Genetics and the
Mechanics of
Natural Selection
ANTH A-103, Human Origins and Prehistory
Larry J. Zimmerman, Ph.D., RPA
Indiana University-Purdue University
Indianapolis
Why study genetics?
Your life and that of offspring may
depend on it!
But it’s hard!
Yes, it does involve some
memorization and thought…
but how much is your life or that of
your child worth?
And…
Of course there are
questions of ethics…
The essentials
Generalized Cell Structure
DNA is in the nucleus; protein synthesis occurs in the ribosomes
The body is composed of cells of two
basic types: Somatic cells and gametes.
Somatic cells are the cellular components of body tissue:
muscle, skin, bone, nerve, heart and brain.
Gametes are sex cells specifically involved in reproduction
and not important structural parts of the body.
The nucleus
Contains two molecules or nucleic acids that contain the
genetic information that controls the cell's function:
DNA (deoxyribonucleic acid)
RNA (ribonucleic acid).
Outside the nucleus
The ribosomes are involved with translating the instructions
coded in the material of inheritance into proteins or
chemical messengers that are used by the body's cells.
Outside the nucleus
Mitochondria are involved with energy transfer within the
cell, and are interesting because they contain their own set of
hereditary material independent of that contained in the
nucleus.
Mitochondrial DNA can be traced through females and
dated back into time.
Chromosomes and DNA
The material of inheritance within the nucleus of a cell is
arranged in long strands called chromosomes.
On the molecular level the chromosomes are actually
nothing more than two long strings of DNA wound
together in a spiral-like structure called a double helix.
The bases and genes
Each of the two strands of DNA in a chromosome are
composed of varying combinations of 4 simple molecules
called bases.
The four bases are adenine, cytosine, guanine, and
thymine labeled A, C, G, and T. A single DNA strand is
composed of a string of bases, each of which can be one
of 4 types: A, C, G, or T.
The order in which the bases occur on the DNA strand is
not random.
Genes are nothing more or less than unique, specific
sequences of the 4 bases.
Allele
"Gene" is a layman's term.
Scientists avoid the use of the word gene, because it is
not very specific.
Locus (plural = loci) and allele are used instead.
Locus
A locus is the section of a DNA strand that contains the
instructions for one specific product (eye color, tongue rolling,
blood serum protein albumin).
A locus is simply a section of the chromosome that holds a
sequence of bases and is like an address.
The particular base sequence that resides at a given locus is
called an allele. Only one allele can reside at a given locus in any
one DNA strand.
Gene Expression
Alleles code for proteins
which are either the desired
product
or are messengers or
controlling substances that
produce the desired result.
A protein is simple a chain of
amino acids. Just like a DNA
strand is a chain of bases, a
protein is a chain of amino
acids.
There are 4 different bases
that can be chained together
to form a DNA strand
There are about 20 different
amino acids that can be
chained together to form a
protein.
Chromosome Number
Each cell of the body (except sperm and ova) has two of
each chromosome or diploid.
One chromosome came from the mother and one came
from the father.
So, the reproductive cells, the sperm and ova, must have
only one chromosome or haploid.
Haploid reproductive cells are called gametes.
The process
Another way to
diagram it
Human Genetic Structure
Humans have 23 pairs of chromosomes, 46 altogether.
For 22 of these chromosomes the two members of the pair are
pretty much identical or autosomal chromosomes.
23rd pair of chromosomes is the sex chromosomes.
The sex chromosomes come in two varieties, X and Y.
Females have two X chromosomes
Males have one X chromosome and one Y chromosome.
Generally speaking, if you have a Y chromosome you are a
male.
The Human Karyotype
46 chromosomes
in 23 pairs
Pair 23 is the sex
chromosome, in
this case a male
Chromosomes
have two arms that
are joined at the
centromere.
The short arm is
called the p arm
for the French
word petit, which
means small or
short.
The long arm is
called the q arm
because q comes
after p in the
alphabet.
The ends of the
chromosomes are
called telomeres.
Detail on Chromosome Structure
Sex and the single chromosome
All the gametes produced by
females carry only X
chromosomes.
But, half of the gametes
produced by males carry the X
chromosome and half carry the Y
chromosome.
Whether a given offspring is
female or male depends on
whether the father's sperm
happens to have an X or a Y
chromosome.
