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Chapter 14: Mendel and The Gene Thing
Points of Emphasis
Know:
1. all the bold-faced terms
2. the basic crosses, especially a test cross
3. Know something about each genetic disorder
4. Terms you are expected to know: true-breeding, P
generation, F1 generation, F2 generation, alleles,
dominant and recessive alleles, homozygous,
heterozygous, phenotype and genotype, monohybrid and
dihybrid
Figure 14.2 Mendel tracked heritable characters for three generations
Law of Segregation
There was no “blending” or mixing of the traits that were being studied by
Mendel. Pale purple flowers were not produced in the F1
And the trait for white flowers was not lost since it reappeared in the F2.
So the trait was not “diluted.”
Mendel was fortunate to pick traits that showed dominance and
recessiveness and were determined by one allelic.
Law of Segregation, cont’d
Mendel’s Basic Ideas From His Experimentation
(with a little help from modern terminology)
1.
Alternative forms of genes are called alleles; genes reside at a
particular locus on a particular chromosome. And yes, if we have
different forms of a gene, the DNA is different at these loci.
2.
Each parent contributes one allele to its offspring, therefore the
offspring inherits two alleles.
3.
Dominant alleles and recessive alleles exist.
4.
Law of Segregation: each of the alleles segregates or separates during
the formation of sperm or egg cells. So one sperm cell or egg cell has
one of the alleles and the other allele is located in another sperm or egg
cell.
Figure 14.3 Alleles, alternative versions of a gene
Table 14.1 The Results of Mendel’s F1 Crosses for Seven Characters in Pea Plants
The Testcross
Purpose: to determine the genotype of the dominant allele
expressing offspring.
It is always a cross with heterozygous individual for that same
trait.
Example:
P_ x pp will produce all offspring that are
expressing the dominant phenotype if the
genotype in question is PP.
P_ x pp will produce offspring that are both
expressing dominant and recessive phenotype if
the genotype in question is Pp
Therefore you know if the genotype if PP or Pp
Figure 14.6 A testcross
Figure 14.7 Testing two hypotheses for segregation in a dihybrid cross
Law of Independent Assortment
Applies to different traits, therefore 2 different sets of alleles
The dihybrid cross is the typical example.
Law of Independent Assortment: Alleles of different traits will separate
and assort themselves independently of each other. The alleles are put
into the gametes in all possible combinations as long as a gamete has one
allele for each gene. All of “mom’s” alleles don’t have to segregate
together. In a sperm or egg cell you have some of mom’s and some of
dad’s and each sperm and egg differs in which of mom’s and which of
dad’s alleles it got. That’s why siblings may look really alike (lots of
similar allelic combinations), kind of alike (some similar combos) or not
very much alike at all (not many similar allelic combinations)
Rules of Probability
Rule of Multiplication
1. First make sure the two events are independent of each
other. For example, two tosses of a coin or two alleles
from different parents.
2. Compute the probability for each independent event
a) ½ for getting a “heads” and ½ for getting a “tails.”
b) ½ for getting the dominant form, P from the parent;
and ½ for getting the recessive form p.
3. Multiply the probabilities.
a) So if we cross Pp x Pp, what is the chance of the
offspring being PP? pp?
Rules of Probability (cont’d)
Rule of Addition
1. So this applies if there is more than one way to get a
specific outcome, like getting Pp or pP
2. Then we add the separate probabilities.
So:
Pp is ½ x ½ or ¼
pP is ½ x ½ or ¼
Then Pp is likely ¼ + ¼ or ½.
Figure 14.8 Segregation of alleles and fertilization as chance events
Incomplete Dominance
The offspring have an appearance in between the two parents.
This is not “blending.”
Classic example: Snapdragons
Tay-Sachs Disease: accumulation of lipid in the brain cells
because of a lack of an enzyme. The heterozygote produces an
intermediate level of this enzyme to prevent lipid accumulation.
Heterozygotes lack the disease even though at the molecular level
they do produce some dysfunctional enzymes.
Figure 14.9 Incomplete dominance in snapdragon color
Figure 14.9x Incomplete dominance in carnations
What is a Dominant Allele?
“Dominance” does not mean it overcomes another allele.
The two forms of a gene really don’t interact. It is just that the dominant gene
codes for an enzyme producing a certain trait and the recessive allele does not.
Two dominant alleles will form more enzyme, let’s say, than a heterozygote.
A dominant allele does not mean it is more frequent in the population.
The allele for polydactyly, extra fingers or toes, is a dominant allele
but 399 out of 400 people are recessive homozygotes so they show
no polydactyly
Dominant traits can be demonstrated as completely dominant, incomplete
dominance or codominance.
Codominance
Definition: where both alleles are observed phenotypically to
some degree in the heterozygote condition. So the heterozygote is
distinctly different from either of the parents but possesses
characteristics of each.
For codominant alleles, all uppercase base symbols are used with
different superscripts.
LM LM
LM LN
LN LN
Codominant Example
M-N Blood Groups: M represents an M protein on the RBC and
therefore anti-M serum will interact with this protein and give a
positive or (+) reaction.
Genotype
Anti-M
Anti-N
Blood Group
LM LM
+
O
M
LM LN
+
+
MN
LN LN
O
O
N
Figure 14.10 Multiple alleles for the ABO blood groups
Figure 14.10x ABO blood types
Pleiotropy
All of the phenotypic manifestations of a single gene are described as a
Pleiotropic gene effect.
Many of the biochemical pathways are interconnected and interdependent so the
phenotypic expression of one gene can effect more than one trait by influencing
a multitude of pathways.
Some of the traits are “major”; some have secondary effects that may be less
evident. A number of these related changes could be called a syndrome.
Sickle-cell anemia is due to abnormal Hb. This is the primary effect of a mutant
gene. Other effects include the clumping of RBCs and the clogging of blood
vessels in heart, kidney, spleen and brain. Defective RBCs are destroyed by the
body causes anemia.
Figure 14.15 Pleiotropic effects of the sickle-cell allele in a homozygote
Epistasis
Epistasis is the interaction between genes that causes the masking of one of the
genes effects.

