1. Chapter 9: Introduction to Genetics
Section 1:
The Work of Gregor Mendel

2. The Work of Gregor Mendel
Biological inheritance, or heredity, is the key to differences between species
Heredity is much more than the way in which a few characteristics are passed from one generation to another
Heredity is at the very center of what makes each species unique, as well as what makes us human
The branch of biology that studies heredity is called genetics

3. Early Ideas About Heredity
Until the 19th century, the most common explanation for family resemblances was the theory of blending inheritance
Because both male and female were involved in producing offspring, each parent contributed factors that were “blended” in their offspring
But, in the last century biologists began to look at the details of heredity
They began to develop a very different view
The work of the Austrian monk Gregor Mendel was particularly important in changing people’s views about how characteristics are passed from one generation to the next

4. Gregor Mendel
Born in 1822 to peasant parents in what is now the Czech Republic
Entered a monastery at the age of 21
Four years later he was ordained a priest
In 1851, Mendel was sent to the University of Vienna to study science and mathematics
He returned 2 years later and spent the next 14 years teaching high school
In addition to his duties, Mendel was in charge of the monastery garden
This is where he did his work that revolutionized biological science

5. Gregor Mendel
From his studies, Mendel had gained an understanding of the sexual mechanisms of pea plants
Pea flowers have both male and female parts
Normally, pollen from the male part of the pea flower fertilizes the female egg cells of the very same flower
Self-pollination
Seeds produced by self-pollination inherit all of their characteristics from the single plant that bore them

7. Gregor Mendel Mendel learned that self-pollination could be prevented
He was able to pollinate the two plants by dusting the pollen from one plant onto the flowers of another plant
Cross-pollination
Produces seeds that are the offspring of two different plants
Mendel was able to cross plants with different characteristics

9. Gregor Mendel Mendel started his studies with peas that were purebred
If they were allowed to self-pollinate, the purebred peas would produce offspring that were identical to themselves
These purebred plants were the basis of Mendel’s experiments

10. Gregor Mendel
In many respects, the most important decision Mendel made was to study just a few isolated traits, or characteristics, that could be easily observed
He chose seven different traits to study
By deciding to restrict his observations to just a few traits, Mendel made his job of measuring the effects of heredity much easier

12. Genes and Dominance
Mendel decided to see what would happen if he crossed pea plants with different characters for the same trait
A character is a form of a trait
For example, the plant height trait has two characters: tall and short
Mendel crossed the tall plants with the short ones
From these crosses, Mendel obtained seeds that he then grew into plants
These plants were hybrids, or organisms produced by crossing parents with different characters

13. Genes and Dominance What were those hybrid plants like?
Did the characters of the parent plants blend in the offspring?
To Mendel’s surprise, the plants were not half tall
Instead, all of the offspring had the character of only one of the parents (They were all tall)
The other characteristic had apparently disappeared

14. Genes and Dominance
From this set of experiments, Mendel was able to draw two conclusions
Individual factors, which do not blend with one another, control each trait in a living thing
Merkmal – German for character
Today, the factors that control traits are called genes
Each of the traits Mendel studied was controlled by one gene that occurred in two contrasting forms
The different forms of a gene are now called alleles
2. Principle of dominance
Some alleles are dominant, whereas others are recessive

15. Segregation Mendel did not stop his experimentation at this point
What happened to the recessive characters?
To answer this question, he allowed all seven kinds of hybrid plants to reproduce by self-pollination
P Generation
Purebred parental plants
F1 Generation
First filial generation
F2 Generation
Second filial generation

16. The F1 Cross The results of the F1 cross were remarkable
The recessive characters reappeared in the F2 generation
This proved that the alleles responsible for the recessive characters had not disappeared
Why did the recessive alleles disappear in the F1 generation and reappear in the F2?

