Early Ideas about Heredity

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Early Ideas about Heredity
Chapter 9, Section 1 Pre-Mendel

• The Blending Theory
• Main theory of inheritance until the late 19th
  century
• Each parent contributed factors that were
  blended in the offspring
• What was the problem with this theory?
• All individuals of a population would eventually
  look the same.
• Once blended, traits would never separate and
  show up in later generations.

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Gregor Mendel (1822 1884)
Chapter 9, Section 1 Mendel

• Augustinian monk who studied pea plants
• Established the particulate theory of heredity
• Significance
• Developed pure lines
• Counted results and kept statistical notes
• His work remained undiscovered until 1903.

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Character
Dominant Trait
Recessive Trait
Mendels Research
Flower Color
Why did he research pea plants?
Purple
White

• They are normally self-pollinated, but can be
  cross-pollinated.
• They have several qualitative traits that are
  easy to distinguish
• i.e.,Tall vs. Short

Flower Position
Side
Top
Seed Color
Yellow
Green
Seed Shape
Round
Wrinkled
Pod Shape
Inflated
Constricted
Pod Color
Green
Yellow
Stem Length
Tall
Dwarf
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Mendels Research

• Removed stamens from purple flower.
• Transferred pollen from stamens of white flower
  to pistil of purple flower.
• Pollinated flower matured into a pod.
• Planted seeds from pod.
• Examined offspring All purple flowers
• Watch this video clip, and seeif you can explain
  why

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Mendels Conclusions

• Law of Segregation
• Factors (genes) for a particular trait occur in
  pairs
• For each trait, an organism inherits two genes,
  one from each parent.
• Dominant alleles mask recessive ones
• Exception 1 Incomplete Dominance
• Exception 2 Co-dominance
• Two alleles for each trait segregate during
  gamete production

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Homologous pair of Chromosomes
Genes The factors that control traits.
Law of Segregation Factors for a particular
trait occur in pairs
Alleles Different forms of a gene.
Locusfor FlowerColorGene
Allele for White Flowers
Allele for Purple Flowers
Back to Mendels Conclusions
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W
w
or
or
w
w
Ww
ww

• Each individual is diploid
• Diploid Containing a double-set of chromosomes
  (2n)
• Each gamete is haploid
• Gamete Reproductive cell (egg or sperm)
• Haploid Containing a single-set of chromosomes
  (n)

Law of Segregation One version of each gene is
inherited from each parent
Back to Mendels Conclusions
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Dominant alleles mask recessive ones
P (Parent) Generation True-breeding parents
F1 (1st Filial) Generation Hybrid Offspring
What happened to the recessive traits?
705 plants had purple flowers
F2 (2nd Filial) Generation
224 plants had white flowers
What is the F2 ratio?
705224 ? 31
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Dominant alleles mask recessive ones
P Generation
Phenotype (Appearance)
Purple Flowers
White Flowers
pp
Genotype (Genetic Makeup)
PP
Gametes
P
p
F1 Generation
Phenotype (Appearance)
Purple Flowers
Genotype (Genetic Makeup)
Pp
p
P
Gametes
Punnett Square
F2 Generation
P
P
PP
What is the Genotypic Ratio of the F2 Generation?
p
p
Pp
Pp
PPPppp ? 121
pp
What is the Phenotypic Ratio of the F2
Generation?
3
1
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Dominant alleles mask recessive ones
Genotype
Phenotype
PP
Purple
1
(homozygous)
Pp
Purple
3
(heterozygous)
2
Pp
Purple
(heterozygous)
pp
1
1
White
(homozygous)
Ratio 121
Ratio 31
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Dominate alleles mask recessive ones
If you have a dominant phenotype (like purple
flowers) how would you design an experiment to
determineif it was homozygous (PP) or
heterozygous (Pp)?
Dominant phenotype, unknown genotype PP or Pp?
Recessive phenotype, known genotype pp
What would your hypothesis be if the genotype was
PP?
What would your hypothesis be if the genotype was
Pp?
If PP, then all offspring purple
If Pp, then ½ offspring purpleand ½ offspring
white
P
P
p
p
Pp
Pp
P
p
p
p
Pp
pp
Pp
Pp
pp
Pp
This is called a test cross
Back to Mendels Conclusions
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P Generation
Phenotype(s) Red and White
Genotype(s) CRCR and CWCW
Gametes CR and CW
CR
CW
F1 Generation
Phenotype Pink
Genotype CRCW
Gametes CR and CW
½
½
CR
CW
F2 Generation
½
CR
CR
½
CRCR
½
Exception to Dominant Alleles Masking Recessive
Alleles Incomplete Dominance Pink Snapdragons
CW
CW
½
CRCW
CRCW
CWCW
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Exception to Dominant Alleles Masking Recessive
AllelesCo-dominance Blood Types
Back to Mendels Conclusions
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Back to Mendels Conclusions
Law of Segregation Two alleles for each trait
segregate during gamete production

