1. Chapter 10

2. 10.1 Mendel’s Laws of Heredity

3. Why Mendel Succeeded
Heredity = the passing on of characteristic from parents to offspring
Genetics = the branch of biology that studies heredity
Characteristics that are inherited are called traits.
Gregor Mendel was the first person to succeed in predicting how traits would be transferred from one generation to the next.

4. Mendel chose his subject carefully
Mendel studied many plants before deciding to use the garden pea in his experiments
Garden pea plants reproduce sexually
Have two distinct sex cells – male and female
Sex cells are called gametes
Both gametes are in the same flower
Male = pollen grain
Female = ovule (located in the pistol)

5. Fertilization = the uniting of male and female gametes
Pollination = the transfer of the male pollen grains to the pistil of a flower
Fertilization = the uniting of male and female gametes
After the ovule is fertilized, it matures into a seed
Peas normally reproduce from self pollination
Cross-pollination occurs when one plant fertilizes another plant

6. Mendel was a careful researcher
Carefully controlled his experiments
Only studied one trait at a time

7. Mendel’s Monohybrid Crosses
A hybrid is the offspring of parents that have different forms of a trait
Mendel’s crosses used two parent plants that different by a single trait – height.

8. The first generation Tall pea plant crossed with short pea plant
Resulting generation = all tall pea plants

9. The second generation
Allowed the resulting first generation plants to self pollinate
Resulting generation =
Three fourths of the plants were as tall as the tall plants in the parents and first generations
One fourth of the offspring were as short as the short plants in the parent generation

10. Generation designations
Original parents = P1 generation
Offspring of parents = F1 generation
Offspring of F1 plants = F2 generation

11. The rule of unit factors
Mendel studied a total of seven pairs of traits:
Seed shape
Seed color
Flower color
Flower position
Pod color
Pod shape
Plant height

12. Mendel concluded that each organism has two factors that control each of its traits
The factors are genes located on chromosomes
Genes exist in alternative forms: alleles
Two alleles exist for each trait and are located on different copies of a chromosome – one inherited from each parent

13. The rule of dominance
Mendel called the observed trait dominant and the trait that disappeared in the F1 generation recessive.
In Mendel’s pea plants the allele for tall plants is dominant to the allele for short plants
Plants that had one allele for tallness and one allele for shortness ­→ tall
Plants that had two alleles for tallness ­→ tall
Plants that had two alleles for shortness ­→ short

14. When recording the results of crosses, it is customary to use the same letter for different alleles of the same gene
Uppercase letter for dominant (ex: T)
Lowercase letter for recessive (ex: t)

15. The law of segregation
Mendel concluded that each tall plant in the F1 generation carried one dominant allele for tallness and one unexpressed recessive allele for shortness
Because each F1 plant has two different alleles, it can produce two different types of gametes
Law of segregation = because each plant has two different alleles, it can produce two different types of gametes. During fertilization, male and female gametes randomly pair to produce four combinations of alleles.

16. Phenotypes and Genotypes
Mendel showed that all tall plants are not the same
The way an organism looks and behaves is called its phenotype
The phenotype of a tall plant is tall, whether it is TT or Tt
The gene combination an organism contains is known as its genotype
You can’t always know an organism’s genotype simply by looking at its phenotype

17. An organism is homozygous for a trait if its two alleles for the trait are the same.
Ex: TT or tt
Recessive traits will always be homozygous
An organism is heterozygous for a trait if its two alleles for the trait differ from each other.
Ex: Tt

18. Genotype or Phenotype? LL Blonde hair Dimpled chin Blue eyes Dd Ss
White and green leaves RR
Purple flower Tt

19. G = green pea pod, g = yellow pea pod
What are all the possible genotypes?
What is the phenotype for each of the genotypes?
Label the genotypes as either homozygous or heterozygous.

20. Mendel’s Dihybrid Crosses
Used peas that differed from each other in two traits
The first generation
Mendel took true-breeding peas that had round yellow seeds (RRYY) and crossed them with true-breeding peas that had wrinkled green seeds (rryy)
F1 generation had the two dominant traits = round and yellow seeds

21. The second generation Mendel let the F1 plants pollinate themselves
F2 generation =
Round yellow: 9
Round green: 3
Wrinkled yellow:3
Wrinkled green:1

22. The law of independent assortment
Law of independent assortment = genes for different traits are inherited independently of each other
Ex: a round seed does not have to be yellow
When a pea plant with the genotype RrYy produces gametes, the alleles R and r will separate from each other as well as from the alleles Y and y.
These alleles can recombine in four different ways

23. Punnett Squares
Reginald Punnett devised a short-hand way of finding the expected proportions of possible genotypes in the offspring of a cross

24. Monohybrid crosses Two F1 tall pea plants (Tt) are crossed
A Punnett square for this cross is two boxes tall and two boxes wide because each parent can produce two kinds of gametes for this trait
The two kinds of gametes from one parent are listed on top of the square, and the two kinds of gametes from the other parent are listed on the left side. T t

25. Each box contains two alleles → one possible genotype
To determine the possible genotypes of the offspring fill in the boxes with the gametes above and to the left side of the box
Each box contains two alleles → one possible genotype
Used to determine the phenotype
Three fourths will be tall
One third homozygous
Two thirds heterozygous
One fourth will be short T t TT Tt tt

