Exploring Mendelian Genetics

Section 11-3

Exploring Mendelian Genetics

Section 11-3

Objectives:7.0 Apply Mendel's law to determine phenotypic

and genotypic probabilities of offspring.7.1 Defining important genetic terms, including dihybrid cross, monohybrid cross, phenotype,

genotype, homozygous, heterozygous, dominant trait, recessive trait, incomplete dominance,

codominance, and allele 7.2 Interpreting inheritance patterns shown in

graphs and charts 7.3 Calculating genotypic and phenotypic

percentages and ratios using a Punnett square

Objectives:7.0 Apply Mendel's law to determine phenotypic

and genotypic probabilities of offspring.7.1 Defining important genetic terms, including dihybrid cross, monohybrid cross, phenotype,

genotype, homozygous, heterozygous, dominant trait, recessive trait, incomplete dominance,

codominance, and allele 7.2 Interpreting inheritance patterns shown in

graphs and charts 7.3 Calculating genotypic and phenotypic

percentages and ratios using a Punnett square

Mendel’s Law of Independent Assortment

• Def: When the segregation of alleles for two different traits occur independently of each other

• EX: Mendel found that peas of either color (yellow or green) could be either round or wrinkled.

• This happens when the alleles for the traits are found on diferent chromosomes.

Mendel’s Two-Factor Cross (F1 Results)

• Mendel took two purebred parent plants with differences in two different traits, and cross-bred them.• One plant had round(R), yellow peas(Y); the other had wrinkled(r), green peas(y).• What is the phenotype of the offspring?

Mendel’s Two-Factor Cross (F2 Results)

• Crossed two, identical appearing F2 plants to produce 556 seeds.

• 315 seeds were round and yellow, and 32 were wrinkled and green - the two parental phenotypes

• However, 209 of the seeds had combinations of phenotypes—and therefore combinations of alleles—not found in either parent.

• MEANING? The alleles for seed shape segregated independently of those for seed color—a principle known as independent assortment

Mendel’s Two-Factor Cross (F2 Results)

• Genotype ratio is 9:3:3:1.

• This is very close to Mendel’s results!

• These results can also be reached by doing 2 single-factor crosses.

Beyond Dominant and Recessive

• Incomplete dominance: – produces a blending of traits– EX: red flowers(R) x white flowers(W)

produce pink flowers(RW)• Codominance:

– both traits show up– EX: black-feathered chicken x white-

feathered chicken produces chickens with both black feathers and white feathers

Beyond Dominant and Recessive

• Multiple alleles: – more than two alleles in the population– EX: coat color in rabbits have 4 possible

alleles– EX: human blood types

• Polygenic traits: – Traits produced by the interactions of

many alleles– EX: At least three genes are involved in

making the eye color of fruit flies– EX: Human skin color is controlled by

more than four different genes