Transmission of Genomes meiosis and Mendel

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Transmission of Genomes (meiosis and Mendel)

• ...Meiosis the process of two consecutive cell
  divisions in the diploid progenitors of sex
  cells,
• results in the production of haploid gametes,
• Mendelian genetics in many ways, applied
  meiosis,
• or, meiosis revealed.

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Meiosis is critical for sexual reproduction in
all diploid organisms

• ...meiosis leads to the formation of gametes,
• one gamete (carrying one genome) from each parent
  forms a zygote,
• ...meiosis is the basis for extensive variation
  among members of a population.

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Recombination II
2n combinations of chromosomes, n number of
chromosomes.
Random Assortment of Chromosomes.
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2n combinations of chromosomes n number of
chromosomes

• n 1, 2n 2
• n 2, 2n 4
• n 3, 2n 8
• n 5, 2n 32 Arabidopsis
• n 23, 2n 8,388,608 H. sapiens
• n 39, 2n yip! dog

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Mendelian Genetics

• Gregor Mendel (1822-1884),
• Augustinian monk,
• Botanist,

• Pisum sativa,
• Garden pea,
• 1st Model Organism.

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Model Organisms

• Ease of cultivation,
• Rapid Reproduction,
• Small size,
• Fecund (large brood size),
• Mutants are available and easy to identify,
• Broad literature and experimental background
  available.

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Model Organisms
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Brief History of Genetics

• prehistory,
• artificial selection non-random breeding with
  no guarantee of results,
• human mediated natural selection
• Canis lupis (wolf) to Canis domesticus (dog),
• savage, misbehaving wolves stewpot,
• helpful, friendly companions lived to breed,
• oldest undisputed dog bones, 20,000 years at an
  Alaskan settlement.

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Gourds!Lagenaria vulgaris
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Why I Am a Biologist?
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Prehistory of Genetics

• By around 10,000 years ago, the same approach
  yielded,
• reindeer, sheep, goats, pigs, cattle, fowl, etc.
• rice, barley, wheat, lentils, corn, squash,
  tomatoes, potatoes,
• peppers, yams, peanuts, gourds, etc.
• yeast and bacteria for fermentation, etc.

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History of Modern Genetics

• By the 19th century, precise techniques for
  selective breeding allowed the systematic
  creation of strains in which offspring often had
  prized traits.
• However, the traits would unpredictably disappear
  in some generations and return in others.

Moravian Sheep Breeders Association (1837) One
breeders dilemma I have an outstanding ram
that would be priceless if its advantages are
inherited by its offspringif they are not
inherited, then it would be worth no more than
its wool, meat and skin.
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Abbot Cyril Napp

• In concluding remarks to the Moravian Sheep
  Breeders Society, Abbot Cyril Napp proposed that
  breeders could improve predictions of traits in
  offspring if they determined the answers to three
  basic questions
• What is inherited?
• How is it inherited?
• What is the role of chance in heredity?

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Monastery of St. ThomasBrno, 1843

• Abbot Cyril Napp, master of the monastery
  admitted Johann Mendel, a gifted student from a
  poor peasant family,
• Johann changed his name to Gregor,
• was sent to the University of Vienna,
• studied physics, chemistry, botany, paleontology
  and plant physiology,
• resolved to answer Abbot Napps three questions.

Also, Mendel had test anxiety issues, and
couldnt pass exams.
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Prevailing Genetic Philosophies

• Philosophy 1 one parent contributes most to the
  offspring,
• an homunculus did it,
• Aristotle contended that it was the male, via a
  fully formed being in the sperm,
• Respected 19th microscopists staked their
  reputation that they could see the homunculus in
  sperm.

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Prevailing Genetic Philosophies

• Philosophy 2 blended inheritance,
• parental traits are mixed and become forever
  changed in the offspring.

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To begin a Science of Genetics

• careful observation, over time, of groups of
  organisms,
• rigorous (i.e. mathmetical) analysis of these
  observations,
• development of a theoretical framework to explain
  these observations.

