Mendelelian Genetics

1
Mendelelian Genetics
2
Gregor Mendel(1822-1884)
Responsible for the Laws governing Inheritance of
Traits
3
Gregor Johann Mendel

• Austrian monk
• Studied the inheritance of traits in pea plants
• Developed the laws of inheritance
• Mendel's work was not recognized until the turn
  of the 20th century

4
Gregor Johann Mendel

• Between 1856 and 1863, Mendel cultivated and
  tested some 28,000 pea plants
• He found that the plants' offspring retained
  traits of the parents
• Called the Father of Genetics"

5
Site of Gregor Mendels experimental garden in
the Czech Republic
6
Particulate Inheritance

• Mendel stated that physical traits are inherited
  as particles
• Mendel did not know that the particles were
  actually Chromosomes DNA

7
Genetic Terminology

• Trait - any characteristic that can be passed
  from parent to offspring
• Heredity - passing of traits from parent to
  offspring
• Genetics - study of heredity

8
Types of Genetic Crosses

• Monohybrid cross - cross involving a single
  traite.g. flower color
• Dihybrid cross - cross involving two traits e.g.
  flower color plant height

9
Punnett Square

• Used to help solve genetics problems

10
(No Transcript)
11
Designer Genes

• Alleles - two forms of a gene (dominant
  recessive)
• Dominant - stronger of two genes expressed in the
  hybrid represented by a capital letter (R)
• Recessive - gene that shows up less often in a
  cross represented by a lowercase letter (r)

12
More Terminology

• Genotype - gene combination for a trait (e.g. RR,
  Rr, rr)
• Phenotype - the physical feature resulting from a
  genotype (e.g. red, white)

13
Genotype Phenotype in Flowers
Genotype of allelesR red flowerr yellow
flower All genes occur in pairs, so 2 alleles
affect a characteristic Possible combinations
are
Genotypes RR Rr rr Phenotypes RED RED
YELLOW
14
Genotypes

• Homozygous genotype - gene combination involving
  2 dominant or 2 recessive genes (e.g. RR or rr)
  also called pure 
• Heterozygous genotype - gene combination of one
  dominant one recessive allele    (e.g. Rr)
  also called hybrid

15
Genes and Environment Determine Characteristics
16
Mendels Pea Plant Experiments
17
Why peas, Pisum sativum?

• Can be grown in a small area
• Produce lots of offspring
• Produce pure plants when allowed to
  self-pollinate several generations
• Can be artificially cross-pollinated

18
Reproduction in Flowering Plants

• Pollen contains sperm
• Produced by the stamen
• Ovary contains eggs
• Found inside the flower

Pollen carries sperm to the eggs for
fertilization Self-fertilization can occur in the
same flower Cross-fertilization can occur between
flowers
19
Mendels Experimental Methods

• Mendel hand-pollinated flowers using a paintbrush
• He could snip the stamens to prevent
  self-pollination
• Covered each flower with a cloth bag
• He traced traits through the several generations

20
How Mendel Began
Mendel produced pure strains by allowing the
plants to self-pollinate for several generations
21
Eight Pea Plant Traits

• Seed shape --- Round (R) or Wrinkled (r)
• Seed Color ---- Yellow (Y) or  Green (y)
• Pod Shape --- Smooth (S) or wrinkled (s)
• Pod Color ---  Green (G) or Yellow (g)
• Seed Coat Color ---Gray (G) or White (g)
• Flower position---Axial (A) or Terminal (a)
• Plant Height --- Tall (T) or Short (t)
• Flower color --- Purple (P) or white (p)

22
(No Transcript)
23
(No Transcript)
24
Mendels Experimental Results
25

• Did the observed ratio match the theoretical
  ratio?

The theoretical or expected ratio of plants
producing round or wrinkled seeds is 3 round 1
wrinkled Mendels observed ratio was 2.961 The
discrepancy is due to statistical error The
larger the sample the more nearly the results
approximate to the theoretical ratio
26
Generation Gap

• Parental P1 Generation the parental generation
  in a breeding experiment.
• F1 generation the first-generation offspring in
  a breeding experiment. (1st filial generation)
• From breeding individuals from the P1 generation
• F2 generation the second-generation offspring
  in a breeding experiment. (2nd filial
  generation)
• From breeding individuals from the F1 generation

27
Following the Generations
Cross 2 Pure PlantsTT x tt
Results in all HybridsTt
Cross 2 Hybridsget3 Tall 1 ShortTT, Tt, tt
28
Monohybrid Crosses
29
P1 Monohybrid Cross

