Chapter 14 Part I: Mendel and
the Gene Idea
• This chapter details Mendel’s discovery of
general laws of heredity as well as
monohybrid dihybrid genetic crosses.
A. Gregor Mendel
1 Austrian Monk
2 Formulated two laws of
inheritance in 1860s.
3 Previously studied math
and science at University of
Vienna.
B. Blending Concepts of
Inheritance
1 Theory stated that offspring would possess
traits intermediate between those of
different parents.
2 Red and White flowers make Pink flowers.
3 Darwin wanted to develop a theory of
evolution based on Mendel’s heredity
principles.
C. Mendel’s Experimental
Procedure
1 Mendel did a statistical study
2 Prepared experiments carefully
A. Garden Pea
B . 22 true-breeding varieties
C . Simple traits were studied
3 Mendel traced inheritance of individual
traits.
4 Principles of probability used for
interpretation.
11.2 Monohybrid Inheritance
A . Cross-pollination Monohybrid Crosses
1 A hybrid is result of 2 true-breeding parents
2 A monohybrid cross is between two true-breeding parents for
two distinct forms of a trait.
3 Mendel tracked 2 generations
A . P Generation
B . F1 Generation
C . F2 Generation
B. Mendel’s Results
1 Contrary results to predictions
2 F1 resembled one parent
3 1/4 of F2 resembled one parent; 3/4 resembled other
parent like F1 generation.
4 3:1 ratio results
A. Dominant/recessive shown in F2.
B . Factors separated when gametes were formed.
C . Random fusion upon fertilization.
B. Mendel’s Results Continued
5 Mendel’s First Law of Inheritance: Law of
Segregation
A. Each organism contains 2 factors for each
trait; factors segregate in formation of gametes;
each gamete contains one factor for each trait.
B . Factors passed from generation to generation
C. As Viewed by Modern
Genetics
1 Traits controlled by two alleles, alternate
forms of trait found at the same gene locus.
2 Gene locus is a specific location on a gene
3 Homozygous is two identical alleles for a
trait.
A. Homozygous dominant - 2 dominant alleles
B . Homozygous recessive - 2 recessive alleles
4 After cross-pollination, all F1 are
heterozygous genotypes.
D. Genotype Vs. Phenotype
1 2 organisms with different allele combinations can
have same outward appearance (TT & Tt)
2 Genotype refers to alleles at fertilization.
3 Phenotype refers to physical appearance.
E. Laws of Probability
1 Probability is the likely outcome a given
event will occur from random chance.
2 Multiplicative law of probability = chance
of 2 or more independent events occurring
together is the product of the probability of
the events occurring separately
3 Additive law of probability = probability of
an event that occurs in 2 or more
independent ways.
F. The Punnet Square
1 Provides simple method to calculate
probable results of genetic cross.
2 Sperm cells lined up vertically; egg cells
lined up horizontally.
3 Larger sample sizes give better outcomes
for predicted ratios.
4 Humans use phenotypic rations for
predictions.
G. One-Trait Testcross
1 Mendel performed testcrosses by crossing F1 to
homozygous recessive.
2 Results indicated recessive factor in F1.
3 Testcross = dominant phenotypic individual crossed with
recessive individual.
11.3 Dihybrid Inheritance
A . Dihybrid Crosses
1 Dihybrid cross is an experimental
cross between 2 true-breeding
parents for 2 different traits.
B . Plants Self-Pollinate
1 Mendel observed 4 phenotypes
among F2.
2 This gave him Second Law of
Heredity: law of independent
assortment.
A . Members assort independently
of each other.
C. Dihybrid Genetics Problems
1 Laws of probability indicate a 9:3:3:1
phenotypic ratio of F2.
A. 9/16 dominant for both traits
B . 3/16 dominant for 1 trait, recessive for other
C . 3/16 dominant/recessive opposite of previous
D. 1/16 recessive for both traits
2 9:3:3:1 ratio expected when heterozygous
for 2 traits are crossed.
D. Two-Trait Test Cross
1 Dihybrid Test Cross - homozygous
dominant traits or heterozygous.
2 Dihybrid genetic problems: 4 alleles for 2
traits.
END OF CHAPTER 11