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Napps Questions

• Napp
• What is inherited?
• How is it inherited?
• What is the role of chance in heredity?

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Mendel Insight 1

• Use the pea,

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Insight 2

• alternate forms,

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Insight 3

• True breeding lines,
• Permit me to state that, as an empirical worker,
  I must define constancy of type as the retention
  of character during the period of observation.
  -Mendel
• Mendel observed his true-breeding lines for up
  to 8 generations.
• Used the pure-breeding line to form hybrid lines,
• offspring of genetically dissimilar parents.

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Insight 4

• Expert plant breeder,
• carefully controlled the matings,
• prevented the intrusion of any pollen foreign to
  the desired mating,
• made reciprocal crosses
• reversing the traits of the male and female
  parents,
• male wrinked x female smooth,
• female wrinkled x male smooth.

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Insight 5

• Used large numbers of subjects,
• applied statistical analysis to his data!
• uncovered the patterns of transmission that we
  will take for granted.

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Insight 6

• Controlled for environmental factors,
• for example, when looking at the short and tall
  plants, he made sure that all subjects received
  equal light,
• from his studies of plant physiology, he knew
  that light mediates stem elongation.

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Insight Summation

• Used the pea,
• Identified alternate forms,
• Identified and used true breeding lines,
• Expert plant breeder,
• Used statistical analysis,
• Controlled for environmental factors.
• Set up a simple black and white system, and
  then figured out how it worked.

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Monohybrid Cross

• Mating between individuals that differ in only
  one trait,
• yellow pea x green pea,
• violet flower x white flower
• tall x dwarf
• round seed x wrinkled seed
• full pod x constricted pod
• etc.

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Monohybrid Cross
Generation Parental (P) First Filial
(F1) Second Filial (F2)
yellow pea green pea (pollen)
(eggs)
x
grow plants, cross pollinate
grow, allow to self-fertilize
all yellow
6022 yellow 2001 green 3 1
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Reappearance of Trait in F2 Generation Disproves
Blending

• Blending did not occur, in fact over 2000 peas
  retained the information necessary to make green
  peas,
• Mendel concluded that there must be two types of
  yellow peas,
• those that breed true like the parent plant,
• those that can yield some green peas, like some
  of the F1 hybrids.

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Generations
Generation Parental (P) First Filial
(F1) Second Filial (F2)
yellow pea green pea (pollen)
(eggs)
x
grow plants,cross pollinate
grow, allow to self-fertilize
all yellow
6022 yellow 2001 green 3 1
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Reciprocal Crosses Disproved Influential Parent
Myths

• In all monohybrid crosses, the ratio of
  contrasting traits was approximately 31,
• in the yellow(male) x green (female) pea cross,
  three yellow peas were produced for every green
  pea in the F2 generation,
• Independent of which parent carried the dominant
  trait...

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Dominant vs. Recessive Traits
x
P
F1
The trait that appears in the F1 generation is
the DOMINANT trait. The trait that disappears in
the F1 generation is termed RECESSIVE.
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Nomenclature

• Dominant unit factors are designated with a
  capital letter, often (but not always) with the
  first letter of the description,
• Y yellow,
• V violet,
• T tall,
• Recessive unit factors are represented by small
  letters,
• y green,
• v white,
• t dwarf,

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Nomenclature IIits not my fault.

• Dominant unit factors are designated with a
  capital letter,
• G yellow,
• W violet,
• D tall,
• Recessive unit factors are represented by small
  letters, often (but not always) with the first
  letter of the description,
• g green,
• w white,
• d dwarf,

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Mendels First PostulateUnit Factors in Pairs

• Genetic characteristics are controlled by unit
  factors that exist in pairs in individual
  organisms,
• each individual receives one unit factor from
  each parent,
• in a monohybrid cross, three combinations of unit
  factors are possible,

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First Postulate

• in a monohybrid cross, three combinations of
  factors are possible,
• YY
• Yy
• yy

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Definitions to Know

• Homozygous the unit factors that determine a
  particular trait are the same,
• YY homozygous dominant,
• yy homozygous recessive,
• Heterozygous the unit factors that determine a
  particular trait are different,
• Yy heterozygous.