17. Explaining the F1 Cross
Mendel assumed that the presence of the dominant tall allele had masked the recessive short allele in the F1 generation
But the fact that the recessive allele was not masked in some of the F2 plants indicated that the short allele had managed to get away from the tall allele
Segregation
During the formation of the reproductive cells, the tall and short alleles in the F1 plants were segregated from each other

18. Explaining the F1 Cross
The possible gene combinations in the offspring that result from a cross can be determined by drawing a diagram known as a Punnett square
Represent a particular allele by using a symbol
Dominant = capital letter
Recessive = lowercase letter
Punnett squares show the type of reproductive cells, or gametes, produced by each parent
Punnett square results are often expressed as ratios

19. Explaining the F1 Cross Phenotype Physical characteristic Genotype
Genetic makeup
Homozygous
Two identical alleles for a trait
Purebred
Heterozygous
Two different alleles for a trait
Hybrid

22. Independent Assortment
After establishing that alleles segregate during the formation of gametes (reproductive cells), Mendel began to explore the question of whether they do so independently
In other words, does the segregation of one pair of alleles affect the segregation of another pair of alleles?
For example, does the gene that determines whether a seed is round or wrinkled in shape have anything to do with the gene for seed color?

23. The Two Factor Cross: F1
In this cross, the two kinds of plants would be symbolized like this:
Round yellow seeds
RRYY
Wrinkled green seeds
rryy

25. The Two Factor Cross: F1
Because two traits are involved in this experiment, it is called a two-factor cross
The plant that bears round yellow seeds produces gametes that contain the alleles R and Y, or RY gametes
The plant that bears wrinkled green seeds produces ry gametes
An RY gamete and an ry gamete combine to form a fertilized egg with the genotype RrYy

26. The Two Factor Cross: F1
Thus, only one kind of plant will show up in the F1 generation – plants that are heterozygous, or hybrid, for both traits
Remember that the concept of dominance tells us that the dominant traits will show up in a hybrid, whereas the recessive traits will seem to disappear

27. The Two Factor Cross: F1
This cross does not indicate whether genes assort, or segregate independently
However, it provides the hybrid plants needed for the next cross – the cross of F1 plants to produce the F2 generation
The seeds from the F2 plants will show whether the genes for seed shape and seed color have anything to do with one another

28. The Two Factor Cross: F2
What will happen when F1 plants are crossed with each other?
If the genes are not connected, then they should segregate independently, or undergo independent assortment
This produces four types of gametes RY, Ry, rY, and ry
Mendel actually carried out this exact experiment
Concluded that genes could segregate independently during the formation of gametes
In other words, genes could undergo independent assortment

29. A Summary of Mendel’s Work
Mendel’s work on the genetics of peas can be summarized in four basic statements:
The factors that control heredity are individual units known as genes. In organisms that reproduce sexually, genes are inherited from each parent.
In cases in which two or more forms of the gene for a single trait exist, some forms of the gene may be dominant and others may be recessive.
The two forms of each gene are segregated during the formation of reproductive cells.
The genes for different traits may assort independently of one another.

30. Chapter 9: Introduction to Genetics
Section 2: Applying Mendel’s Principles

31. Genetics and Probability
Mendel applied the mathematical concept of probability to biology
Probability is the likelihood that a particular event will occur
Probability = the number of time a particular event occurs divided by the number of opportunities for the event to occur

32. Genetics and Probability
Flipping a coin
One of two possible events can occur
Heads up or tails up
The probability of the coin coming up heads is ½, or 1:1
The larger the number of trials, the closer you get to the expected ratios

33. Using the Punnett Square
The Punnett square is a handy device for analyzing the results of an experimental cross

34. One-Factor Cross In pea plants, tall (T) is dominant over short (t)
You have a tall plant
Design a cross to see if this plant is homozygous (TT) or heterozygous (Tt)

35. One-Factor Cross Solution: Cross your tall plant with a short plant
The cross of an organism of unknown genotype and a homozygous individual is called a test cross
As you can see in the Punnett squares, if any of the offspring resulting from a test cross shows the recessive phenotype, then the unknown parent must be heterzygous

36. TT X tt
Tt X tt T t Tt T t Tt tt
T = tall t = short

37. Two-Factor Cross
In pea plants, green pods (G) are dominant over yellow pods (g), and smooth pods (N) are dominant over constricted pods (n)
A plant heterozygous for both traits (GgNn) is crossed with a plant that has yellow constricted pods (ggnn)

38. GN Gn gN gn GgNn Ggnn ggNn ggnn GgNn X ggnn G = green g = yellow
N = smooth n = constricted

39. Chapter 9: Introduction to Genetics
Section 3:
Meiosis

40. Meiosis How are gametes formed?
In Chapter 8 you learned abut mitosis, a process that involves the separation of chromosomes and the formation of new cells
Could gametes be formed by mitosis?
The answer to this question is no
If gametes were formed by mitosis, when sperm and egg fuse during fertilization, the number of chromosomes would double in each generation
Before long the cells would contain a very large number of chromosomes

