Chapter 14: Mendel

1
Chapter 14 Mendel The Gene - Basic question

• What is the genetic basis of VARIATION among a
  populations individuals?or

2

• What principles account for the transmission of
  these variations from parents to offspring?

3
Chapter 14 Mendel The Gene-Basic question

• Possible AnswersBlending Hypothesis - genetic
  material is blended (e.g. blue yellow make
  green).

4
Blending Theory


5

• Problems with this hypothesis- colors would
  be inseparable- traits skipping generations ?

6
Chapter 14 Mendel The Gene -Basic question

• Alternate Hypothesis Particulate Model of
  Inheritance - Parents pass on discrete heritable
  units (genes) that retain their separate
  identities in offspring.

7
Mendel The GeneBasic question

• More like a bucket of marbles, rather than a
  bucket of paint.

8
Johann (Gregor) Mendel Mid 1800s Old
questions, fresh approach
9
Mendel The Gene

• Gregor Mendel - grew up in the Austrian region
  (Czech Rep.) in an agricultural setting.

10

• In a monastery he received theological training
  but failed the teachers exam...failure sometimes
  guides us in a new, better direction.

11
Gregor Mendel Factoids

• Went to Vienna where he studied with a 2
  teachersPhysicist (Doppler) science thru
  experiments math Botanist plant variation
• Science in the pea garden.

12
Gregor Mendel Factoids

• Mendel took a fresh approach to study very old
  questions about heredity.

13

• Abbey pea garden had numerous varietiescharact
  ers trait purple or white

C o l o r
14
A Genetic Cross
15
Mendels Pea Characters Occurred in Two
Alternative Forms

• Flower Color (purple or white)
• Flower Position (axial / terminal)
• Stem Length (tall / dwarf)
• Pod Shape (inflated / constricted

16
Pea Characteristics

• Pod Color (green or yellow)
• Seed Color (yellow or green)
• Seed Shape (round or wrinkled)

17
Results Mendels F1 CrossesUsing 7 Characters
in Peas
RATIO
SEED COLOR
3.011
X
GREEN
YELLOW
SEED SHAPE
X
2.961
ROUND
WRINKLED
18
Mendels Pea System

• Mendel could always be sure of the parentage of
  new seeds.

19

• He used characters that were either-or rather
  than more-or-less, which allowed him to
  discover PARTICULATE INHERITANCE

20
Mendels Pea System

• Mendel started all experiments with varieties
  that were true-breeding ....self-pollinated
  plants where all offspring are the same variety
  (e.g. all PURPLE flowers or all WHITE flowers).

21
True-Breeding Plant Varieties

• True-breeding parental plants of such a cross
  are called the P generation ( parental).
• Hybrid offspring of the P generation are called
  the F1 generation (first filial).

22

• F1 generations that self-pollinate produce the
  next generation F2 generationsecond filial

23
Mendels Pea System

• Mating or cross-breeding of two varieties is
  called HYBRIDIZATION.

24
Test

• MONOHYBRID CROSS tracks the inheritance of a
  single character (flower color), usually for at
  least 3 generations (P, F1, F2).

25
MONOHYBRID CROSS Tracks the inheritance of
a single character for 3 generations
26
Two Principles of Heredity Found Tracking F2
Generation

• Law of Segregation
• Law of Independent Assortment

27
Law of Segregation

• The two alleles for a character (e.g. flower
  color) are packaged into separate gametes.
• How do we know this?
• Mendel did an experiment.

28
Law of Segregation Mendel Crossed True-breeding
Plants with Different Traits

• Crossed purple with white flowers.Produced F1
  progeny (offspring) with purple flowers only.

29

• Hypothesis If inheritable factor for white
  flowers had been lost, then a cross between F1
  plants should produce only purple-flowered plants.

30
Law of Segregation Mendel Crossed True-breeding
Plants with Different Traits

• Experiment allowed F1 plants to self-pollinate.
• Results there were 705 purple- 224
  white-flowered plants in the F2 generation - a
  ratio of 31.

31

• The inheritable factor for white flowers was not
  lost, so the hypothesis was rejected.

32
Results of Mendels F1 CrossesUsing Seven
Characters in Peas
RATIO
X
Flower Color
3.151
PURPLE
WHITE
Flower Position
X
3.141
AXIAL
TERMINAL
33
Law of SegregationConclusion

• Since the inheritable factor for white flowers
  was not lost in the F2 generation, it must have
  been masked by the presence of purple-flower
  factor.

34

• Mendels factors are now called GENES.

