Patterns of Inheritance
Chapter 9
Gregor Mendel
• Austrian monk
• “Father of Modern
Genetics”
• Famous for his
work with peas
Mendel’s Peas
Cross-Pollinating
General Background Vocabulary
 Self-pollination: pollen from flower fertilizes the same
plant
 True-breeds: pure gene lines – offspring match parent
due to self-fertilization)
 Cross-pollination: pollen will fertilize a different plant
 Hybrid: Offspring that result from a cross between
organisms with different traits; express a combination of
traits from
 Character: heritable feature that varies among
individuals (ex. seed color)
 Trait: possible variations for a particular character (ex.
yellow seeds vs. green seeds)
Mendel’s Crosses
• Mendel noticed that one trait was always expressed
over the other in the F1 offspring
• However, trait that disappeared always reappeared
in about 25% of the F2 offspring.
Mendel’s Initial Conclusion
• Biological inheritance is determined by chemical
factors passed from one generation to the next
– Geneticists now refer to these heritable factors as genes
– Genes can come in more than one form, each form is an
allele ex. B or b (The “B” gene w/ 2 alleles)
• Additional relevant terminology:
– Homozygous: two identical alleles (AA or aa)
– Heterozygous: two different alleles (Aa)
– Phenotype: Physical appearance
– Genotype: Genetic make-up
• Homozygous dominant (AA)
• Homozygous recessive (aa)
• Heterozygous (Aa)
The Principles
Dominance & Segregation
• Dominance: Certain alleles will be expressed
over others, the expressed alleles are
dominant to the unexpressed recessive alleles
• Segregation: Each parent carries two alleles
for each gene. During meiosis, the pairs are
separated to that only one allele is sent to the
offspring in the gamete from each parent.
Genetics & Probability
• Probability is the likelihood of
an event happening
• Consider flipping a coin
– Likelihood of flipping heads =
50%
– (1 of 2 possibilities)
– Likelihood of flipping heads
twice?
• 50% x 50% = 25%
• To predict outcomes of genetic
crosses we use punnett
squares
The Test Cross
Exploring Mendelian Genetics
• Does segregation of one set of alleles influence
the segregation of another pair of alleles?
• Mendel’s Two Factor (dihybrid) Crosses
– Followed two traits at a time.
– Same method as his original single-factor crosses
– Cross-pollinated to produce the F1 and allowed
them to self-pollinate
• Mendel found that alleles for different traits
did not influence each others segregation.
This is referred to as the principle of
independent assortment.
Pedigrees
• Pedigrees are like genetic “family trees”. They are used
to show the inheritance of traits within families and to
predict genotypes and/phenotypes of certain
individuals.
• The following key explains the symbols and layout of a
typical pedigree:
Autosomal Dominant Pedigree
Autosomal Recessive Pedigree