1. Section 1 Mendel and His Peas
Objectives
Content
Explain the relationship between traits and heredity.
Explain the difference between dominant and recessive traits.
Language
Describe the experiments of Gregor Mendel.

2. Section 1 Mendel and His Peas
Who Was Gregor Mendel?
Gregor Mendel was born in 1822 in Heinzendorf, Austria.
At age 21, Mendel entered a monastery. He performed many scientific experiments in the monastery garden.
Mendel discovered the principles of heredity, the passing of traits from parents to offspring.

3. Unraveling the Mystery
Section 1 Mendel and His Peas
Unraveling the Mystery
Mendel used garden pea plants for
his experiments.
Self-Pollinating Peas have both male and female reproductive structures. So, pollen from one flower can fertilize the ovule of the same flower.
When a true-breeding plant self pollinates, all of the offspring will have the same trait as the parent.

4. The image below shows cross-pollination and self-pollination.
Section 1 Mendel and His Peas
Pea plants can also cross-pollinate. Pollen from one plant fertilizes the ovule of a flower on a different plant.
The image below shows cross-pollination and self-pollination.

5. Section 1 Mendel and His Peas
Characteristics Mendel studied only one pea characteristic at a time. A characteristic is a feature that has different forms in a population.
Different forms
of a characteristic
are called traits.

6. Section 1 Mendel and His Peas
Mix and Match Mendel was careful to use plants that were true breeding for each of the traits he was studying. By doing so, he would know what to expect if his plants were to self-pollinate.

7. Mendel’s First Experiments
Section 1 Mendel and His Peas
Mendel’s First Experiments
Mendel crossed pea plants to study seven different characteristics.
Mendel got similar results for each cross. One trait was always present in the first generation, and the other trait seemed to disappear.
Mendel called the trait that appeared the dominant trait. The trait that seemed to fade into the background was called the recessive trait.

8. Mendel’s Second Experiments
Section 1 Mendel and His Peas
Mendel’s Second
Experiments
To find out more about recessive traits, Mendel allowed the first-generation plants to self-pollinate.
In each case some of the second-generation plants had the recessive trait.

9. Section 1 Mendel and His Peas
Ratios in Mendel’s Experiments The recessive trait did not show up as often as the dominant trait.
Mendel decided to figure out the ratio of dominant traits to recessive traits.

10. In all cases the ratio was about 3:1 dominant : recessive.
Section 1 Mendel and His Peas
In all cases the ratio was about 3:1 dominant : recessive.

11. Mendel’s Second Experiments
Section 1 Mendel and His Peas
Mendel’s Second Experiments
Gregor Mendel – Gone But Not Forgotten Mendel realized that his results could be explained only if each plant had two sets of instructions for each characteristic.
Mendel’s work opened the door to modern genetics.

12. Section 2 Traits and Inheritance
Bellringer
If you flip a coin, what are the chances that it will land on heads? tails? Suppose that you flipped the coin and got heads. What are the chances that you will get heads again?
Record your answers in your science journal.

13. Section 2 Traits and Inheritance
Objectives
Explain how genes and alleles are related to genotype and phenotype.
Use the information in a Punnett square.
Explain how probability can be used to predict possible genotypes in offspring.
Describe three exceptions to Mendel’s observations.

14. Section 2 Traits and Inheritance
A Great Idea
Mendel knew that there must be two sets of instructions for each characteristic.
The instructions for an inherited trait are called genes.
The different forms (often dominant and recessive) of a gene are alleles.
Phenotype An organism’s appearance is known as its phenotype. Genes affect the phenotype.

15. Section 2 Traits and Inheritance
A Great Idea, continued
Genotype The combination of inherited alleles together form an organism’s genotype.
Punnett Squares are used to organize all the possible genotype combinations of offspring from particular parents.

16. Chapter 4
Section 2 Traits and Inheritance

17. Section 2 Traits and Inheritance
What Are the Chances?
Probability is the mathematical chance that something will happen.
Probability is most often written as a fraction of percentage.