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Mendels Second PostulateDominance/Recessiveness

• When two unlike unit factors are present in a
  single individual, one unit factor is dominant to
  the other, which is said to be recessive.

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Unlike Unit Factors Alternate Forms of the
Same Gene Alleles
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Unit Factors Genes

• three combinations of alleles are possible,
• YY
• Yy
• yy

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Molecular Alleles
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Mendels Third PostulateSegregation

• During the processes of heredity, the paired unit
  factors separate so that the offspring receives
  one unit factor from each parent,
• The unit factors segregate to offspring randomly.

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When Unit Factors Separate

• Two Unit Factors Diploid
• One Unit Factor Haploid
• During Gamete formationUnit Factors Separate

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Mendels First Three Postulates

• Unit Factors in Pairs
• Dominance/Recessiveness
• Segregation

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More Definitions to Know

• Phenotype observable trait(s),
• Genotype the actual alleles present in an
  individual.

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Postulates 1-3 AppliedP - F1 Generation

• P Yellow Green Phenotype
• YY yy Genotype
• Gametes Y y
• Yellow Phenotype
• F1 Yy Genotype

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Postulates 1-3 AppliedF1 - F2 Generation

• Yellow Phenotype
• F1 Yy Genotype
• F1 Self-Cross Yy (microsporocyte) Yy
  (megasporocyte)
• Gametes Y or y Y or y
• F2

Yy
Yy
yy
YY
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3 1

• 1 1 1
  1
• Yellow Yellow Yellow
  Green
• F2 YY Yy Yy yy
• homozygous heterozygous
  heterozygous homozygous
• dominant recessive

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Generations
Generation Parental (P) First Filial
(F1) Second Filial (F2)
yellow pea green pea (pollen)
(eggs)
x
grow plants,cross pollinate
grow, allow to self-fertilize
all yellow
6022 yellow 2001 green 3 1
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Punnett Squares
gametes Parent 1
YY
Yy
YY
Yy
Predicted Offspring In Squares
gametes Parent 2
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Back to the Moravians

• So, youve got a prize ram, how do you tell its
  not a dud dad?
• S stud
• s dud
• SS or Ss?

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Test Cross

• Your ram has a stud phenotype, but unknown
  genotype,
• cross it to a homozygous recessive individual,

Ss
Ss
Ss
Ss
ss
ss
Ss
Ss
all studs
half studs, half duds
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Test Cross

• Your ram has a stud phenotype, but unknown
  genotype,
• cross it to a homozygous recessive individual,

SS x ss
Ss x ss
The phenotypic ratio is the same as the allele
ratio in the tested parent!
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Ram x Ewe yields 1-2 Lambs

• test cross on a ram of unknown genotype,
• if one offspring is a Dud, you should know the
  Rams genotype,
• if first offspring is a Stud, what do you know?

...nothing! - its a 50 / 50 expected
outcome if Ram is Ss, second offspring 50 / 50
too, if Ss, etc.
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Probability and Chance

• Chance 100 50
  0
• Probability 1
  0.5 0

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Laws of Probability

• Product Law the probability of two or more
  independent outcomes occurring is equal to the
  product of their individual probabilities.

P(two straight Heads) p(H) x p(H) .25
If independent, then p(seeing a Professor w/
Gourd) .009
HoweverI am dependent (linked) to Gourds, so
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Laws of Probability

• 1 Stud sheep offspring .5 ...if heterozygote
  parent.
• 2 Stud sheep offspring .5 x .5 .25
• 3 Stud sheep offspring .5 x .5 x .5 .125
• 8 Stud sheep offspring .5 x .5 x .5 x .5 x .5 x
  .5 x .5 x .5 .004
• 10 stud sheep offspring .001 0.1
• or, you have a 99.9 chance that your Ram is a
  Stud.

