Genotype vs Phenotype
 Genotype: An organism’s genetic makeup which
consists of the alleles that an organism inherits from
it’s parents (ex: Ee, EE, or ee)
 Phenotype: The physical appearance of an organism’s
genotype.
Homozygous vs Heterozygous
 Homozygous: When both alleles of a pair are alike for a
specific characteristic.
 An organism can be homozygous dominate (EE) or
homozygous recessive (ee)
 Heterozygous: When both alleles in the pair are
different. (Ee)
Probability
 Probability = # of times expected to happen
# of times could happen
 Example:
 In Mendel’s experiment the tominate trait of yellow seed
color appeared in F2 generation 6,022 times. The
recessive trait of green seed color appeared 2,001 times.
The total number of individuals was 6,022+2001 = 8,023.
 What is the probability of the dominate trait?
Probability
 Probability of dominate trait = 6,022 = .75 or 75%
8,023
 This can be expressed in either a percentage, fraction, or a
ratio
 Probability tells us that there are three chances in four that
an offspring of two heterozygous individuals will have the
dominate trait and one chance in four that the offspring
will have the recessive trait.
Monohybrid Crosses
 Monohybrid Cross: a cross that only one characteristic is tracked
is monohybrid.
 Punnett Square: a diagram used to aid biologists in predicting
the probable distribution of inherited traits in offspring.
 Six Examples:
1.
Homozygous X Homozygous
2. Homozygous X Heterozygous
3. Heterozygous X Heterozygous
4. Testcross
5. Incomplete Dominance
6. Codominance
Example One:
Homozygous X Homozygous
 There is a 100% probability that the offspring will have
one specific genotype, thus producing the same
phenotype.
Example Two:
Homozygous X Heterozygous
 Will produce an outcome 50% probability dominate or
recessive.
Example Three:
Heterozygous X Heterozygous
 ¼ BB, ½ Bb, ¼ bb
 Genotypic Ratio: 1BB: 2Bb: 1 bb
 Phenotypic Ratio: 3 Brown, 1 Blue
Example Four:
Testcross
 How can you determine if a guinea pig is homozygous
(BB) or heterozygous (Bb)?
 Perform a Testcross: taking an individual with an
unknown genotype and crossing it with another
individual who is homozygous
Example Five:
Incomplete Dominance
 Incomplete dominance occurs when the phenotype of a
heterozygote is intermediate between the phenotypes
determined by the dominant and recessive traits.
 Pink flowers (Rr) come from a dominant homozygous red
parent (RR) and a recessive homozygous white parent (rr).
Example Six:
Codominance
 Codominance occurs when both alleles for a gene are
expressed in heterozygous offspring.
 Neither allele is dominant or recessive, nor do the alleles
blend in the phenotype (as in incomplete dominance).
Dihybrid Cross
 Dihybrid cross: is a cross which two characteristics
(genes) are tracked.
 The offspring of the animal being crossed in a dihybrid
cross is called a dihybrid.
 Two examples:
1. Homozygous X Homozygous
2. Heterozygous X Heterozygous
Example One:
homozygous X homozygous
 Review to page 185
 Two seeds with two different genes being expressed:
(1) round, yellow seed (RRYY)
(1) wrinkled, green (rryy)
Example Two:
Heterozygous X Heterozygous
 Use the same procedure as in homozygous x
homozygous.