26. Dihybrid crosses
When two traits are compared four boxes are needed on each side for a total of 16 boxes.
Ex: F1 parents (RrYy)

27. RY Ry rY ry
RRYY
RRYy
RrYY
RrYy
RRyy
Rryy
rrYY
rrYy
rryy

28. Probability
Punnett squares are good for showing all the possible combinations of gametes and the likelihood that each will occur but in reality we do not get the exact ratio of results in the square.
Genetics follows the rules of chance
Probability or chance that an event will occur can be determined by dividing the number of desired outcomes by the total number of possible outcomes

29. 10.2 Meiosis

30. Genes, Chromosomes, and Numbers
Genes are lined up on chromosomes
Typically, a chromosome can contain a thousand or more genes

31. Diploid and haploid cells
In the body cells of animals and most plants, chromosomes occur in pairs
One chromosome from each parent
A cell with two of each kind of chromosome is called a diploid cell
Contains a diploid, or 2n, number of chromosomes
One allele is located on each of the paired chromosomes

32. A cell with one of each kind of chromosome is called a haploid cell
Contains a haploid, or n, number of chromosomes
Each species has a characteristic number of chromosomes that it contains

33. Homologous chromosomes
Paired chromosomes with genes for the same traits arranged in the same order are called homologous chromosomes
There are different possible alleles for the same gene
The two chromosomes in a homologous pair are not always identical to each other

34. Why meiosis?
This kind of cell division produces gametes containing half the number of chromosomes as a parent’s body
Occurs in the specialized body cells of each parent that produces gametes

35. Consists of two separate divisions, known as meiosis I and meiosis II
Meiosis I begins with one diploid (2n) cell
By the end of meiosis II, there are four haploid (n) cells
Called sex cells: gametes
Male gametes are called sperm
Female gametes are called eggs
When a sperm fertilizes an egg, the resulting cell, called a zygote, once again has the diploid number of chromosomes
The pattern of reproduction that involves the production and subsequent fusion of haploid sex cells is called sexual reproduction.

36. The Phases of Meiosis Interphase
During interphase that precedes meiosis I, the cell replicates its chromosomes
After replication, each chromosome consists of two identical sister chromatids, held together by a centromere

37. Prophase I Chromosomes coil up Spindle forms
Each pair of homologous chromosomes comes together, matched gene by gene, to form a four-part structure called a tetrad
Consists of two homologous chromosomes (four sister chromatids)
Chromatids in a tetrad pair so tightly that nonsister chromatids from homologous chromosomes sometimes actually exchange genetic material in a process known as crossing over.

38. Can occur at any location on a chromosome
Can occur at several locations at the same time
Results in new combinations of alleles on chromosomes

39. Metaphase I The centromere becomes attached to a spindle fiber
Spindle fibers pull the tetrads into the middle
Homologous chromosomes are lined up side by side as tetrads

40. Anaphase I
Begins as homologous chromosomes separate and move to opposite the ends of the cell
Centromeres do not split
Ensures that each new cell will receive only one chromosome from each homologous pair

41. Telophase I
Events occur in the reverse order from the events of prophase I
Spindle is broken down
Chromosomes uncoil
Cytoplasm divides to yield two new cells
Each cell has half the genetic information of the original
Each chromosome is still doubled, consisting of two sister chromatids

42. The phases of meiosis II
Prophase II
Spindle forms in each of the two new cells
Fibers attach to chromosomes
Metaphase II
Chromosomes pulled to the middle
Anaphase II
Centromere splits
Sister chromatids separate and move to opposite poles

43. Same process as mitosis
Telophase II
Nuclei re-form
Spindles break down
Cytoplasm divides
Same process as mitosis
Four haploid sex cells have been formed from the one original diploid cell
Each haploid cell contains one chromosome from each homologous pair
Will become gametes

44. Meiosis Provides for Genetic Variation
Genetic recombination
By shuffling the chromosomes, genetic variation is produced
The gene combinations in the gametes vary depending on how each pair of homologous chromosomes lines up during metaphase I, a random process
In humans, n = 23, so the number of different kinds of eggs or sperm a person can produce is more than 8 million (223)
An almost endless number of different possible chromosomes can be produced by crossing over
Reassortment of chromosomes and the genetic information they carry, either by crossing over or independent segregation of homologous chromosomes, is called genetic recombination.

45. Meiosis explains Mendel’s results
The segregation of chromosomes in anaphase I explains Mendel’s observation that each parents gives one allele for each trait at random to each offspring

46. Mistakes in Meiosis Trisomy, monosomy, and triploidy
The failure of homologous chromosomes to separate properly during meiosis is called nondisjunction.
The effects of nondisjunction are seen after gametes fuse

47. In one form of nondisjunction two kinds of gametes result
One has an extra chromosome
organisms with extra chromosomes often survive
One is missing a chromosome
organisms lacking one or more chromosomes usually do not survive
When a zygote lacks a chromosome this condition is called monosomy

48. Another form of nondisjunction involves the total lack of separation of homologous chromosomes
A gamete inherits a complete diploid set of chromosomes, and when fertilized with a normal haploid gamete the offspring has three sets of chromosomes and is triploid.
Organisms with more than the usual number of chromosome sets are called polyploids
Rare in animals and almost always results in the death of the zygote
Frequently occurs in plants
Fruits and flowers are larger and plants are healthier