Mendelian Genetics
Standard 3.2: Determine the probability of a particular trait in an offspring
based on the genotype of the parents and the particular mode of inheritance.
Chapter 10.2
How Genetics Began…
• Gregor Mendel:
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“Father of Genetics”
1866-Austrian monk
Plant breeder (true-breeding pea plants used for experiments)
Published his findings on the method of inheritance
• Inheritance: The passing of traits to the next generation.
• Also called heredity
• True-breeding: consistently produce offspring with only one form of a
trait.
Gregor Mendel’s Experiments
• Performed cross-pollination:
• Transferring a male gamete from one flower to a female flower.
• Analyzed his results and formed hypotheses on how traits are
inherited.
• Genetics: the science of heredity
Inheritance of Traits
• P generation: Parent Generation
• In Mendel’s experiments, the P generation consisted of a
true-breeding green-seed plant and a true-breeding
yellow seed plant.
Inheritance of Traits
• F1 Generation: First Filial Generation
• The offspring of the P generation
• In Mendel’s experiments, the cross between the P generation
produce all yellow seeded plants in the F1 generation.
• Green seeds were not present.
Inheritance of Traits
• F2 Generation: Second Filial Generation
• The offspring of the F1 generation
• In Mendel’s experiments, he crossed two pea plants from the F1
generation.
• He developed a 3:1 ratio of yellow to green seeds.
• 75% were yellow seeds
• 25% were green seeds
Mendel’s Conclusion:
• There must be two forms of the seed trait in the pea plants:
• Yellow-seed and green-seed
• Each are controlled by a factor called an allele.
• Allele: Single gene passed from generation to generation.
• Y for yellow (dominant)
• y for green (recessive)
• The 3:1 ratio observed in his experiments could be explained
if the alleles were paired in each of the plants.
Mendel’s Conclusion:
• Law of Dominance:
• There are dominant alleles.
• Masks the recessive alleles (i.e. hides them)
• There are recessive alleles.
• Alleles that are masked (hidden) by the dominant alleles.
• In Mendel’s experiments, which color was dominant?
• Recessive?
• Answer: F1 produced all yellow seeds, but in F2 green seeds reappeared.
• Yellow must be dominant, and green recessive.
Dominance
• When modeling inheritance:
• The dominant allele is represented by a capital letter (A).
• The recessive allele is represented by a lower-case letter (a).
• Homozygous: two of the same alleles for a certain trait.
• Homozygous Dominant: AA
• Homozygous Recessive: aa
• Heterozygous: two different alleles for a certain trait.
• Aa
• These organisms are called hybrids.
Genotype vs Phenotype
• Genotype: the organism’s allele pairs.
• AA
• Aa
• aa
• Phenotype: the observable characteristic or outward expression of an
allele pair.
• Hint: what does it look like??
• Yellow? Green?
Mendel’s Law of Segregation
• Two alleles for each trait separate during meiosis.
• During fertilization, two alleles for the trait unite.
• Hint: One allele comes from the male, and one from
the female in the crossed plants.
Monohybrid Cross
• Crossing of two organisms for one trait.
• Example: crossing plants for flower color.
P
p
p
P
Pp
Pp
Pp
Pp
1. Which color is
dominant?
2. Which color is recessive?
3. What is the phenotype
for all of the offspring?
Law of Independent Assortment
• A random distribution of
alleles occurs during
gamete formation.
• Genes on separate
chromosomes sort
independently during
meiosis.
• Predictable Phenotypic
Ratios for two
heterozygous traits:
• 9:3:3:1
Dihybrid Crosses