Mendel and Monohybrid Crosses Notes

1
Mendel and Monohybrid Crosses Notes
2
Gregor Mendel

• Mendel was an Austrian monk.
• Mendel formulated two fundamental laws of
  heredity in the early 1860's.
• He had previously studied science and mathematics
  (including Statistics) at the University of
  Vienna. Mendels knowledge of statistics later
  proved valuable in his research on Heredity the
  transmission of characteristics from Parent to
  Offspring.
• Mendel's work was unrecognized until 1900.

3
(No Transcript)
4
Mendels Experiments

• A hybrid is the product of parent organisms that
  are True-Breeding or pure-breeding for different
  forms of One Trait. Plants that are PURE for a
  trait always produce offspring with that Trait.
  The term STRAIN denotes plants that are Pure for
  a Specific Trait.
• 1. Mendel produced strains by allowing the plants
  to Self-Pollinate for several generations.
• 2. He eventually obtained 14 strains, one for
  each of the 14 traits he observed. He called each
  strain a Parental Generation or  P1 generation.
  This is the parental generation in a breeding
  experiment.

5

• 3. He used Cross-pollination between two strains
  and tracked each trait through two generations.
  (Monohybrid Cross)
• 4. F1 generation is the first-generation
  offspring in a breeding experiment. The First
  Filial Generation.
• 5. F2 generation is the second-generation
  offspring in a breeding experiment. The Second
  Filial Generation.
• 6. He also performed reciprocal crosses of pollen
  on stigmas (e.g. tall-with-short and short-with
  tall).

6
Mendels Conclusions

• In his crosses each trait had a Dominant Factor
  which masked, or dominated the other factor for a
  specific trait the other we call the Recessive
  Factor.
• In a true-breeding cross (BBxbb) the recessive
  factor is masked in F1 but appears in F2

7

• Law of Segregation Each organism contains two
  factors for each trait factors segregate in the
  formation of gametes. When two gametes combine
  during fertilization, the offspring have two
  factors controlling a specific trait.
• Law of Independent Assortment states that factors
  for different characteristics are distributed to
  gametes independently.

8
Punnett Squares

• Punnett sqaures are used to aid in predicting the
  probability that certain traits will be inherited
  by offspring
• Probability is the likelihood that a specific
  event will occur or is the likely outcome a
  given event will occur from random chance

9
Monohybrid Crosses (only one trait)

• 1. Determine the dominant and Recessive trait

Will be stated in the problem ie. Black is
dominant to white in bunnies
10

• 2. Assign letters for the trait

Dominant trait gets the capital letter Recessive
traits gets the lower case letter B black b
white
11

• 3. Determine genotype for parents

White male bb Heterozygous black female Bb
12

• 4. Put parents on the square

Bb x bb B b b b
13
Punnett square holds offspring genotypes
B b b b Genotype
letter code

• 5. Determine genotype of offspring

14

• 6. Determine genotype ratio

Count offspring in the Punnett square genotype
ratio 22 0/4 BB 2/4 Bb 2/4 bb
15

• 7. Determine phenotype ratio

Phenotype what they look like Count offspring
in the Punnett square phenotype ratio 22 If
ratio doesnt add up to total number of boxes in
the punnett square youve made a mistake
16
Practice Problems

• A pure-breed white flower crosses with a
  pure-breed purple flower. Purple is dominant to
  white.
• Show the punnet square
• What is the genotype of the F1 generation
• What is the phenotype of the F1 generation

17
Genotype Pp Phenotype Purple
18
More practice

• A homozygous black rat is crossed with a
  heterozygous black rat.
• B black b brown
• Show the punnett square
• What are the genotypes?
• What are the genotype ratio?

19
Genotypes BB and Bb Genotype Ratio 2BB2Bb
20
More practice

• If you had a black rat how could you tell its
  genotype was homozygous and heterozygous?

21
Use a test cross cross with a pure breed
recessive and look at the F1 generation