The first scientists to study the laws of heredity
had some difficult initial problems to solve
Two parents have to contribute equally to make one
child
Sometimes offspring show similar traits to their
parents while in other ways they show traits that don’t
appear related to their parents in any way
Mixed breeds: two different species can sometimes
produce offspring
Laws of heredity must explain how two parents
can allow their traits to mix with each other and
make a child (the process is stable, with chaos).
When Charles Darwin wrote Origin of Species he was attempting to explain how species evolve from one generation to the next He had to explain how traits are passed for this to make
sense
Darwin proposed a “blending” theory, which says that parent genes (called particles at the time) blend traits to produce an offspring
In the meantime an Austrian monk named Gregor Mendel attempted to mathematically explain the change from one generation to the next (Mendel was able to explain “how” genes pass from
parent to offspring even though “genes” as we know them wouldn’t be officially discovered for another 100 years)
Mendel was a mathematician, so he attempted to
explain heredity using statistics and data
He used the common garden pea for his experiments
The pea is easy to cultivate, has a short generation time, and
can be self-pollinated or cross-pollinated
The first experiment involved crossing (mating) gametes
of a tall pea plant with gametes of a short pea plant.
Gametes are male and female sex cells (egg/sperm, etc)
This first generation is called the P generation
Hypothesis: “If the blending theory is correct, then all
offspring should be medium length because each
receive an equal share of genes from their parent”
The first offspring generation is called the F1 generation
Mendel crossed plants thousands of times to ensure
the accuracy of his data
When the F1 generation grew, his results were
contrary to his hypothesis: 100% of the plants were
tall plants. Only one parental trait was passed on.
Did the “short” parental genes disappear?
Mendel then crossed members of the F1 generation
with each other to produce the F2 generation.
The results: 787 tall plants and 277 short plants
The ratio was pretty close to 3:1 (74% : 26%)
The “short” trait had disappeared for a generation,
but reappeared later
Perhaps, however, this was just a trait with the
height of plants
Mendel repeated the same experiments over the
next few months with the following traits: pea pod
shape, seed shape, pod color, flower color, seed
color, flower position.
Each time, the same results: 100% of one trait in
the F1 generation, a 3:1 ratio in the F2.
*Note: this ratio represents the simplest type of gene.
Only a small percentage of all genes in all organisms are
actually this simple and easy to calculate. And Mendel
happened to pick those genes.
In other words…Mendel got really lucky.
After analyzing all possible mathematical
explanations for his results, Mendel wrote his first
of two laws: The Law of Segregation
Each organism has two factors for each trait
When gametes form in the organism, each gamete
contains only one of the two factors
When gametes fertilize, each new organism contains
one factor from each parent for each trait
We now know that these “factors” are the strands
of DNA that contain our genes
Each gene has a minimum of two possible alleles
An “allele” is an alternate form of the same gene
Gene: plant size. Alleles: tall and short
One of these genes is a dominant allele and the
other is recessive
Dominant alleles means the trait they code for will
always appear in an organism
Recessive alleles can be masked (covered, but not
absent) by the dominant allele
Because you receive a set of genes from each parent, eukaryotic organisms all have two alleles for each gene (one from mom, one from dad)
The combination of alleles an organism has is called their genotype Each allele in a genotype is given a single-letter label
Capital letters are dominant, lower-case are recessive
The actual trait that appears in an organism is called a phenotype T=tall plants, t=short plants
TT or Tt genotypes = tall phenotypes
tt is only genotype that codes for a short phenotype
Genotypes with the same allele are called homozygous. Different alleles are called heterozygous
Mendel noticed that in his crosses different
combinations of genes always occurred
You never ALWAYS had green peas with tall plants. You could also
have yellow/tall, green/short, yellow/short
This must mean that genes are not connected to each
other.
This led to Mendel’s second law, the Law of Independent
Assortment
Each gene separates independently from itself
Every theoretical combination of alleles is possible within an
individual organism
Mendelian, or Simple Heredity traits, are traits with the
following three rules: the trait is found on only one
gene, has only two alleles, and one allele is dominant
over the other
There are many traits that break one of the three
Mendelian rules
Incomplete dominance
In incomplete dominance, the heterozygote phenotype
actually is a blending of the two alleles
Snapdragon flower color is an incomplete dominance
trait
R=red flowers
R’=white flowers
RR= red flowers
R’R’= white flowers
RR’= pink flowers
Hair style (straight, wavy, curly) is an incomplete
dominance trait in humans
Codominance
Codominant traits are traits that show both alleles
equally in the genotype
Dairy cow fur has is a codominant trait
B = Black fur
W = White fur
BB = All black
WW = All white
BW = Black and White spotted
A human example of codominance is sickle blood
cells
The two traits are round blood cells and sickle-shaped
blood cells
Multiple Allele Traits
Multiple allele traits are traits that have three or more
alleles
The alleles all have an order of dominance
Labrador fur shows multiple alleles
Y=Yellow
Y1=Black
Y2=Chocolate
YY; YY1; YY2= Yellow
Y1Y1; Y1Y2 = Black
Y2Y2 = Chocolate
For humans, a multiple allele trait is blood type
Your immune system must recognize the difference between foreign substances and your own blood
To do this, your blood has specific proteins called antigens on its plasma membrane. Antigens are glycolipids
Your immune system recognizes these proteins and knows that the blood cell belongs to you and isn’t an intruder
The different antigens are labeled A and B
Alleles: IA=A-Type Blood; IB=B-Type Blood;
i=Neither type
There are 4 possible phenotypes of blood, arising
from 6 possible genotypes
Genotype Antigens Present Phenotype (Blood Type)
IAIA (AA); IAi (AO) A-Type only Type A
IBIB (BB); IBi (BO) B-Type only Type B
IAIB (AB) Both A and B Types Type AB
ii (O) Neither A or B Types Type O
It is important for you to know what your blood type is BEFORE you get a blood transfusion
If you get blood with a different protein than what your immune system is used to, it will attack the blood
This results in blood clots and, usually, is deadly
(Because “O” type blood has NO proteins on it, your cells won’t recognize the WRONG proteins.)
