rather than a blending.

41. Sometimes mistakes happen in the genetic code.
Mutations
Without correctly coded proteins, an organism can’t grow, repair, or maintain itself.
Sometimes mistakes happen in the genetic code.
Any permanent change in a gene or chromosome of a cell is called a mutation.

42. Mutations can:
*be harmless, beneficial, or harmful
*happen randomly or be caused by an
outside factor (x-rays, radiation,
chemicals etc.)
*add variety to a species over time

43. Three Types of Mutations
*Substitution—one base pair is
substituted for another.
*Deletion—one base pair is omitted
*Addition—one base pair is added

44. Mutations - The DNA Connection
Mutations can cause a cell to produce an incorrect protein during protein synthesis. As a result, the organism’s trait, or phenotype, may be different from what it normally would have been.

45. Data Sharing Lab - Mendel’s Work
Click the PHSchool.com button for an activity about sharing data for the Skills Lab Take a Class Survey.

46. Links on Probability and Genetics
- Probability and Heredity
Links on Probability and Genetics
Click the SciLinks button for links on probability and genetics.

47. Click the SciLinks button for links on meiosis.
- The Cell and Inheritance
Links on Meiosis
Click the SciLinks button for links on meiosis.

48. Protein Synthesis Activity
- The DNA Connection
Protein Synthesis Activity
Click the Active Art button to open a browser window and access Active Art about protein synthesis.

49. Click the Video button to watch a movie about protein synthesis.
- The DNA Connection
Protein Synthesis
Click the Video button to watch a movie about protein synthesis.