Mendel and his Peas

1
Mendel and his Peas

• Chapter 9

2
State Objectives

• CLE 3210.4.5 Recognize how meiosis and sexual
  reproduction contribute to genetic variation in a
  population.
• CLE 3210.4.3 Predict the outcome of monohybrid
  and dihybrid crosses.
• SPI 3210.4.4 Determine the probability of a
  particular trait in an offspring based on the
  genotype of the parents and the particular mode
  of inheritance.
• CLE 3210.4.4 Compare different modes of
  inheritance sex linkage, codominance, incomplete
  dominance, multiple alleles, and polygenic traits.

3
Sub-Objectives

• Explain the experiments of Gregor Mendel
• Explain how genes and alleles are related to
  genotypes and phenotypes
• Use a Punnett square to predict genotypes and
  phenotypes

4
Gregor Mendel

• A monk
• Worked in the garden at the monestary
• Wrote Experiments in Plant Hybridization in
  1866.
•   Experiments unnoticed until 1900.

5
Gregor Mendel

• Wanted to know how traits are passed from one
  generation to the next
• How some traits seem to skip a generation and
  show up in the next
• Chose the pea plant to study

6
Why Peas?

• Grow quickly
• Self pollinating
• Each flower contained both male and female parts

Image from http//www.jic.bbsrc.ac.uk/germplas/pi
sum/zgs4f.htm
7
Sexual Reproduction in Peas

• Pollen from the anther of one plant is
  transferred to the stigma of another. Pollen
  travels down to to egg cell

8
Image from http//anthro.palomar.edu/mendel/mende
l_1.htm
9
Mendels Peas

• Mendel began by studying one trait at a time.
• That way, he could understand the results
• Some of the traits he observed
• Plant height, seed shape, flower color

10
How did he start?

• He crossed pure-bred tall plants with pure bred
  tall plants
• Results All tall plants
• He crossed pure-bred short plants with pure bred
  short plants
• Results All short plants

11
Cross-pollination

• Anthers of one plant are removed so it can not
  self pollinate
• Pollen from another plant is used to pollinate
  the flower

12
Results from cross-pollination

• When the peas were cross pollinated, they
  produced offspring.
• These offspring are the first generation
• In the case of tall and short plants, all the
  offspring came out tall

13
Dominant and recessive traits

• The trait that appeared in that first generation
  was called the dominant trait
• Dominant trait masks the presence of other
  traits
• The trait that did not show up he called the
  recessive trait

14
Mendels second experiment

• Mendel crossed the individuals from the first
  generation with each other
• The next offspring are called the 2nd generation
• In the second generation, the recessive trait
  reappeared

15
Counting the offspring

• Mendel counted the offspring in the second
  generation with each trait to determine the ratio
  of individuals with the dominant trait to those
  with the recessive trait.

16
Mendels peas
17
Results of Mendels experiment
18
Results of Mendels experiment
19
Of Genes and Alleles

• Mendel looked at the math and decided for each
  trait offspring had to have two factors one
  from their mother and one from their father
• These factors that coded for the same trait are
  called genes

20
So whats a trait?

• A specific characteristic that varies from one
  individual to another
• Mendel looked at seven
• Seed shape, seed color, seed coat color, pod
  shape, pod color, flower position and plant height

21
Of Genes and Alleles

• For each gene, there may be more than one form
• These different forms of genes are called alleles

22
Terms

•       Gene a unit of heredity on a chromosome.
•       Allele alternate state of a gene.
•       Dominant an allele that masks the
  expression of other alleles.
•       Recessive an allele whose expression is
  masked by dominant alleles.

23
Linking with Meiosis

• How does the information of two alleles for each
  gene compare with what we know from meiosis?
• What does each zygote get when sperm and egg
  fertilize?

24
What we know from meiosis

• Principle of segregation
• Alleles on homologous chromosomes separate during
  the process of meiosis
• Only one allele from each parent is passed to the
  offspring

25
Segregation
Image from http//anthro.palomar.edu/mendel/mende
l_1.htm
26
Probability

• The mathematical chance that an event will occur
• If you flip a coin, whats the chance it will
  come up heads?
• What the chance of three tails in a row?
• Biologists use probability to predict the outcome
  of genetic crosses

27
Punnett Square

• To understand Mendels conclusions, we use a
  diagram called a Punnett square
• Dominant alleles are symbolized with capital
  letters
• Recessive alleles are symbolized with lower case
  letters

28
Remember for each trait there are two alleles

• So a cross from a true breeding tall plant will
  produce a tall offspring whose alleles are
  written, TT
• A true breeding short plant would be tt

29
Genotype
TT, tt, or Tt

• The actual letters represent the alleles this
  combination of alleles is called the genotype
• Genotype alleles present in the organism

30
Back to our pea plants
31
Genotype

• Two possibilities
• Homozygous contains identical alleles (TT or tt)
• Heterozygous contains different alleles (Tt)

