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 states that genetic material from the two parents blends together
Example: Blue and yellow paint blend to make green
The states that parents pass on discrete heritable units
These heritable units are that can be passed on to the next generation
Gregor Mendel documented a particulate mechanism through his experiments with garden peas

4. Gregor Mendel Austrian monk Analyzed Asked why
Tested his theories making predictions based on math
Worked with garden peas

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
Mating of plants can be controlled
Each pea plant has sperm-producing organs (stamens) and egg-producing organs (carpels)
Cross-pollination (fertilization between different plants) easily done

6. Traits Mendel used

7. True breeding plants Began with true-breeding varieties Self-pollinate
Always produce same offspring
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 involving only
Trait - plant height
Two variations - tall and short
Generations
P - parental (true-breeding): tall & short
F1 - first filial or son (children): all tall
F2 - next generation (grandkids): tall & short
What happened? How did the short plants reappear?

11. Mendel’s Conclusions
1. Alternative versions of genes account for variations in inherited characters
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 (location on a specific chromosome)
Therefore, we distinguish between an organism’s
or physical appearance
or genetic makeup

12. Alleles

13. Genotype Terminology If alleles are identical =
If both alleles are recessive =
Genotype is aa
If both alleles are dominant =
Genotype is AA
If both alleles are different =
Dominant phenotype expressed
One dominant allele and one recessive allele (Aa)
An organism’s traits do not always reveal its genetic composition

15. Mendel’s Conclusions
For each character, an organism inherits two alleles, one from each parent
Factors (genes) that determine traits can be hidden or unexpressed.
Alleles may be identical (true-breeding plants) or different (F1 hybrids)
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 gene

16. Mendel’s Conclusions
3. If two alleles at a locus differ, then one (dominant allele) determines the organism’s appearance, and the other (recessive allele) has no noticeable effect on appearance
When Mendel crossed contrasting, true-breeding white and purple flowered pea plants, all of the F1 hybrids were purple
When Mendel crossed the F1 hybrids, many of the F2 plants had purple flowers, but some had white
Mendel discovered a ratio of about three to one, purple to white flowers, in the F2 generation

18. Mendel’s Law of Segregation
4. The law of segregation states that the two alleles for a heritable character segregate (separate) during gamete formation and end up in different gametes
Thus, an egg or a sperm gets only one of the two alleles that are present in the somatic cells of an organism
This segregation of alleles corresponds to the distribution of homologous chromosomes to different gametes in meiosis

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

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

22. Law of Independent Assortment
The law of independent assortment states that each pair of alleles segregates independently of each other pair of alleles during gamete formation
This law applies only to genes on different, nonhomologous chromosomes
Genes located near each other on the same chromosome tend to be inherited together

23. Probability Rules Mendel’s laws reflect the rules of probability
When tossing a coin, the outcome of one toss has no impact on the outcome of the next toss
In the same way, the alleles of one gene separate into gametes independently of another gene’s alleles
The states that the probability that two or more independent events will occur together is the product of their individual probabilities
Example: probability of 2 coins landing heads up is 1/4 (1/2 x 1/2 = 1/4)

24. Probability Rules
Each gamete has a chance of carrying the dominant allele and a chance of carrying the recessive allele
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 states that the probability that any one of two or more exclusive events will occur is calculated by adding together their individual probabilities
Example: probability of one heads & one tails is 1/2 (1/4 + 1/4 = 1/2)

25. Probability Rules
These rules can be used to predict the outcome of crosses involving multiple characters
A dihybrid or other multicharacter crosses are equivalent to two or more independent monohybrid crosses occurring simultaneously
In calculating the chances for various genotypes, each character is considered separately, and then the individual probabilities are multiplied together

26. Punnett squares Probability can be depicted through the use of a
A diagram for predicting the results of a genetic cross between individuals of known genetic makeup
Predicts all possible offspring for all possible gametes with random fertilization
Same letter used for trait
Capital letter = dominant allele (A)
Lower case letter = recessive allele (a)

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 exact genotype
Individual expressing dominant phenotype
Could be heterozygous or homozygous dominant
Cross with
Make prediction with Punnett square
If homozygous dominant:
TT x tt
All offspring Tt or tall plants
If heterozygous:
Tt x tt
Offspring - half Tt (tall), half tt (short)

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.