Genetic Crosses

Genotype vs PhenotypeGenotype: 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 HeterozygousHomozygous: 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)

ProbabilityProbability = # of times expected to happen

# of times could happenExample:

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?

ProbabilityProbability of dominate trait = 6,022 = .75 or

75% 8,023This 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 CrossesMonohybrid 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 Homozygous2. Homozygous X Heterozygous3. Heterozygous X Heterozygous4. Testcross 5. Incomplete Dominance6. Codominance

Example One:Homozygous X HomozygousThere is a 100% probability that the offspring

will have one specific genotype, thus producing the same phenotype.

Example Two:Homozygous X HeterozygousWill produce an outcome 50% probability

dominate or recessive.

Example Three:Heterozygous X Heterozygous¼ BB, ½ Bb, ¼ bbGenotypic Ratio: 1BB: 2Bb: 1 bbPhenotypic Ratio: 3 Brown, 1 Blue

Example Four:TestcrossHow 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 DominanceIncomplete 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:CodominanceCodominance 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 CrossDihybrid 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 Homozygous2. Heterozygous X Heterozygous

Example One:homozygous X homozygousReview to page 185

Two seeds with two different genes being expressed:

(1) round, yellow seed (RRYY)(1) wrinkled, green (rryy)

Example Two:Heterozygous X HeterozygousUse the same procedure as in homozygous x

homozygous.