Monohybrid Notes
Gregor Mendel
• Mendel was an Austrian monk.
• Mendel formulated two fundamental laws of
heredity in the early 1860's.
• He had 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.
• 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.
Genetics Vocabulary
• Gene - the unit of heredity; a section of DNA
sequence encoding a single protein
• Alleles - two genes that occupy the same
position on homologous chromosomes and cover
the same trait
• Locus - a fixed location on a strand of DNA where
a gene or one of its alleles is located
Genetics Vocabulary
• Homozygous - having identical genes (one from
each parent) for a particular characteristic.
• Heterozygous - having two different genes for a
particular characteristic.
Genetics Vocabulary
• Phenotype - appearance (description)
• Genotype - genetic makeup (letter code)
Genetics Vocabulary
• Dominant - the trait that appears in the
heterozygous condition.
• Recessive - the trait that is masked in
the heterozygous condition.
Genetics Vocabulary
• Incomplete Dominance - a form of
intermediate inheritance in which one
allele for a specific trait is not completely
dominant over the other. If the trait is
incomplete it will be stated in the problem.
Genetics Vocabulary
• Codominance - A condition in which both
alleles of a gene pair in a heterozygote are
fully expressed, with neither one being
dominant or recessive to the other. If the
trait is codominant it will be stated in the
problem.
RR
RW
WW
Genetics Vocabulary
• Monohybrid cross - cross involving a
single pair of genes, one trait
• P – “parents”
• F1 – “kids”
• F2 – “grandkids”
Genetics Vocabulary
• Punnett squares - 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
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 =
2Bb:2bb
7. Determine
phenotype ratio
Phenotype = what
they look like
Count offspring in the
Punnett square
phenotype ratio =
2 black:2 white
*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 punnett 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 of F1?
• What are the genotype ratio of F1?
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 or
heterozygous?
Use a test cross:
cross with a
pure breed
recessive and
look at the F1
generation
Possible Answer Formats
• % - chance out of total possible
– 1 out of 4 = 25%
• Fraction
– 1 out of 4 = 1/4
• Ratio – ratio adds up to total possible
– 1 out of 4 = 1:3