32
Phenotype

• An organisms appearance, what the gene looks like
  is the phenotype
• Phenotype the physical appearance of the trait
• what it looks like

33
Objective

• 3210.4.3 Predict the outcome of monohybrid and
  dihybrid crosses

34
Making a Punnett Square
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Baby Steps to a Punnett square

• 1. determine the genotypes of the parent
  organisms 2. write down your "cross" (mating)
  3. draw a p-square 4. "split" the letters of
  the genotype for each parent put them "outside"
  the p-square

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Baby Steps to a Punnett square

• 5. determine the possible genotypes of the
  offspring by filling in the p-square 6.
  summarize results (genotypes phenotypes of
  offspring) 7. bask in the glow of your
  accomplishment !

37
Objective

• 3210.4.3 Predict the outcome of monohybrid and
  dihybrid crosses

38
Making gametes

• Remember the principle of segregation, especially
  with dihybrid crosses, Each gamete only gets one
  allele for each trait!!!!

Image from http//arbl.cvmbs.colostate.edu/hbooks
/pathphys/reprod/fert/gametes.html
39
Are all wrinkled peas yellow??

• Once Mendel found how traits are passed, he
  wanted to know if the segregation of one pair of
  alleles had anything to do with the segregation
  of another pair
• In other words, are all wrinkled peas yellow???

40
Principle of Independent Assortment

• Genes for different traits do not affect each
  other in segregation
• Works for most traits unless they are linked
  close together on the same chromosome

41
Dihybrid cross

• Use a Punnett square to track two traits at once
• Works like a monohybrid cross, but you have to
  take care in forming your gametes

42
Objective

• 3210.4.3 Predict the outcome of monohybrid and
  dihybrid crosses

43
Objective

• 3210.4.4 Compare different modes of inheritance
  sex linkage, codominance, incomplete dominance,
  multiple alleles, and polygenic traits

44
Other exceptions to Mendel

• Incomplete dominance
• Codominance
• Multiple Alleles
• Polygenic traits

45
Incomplete Dominance

• With incomplete dominance, a cross between
  organisms with two different phenotypes produces
  offspring with a third phenotype that is a
  blending of the parental traits.

46
Incomplete dominance

• Neither allele is completely dominant over the
  other
• Example White and pink four oclocks
• WhiteW
• Red R

47
A Classic Example Snapdragons

• R allele for red flowers W allele for white
  flowers
• red x white ---gt pink RR x WW ---gt 100 RW

48
Recognizing Incomplete Dominance

• Two steps 1) Notice that the offspring is
  showing a 3rd phenotype.  Not shown in the
  parents 2) Notice that the trait in the
  offspring is a blend (mixing) of the parental
  traits.

49
Practice problems

• Try problems one and two on your practice
  problems sheet

50
Codominance

• In Codominance, the "recessive" "dominant"
  traits appear together in the phenotype of hybrid
  organisms.
• red x white ---gt red white spotted

51
Another classic example Cows

• In cows if you cross a pure bred red cow with a
  pure bred white cow, the offspring are roan
• The color difference in their coats is because
  they have both red and white hairs together

52
Practice problems

• Try problems three through five on your practice
  problems sheet

53
Multiple alleles

• Genes that have more than two alleles for a trait
• Each individual can only have two, but in the
  population more than two exist

54
Another classic example Blood Type

• Humans have three alleles for blood type
• IA Type A
• IB Type B
• I Type O
• A and B are both dominant over O, but are
  codominant with each other

Image from http//www-micro.msb.le.ac.uk/MBChB/bl
oodmap/Blood.html
55
Polygenic traits

• Some traits are determined by the interaction of
  many traits
• Examples
• Height Hair color
• Skin color

56
Objective

• 3210.4.4 Compare different modes of inheritance
  sex linkage, codominance, incomplete dominance,
  multiple alleles, and polygenic traits

57
Studying genetics

• Thomas Hunt Morgan
• Uses fruit flies Drosophila melanogaster
• Breed a new generation every 14 days
• Used because short generation time allows
  production of many generations

Image from http//www.ceolas.org/fly/intro.html
58
Environmental Influences

• Our genes arent all of what we are
• Our environment has influences as well
• Example Heart disease
• People with poor diets have higher incidences of
  heart disease

59
State Objectives

• CLE 3210.4.5 Recognize how meiosis and sexual
  reproduction contribute to genetic variation in a
  population.
• CLE 3210.4.3 Predict the outcome of monohybrid
  and dihybrid crosses.
• SPI 3210.4.4 Determine the probability of a
  particular trait in an offspring based on the
  genotype of the parents and the particular mode
  of inheritance.
• CLE 3210.4.4 Compare different modes of
  inheritance sex linkage, codominance, incomplete
  dominance, multiple alleles, and polygenic traits.