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Mendelian Genetics
Two Hypotheses of Heredity
1. Blending Hypothesis – genetic material from both parents “mixes” like red
and blue make green.
2. Particulate Hypothesis – parents pass on distinct units (genes) that retain
their separate identity in an undiluted form
a. This is the hypothesis that is accepted today
b. Gregor Mendel developed his theory in the mid 1800’s before
chromosomes had been viewed under the microscope
c. He was a Monk who studied math and crossing pea plants to study
inheritance
Terms for Studying Genetics
Character – a heritable feature that varies among individuals, such as flower color.
Trait – each variant for a character, such as white or purple flowers
Hybridization – crossing of true-breeding varieties
True-breeding – over many generations, the offspring produces the same variety as
the parent.
True breeding parents are called the P or Parent Generation
The offspring of the P generation are called F1 or First Filial (meaning son)
Generation
The offspring of F1 breedings are called F2
Mendels Laws and His Model:
Mendel found that breeding F1 plants always lead to an approx 3:1 ratio of purple to
white flowers
He called the purple flower the Dominant trait because when this “factor” was
present, the purple color always resulted
He called the white flower the Recessive trait because the “factor” was somehow
hidden or masked when the purple factor was present.
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He also noted that when the white flower re-appeared in the F2 generation, it was
not because it was diluted by the purple.
Mendel’s Model: (He developed this model to explain the 3:1 ratio of purple to
white)
1. Alternative versions of genes account for variations in inherited characters.
The gene for flower color exists in two forms, purple or white, called Alleles.
Today we know that alleles are present on homologues at the same location
or Locus.
2. For each character, an organism inherits 2 alleles, one from each parent
3. If two alleles at a locus differ, the Dominant allele determines appearance
and the Recessive allele has no noticeable effect on the organisms
appearance.
4. Law of Segregation – 2 alleles for a heritable character segregate (or
separate) during gamete (egg or sperm) formation and end up in different
gametes. Egg or sperm ends up with only one allele that way at fertilization,
there are 2 alleles present.
Law of Independent Assortment – Each pair of alleles segregates
independently from each other during gamete formation (meiosis)
A Punnet Square is used to predict allele composition of offspring of a cross
between individuals of known genetic makeup.
A capital letter symbolizes the dominant allele (P for purple)
A lower case letter symbolizes the recessive allele (p for white)
Homozygous – a pair of identical alleles controlling that character (PP or pp)
Heterozygous – two different alleles for a gene (Pp; also purple)
Phenotype – an organisms observable trait (purple or white)
Genotype – an organisms genetic makeup (PP, Pp, or pp)
Monohybrid cross – cross following a single character like flower color
Dihybrid cross – cross following more than one character like flower color and seed
shape. Alleles for different characters sort independently from one another
following Mendel’s Law of Segregation
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Example Monohybrid Cross:
Parent Generation:
PP purple x white pp
F1:
P
P
Pp Pp
Pp Pp
Cross F1 offspring Pp x Pp
P
p
PP Pp
Pp pp
3:1 purple to white
What if you do not know the genetic makeup of the plant or animal?
Test Cross:
Unknown plant x recessive (white) (always cross unknown with recessive)
Purple (could be PP or Pp) x White (pp)
P
P
Pp
Pp
Pp
Pp
All purple lets you know the parent is PP because it donates a dominant allele to
each offspring
P
p
Pp
Pp
pp
pp
One half purple, one half white lets you know the parent is Pp and donates either a P
or a p to offspring
More Complex Genetic Patterns
Complete Dominance – Follows the classic Mendel’s Laws. F1 offspring always
look like one of the two parental varieties
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Incomplete Dominance – Neither allele is completely dominant. F1s have a
phenotype that is somewhere between the two parental varieties.
Example: Snapdragons
Red x white parents = pink hybrids
RR x rr = Rr is pink instead of red.
At first this looks like blending. If it were blending then the crosses of pink would
always remain pink. Always. Not blending because the red and white phenotypes
can be retrieved from pink.
Rr x Rr = RR, Rr, and rr = Red and pink and white (1:2:1)
Codominance – the two alleles both affect the phenotype in separate,
distinguishable ways. Example: Multiple Alleles
Multiple Alleles – genes that exist in more than two allelic forms
Example: ABO blood groups in humans
Three alleles of a gene:
IA, IB, i
Blood types:
A = IAIA or IAi
B = IBIB or IBi
AB = IAIB (codominant so both alleles are expressed)
O = ii
The blood types represent carbs on the surface of RBCs.
Therefore, in the AB bloodtype, both carbs are found on the RBC surface
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Another Example:
Coat Color in Rabbits
C = brown
cch = chinchilla
ch = Himalayan
cw = albino
brown is most dominant and albino most recessive
Pleiotropy – when a gene has multiple phenotypic effects
Example – when there are multiple symptoms associated with certain hereditary
diseases.
Cystic Fibrosis –
Sickle Cell disease –
Epistasis – a gene at one locus alters or controls the phenotypic expression of a
gene at another locus
Example – one trait is dependent on the expression of another.
