Biology 101
Section 6 Notes
Patterns of Inheritance
These notes will correspond to Chapter 9 in the textbook.
These are the notes on genetics. Many of these notes will be familiar; they are closely related to
cell division. Several terms, concepts and points of interest are shared between the two tests.
Hence, understanding cell division is important for this section. However, it also makes this
section partly review and easier, by comparison. So here we go.
Since this section deals with genetics, the best place to start is to define genetics. Many people
today either get confused, or misled, or daunted by the idea of “genetics.” Many people’s view
of genetics is highly flavored by the media, TV and Movies like “The X-Men”. However, the
definition of genetics is very simple, and incredibly old.
Genetics = science of heredity.
Genetics have been practiced by humans since we first domesticated animals. It is not something
only practiced by weird people in white lab coats in strange labs. Anytime someone works out
the heredity of an animal or plant, such as professional horse breeders, they practice genetics.
So, what is heredity?
Heredity: passing of characteristics from parents to offspring, from one generation to the next.
Since genetics predates modern science, the original definition of heredity does not directly deal
with genes.
Wildtype vs. Variant
In genetics, we often deal with what is referred to as the wildtype, a sort of generic, all-purpose
version of an animal or plant. Something plain that contains the basic traits for that species being
studied. A representative sample for that species. However, if there are members of a species
with drastic variation from the norm, mainly some sort of mutation or aberration, they are
referred to as a Variant. For example, humans normally have some degree of pigmentation due
to melanin. Practically any human picked from around the globe could serve as a wildtype.
However, someone with albinism (an albino) does not produce pigment. This is due to a
mutation in the melanin making gene, and would be considered a variant, though generally not a
harmful one.
Roots of Genetics
The roots, or beginnings, of genetics dates all the way back to the ancient Greeks. Two of the
most influential predecessors of genetics are Hippocrates and Aristotle. They are the first to
suggest that organisms inherit traits specifically from their parents and are not a completely new
organism at birth, that traits can be inherited in small, discrete packets (which we recognize as
genes today), and that organisms can change over time by selection of certain traits over others.
Remember, these individuals are also given credit for the first hypotheses on Evolution.
However, they did not fully understand the behavior of traits, knew nothing of DNA or cells, and
could not test their ideas well. The thinking on genetics stopped until the mid 19th century, when
a Franciscan monk reinvigorated research into genetics and developed most of our modern
understanding of how genes behaved and the mathematics of genetic probability. His name is:
The Father of Modern Genetics
 Began in 1860s with Gregor Mendel
 Discovered fundamental principles of genetics
 Parents pass on to offspring discrete heritable factors (genes)
Gregor Mendel is known as the Father of Modern Genetics (at least the math based part). He
developed several rules to determine how genes behaved. These are covered well in the biology
book, so I would read over the material about him closely. Also note the remarkable close
timing of his development of genetics with Darwin’s release of his book On the Origin of
Species. These two events occurred within a couple of years of each other, purely by
coincidence.
The following list is a list of terms or words that you should look up and write the definition for
them in your notes.
Self-fertilize
Cross-fertilize
Gene
Allele
Terms to Know
Cross (genetic)
Hybrids
Monohybrid cross
Dihybrid cross
P-generation
F1-generation
F2-generation
Punnett Square
Mendel developed several principles for genetics, which here I have divided into three main
ones. The book lists only two; it includes Mendel’s Principle of Dominance with his Principle of
Segregation.
Principle of Segregation
- Mendel's hypotheses (review them)
- Alleles = alternative forms of genes
- Dominance and Recessiveness
Pairs of genes, on homologous chromosomes, separate during gamete formation, and are paired
again at fertilization, with one half coming from each parent.
More Terms to Look Up
Homozygous (homozygous dominant versus homozygous recessive)
Heterozygous
Phenotype
Genotype
More notes on these terms and performing crosses are included as an additional note.
Genes and Chromosomes
Now, we understand DNA and the passage of genetic material in the context of cell-division.
When cells divide, they replicate, separate and pass on chromosomes. How are genes and
chromosomes related? A chromosome is a fairly large, long piece of DNA, 10s of millions of
bases long (DNA base pairs used in the genetic code). A typical gene is approximately 1500
bases long. Several thousand genes will fit on one chromosome. From the ancient Greeks up to
Mendel scientists always believed that genes or traits were inherited separately from one another,
as individuals. Mendel even points this out strongly with his Principle of Independent
Assortment. However, we know better today. Genes are not fully independent, they occur as
bundles on chromosomes, and we inherit chromosomes from our parents, not individual genes
one at a time. This phenomenon is described by the theory of Linked Genes. Modern genetics
has determined a way for genes to get around being “linked” and is related to another
phenomenon described in the previous section, Crossing Over. For the sake of this class we will
treat genes like they are inherited separate from one another.