• Trait Seed Shape
• Alleles R Round r Wrinkled
• Cross Round seeds x Wrinkled seeds
• RR x rr

Genotype Rr Phenotype Round GenotypicRatio
100 Rr PhenotypicRatio 100 round
r
r
Rr
Rr
R
R
Rr
Rr
30
P1 Monohybrid Cross Review

• Homozygous dominant x Homozygous recessive
• Offspring all Heterozygous (hybrids)
• Offspring called F1 generation
• Genotypic Phenotypic ratio is ALL ALIKE

31
F1 Monohybrid Cross

• Trait Seed Shape
• Alleles R Round r Wrinkled
• Cross Round seeds x Round seeds
• Rr x Rr

G.Ratio 121 RRRrrr P.Ratio
31 roundwrinkled
r
R
RR
Rr
R
r
rr
Rr
32
F1 Monohybrid Cross Review

• Heterozygous x heterozygous
• Offspring25 Homozygous dominant RR50
  Heterozygous Rr25 Homozygous Recessive rr
• Offspring called F2 generation
• Genotypic ratio is 121
• Phenotypic Ratio is 31

33
What Do the Peas Look Like?
34
And Now the Test Cross

• In order to find the genotype of a
  dominant-showing trait, the organism must be
  crossed with a homozygous recessive organism.
• There are two possible testcrossesHomozygous
  dominant x homozygous recessiveHeterozygous x
  Homozygous recessive

35
Test Cross (1st)

• Trait Seed Shape
• Alleles R Round r Wrinkled
• Cross Round seeds x wrinkled seeds
• RR x rr

Genotype RR, Rr Phenotype Round GenotypicRatio
100 Rr PhenotypicRatio 100 round
r
r
Rr
Rr
R
R
Rr
Rr
36
F2 Test Cross (2nd)

• Trait Seed Shape
• Alleles R Round r Wrinkled
• Cross Wrinkled seeds x Round seeds
• rr x Rr

r
R
Genotype Rr, rr Phenotype Round Wrinkled G.
Ratio 11 P.Ratio 11
Rr
rr
r
r
rr
Rr
37
F2 Monohybrid Cross Review

• Homozygous x heterozygous(hybrid)
• Offspring50 Homozygous RR or rr50
  Heterozygous Rr
• Phenotypic Ratio is 11
• Called Test Cross because the offspring have SAME
  genotype as parents

38
Mendels Laws
39
Results of Monohybrid Crosses

• Inheritable factors or genes are responsible for
  all heritable characteristics
• Phenotype is based on Genotype
• Each trait is based on two genes, one from the
  mother and the other from the father
• True-breeding individuals are homozygous ( both
  alleles) are the same

40
Law of Dominance
One allele overshadows the other allele for a
trait. The allele that is overshadowed is called
the recessive allele. The one that does the
overshadowing is the dominant allele.

41
Law of Dominance
42
Law of Segregation

• During the formation of gametes (eggs or sperm),
  the two alleles responsible for a trait separate
  from each other.
• Alleles for a trait are then "recombined" at
  fertilization, producing the genotype for the
  traits of the offspring.

43
Applying the Law of Segregation
44
Law of Independent Assortment

• Alleles for different traits are distributed to
  sex cells ( offspring) independently of one
  another.
• This law is illustrated by the meiosis babies
  activity and dihybrid crosses.

45
Dihybrid Cross

• A breeding experiment that tracks the inheritance
  of two traits.
• Mendels Law of Independent Assortment
• a. Each pair of alleles segregates independently
  during gamete formation
• b. Formula 2n (n of heterozygotes)

46
QuestionHow many gametes will be produced for
the following allele arrangements?

• Remember 2n (n of heterozygotes)
• 1. RrYy
• 2. AaBbCCDd
• 3. MmNnOoPPQQRrssTtQq

47
Answer
1. RrYy 2n 22 4 gametes RY Ry rY
ry 2. AaBbCCDd 2n 23 8 gametes ABCD
ABCd AbCD AbCd aBCD aBCd abCD abCd
3. MmNnOoPPQQRrssTtQq 2n 26 64 gametes
48
Dihybrid Cross

• Traits Seed shape Seed color
• Alleles R round r wrinkled Y
  yellow y green

RrYy x RrYy
RY Ry rY ry
RY Ry rY ry
All possible gamete combinations
49
Dihybrid Cross
50
Dihybrid Cross
51
Dihybrid Cross
Round/Yellow 9Round/green
3wrinkled/Yellow 3wrinkled/green 1 9331
52
Test Cross