Pigment vs color in an animal:
B=black
b=brown
P=pigment
p=no pigment
BBpp=white
the pigment gene controls the color
No pigment then the animal is white
BbPp=brown
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Environmental Impact on Phenotype (Nature vs Nurture controversy)
Even though the genes are distinct, environment somehow affects phenotype
Called Multifactorial – many factors, genetic and environmental, affect phenotype
Examples –
o Identical twins develop their own personalities
o Hydrangea plants develop either blue or pink flowers depending on
soil acidity
o Exposure to the sun gives a continuous variation in skin color
Recessively Inherited Disorders
Thousands of genetic disorders arise from simple recessive traits.
For example – albinism, cystic fibrosis
Behavior of Recessive Alleles
Recessive alleles in disease conditions produce either a malfunctioning protein or
no protein at all.
Heterozygotes do not exhibit the disease because the normal allele produces enough
of the necessary and normal protein.
Therefore, the disease shows up in homozygous recessive genotypes.
Heterozygotes may transfer a disease allele to their offspring. They are called
Carriers.
Many recessively inherited disorders are not evenly distributed but found in certain
cultures
Examples of recessively inherited disorders:
Cystic Fibrosis – most common lethal genetic disorder; one of every 2500 people of
European descent. 4 % are carriers.
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The normal allele produces a protein that transports chloride ions between cells and
extracellular fluid. In children with the disease, the transporters are defective or not
present at all resulting in a mucus coat on cells that is thicker than normal. Mucus
builds up in many organs including the lungs. If untreated they usually die by age 5
from infections and clogged airways. Now, antibiotics and gently chest pounding
can boost survival to the 20s and 30s.
Sickle Cell Disease
Affects one of 400 African-Americans
Caused by a substitution of a single amino acid in the hemoglobin protein contained
in red blood cells.
In the diseased state, hemoglobin forms abnormal shapes making the RBC sickle
shaped. This causes the RBCs to clump and clog small blood vessels. This leads to
many symptoms such as weakness, pain, organ damage and paralysis.
Heterozygotes are said to have sickle cell trait and may suffer some problems.
Homozygotes result in full blown sickle cell disease.
Blood transfusions and new drugs can help symptoms but there is no cure.
In tropical Africa, sickle cell disease is both bad and good. Good because the malaria
parasites common to that area have a hard time dividing in red blood cells.
Many recessive diseases are sex linked:
Such as:
Red-Green Colorblindness
Duchenne’s Muscular Dystrophy – progressive weakening of muscles
Hemophilia – absence of a protein required for blood clotting
Sex-linked genes are on the X chromosome:
Fathers donate the disease only to daughters, because of XX
To be female one X from mom and the other X from dad
Mothers can donate disease to sons or daughters
X from mom and X or Y from dad
Example: color blindness:
XCXc = carrier (Xc is the recessive gene that combined with another Xc causes
colorblindness)
XCXc x XcY
¼ female normal, ¼ female colorblind, ¼ male normal, ¼ male colorblind
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Dominantly Inherited Disorders
Some disorders are the result of dominant alleles.
Much less common than diseases caused by recessive alleles.
Examples:
Achondroplasia or a certain type of dwarfism. Uncommon because most people
are homozygous recessive.
Huntington’s disease – degenerative disease of the nervous system. The person
survives just past reproductive age (35-45) and the symptoms don’t show up until
around that age. Of ten times the person has already had children and the disease
gene is passed on to them.
Multifactorial Diseases – a disease with a genetic component in addition to a
significant environmental influence.
Heart disease, cancer, diabetes, alcoholism and certain mental disorders.
Many of these diseases are polygenic – controlled by many genes.
Genetic Testing –
Used in people who have a family history of a genetic disease.
Fetal Testing –
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Amniocentesis – needle into uterus to extract small amount of amniotic fluid
(bathes fetus). Presence or absence of the disease is determined by certain
chemicals present or by karyotypes done from cells sloughed off from the
fetus.
Chorionic Villus Sampling – narrow tube through cervix and takes tiny tissue
sample from the placenta (membranous structure that unites the fetus and
uterus for nutrition and waste elimination). The placenta has the same
genetic makeup as the fetus. These cells can be tested right away, while the
cells of amnion must be cultured for several weeks before answer. Chorion is
the outermost membrane while the amnion is the innermost membrane and
the fluid inside is the amniotic fluid. Villi are structures that protrude out
from the chorion.
Ultrasound Technique – soundwaves produce an image of the fetus.
Noninvasive.
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Fetoscopy – needle thin tube with a light and a scope is inserted into the
uterus. Fetoscopy is also used in in utero surgical procedures like removal of
fetal tumors and correction of spina bifida.
o In utero – in the uterus
o In vivo – in the organism
o In situ – in the situation (like studying an organ while its working)
o In vitro – in an “artificial” environment (like petri dish)
o Ex vivo – conservation term for preserving a species outside of its
environment.
Newborn Testing
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PKU (phenylketonuria)
o Recessively inherited in 10-15,000 births
o Cannot properly metabolize phenylalanine
o This and its by product accumulates in the blood and causes mental
retardation
o A special diet low in phenylalanine can be used.