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LESSON 2: INHERITANCE
AND HEREDITY PATTERNS
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Lessons:
1 | 2 |
Overview
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 In
a previous lesson, we learned how gametes are formed through the
process of
meiosis. We also know that the fusion of two gametes
is the beginning of the formation of a new organism. If the DNA
in the gametes is the blueprint for the new individual, how are
traits passed on to offspring? Why do children look similar to
their parents, but not identical? Why do some diseases get
passed on to all of the offspring while others don’t? These are
the questions that will be answered in the next few lectures.
Gregor Mendel set most of the groundwork that is the foundation
of modern genetics today. But many discoveries have been made
since his time. For instance, some genes are usually inherited
together because they are close to each other on a chromosome.
This is called
linkage. Additionally, some genes are only passed on to offspring
of a certain sex. These are
sex-linked genes. We will see many other examples of types of
genes, how offspring inherit them from their parents, and how
this affects their ultimate
phenotype.
Learning Objectives
Topics covered in this Lesson
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 Gregor
Mendel was an Austrian monk who lived in the middle of the 19th
century. Mendel performed breeding experiments on bean plants by
measuring the heredity patterns of a number of different
characters of the plants, such as the color of fruits and
flowers, the shape of seeds and fruits, and the size of plants.
Mendel’s studies led him to propose two fundamental laws that
govern the heredity of many characters in organisms – the law of
segregation and the law of independent assortment. Although
Mendel’s work was largely ignored for over 30 years after its
completion, it ultimately has provided a strong foundation for
the understanding of inheritance and
gene
expression.
Learning Objectives
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Understand Mendel’s first and
second laws and how they apply to the heredity patterns
Mendel observed in pea plants.
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Understand the term allele and
the relationship between dominant and recessive alleles.
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Know the difference between
monohybrid and dihybrid crosses.
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Understand the link between
genotype and phenotype.
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Understand the difference
between a
character and a
trait.
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 While Mendel’s experiments revealed
systematic laws involved in patterns of heredity, they certainly
didn’t explain everything. As usually happens in scientific
research, finding the answer to one question raises at least ten
more. Clearly not every inherited trait follows Mendel’s laws,
so what else is involved? In this lesson we will investigate
some other types of inheritance that take Mendel’s laws another
level deeper or put a twist on them.
While not all aspects of heredity
follow strict Mendelian laws, Mendelian inheritance patterns
have turned out to be very important and useful in fields such
as plant and animal breeding and understanding and diagnosing
inherited diseases in humans.
Learning Objectives
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Understand that not all genetic traits are inherited in
Mendelian fashion.
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Keep thinking about how genotype affects phenotype.
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Understand how pedigrees are used to track inherited
characteristics.
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Understand how alleles for genetic diseases are passed
down through families and how this process is different
in dominant versus recessive diseases.
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 In
eukaryotic organisms, the majority of
genes
are located on
autosomes, or non-sex chromosomes. While many of these
genes have Mendelian inheritance patterns, there are also many that
do not. Inheritance patterns that differ from Mendelian patterns can
occur for a number of reasons, including location of the gene(s) on
the chromosome, environmental effects, and varying characteristics
of the protein made by different
alleles
of the gene. Although these types of relationships lead to
heredity patterns that would seem to disprove Mendel’s laws, this is
not exactly the case. Rather, instances of non-Mendelian inheritance
provide us with further insight into the link between genotype and
phenotype, and patterns of heredity.
Learning Objectives
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Understand and be able to define the
following terms:
incomplete dominance,
codominance,
epistasis,
pleiotropy.
- What makes alleles different? Do some
genes have more than two alleles? Do some only have one?
Where do alleles come from?
- How can the location of genes on a
chromosome affect heredity?
- Be able to describe the concept of a
genetic map.
- What other types of DNA (besides
nuclear DNA) are in eukaryotic cells? How are they
inherited?
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 As humans, we are very accustomed to
considering others as male or female, girl or boy, or man and
woman. It may be surprising to learn, however, that the majority
of sexually reproducing organisms on earth, including many
plants, algae, and even some animals, are functionally both male
and female at the same time.
In species that do produce male and
female individuals, sex may be determined in a number of
different ways. In some cases, such as humans, sex is determined
by special
chromosome, called a
sex chromosome. Often, genes for non-sexual
characters are also found on sex chromosomes. In such cases,
inheritance patterns of these characters may vary from typical
Mendelian inheritance.
Learning Objectives
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Are all species classified into
‘male’ and ‘female’ sexes? Why or why not?
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What is a sex chromosome? What does
it mean for an organism to be dioecious?
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Understand how sex is determined
and know some specific examples.
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Be able to describe how X and Y
chromosomes apply to the determination of human sexes.
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Understand the inheritance patterns
of sex-linked genetic diseases in humans.
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 Prokaryotic
organisms, the Bacteria and Archaea, are found just about
everywhere on earth and were the first organisms to appear in
the geologic record. They have been tremendously successful
despite the fact that they reproduce asexually, producing
clones with exactly the same genetic makeup as the
original cell. This reproductive strategy seems like it would
hinder a species in adapting to environmental changes, but
prokaryotes have developed several types of horizontal
gene transfer to acquire and exchange genetic
material.
Learning Objectives
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Understand the difference
between vertical and horizontal gene transfer.
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Be able to describe the three
main types of horizontal gene transfer in prokaryotes.
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Know what a
plasmid is and what types of genes they typically carry.
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Understand how the process of
conjugation works.
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Know what
transposable elements are and what effects they may have on the
evolution of a species.
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