If you have this blood type… …You can receive these blood types
Type A Type A or Type O
Type B Type B or Type O
Type AB Type A or Type B or Type O
Type O Type O only
Polygenic Traits
Polygenic traits follow all normal Mendelian rules, but
are combinations of multiple different genes
Seed color in wheat: three different genes
Human height and skin color: an unknown number of
genes, but we think its between 5 and 12.
Polygenic traits tend to result in what appears to be
multiple or infinite different phenotypes
This is Dolly and Wally. Dolly and Wally have brown hair.
Each of them have six genes for hair color, a total of 12
alleles. Each of them also has three alleles for each gene
(black, brown, red and blonde).
If they have three children, is it possible for these children to
have black, red, or blonde hair?
Wally Dolly
Dolly Wally
Their first child is Holly. She has black
hair, because she received a total of six
black alleles, four brown alleles, and one
each of red and blonde. Holly
Wally Dolly
Holly Ollie
Their second child is
Ollie. He’s a redhead,
because his parents gave
him a total of six red
alleles, four blonde
alleles, and two brown
alleles
Wally Dolly
Holly Ollie
Molly
Their third child is Molly. She has blonde hair because
her parents gave her six blonde alleles, three brown
alleles, two red alleles and only one black allele.
Oh what a diverse family of hair color!
Epistasis
Epistasis is when one gene masks the effect of another
gene.
Albinism in humans is an example of epistasis
Humans have multiple genes for what their skin color
will be.
The different tones of color are controlled by how much
melanin is produced by skin cells
The more melanin, the darker the skin
They have another gene elsewhere that controls whether
or not melanin will be produced
If no melanin is produced, then the organism will be an albino
and their skin color genes no longer matter
These diseases are recessive alleles that can be
passed from parents to offspring
Tay-Sachs disease
Neurological impairment at 7-8 months old
Blindness, seizures, paralysis possible
Cystic Fibrosis
Mucus forms in the bronchial tubes and prevents lungs
from working properly
CF children develop more slowly and only live to 20-30
years
Phenylketonuria
Cannot digest the amino acid phenylalanine
Results in severe mental retardation
Neurofibromatosis
Tumors covering the nerve endings may cause
deformations in bone and tissue structure
Huntington’s Disease
Brain cells begin to deteriorate at age 40
Victims lose motor and cognitive function as well
You can be tested to see if you have the gene for
Huntington’s, but there is no cure
Question: If these diseases are dominant, how come
hardly anyone ever get’s them?
Humans have a total of 46 chromosomes (23 from
mom, 23 from dad
Homologous chromosomes #1-22 are all called
autosomes. These chromosomes contain the same
genes no matter which parent they came from
Not necessarily the same alleles.
There is one set of homologous chromosomes that
may be different. These are the sex chromosomes
Two “X” chromosomes (Female—XX)
One “X” chromosome, one “Y” chromosome (Male—XY)
These chromosomes obviously contain gender-
determination genes, but they have other genes as well
that don’t relate to gender.
Any trait controlled by one of these chromosomes is
called a sex-linked trait.
Females have two X chromosomes. Therefore,
mothers can only donate an X chromosome to
their offspring
Males have both an X and Y chromosome.
If Dad donates an X chromosome, the offspring will be
female.
If Dad donates a Y chromosome, the offspring will be
male.
A father cannot pass a sex-linked Y-chromosome
trait to his daughter OR an X-chromosome trait to
his son.
A female will never show a phenotype from a Y-
chromosome’s gene
X-chromosome sex-linked traits are harder to track
Males only have one X chromosome. They will only have
one allele for any X-linked trait
Dominance/recessiveness doesn’t apply.
Females have two X chromosomes, so they will have two
alleles for the trait.
Dominance/recessiveness still applies
It is harder for females to have a recessive X-linked
phenotype than it is for males. She needs 2, he only
needs one
Phenotype ratios for sex-linked traits are
different depending on the gender of the
offspring
In punnett squares, sex-linked traits are expressed as an X or a Y.
There is also a superscript to describe which allele is being represented Fruit flies: XR=Red eyes. Xr=white eyes. Y=Y
chromosome, so no gene is present on the chromosome
Muscular Dystrophy
Muscles are weak, to the point where the victim loses
almost all use of their muscles
Death usually results by age 20
Color Blindness
Color-blind people have difficulty distinguishing colors,
particularly in the red/green spectrum
Hemophilia
Hemophilia is an absence of the ability to clot blood.
Fragile X syndrome
A form of mental retardation, but victims are able to
live to become a grandfather
Sometimes organisms have a gene for a specific
trait, but the trait is not expressed because of
the organism’s gender.
A sex-determined trait is when a trait only
appears in a certain gender
Hormones produced by other genes block sex-
determined genes from expressing in an
organism
Reason: Parents of one gender may not express the
same traits as children of the opposite gender.
However, parents still need to pass all necessary
genes to their child regardless of age.
Both men and women produce testosterone and estrogen.
Around puberty, the body begins to produce higher amounts of one or the other
Your gender determines which structures your body will form (testes or ovaries), and these structures produce high quantities of testosterone and estrogen, respectively
Therefore, even though you have the gene to produce both hormones, your gender decides which you will produce more of.