Gene: a segment of DNA that codes for a heritable trait
- located at gene loci or loci on chromosomes
- chromosomes are 1000's of genes in size
Principle of Independent Assortment
- Each trait's (gene's) separate copies, alleles, segregate, or split to opposite sides of the cell,
independently from one another during gamete formation (shown with a dihybrid cross).
This means that the inheritance of one gene does not determine the inheritance of another.
Just because you might inherit blue eyes does not mean that you will necessarily inherit light
hair. People come with blue eyes and dark hair and dark eyes with light hair. Useful with a
test cross: a cross of an unknown genotype with a homozygous recessive to identify the
unknown genotype.
Principle of Dominance
- Each trait has at least two different versions, or alleles. One is dominant and one is recessive.
Whenever the dominant is present it will completely mask or inhibit the expression of the
recessive. This is called complete dominance.
- The dominant allele masks the effects of the recessive.
- Anytime you have the dominant allele, that particular phenotype is expressed.
Note: two of the most important words to ever learn with genetics are Genotype and Phenotype.
I would study these repeatedly until you have their definitions down.
The Relationship of Genotype to Phenotype is Not a Simple One
NOW!! EXCEPTIONS TO MENDEL'S PRINCIPLES
Mendel, after all, was still human. He made mistakes, assumptions, and technology has
advanced remarkably since his time. Here are a few exceptions to Mendel’s general rules.
Incomplete Dominance:
One allele in a pair of genes is not fully dominant over the other, resulting in a middle
phenotype that is a mixture of the other two: also sometimes called a weak dominant.
Ex. snapdragons
Codominance:
Pair of non-identical dominant alleles that specify two wholly different phenotypes are
present in the person and both are expressed at the same time in the heterozygote condition.
Ex. blood type
Multiple Alleles:
A gene may actually have more than two alleles; three or four, or sometimes more.
Ex. blood type
Pleiotropy:
A single pair of genes affects many phenotypic characteristics in the organism.
Ex. growth hormone
Epistasis:
One set of alleles (a gene) may mask or inhibit the expression of another gene's alleles.
Ex. coat color in Labrador Retrievers
Polygenic Inheritance:
This is a condition of continuous variation in a trait, often over a wide spectrum of
variation that is the result of the additive effects of two or more genes on a single phenotypic
trait.
Ex. human height
Linked Genes:
Genes located close together on the same chromosome tend to not separate during
crossing over and the formation of the gametes and to be inherited together.
Problems of Genes
The problems of genes, or more precisely genetics, falls into one of three categories: mutations,
chromosomal aberrations, and abnormal numbers of chromosomes. To understand these
problems, some general background on a few terms is needed.
Pedigree = family history of genetic traits
Pedigree is a term most people apply to dogs, cattle and horses. Many feel uncomfortable with
applying the term to humans. But every time someone goes to the doctor and the doctor asks for
their family history of cancer or heart disease, the doctor is asking for their pedigree and people
will answer this question quite easily.
Mutation = a change in the base code of DNA for a gene
A mutation is actually a rather narrow phenomenon. It is the change or alteration of the base
sequence in a strand of DNA. Technically speaking, Down’s Syndrome (for those who might
know what it is) is not a mutation. Mutations typically are not that bad, since they are restricted
to only one gene at a time. And no, mutations will not lead to super human abilities or powers.
The majority of mutations, if they do anything, are fatal.
Genetic Abnormality vs. Genetic Disorder
(genetic "disease" not correct)
The homework answers this one some. Most geneticists do not like the word “disease” being
applied to people with genetic problems. Generally disease refers to an infection with either a
bacterium, virus, fungus or a parasite. Since genetic problems are inherited, intrinsic and a part
of the person (in every cell), and are not a foreign agent, they use the terms Abnormality or
Disorder to describe them. Genetic abnormalities generally are not life threatening, require no
long term medical treatment or hospitalization, and require a minimum medical treatment or
medication. People with genetic abnormalities usually lead a full and normal life. An example
is red-green colorblindness. A genetic disorder, on the other hand, can be quite severe, needs
constant treatment or medication, may require hospitalization, and can be potentially fatal. An
example is hemophilia.
Disorders come in two varieties, depending on whether or not the allele that causes the disorder
is recessive (which is typical) or is dominant. Also, disorders are separated based on whether
they occur on the regular first 22 pairs of chromosomes called autosomes or on the 23 pair called
the Sex Chromosomes. More on this is covered in an additional note.
