Chapter 9
Patterns of Inheritance
Overview
In the previous chapters we have explored the principles of sexual and asexual reproduction, and the role each
has in the variation of an organism. In the discussion of meiosis we examined the role that independent
assortment has in increasing variation. In this chapter, we will examine the work of Gregor Mendel, who is
considered the father of modern genetics for his work in explaining observable patterns of inheritance.
For additional practice in this unit, you should work some of the genetics problems in the Lab Manual. An
understanding of this chapter is need for the discussion on human genetics.
Assigned Reading
Text, Pages 153-177
PowerPoint Presentation
Chapter Review, Page 178
Testing Your Knowledge, Page 179
Key Terms
 Gene
 Allele
 Locus
 Homozygous, heterozygous
 Dominant, recessive
 Genotype, phenotype
 Gregor Mendel
 Homologous chromosomes
 Codominance
 Incomplete dominance
 Monohybrid cross, dihybrid cross
 Law of segregation
 Law of independent assortment
 Punnett square
 Autosomal dominant, autosomal recessive
 Amniocentesis
 Pleiotropy
 Polygenic inheritance
 Linked genes
 Sex-linked (X-linked) inheritance
Introduction
After examining the process of meiosis in the previous lesson, we know that a system exists to introduce
variation into the reproductive process. Previous discussions on Charles Darwin have indicated that natural
selection acts on this variation in the "survival of the fittest" scenario.
In this chapter we are going to explore the works of Gregor Mendel, considered by many to be the "father of
genetics". What is important to realize here is that even in an introductory biology class, such as BIOLOGY 1010,
we have much more knowledge of cellular functions than Mendel did. During Mendel's time the microscope
was a primitive instrument, DNA had not yet been detected in cells and the processes of mitosis and meiosis
were unknown. But a few early scientists observed that there appeared to be patterns in the family trees of
organisms, and they attempted to explain these mathematically.
Key Genetic Concepts
Section 9.2 of your textbook does a good job of describing Mendel's approach to genetic experiments. A few
key items to note: 1) Mendel had the ability to form "true-breeding" plant lines - by doing so he was able to limit
the variation in a single plant line to a specific trait, a useful tactic in genetics. 2) You need to take a good look at
the Figure 11.4 and understand the idea of gene locus and alleles.
The locus is the physical location of a gene on a chromosome - and is dependent on the species (not the
individual). Thus all Homo sapiens have the gene for eye color at one location. Alleles are variations of genes.
There may be many variations for a gene ( eye color) or very few (metabolic pathways for example).
Monohybrid Crosses
Prior to Mendel's time it was believed that traits
were blended from one generation to another.
However, one of the first things that Mendel
noticed when crossing his pea plants was that
some traits (flower color, height of plant, etc)
did not blend over generations. To test this,
Mendel used a number of lines of pea plants
which bred true for a single trait. He then
designed crosses between these traits. We call
these types of crosses monohybrid crosses since
they test for a single trait.
We will use a procedure called a Punnett Square
to do out genetic analyses. You will become
very familiar with these in lab, but for now click
on details on how to construct these squares.
How to make a Punnett Square (Monohybrid Cross)
Mendel constructed lots of these crosses (see Figures 9.3A and 9.3B) and always ended up with a 3:1 phenotypic
ratio in the F2 generation. This told Mendel that the traits were not blended between generations, but rather
were distinct inheritable components which segregated mathematically. We know now that this is DNA, but
that would take decades to establish.
Test Cross
We can use Mendel's monohybrid cross to do some very elementary, but interesting, genetic analyses. The
question is, how do we know if an individual which displays a dominant trait is homozygous dominant (2
dominant alleles) or heterozygous (one dominant and one recessive allele). See the link to construct this cross:
How to do a testcross.
Dihybrid Crosses
The next question that Mendel faced was whether there was any interaction between different traits in an
organism. In other words, did the ratio of one trait influence the ratio of another unrelated trait? To do this
Mendel constructed a dihybrid cross which used two traits. Figure 9.5A diagrams this cross very well.
Note that we set up a dihybrid in much the same manner as a monohybrid cross. The possible gametes from
each parent go along the top and left edges of the cross and the possible zygotes are located inside the cross.
Important Note: Gametes are haploid and must have a single copy of each trait. Zygotes are diploid and must
have two copies of each trait.
additional example of a dihybrid cross
A few very important items to be considered:
If the F1 parents are heterozygous for each trait, then the F2 generation will have a 9:3:3:1 phenotypic ratio. In
a monohybrid cross the chances of getting a homozygous recessive individual were 1/4. In a dihybrid cross the
chances are 1/4 x 1/4 or 1/16, which is shown in Figure 9.5A as the “aabb” offspring in the lower right of the
cross
Variations in Mendelian Genetics
If only genetics were as simple as Mendel indicated! Unfortunately, there are many factors which cause the
ratios described above to deviate from what Mendel predicted. The closing sections of this chapter should give
you an idea of the complexity of genetics. Many of these will be explored in lab, but for the test you should be
able to define the following: codominance, incomplete dominance, pleiotropy, epistasis, continuous variation.
Links of Interest
The Biology Project - site maintained by the University of Arizona, link to the genetics pages
Mendelweb.Org - A website about Gregor Medel
Concepts
 Understand the work of Gregor Mendel and his contribution to genetics.
 Understand the principles of dominance, segregation, and independent assortment.
 Be able to construct Punnett squares and solve genetic probability questions. Specifically be able to tell
the numbers and percentages of heterozygous and homozygous genotypes, as well as dominant and
recessive phenotypes. You will have to answer questions based on these numbers on the exam!
 Understand the ratios from simple Mendelian crosses.
 Understand the principles of monohybrid and dihybrid crosses.
 Understand autosomal inheritance and how this can cause disease.
 Understand how to determine a human’s blood type based on genotype.
 Understand the different methods of determining sex/gender based on chromosomes.
 Understand sex-linked inheritance, and be able to solve problems involving Punnett squares with this
method of inheritance.
Review Material
MyBiology.com—Study guides and resource for this text. Specifically look at MP3 Tutor and all Web Activities