18. Calculating Probability
Section 2 Traits and Inheritance
Calculating Probability

19. What Are the Chances?, continued
Section 2 Traits and Inheritance
What Are the Chances?, continued
Genotype Probability To have white flowers, a pea plant must receive a p allele from each parent. Each offspring of a Pp Pp cross has a 50% chance of receiving either allele from either parent. So, the probability of inheriting two p alleles is 1/2  1/2, which equals 1/4, or 25%.

20. Section 2 Traits and Inheritance
More About Traits
Incomplete Dominance Researchers have found that sometimes one trait is not completely dominant over another.
One Gene, Many Traits Sometimes one gene influences more than one trait.
Many Genes, One Trait Some traits, such as the color of your skin, hair, and eyes, are the result of several genes acting together.

21. More About Traits, continued
Section 2 Traits and Inheritance
More About Traits, continued
The Importance of Environment Genes aren’t the only influences on traits. A combination of things determine an individual’s characteristics.
Your environment also influences how you grow.
Lifestyle choices can also affect a person’s traits.

22. Section 3 Meiosis
Bellringer
Write a sentence to describe each of the following terms: heredity, genotype, and phenotype. Note how genotype and phenotype are related, and how they are different. Is heredity necessarily a factor in both genotype and phenotype? Why or why not?
Record your answers in your science journal.

23. Objectives Explain the difference between mitosis and meiosis.
Section 3 Meiosis
Objectives
Explain the difference between mitosis and meiosis.
Describe how chromosomes determine sex.
Explain why sex-linked disorders occur in one sex more often than in the other.
Interpret a pedigree.

24. Section 3 Meiosis
Asexual Reproduction
In asexual reproduction, only one parent cell is needed. The structures inside the cell are copied, and then the parent cell divides, making two exact copies.
This type of cell reproduction is called mitosis. Most of the cells in your body and most single-celled organisms reproduce this way.

25. Section 3 Meiosis
Mitosis

26. Section 3 Meiosis
Sexual Reproduction
In sexual reproduction, two parent cells (sex cells) join together to form offspring that are different from both parents.
Chromosomes that carry the same sets of genes are called homologous chromosomes.
Each sex cell has only one of the chromosomes from the homologous pair.

27. Sexual Reproduction, continued
Section 3 Meiosis
Sexual Reproduction, continued
Meiosis Sex cells are made during meiosis.
Meiosis is a copying process that produces cells with half the usual number of chromosomes.

28. Meiosis Section 3 Meiosis Click below to watch the Visual Concept.

29. Genes are located on chromosomes.
Section 3 Meiosis
Sexual Reproduction, continued
Genes and Chromosomes Walter Sutton studied meiosis in sperm cells in grasshoppers.
Using his observations and his knowledge of Mendel’s work, Sutton proposed that:
Genes are located on chromosomes.

30. Section 3 Meiosis
The Steps of Meiosis
During meiosis, chromosomes are copied once, and then the nucleus divides twice.
The resulting sex cells (sperm and eggs) have half the number of chromosomes of a normal body cell.

31. Chapter 4
Section 3 Meiosis

32. Chapter 4
Section 3 Meiosis

33. Section 3 Meiosis
Meiosis and Mendel
The steps of meiosis explain Mendel’s results. The following slide shows what happens to a pair of homologous chromosomes during meiosis and fertilization.

34. Chapter 4
Section 3 Meiosis

35. Meiosis and Mendel, continued
Section 3 Meiosis
Meiosis and Mendel, continued
Sex Chromosomes carry genes that determine sex.
Human females have two X chromosomes.
Human males have one X chromosome and one Y chromosome.

36. Meiosis and Mendel, continued
Section 3 Meiosis
Meiosis and Mendel, continued
Sex-Linked Disorders The genes for certain disorders, such as colorblindness, are carried on the X chromosome.
Genetic Counseling Genetic counselors use pedigrees to trace traits through generations of a family. These diagrams can often predict if a person is a carrier of a hereditary disease.
Selective Breeding In selective breeding, organisms with desirable characteristics are mated.

37. Heredity
Concept Mapping
Use the terms below to complete the concept map on the next slide.
alleles offspring
parents genotype
phenotype characteristics
genes dominant

38. Chapter 4
Heredity

39. Chapter 4
Heredity