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Mendels Forth PostulateIndependent Assortment

• How do two pairs of alleles segregate in the
  offspring of an individual that is heterozygous
  for two genes at the same time?

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Dihybrid Crosses

• Monohybrid Cross,
• one set of contrasting traits,
• Y versus y.
• R (round) versus r (wrinkled).
• Dihybrid Cross,
• YYRR x yyrr,

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Dihybrid Cross,
phenotype genotype gametes genotype

• yellow/round
  green/wrinkled
• YYRR x yyrr
• YR yr
• YyRr

P F1
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Dihybrid Cross,

• yellow/round
• YyRr
• YR Yr yR yr

phenotype genotype gametes genotype
F1 F2?
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Gamete Formation in F1 Dihybrids
Y y R
r
genotype gametes
YR
Yr
yR
yr
probability
.25
.25
.25
.25
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Mendels Results(phenotypes)
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Probability

• Chance 100 50
  0
• Probability 1 0.5 0

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Laws of Probability

• Product Law the probability of two or more
  independent outcomes occurring is equal to the
  product of their individual probabilities.

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Monohybrid Cross

• P YY x yy
• F1 Yy
• Gametes Y y Y
  y
• 1/2 1/2 1/2 1/2
• F2

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Sum Law

• The probability of an outcome that can occur in
  more than one way is the sum of the probabilities
  of the individual events.

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Random Segregationsum law

• F2 YY Yy yY yy
• 1/4 YY 1/4 Yy 1/4 yY
  1/4 yy

1/4 YY 1/2 Yy 1/4 yy
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Problem

• You cross YYRr x YyRr individuals.
• What are the expected phenotypic ratios?
• use a Punnett Square, and/or probability
  calculations to get your answer.

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YYRr x YyRr
YR
YR
Yr
Yr
YR
YR
YR
YR
YR

• Y_ R_
• yy R_
• Y_ rr
• yy rr

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YR
YR
Yr
Yr
YR
YR
Yr
Yr
Yr
Yr
Yr
Yr
Yr
YR
YR
Yr
Yr
yR
4
yR
yR
yR
yR
YR
YR
Yr
Yr
yr
yr
yr
yr
yr
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The Key
1
1/2
1/2
1
1/4
1/2
1/4
1/2
1/2
1
Assignment Correlate this with the observed
phenotype.
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Using Probability
YYRr x YyRr
YY x Yy Rr x Rr
YY or Yy RR Rr rr
(p) Y_ 1
(p) R_ .75
Product Rule (p) Y_R_ .75
(p) ss .25
Product Rule (p) Y_rr .25
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Forked-Line MethodYyRr x YyRr (Genotypes)
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Why use Forked-Line Method?

• Based on a classic dihybrid cross (YyRr x YyRr),
  what is the probability that an organism in the
  F2 generation will have round seeds and breed
  true for green cotyledons?

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OK?
YR
Yr
yR
yr
YYRR
YYRr
YyRR
YyRr
YR
YYrr
Yyrr
YYRr
YyRr
Yr
yyRR
yyRr
YyRR
YyRr
yR
YyRr
Yyrr
yyRr
yyrr
yr
3/16 p 0.1875
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Better
3/16 p 0.1875
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Best (?)
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Forked-Line Method(phenotypes)
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Example

• P Rr YY x rrYy
• Probability Rr YY in offspring

1/2 Rr
1/2 rr
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Example

• P Rr Yy x RRYy
• Probability of Rr Yy in offspring

1/2 Yy
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Using Probability
YYRr x YyRr
YY x Yy Rr x Rr
YY or Yy RR Rr rr
(p) Y_ 1
(p) R_ .75
Product Rule (p) Y_R_ .75
(p) ss .25
Product Rule (p) Y_rr .25