Chapter 14 Mendel and The Gene Idea

1
Chapter 14Mendel and The Gene Idea
2
Gregor Mendel and Modern Genetics

• Inherited Traits
• transmitted from parents to offspring
• color of hair, eyes, skin height
• Blending hypothesis
• genetic material mixes like paints
• would result in a uniform population
• questioned by Mendel

3

• Mendels Experiments
• Particulate Hypothesis
• genes - discrete heritable units
• retain their own identity
• bucket of marbles
• Advantages of Pea plants
• Distinct heritable features - characters
• Different variants - traits
• Experimental Control

4

• Pea plant has carpal and stamens (female male)
• Typically self-fertilize
• Mendel used cross-pollination
• True-breeding plants
• Characters were either-or
• not more-or-less
• 3 Generations
• P, F1, F2

5

• P generation, true-breeding
• 
  cross-pollinate
• F1 generation, hybrid
• self-fertilize
• F2 generation

6

• First Experiment
• What color would F1 plants be?
• Purple, white, light purple
• light purple Blending
• ALL PURPLE
• What happened to white?
• If white is lost, then F2
  would
  all be purple

7

• F2 generation purple and white
• white trait reappeared
• 75 purple
• 25 white
• 31 ratio important
• Importance of F2
• white not blended
  or
  diluted
• skipped a generation
• Dominant vs Recessive

8
Table 14.1
9

• Mendels Hypotheses
• 1. Alternative versions of genes - alleles
• some variation in nucleotide sequence
• purple-flower allele and white-flower allele are
  two DNA variations at the flower-color locus
• Character - color
• Traits - purple, white
• Alleles
• Locus

10

• Mendels Hypotheses
• 2. An organism inherits 2 alleles, 1 from each
  parent
• pair of homologous chromosomes
• two copies of each locus
• homologous loci may be identical
• Mendels true-breeding plants
• two alleles may differ
• F1 generation plants

11

• Mendels Hypotheses
• 3. The dominant allele is fully expressed
• recessive allele has no effect
• F2 had purple flowers
• 4. The 2 alleles segregate during gamete
  production
• distribution of homologous chromosomes
• identical alleles - in all gametes
• different alleles - 50 of gametes receive one
  allele and 50 receive the other

12

• Mendels Law of Segregation - separation of
  alleles
• into separate gametes
• accounts for the 31 ratio F2 generation
• F1 hybrids produce 2 types of gametes
• gametes combine randomly during fertilization
• result in 4 possible combinations
• Punnett Square

13

• Punnett square
• predicts results of a genetic
  cross
  between individuals
  
  of a known genotype

14

• Homozygous - organism with two identical alleles
  for a character
• Heterozygous - organism with two different
  alleles for a character
• Phenotype - description of an organisms traits.
• Genotype - description of its genetic makeup
• Phenotype Genotype
• Purple PP, Pp
• White pp

15

• PP and Pp plants
• same phenotype
• different genotypes
• Only pp produces
  white phenotype

16

• Testcross - breeding an individual of unknown
• genotype with a homozygous recessive
• Monohybrid Cross
• inheritance of a single
  
  character
• Dihybrid Cross
• inheritance of 2 different
  characters

17

• Dihybrid Cross Experiment
• Question Do alleles travel together?
• Seed color
• Dominant (Y) yellow
• Recessive (y) green
• Seed shape
• Dominant (R) round
• Recessive (r) wrinkled
• cross YYRR with yyrr

18

• Alternative hypothesis alleles travel separately
• F1 generation gametes
  YR, Yr, yR, yr
• Sixteen equally likely
  combinations in F2
• Four phenotypes in a
  9331
  ratio
• 31 ratio in F2 for each
  individual character
  

19

• Mendels Law of Independent Assortment
• independent assortment of each pair of alleles
  during gamete formation
• Mendels Law of Segregation
• allele pairs separate during gamete formation
• What is the difference?
• Segregation Y and y go into separate gametes
  you dont get Yy in one egg or sperm
• Independent Assortment Y is not linked to R
  you can get YR, Yr, yR, yr in gametes

20
Rules of Probability

• Probability scale ranges from 0 to 1
• zero - event with no chance of occurring
• one - an event that is certain to occur
• Probability of all possible outcomes adds up to 1
• roll of the die
• Independent Events
• chance of an event not affected by previous
  events
• true for coin toss
• also true for distribution of alleles into gametes

21

• Rule of multiplication
• The chance that two or more independent events
  will
• occur together in some specific combination
• Compute probability of each independent event
• Multiply individual probabilities to obtain
  overall probability of events occurring together
• Probability that two coins tossed at the same
  time will land heads up is 1/2 x 1/2 1/4

22

• Monohybrid Cross
• Probability that a heterogyzous pea plant (Pp)
  will produce a white-flowered offspring (pp)
• Pp x Pp
• chance that gamete will contain p is 1/2
• 1/2 x 1/2 1/4

23

• Dihybrid Cross
• What is the probability that a heterozygous
  parent (YyRr) will have a YYRR offspring?
• Requires YR gamete from each parent
• chance that gamete has Y 1/2
• chance that gamete has R 1/2
• chance that gamete has YR 1/2 x 1/2 1/4
• The probability of a YYRR offspring 1/16
• chance for a YR ovum 1/4
• chance for a YR sperm 1/4
• Eliminates need for complicated Punnett square

24

• Rule of Addition
• The chance of an event that can occur two or more
• different ways is the sum of the separate
  probabilities
• of those ways.
• Probability of rolling a 3 with two dice?
• 1st die - 1 (1/6) OR 1st die - 2
  (1/6)
• 2nd die - 2 (1/6) 2nd die - 1
  (1/6)
• 1/6 x 1/6 1/36 1/6 x 1/6
  1/36
• Overall probability of 3 is 1/36 1/36 2/36

25

• Rule of Addition
• What is the probability of F1 gametes forming a
• heterozygote offspring?
• F1 are heterozygotes (Pp)
• P could come sperm and p from ovum
• 1/2 x 1/2 1/4 OR
• P could come from ovum and p from sperm
• 1/2 x 1/2 1/4
• The overall probability of a heterozygote
• 1/4 1/4 1/2.