41. Chromosome Number
The number of chromosomes in a cell is different from organism to organism
However, it is true that each cell has an equal number of chromosomes from each parent
Each chromosome in the male set has a corresponding chromosome in the female set
These corresponding chromosomes are said to be homologous

42. Chromosome Number
A cell that contains both sets of homologous chromosomes (one set from each parent) is said to be diploid
The diploid number is sometimes represented by the symbol 2N
Contains two complete sets of chromosomes and two complete sets of genes
The gametes of organisms that reproduce sexually contain a single set of chromosomes (and genes)
Haploid (N)
In order for gametes to be produced, there must be a process that divides the diploid number of chromosomes in half

43. The Phases of Meiosis
Haploid gametes are produced from diploid cells by the process of meiosis
Meiosis is a process of reduction division in which the number of chromosomes per cell is cut in half and the homologous chromosomes that exist in a diploid cell are separated
Meiosis takes place in two stages
Meiosis I
Meiosis II

44. Interphase Chromosome replicate during S phase
Each replicated chromosome consists of two genetically identical sister chromatids connected at the centromere

45. Prophase I
Typically occupies more that 90% of the time required for meiosis
Chromosomes begin to condense
Homologous chromosomes loosely pair along their lengths, precisely aligned gene by gene
Tetrad consists of 4 chromatids
In crossing over, the DNA molecules in nonsister chromatids break at corresponding places and then rejoin to the other’s DNA

46. Metaphase I
The pairs of homologous chromosomes, in the form of tetrads, are now arranged on the equator of the cell
Homologous chromosomes are attached to spindle fibers

47. Anaphase I
The chromosomes move toward the poles, guided by the spindles
Sister chromatids remain attached at the centromere and move as a single unit toward the same pole
Homologous chromosomes, each composed of two sister chromatids, move toward opposite poles

48. Telophase I and Cytokinesis
At the beginning of telophase I, each half of the cell has a complete haploid set of chromosomes, but each chromosome is still composed of two sister chromatids
Cytokinesis usually occurs simultaneously with telophase I, forming two haploid daughter cells
No chromosome replication occurs between the end of meiosis I and the beginning of meiosis II, as the chromosomes are already replicated

49. Prophase II A spindle forms
In late prophase II, chromosomes, each still composed of two chromatids, start to move toward the middle of the cell

50. Metaphase II
The chromosomes are positioned on the equator as in mitosis
Because of crossing over in meiosis I, the two sister chromatids of each chromosome are NOT genetically identical

51. Anaphase II
The centromeres of each chromosome finally separate, and the sister chromatids come apart
The sister chromatids of each chromosome now move as two individuals chromosomes to opposite poles

52. Telophase II and Cytokinesis
Nuclei form and cytokinesis occurs
The meiotic division of one parent cell produces four daughter cells, each with a haploid set of chromosomes
Each of the four daughter cells is genetically distinct form the other daughter cells and from the parent cell

53. Meiosis and Genetics
Chromosomes pair and separate during meiosis exactly as Mendel would have predicted for the structures that carry genes
Meiosis I results in segregation and independent assortment
The separation of chromosomes during the first meiotic division is completely random
Which cell receives the maternal copy of a chromosome and which cell receives the paternal copy is strictly a matter of chance

54. Gamete Formation
In male animals, the haploid gametes produced by meiosis are called sperm
In female animals, generally only one of the cells produced by meiosis is used for reproduction
egg
In female animals, the cell divisions at the end of meiosis I and meiosis II are uneven, so that the egg receives most of the cytoplasm
The other three cells produced in the female during meiosis are known as polar bodies and usually do not participate in reproduction

55. Comparing Mitosis and Meiosis
Mitosis results in the production of two genetically identical cells
A diploid cell that divides by mitosis gives rise to two diploid daughter cells
The daughter cells have sets of chromosomes (and genes) identical to each other and to the original parent cell

56. Comparing Mitosis and Meiosis
Meiosis begins with a diploid cell but produces four haploid cells
These cells are genetically different from the diploid cell and from one another
This is because homologous chromosomes are separated during the first meiotic division and because crossing-over results in the production of new gene combinations on the chromosomes