• This is a process called meiosis
• Specifically it is crossing over, which occurs
  very early during Prophase I of Meiosis

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Mendels Conclusions

• Law of Independent Assortment
• The law of segregation followed one single trait
  at a time, such as flower color.
• What if two traits were followed?
• Two hypotheses were written and experimented on
  to determine if traits were
• Dependent (linked), or
• Independent (not linked)
• If the genes are not connected, then they should
  segregate independently.

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Following two traitsSeed shape and seed color
P
yr
YR
Gametes

• Hypothesis If the genes for seed shape and
  color are connected in some way,
• then the dominant R and Y alleles and the
  recessive r and y alleles will be segregated as
  matched sets in the gametes.

F1
YyRr
Ova
Sperm
YR
YR
yr
yr
F2
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What are the genotypes of the P Generation?
Law of Independent Assortment
YYRR
yyrr
What does the P Generation pass on to F1?
yr
Gametes
YR
F1 Phenotype?
YyRr
F1 Genotype?
What does the F1 Generation pass on to F2?
Sperm
Ova
YR
YR
Yr
Yr
Combine the sperm and ova to produce the
offspring in F2
YYRR
yR
yR
YYRr
YYRr
yr
yr
What do you expect the phenotype will be when YR
and YR are combined?
YyRR
YyRR
YYrr
Phenotypic Ratios
YyRr
YyRr
YyRr
YyRr
Yellow Round
9/16
What will the Genotype be when YR and YR are
combined?
yyRR
Yyrr
Yyrr
Green Round
3/16
yyRr
yyRr
3/16
Yellow Wrinkled
Now try to do it on your own!
yyrr
Green Wrinkled
1/16
What are the phenotypic ratios?
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Significance of Mendels Research

• Probability was introduced as a biological
  concept
• The number of times an event occurs The
  number of trials
• Punnett Square was introduced as a tool

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Probability The number of times an event
occursThe number of trials
Segregation of alleles into ova
Segregation of alleles into sperm
Pp x Pp
½
P
P
½
P
P
½
p
p
½
½ x ½
¼
P
p
P
p
¼
¼
p
p
How do you get a genotypic ratio of 121 from
these numbers?
¼
Back to Mendels Principles
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Punnett Square Exercise

• One-trait cross
• The allele for the hairy trait is H, while the
  allele for the hairless trait is h.
• Make a cross between two homozygotes for each of
  these traits.

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Punnett Square Exercise

• One-trait cross
• Henry VIII divorced six of his wives (two
  of whom were executed) for not bearing him
  any sons.
• Use Mendelian Genetics to prove to King Henry
  that it wasnt his wives fault.
• Remember Females are XX and Males are XY.

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Punnett Square Exercise

• One-trait cross
• In a populations of hamsters, brown fur is
  dominant (B), and white fur is recessive (b).
• Predict the offspring from the cross of
  a white hamster and a brown hamster if
  the brown hamster's mother was white.

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Punnett Square Exercise

• Two-trait cross
• Following two traits
• Kernel shape Plump vs. withered (P and p) and
  Kernel color Red vs. yellow (R and r)
• Predict the cross between a homozygousrecessive
  corn plant and a homozygous dominant corn plant.

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Punnett Square Exercise

• Two-trait cross
• Now take the offspring from that cross, and
  self-pollinate that plant.
• What is the phenotypicratio of this cross?

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Punnett Square Exercise

• Two-trait cross
• My pet guinea pigs (Joni and Chachi) are
  going to have little guinea pigs.
• What is the possibility that their offspring
  will have long hair, if I know that Joni and
  Chachi are heterozygous for the following traits?
• Black fur (B) White fur (b)
• Long hair (L) Short hair (l)