26

• Use rules of addition multiplication to solve a
• trihybrid cross.
• PpYyRr x Ppyyrr
• what is the probability of recessive phenotypes
  for at least 2 traits
• what are the possible genotypes?
• ppyyRr, ppYyrr, Ppyyrr, PPyyrr, and ppyyrr
• rule of multiplication - calculate probability
  for each genotypes
• rule of addition to add up the probabilities

27

• PpYyRr x Ppyyrr
• The probability of genotype ppyyRr
• probability of producing pp 1/2 x 1/2 1/4
• probability of producing yy 1/2 x 1 1/2
• probability of producing Rr 1/2 x 1 1/2
• Therefore, the probability of all three being
  present (ppyyRr) in one offspring is 1/4 x 1/2 x
  1/2 1/16
• Must repeat for other 4 genotypes

28

• PpYyRr x Ppyyrr
• For ppYyrr 1/4 x 1/2 x 1/2 1/16
• For Ppyyrr 1/2 x 1/2 x 1/2 2/16
• For PPyyrr 1/4 x 1/2 x 1/2 1/16
• For ppyyrr 1/4 x 1/2 x 1/2 1/16
• Therefore, the chance of at least two recessive
  traits is 6/16

29

• Important Points
• Can only predict probabilities
• Sample sizes must be large
• Mendels laws apply to all diploid organisms that
  reproduce by sexual reproduction (not just peas)

30

• Beyond Mendels Genetics
• Incomplete dominance
• Codominance
• Multiple alleles
• Pleiotropy
• Epistasis
• Polygenic Inheritance
• Environmental Factors

31

• Types of Dominance
• Incomplete dominance
• heterozygotes have
  intermediate phenotype
• not blended inheritance
  
• red white in F2
• 121
• why pink and not red?

32

• Codominance
• both alleles are expressed
• M, N, and MN blood groups
• not recessive or dominant
• MM - synthesize only the M protein
• NN - synthesize only the N protein
• MN - synthesize both M and N proteins

33

• Dominance is a spectrum
• Dominant allele is not a bully
• Dominant not necessarily more common

Complete Dominance
Varying degrees of Incomplete Dominance
Codominance
34

• Multiple Alleles
• ABO blood groups
• A and B are codominant
• O is recessive
• 6 possible genotypes
• 4 possible phenotypes
  (blood types)

Genotype Phenotype AA A AO A
BB B BO B AB AB
OO O
35

• Pleiotropy
• One gene affects more than one character
• Sickle cell anemia

Hemoglobin molecule
(single amino acid substitution)
Pain
Fatigue
Kidney problems
Pneumonia
36

• Epistasis
• Gene at one locus alters expression of gene at a
  second locus
• color depends on 2 genes
• epistatic gene (C or c)
  pigment or no
  pigment
• second gene (B or b)
  black or
  brown
• cc is always white

37

• Polygenic inheritance
• More than 1 gene affects single character
• 3 different genes for skin color
• two alleles for each gene
• one light and one dark
• incomplete dominance
• AABBCC individual is very dark
• aabbcc is very light
• whole spectrum of intermediate colors

38

• Influence of Environment
• nature versus nurture
• nutrition affects height
• studying affects intelligence
• sun exposure affects skin color
• Norm of Reaction
• range of phenotypes for a given phenotype
• wide range (skin color)
• narrow range (blood types)
• Widest for Polygenic Traits

39
Human Inheritance

• Pedigree Analysis
• analysis of matings that have already occurred
• family tree
• Can predict probability of traits in future
  generations

__
Widows peak (ww)
No widows peak (WW or Ww)
ww
ww
ww
40
Genetic Disorders

• Recessive Inheritance
• defective or missing protein/enzyme
• relatively mild or life-threatening
• affected individuals are homozygous recessive
• heterozygotes have normal phenotype
• heterozygotes are carriers

41
Genetic Disorders

• Cystic Fibrosis
• Tay Sachs
• Sickle Cell Disease
• Consanguinous Matings
• recent common ancestor
• increased chance of carrying same recessive allele

42
Genetic Disorders

• Dominant Inheritance
• heterozygotes have disorder
• 50 chance of affected offspring
• not as common
• Achondroplasia
• Huntingtons disease

43
Genetic Disorders

• Multifactorial Genetic Disorders
• genetic component
• environmental influence
• heart disease, diabetes, cancer, alcoholism,
  schizophrenia

44
Genetic Testing/Counseling

• Pedigree Testing
• Clinical tests available for several disorders
• Fetal Testing
• Amniocentesis
• Chorionic villus sampling
• Ultrasound
• Fetoscopy
• Phenyketonuria (PKU)