Transcript Monohybrid Crosses (only one trait)

Mendel and Monohybrid
Crosses Notes
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. Mendel’s
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.
Mendel’s Experiments
A hybrid is the product of parent organisms that are TrueBreeding 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 SelfPollinate 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.
3. He used Cross-pollination between
two strains and tracked each trait
through two generations.
(Monohybrid Cross)
4. F1 generation is the firstgeneration offspring in a breeding
experiment. The First Filial
Generation.
5. F2 generation is the secondgeneration 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).
Mendel’s 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
• 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.
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
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
2. Assign letters for the
trait
Dominant trait gets the
capital letter
Recessive traits gets
the lower case letter
B = black
b = white
3. Determine genotype
for parents
White male = bb
Heterozygous black
female = Bb
4. Put parents on the
square
Bb x bb
B
b
b
b
5. Determine genotype
of offspring
Punnett square holds
offspring genotypes
B
b
b
b
Bb
bb
Bb
bb
Genotype = letter code
6. Determine genotype
ratio
Count offspring in the
Punnett square
genotype ratio = 2:2
0/4 BB
2/4 Bb
2/4 bb
7. Determine phenotype
ratio
Phenotype = what they
look like
Count offspring in the
Punnett square
phenotype ratio = 2:2
*If ratio doesn’t add up
to total number of
boxes in the punnett
square you’ve made a
mistake
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
Genotype = Pp
Phenotype = Purple
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?
Genotypes = BB and Bb
Genotype Ratio =
2BB:2Bb
More practice
If you had a black rat how could you tell it’s
genotype was homozygous and
heterozygous?
Use a test cross:
cross with a pure
breed recessive
and look at the
F1 generation