35
Law of SegregationConclusion

• Purple is the DOMINANT trait White is the
  RECESSIVE trait.

36
Mendels Other Experiments

• Repeated these experiments with the six other
  characters and found a similar 31 ratio in the
  F2 generations (Fig. 13.1).

37
Law of Segregation 4 Parts

• From these results he developed a hypothesis that
  is subdivided into four parts

38
Hypothesis Four related areas

• Alternative versions of genes (different
  alleles) account for variations in inherited
  characters

39
Hypothesis Four related areas

• For each character, an organism inherits two
  alleles, one from each parent. Each parent
  contributes one factor or gene, as we now know.

40
Hypothesis Four related areas

• Each genetic locus is represented twice in
  diploid organisms, which have homologous pairs of
  chromosomes, one set for each parent.

41

• Homologous loci may have the same allele as in
  Mendels true-breeding plants, or they may differ
  as in the F1 hybrids.

42
Hypothesis Four related areas

• If two alleles differ, then the one, the
  DOMINANT ALLELE, is fully expressed in the
  organisms appearancethe other, RECESSIVE
  ALLELE, has no noticeable effect on appearance.

43
Hypothesis Four related areas

• Dominant alleles are designated by a capital
  letter P PURPLE flower color

44

• Recessive alleles are designated by a lowercase
  letter p white flower color

45
Hypothesis Four related areas

• Two alleles for each character segregate during
  gamete production.

46

• Without knowing about meiosis, Mendel deduced
  that a sperm cell or ovum carries only one allele
  for each inherited characteristic........

47
Hypothesis Four related areas

• ......because allele pairs separate (segregate)
  from each other during gamete production
  (meiosis).

48

• Gametes of true-breeding plants all carry the
  same allele.

49

• If different alleles present in the parent, -
  50 chance gamete will receive the dominant
  allele - 50 chance it will get the recessive
  allele.

50

• Sorting of alleles into separate gametes is
  known asMendels Law of Segregation

51
Mendels Law of Segregation

• Allele pairs segregate during gamete formation,
  and the paired condition is restored by the
  random fusion of gametes at fertilization.

52
Mendels Law of Segregation

• This law predicits 31 ratio observed in the F2
  generation of a monohybrid cross.

53

• F1 hybrids (Pp) produce two classes of gametes
  when allele pairs segregate during gamete
  formation.....

54

• .....half receive a purple-flower allele (P) and
  the other half the white-flower allele (p).

55

• During self-pollination, these two classes of
  gametes unite randomly.

56

• Ova containing PURPLE-flower alleles have equal
  chances of being fertilized by sperm carrying
  PURPLE -flower alleles or sperm carrying
  white-flower alleles.

57

• Since the same is true for ova containing
  white-flower alleles, there are four equally
  likely combinations of sperm and ova.

58

• Prediction of outcome using a Punnett Square.

59
MENDELS LAW OF SEGREGATION
Mendels Law of Segregation
P Generation
PP
pp
F1 Generation
Pp
P
p
F2 Generation
3
1
Appearance
60
Useful Vocabulary

• HOMOZYGOUS having two identical alleles for a
  given trait (e.g. PP or pp).- all gametes carry
  that allele- homozygotes are true - breeding.

61

• HETEROZYGOUS having two different alleles for a
  trait (e.g. Pp)- half of the gametes carry one
  allele (P) and the remaining half carry the other
  (p).

62

• - heterozygotes are not true-breeding.

63
Genetic Vocabulary

• GENOTYPE
• PHENOTYPE

64
GENOTYPE vs. PHENOTYPE
True- Breed
True- Breed
PP Homozygous
PURPLE
Pp Heterozygous
PURPLE
Pp Heterozygous
PURPLE
pp Homozygous
True- Breed
WHITE
RATIO 121
RATIO 31
65
The Testcross

• Why do a testcross? To determine whether an
  organism with a dominant phenotype (e.g. purple
  flower color) is homozygous dominant or
  heterozygous.

66
Testcross

• breed an organism of unknown genotype with a
  homozygous recessive (see Figure 14.6)

67

• - if the result was all purple flowers then
  genotype was homozygous PP - if 11
  purplewhite, then heterozygous Pp.

68
Law of Independent Assortment

• Each pair of alleles segregate into gametes
  independently.

69

• Experiments using monohybrid crosses, parental
  varieties differing in a single trait, followed
  by dihybrid cross (two characters differ).

70

• Dihybrid Cross mating between parents that are
  heterozygous for two characters (dihybrids).