• A mating between an individual of unknown
  genotype and a homozygous recessive individual.
• Example bbC__ x bbcc
• BB brown eyes
• Bb brown eyes
• bb blue eyes
• CC curly hair
• Cc curly hair
• cc straight hair

53
Test Cross

• Possible results

54
Summary of Mendels laws
55
Incomplete DominanceandCodominance
56
Incomplete Dominance

• F1 hybrids have an appearance somewhat in between
  the phenotypes of the two parental varieties.
• Example snapdragons (flower)
• red (RR) x white (rr)
• RR red flower
• rr white flower

57
Incomplete Dominance
r
58
Incomplete Dominance
59
Codominance

• Two alleles are expressed (multiple alleles) in
  heterozygous individuals.
• Example blood type
• 1. type A IAIA or IAi
• 2. type B IBIB or IBi
• 3. type AB IAIB
• 4. type O ii

60
Codominance Problem

• Example homozygous male Type B (IBIB)
• x heterozygous female Type A (IAi)

61
Another Codominance Problem

• Example male Type O (ii) x
  female type AB (IAIB)

62
Codominance

• QuestionIf a boy has a blood type O and his
  sister has blood type AB, what are the
  genotypes and phenotypes of their parents?
• boy - type O (ii) girl - type AB (IAIB)

63
Codominance

• Answer

Parents genotypes IAi and IBi phenotypes
A and B
64
Codominance

• Sickle-cell Disease
• -common in people of African descent
• -sickled allele causes hemoglobin to change and
  red blood cell forms a crescent shape cant
  carry oxygen well

65
Codominance

• Genotypes
• BN BN normal blood cells
• BNBS normal blood cells and sickled cells
• BSBS sickled blood cells

66
Sex-linked Traits

• Traits (genes) located on the sex chromosomes
• Sex chromosomes are X and Y
• XX genotype for females
• XY genotype for males
• Many sex-linked traits carried on X chromosome

67
Sex-linked Traits
Example Eye color in fruit flies
Sex Chromosomes
68
Sex-linked Trait Problem

• Example Eye color in fruit flies
• (red-eyed male) x (white-eyed female) XRY
  x XrXr
• Remember the Y chromosome in males does not
  carry traits.
• RR red eyed
• Rr red eyed
• rr white eyed
• XY male
• XX female

69
Sex-linked Trait Solution
50 red eyed female 50 white eyed male
70
Female Carriers
71
Question Is it possible for a female carrier of
hemophilia to have a daughter with hemophilia?
XH Xh
Xh
XH Xh
XhXh
Y
XH Y
Xh Y
72
Epistasis
When one gene hides the effects of another
gene 2 genes determine coat color E gene
can mask B gene
73
Epistasis
Ee or EE Can have dark pigment ee cant have
dark pigment B darker Dark pigment b
lighter Dark pigment
74
Dosage Compensation
Females have 2 doses of X Males only have 1
dose of X To balance this difference one of
the X chromosomes is inactivated or turned off
X chromosome Genes necessary For male/female Y
chromosome Genes necessary For male only
75
Dosage Compensation

• Chromosome inactivation One X in each cell is
  randomly inactivated.
• Orange cells where X chrom. with black allele
  is inactivated
• Black cells where X chrom. With orange allele
  is inactivated

76

• Barr bodies
• Inactivated X
• chromosome forms
• a condensed dark spot
• in nucleus

77
Polygenic TraitsA trait that is controlled by
more than one gene.
78
Polygenic Traits

• So, instead of only 2 alleles contributing to a
  trait, there can be 4 or 6 or 8, etc. alleles
  contributing to a trait.
• Skin color the more dominant alleles for each
  gene, the darker the skin

AABbCc darker skin aaBbCc lighter skin
79
(No Transcript)
80
Gene Linkage

• Mendel studied genes on different chromosomes, so
  they assorted independently many combinations
• Genes located close together on the same
  chromosome are said to be linked they get
  passed down to gametes together dont assort
  independently

81
(No Transcript)
82
Gene Linkage

• A chromosome map shows relative distances between
  2 genes. More distance more likely to cross
  over.

83
Polyploidy

• One or more extra complete sets of chromosomes in
  an organism.
• Triploid 3n Tetraploid 4n
• Rarely in animals, lethal in humans found often
  in plants- wheat(6n), sugar cane (8n)

84
Polyploidy
Diploid

• Increased health and size

Diploid
Polyploid
Polyploid