Autosomal recessive disorders + Autosomal dominant disorders
Carriers = heterozygotes for a recessive disorder
Kinds of Mutations
1) Base Substitutions = replacing a base with another
Example: AATCGAGTTCAGCGT → AATCGAGTTCATCGT
2) Base Insertions = adding extra bases to a sequence
Example: AATCGAGTTCAGCGT → AATCGAGTAGCTCAGCGT
3) Base Deletions = removing bases from a sequence
Example: AATCGAGTTCAGCGT → AATCGAGCAGCGT
Chromosomal Aberrations
Some problems are not mutations per say, but alterations in the gross over-all structure of the
chromosome. These are considered to be substantially worse since any aberration will affect a
host of genes at once, not just one. There are four ways a chromosome can be malformed:
1) Deletions = loss of whole sections of chromosome
2) Duplications = section of chromosome copied
3) Inversions = piece of chromosome reversed
4) Translocation = part of one chromosome is put into another
I would suggest reading the part in the chapter on this closely and paying close attention to the
drawings in the figures. They are very good.
Changes in Chromosome Number
Sometimes, instead a change in the shape or figure of a chromosome, or gaining and losing
pieces, we will gain or lose a whole entire chromosome. This is the worst that can happen
genetically to a person. Nearly every case of this happening is fatal. A person cannot live with
one of the autosomes missing, and there are only two known ways a person can get an extra and
survive: an extra #21 (Down’s Syndrome, also called trisomy 21) and an extra #13. Extra sex
chromosomes seem to occur more frequently and easily. Research Klinefelter’s Syndrome and
Metafemale Condition. Also, it is among the sex chromosomes that the only known case of a
human surviving with a missing chromosome is known. Research Turner’s Syndrome. Now,
when dealing with chromosome numbers (or anything else) there are only two ways to change
number: increase or decrease.
Aneuploidy = typically the loss of a whole chromosome (this term is sometimes used to refer to
the gain of only one extra chromosome too)
Polyploidy = the addition of a whole chromosome or chromosomes
Sex Linked Genes
Autosomes = pairs 1-22
Sex Chromosomes = pair 23
The last pair of chromosomes is called the Sex Chromosomes (and yes both words are
capitalized, it is a formal name). All genes within the body that determine the sex of an
individual, their gender, are located on the Sex Chromosomes and nowhere else. And the only
genes found on these chromosomes, normally, are those involved in sex determination.
Remember, for all chromosomes they occur in pairs, one inherited from each parent. So for the
Sex Chromosomes, they are paired up to actually determine whether you are a female or male.
Sex Chromosomes
 Contain genes that determine gender
 Two of them; X and Y
 All genes that determine the female gender are located on the X chromosome
 All genes that determine the male gender are located on the Y chromosome
 XX is female, XY is male
 The Y chromosome is dominant to the X. If you inherit a Y chromosome you are male
regardless of the number of X chromosomes you may inherit.
 Yes, the male actually determines the gender of the child, though this is chance and not a
deliberate act on the males part
 Sex-linked genes are those genes that are found on the sex chromosome but are unrelated to
sex determination and usually cause disorders of some type
 Most sex-linked genes are found on the X chromosome (80%)
 Passed on maternally from mother to son
! Most disorders occur in males! Why? There are no such things as male carriers for sex-linked
traits.
Ex. colorblindness and hemophilia
Some Final Notes on Probability
Mendel's crosses and rules reflect chance, not certainty. They are a mathematical means
of determining what the probabilities are for certain genetic outcomes. Genetic crosses show
only the odds of getting a particular genotype at any one time in the offspring, not what must be.
Genes, Natural Selection and Adaptation
Most mutations are what we call neutral (no effect). Some mutations are good, but the majority
of the non-neutral mutations are deleterious. Mutations, genetic recombination and crossing over
provide novel new gene combinations.
Genome = the total of all genes in an individual
Gene Pool = the total collection of genes in a population
The Hardy-Weinberg Equilibrium
Equilibrium law. The frequency of each allele in the gene pool will remain constant. If
the frequency of one is known, the other can be determined.
P+q=1
p2 + 2pq + q2 = 1
p and q frequency of alleles for a gene, p = A, q = a
p2, 2pq, q2 are frequencies of genotypes in a population
Molecular Biology of the Gene: Revisited
Genes = short pieces of DNA, are encoded in DNA
 ~30,000 in humans
 Located on chromosomes (loci)
 Many on one chromosome, but lots of empty space
 Have a beginning and end
What are exons and intron?
What is a codon?
What is the Genetic Code?
What is Junk DNA?
Many believe that the extra DNA has regulatory functions
Structure of DNA
Made up of nucleotides
Double stranded
Helix
Sugar-phosphate backbone
Nitrogen bases, face inward
Hydrogen bonds
Ladder-like shape
Complimentary base pairing


4 bases: adenine (A), guanine (G), cytosine (C), thymine (T)
Base sequence determines nature of gene [ATTACGTATCG]