This is the masking effect of one gene locus on another.

This is not dominance which involves intra-allelic gene suppression but
epistasis is interallelic gene suppression.

The typical 9 : 3 : 3 : 1 ratio for a dihybrid cross can become
modified by epistasis into ratios that are various combinations of the
9 : 3 : 3 : 1 grouping.
Figure 14.11 An example of epistasis
B represents coat color;
C represents the deposition of
the pigment.
Polygenic Inheritance
Many characteristics in a population can be found as a continuum or gradation.
This is due to the additive effect of two or more genes on a single phenotype.
Skin pigmentation, height.
Figure 14.12 A simplified model for polygenic inheritance of skin color
Figure 14.13 The effect of environment of phenotype
Hydrangeas and pH of the soil.
Human Disorders
Recessively Inherited Disorders
 Albinism, cystic fibrosis, Tay-Sachs, Sickle-cell
 Shows up in homozygous recessive individuals
 Heterozygote is the carrier
Human Disorders
Dominantly Inherited Disorders
 Achondroplasia
 1/25,000
 all races, sexes
 normal torso; arms and legs are dwarfed in size
 the rate at which the cartilage turns into bones is affected
in the long bones.
Human Disorders
Dominantly Inherited Disorders
 Huntington’s Disease
 lethal dominant
 symptoms set in later in life so the allele has been passed
on to offspring.
Identifying Carriers
Carrier Recognition: identify the heterozygotes
Fetal Testing through Amniocentesis

done at about 4th month

amniotic fluid is removed and lost cells are karyotyped.
Fetal Testing through Chorionic Villus Sampling (CVS)

small sample of the placenta is removed by insertion of a suction tube
through cervix and into uterus.

Karyotype is done on these rapidly dividing cells and so results are
seen sooner than the amnio test (within 24 hours)

CVS can be performed as early as 2nd month.
Identifying Carriers
Fetal Testing through ultrasound

anatomical abnormalities