Autosomal and sex-linked chromosomes
Most traits are autosomal traits, the loci carrying alleles for
them are located on one of the autosomal chromosomes.
A few traits are sex linked, carried on one of the sex
chromosomes - usually the X.
One sex-liked trait is hemophilia, caused by a recessive gene
on the X chromosome.
It occurs mostly in men because they only have one X
chromosome.
If they have the recessive gene, then they will have hemophilia.
Mendelian inheritance
Gregor Mendel
1822-1884
Mendel’s Sweet Pea
Experiments
Punnet Squares
Medel’s Peas
Punnet Squares
Key Mendelian Terms
Genotype: the genetic structure of the
population
Phenotype: the genetic structure of the
individual
Homozygous: both genes are the same
(RR, YY)
Heterozygous: the genes are different (Rr,
Yy)
Dominant: the characteristic shows (R, Y)
Recessive: the characteristic normally is
hidden (r, y)
Dominant shows: RR, Rr, YY, Yy
Recessive shows: rr, yy
Dominant & Recessive
Principle of Segregation
• Genes occur in pairs because chromosomes occur in pairs.
• During gamete production, the members of each gene pair
separate so that each gamete contains one member of a
pair.
• During fertilization, the full number of chromosomes is
restored and members of a gene or allele pairs are reunited.
Principle of
Independent Assortment
• The distribution of one pair of alleles into gametes does not
influence the distribution of another pair.
• The genes controlling different traits are inherited
independently of one another.
Genetic Disorders
Hereditary vs. acquired Mutations
Gene mutations can be either inherited from a parent or acquired. A
hereditary mutation is a mistake that is present in the DNA of virtually
all body cells. Hereditary mutations are also called germline mutations
because the gene change exists in the reproductive cells (germ cells)
and can be passed from generation to generation, from parent to
newborn. Moreover, the mutation is copied every time body cells
divide.
Acquired mutations, also known as somatic mutations, are changes in
DNA that develop throughout a person's life. In contrast to hereditary
mutations, somatic mutations arise in the DNA of individual cells; the
genetic errors are passed only to direct descendants of those cells.
Mutations are often the result of errors that crop up during cell
division, when the cell is making a copy of itself and dividing into two.
Acquired mutations can also be the byproducts of environmental
stresses such as radiation or toxins.
Hereditary mutations are carried in the DNA of the
reproductive cells. When reproductive cells containing
mutations combine to produce offspring, the mutation
will be present in all of the offspring's body cells.
Acquired mutations develop in DNA during a person's
lifetime. If the mutation arises in a body cell, copies of the
mutation will exist only in descendants of that particular cell.
Most mutations are benign, that is, the neither harm or help the
individual. Severe mutations may be deleterious; others may
eventually be adaptive.
In dominant genetic disorders, if one affected parent has a
disease-causing allele that dominates its normal counterpart,
each child in the family has a 50-percent chance of inheriting the
disease allele and the disorder.
Genetic Testing
Different types of genetic tests are used to hunt for
abnormalities in whole chromosomes, in short stretches of
DNA within or near genes, and in the protein products of
genes.
Example: BRCA1 breast cancer/colon cancer
susceptibility genes
As many as 1 in 300 women may carry inherited mutations of breast cancer
susceptibility genes, and approximately the same proportion of Americans
carry mutations that make them susceptible to colon cancer.
Inherited forms of cancer represent perhaps 5 or 10 percent of
all cancers. The great majority of people who get breast cancer
(or colon cancer) acquire mutations during their lifetimes.
Cancer usually arises in a single cell. The cell's progress from
normal to malignant to metastatic appears to follow a series of
distinct steps, each controlled by a different gene or set of genes.
Persons with hereditary cancer already have the first mutation.
What does a predictive gene test tell you?
Scientists looking for a disease
gene often begin by studying DNA
samples from members of 'disease
families' that have numerous
relatives, over several generations,
who have developed an illness.
Women who carry the BRCA1 breast cancer susceptibility gene
have an 80-percent chance of developing breast cancer by the age
of 65; their risk is high but not absolute.
Genetic testing gives you options!
Not interested in genetics?
Maybe you should be?
In your lifetime, it’s just about 100% certain
you will be!