1. Patterns of Inheritance
Chapter 14: Mendel and the Gene Idea

2. Patterns of Inheritance
Parents and offspring often share observable traits.
Grandparents and grandchildren may share traits not seen in parents.
Why do traits disappear in one generation and reappear in another?

3. Possible Hypotheses The “blending” hypothesis states that
Example: Blue and yellow paint blend to make green
The “particulate” hypothesis states that
These heritable units are that can be passed on to the next generation
Gregor Mendel documented a through his experiments with

4. Gregor Mendel Austrian monk Analyzed Asked why Tested his theories
Worked with

5. Pea plants Pisum sativum
Advantages of pea plants for genetic study:
Many varieties with distinct heritable features, or (such as flower color)
Character variants (such as purple or white flowers) are called
Each pea plant has sperm-producing organs (stamens) and egg-producing organs (carpels)

6. Traits Mendel used

7. True breeding plants Began with true-breeding varieties (pure-bred)
Crossed with other true-breeding variety
Offspring called

8. Generations Mendel mated two contrasting, true-breeding varieties
Process called
The true-breeding parents are the
The hybrid offspring of the P generation are called the
Referred to as hybrids
When F1 individuals self-pollinate, the is produced

9. Crosses Had lots of varieties 7 traits with two forms of each trait
Crossing a tall plant with a short plant is a
A monohybrid cross is
Trait -
Two variations -
Generations
P - parental (true-breeding):
F1 - first filial or son (children):
F2 - next generation (grandkids):
What happened? How did the short plants reappear?

11. Mendel’s Conclusions 1. Alternative versions of genes account for
For example, the gene for flower color in pea plants exists in two versions: purple flowers and white flowers
These alternative versions of a gene are now called
Each gene resides at a specific
Therefore, we distinguish between an organism’s

12. Alleles

13. Genotype Terminology If alleles are identical =
If both alleles are recessive =
Genotype is
If both alleles are dominant =
If both alleles are different =
One dominant allele and one recessive allele ( )
An organism’s traits do not always reveal

15. Mendel’s Conclusions For each character, an organism inherits
Factors (genes) that determine traits can be
Alleles may be
traits expressed in the F1 generation
traits not expressed in the F1 generation
Mendel observed the same pattern of inheritance in 7 pea plant characters, each represented by two traits
What Mendel called a “heritable factor” is what we now call a

16. Mendel’s Conclusions
3. If two alleles at a locus differ, then one (dominant allele) determines the organism’s appearance,
When Mendel crossed contrasting, true-breeding white and purple flowered pea plants,
When Mendel crossed the F1 hybrids,
Mendel discovered a ratio of about

18. Mendel’s Law of Segregation
4. The law of segregation states that the two alleles for a heritable character
Thus, an egg or a sperm gets only
This segregation of alleles corresponds to the distribution of

19. Principle of Segregation
Two copies of each trait (gene)
Fully expressed gene -
Other gene -
2. Gametes only have
3. Fertilization restores

21. Monohybrid & Dihybrid Crosses
Mendel derived the law of segregation by following
Mendel identified his second law of inheritance by following
Crosses involving two traits are called
A dihybrid cross can determine whether two characters are transmitted to offspring
Using a dihybrid cross, Mendel developed the

22. Law of Independent Assortment
The law of independent assortment states that each pair of alleles
This law applies only to genes on
Genes located near each other on the same chromosome

23. Probability Rules Mendel’s laws reflect
When tossing a coin, the outcome of one toss has on the outcome of the next toss
In the same way, the alleles of one gene
The multiplication rule states that the
Example: probability of 2 coins landing heads up is

24. Probability Rules
Each gamete has a chance of carrying the and a chance of carrying the
Similar to heads and tails
Another rule is needed to figure out the probability that an F2 plant from a monohybrid cross will be heterozygous rather than homozygous
The rule of addition states that the probability that any one of two or more exclusive events will occur is calculated
Example: probability of one heads & one tails is

25. Probability Rules
These rules can be used to predict the outcome of crosses involving
A dihybrid or other multicharacter crosses are equivalent to two or more
In calculating the chances for various genotypes, each character is

26. Punnett squares Probability can be depicted through the use of a
A diagram for predicting the results of a genetic cross between
Predicts for all possible gametes with
Same letter used for trait

27. Setting up a Punnett Square
Step 1. Designate letters which will represent the genes/traits.
T = tall t = short
Step 2. Write down the genotypes (genes) of each parent. These are often given to you or are possible to determine.
TT (tall) X tt (short) - both homozygous or purebred
Step 3. List the genes that each parent can contribute. Parent 1 Parent 2

28. Setting up a Punnett Square
Step 4. Draw a Punnett square and write the possible gene(s) of one parent across the top and of the other parent along the side.


Step 5. Fill in each box of the Punnett square by transferring the letter above and in front of each box into each appropriate box. As a general rule, the capital letter goes first and a lowercase letter follows.
Step 6. List the possible genotypes and phenotypes of the offspring for this cross.
Genotypic Ratio:
Phenotypic Ratio:

29. Practice!
1. Cross a homozygous tall plant with a short plant. What are the genotypic and phenotypic ratios?
2. Cross a heterozygous tall plant with a homozygous tall plant. What are the genotypic and phenotypic ratios?
3. Cross a heterozygous tall plant with a short plant. What are the genotypic and phenotypic ratios?

30. Testcross Used to determine Individual expressing Could be Cross with
Make prediction with Punnett square
If homozygous dominant:
TT x tt
If heterozygous:
Tt x tt

32. More Practice!!!
1. In rabbits, the allele for black fur (B) is dominant over the allele for brown fur (b). If a heterozygous male mates with a heterozygous female, what are the chances that the offspring will have black fur?
2. In humans, dimples are dominant to no dimples. If a homozygous dominant man reproduces with a heterozygous female, what are the chances of having a child with no dimples?
3. In humans, freckles are dominant over no freckles. A man with freckles reproduces with a woman with freckles, but the children have no freckles. What chance did each child have for freckles?
4. If a man is homozygous for widow’s peak (dominant) reproduces with a woman homozygous for straight hairline (recessive), what are the chances of their children having a widow’s peak? A straight hairline?
5. In humans, pointed eyebrows (B) are dominant over smooth eyebrows (b). Mary’s father has pointed eyebrows, but she and her mother have smooth. What is the genotype of the father?

33. Review Questions
Differentiate between the blending and particulate hypotheses of inheritance.
Explain the importance of Gregor Mendel’s work with garden peas. Also, explain why he used garden peas.
Define the following vocabulary associated with basic genetics: character, trait, hybrid, gene, allele, locus, genotype, phenotype, dominant, recessive, homozygous, & heterozygous.
Differentiate between the P, F1, and F2 generations.
Differentiate between monohybrid and dihybrid crosses.
Explain Mendel’s four basic conclusions regarding inheritance patterns.
Explain the three parts to the law of segregation.
Explain the law of independent assortment.
Properly construct a Punnett square for use in solving a genetics problem involving probability.
Explain the idea of a testcross.