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CH 14: Mendel and the Gene Idea (Probability Laws (complex genetic…
CH 14: Mendel and the Gene Idea
Inheritance Patterns
extending medelian genetics for a single gene
degrees of dominance
codominance
where two alleles both affect the phenotype separately
human MN blood group
determined by codominant alleles for two specific molecules on the surface of red blood cells
at single gene locus,
theres a homozygous M alleles (MM)
and homozygous N alleles (NN)
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complete dominance
is what one of the allele in a pair has
in order for
an offspring,
F1 generation
, to look exactly like one of it's parents
in some cases,
the phenotypes of heterozygous and dominant homozygous are indistinguishable
the alleles of brown eyes
have complete dominance over blue eyes
incomplete dominance
is where none of the alleles are completely dominant
instead,
they're a mix of the two parents
a red snapdragon mated with a white snapdragon
produces a pink
F1 offspring
this comes from
the heterozygotes flowers having less red pigment than the red homozygotes
interbreeding F1 plants
produces
F2 offspring
that are 1/2 pink, 1/4 white, 1/4 red
the pink offspring
confirms that
that alleles for flower colors are heritable factors
which maintain their identity in hybrids
relation between dominance and phenotype
for any character,
the dominant/recessive relationship of alleles depends on the level of how phenotypes are examined
tay-sachs disease
where children who inherit two copies of the tay-sachs alleles (homozygotes)
organismal level
is considered recessive
activity level
of lipid-metabolizing enzyme in heterozygotes
is intermediate between the activity level
in individuals homozygous for the normal alleles
biochemical level
the intermediate phenotype
is of incomplete dominance of either allele
molecular level
the normal allele and tay-sachs allele are codominant
frequency of dominant alleles
dominant alleles aren't always the more present
polydactyly
is where a baby has an extra finger or toe
occurs when a rare dominant gene is present
the rarity shows that
there's more recessive alleles
that prevents this
multiple alleles
exists in more than two allelic forms
ABO blood types in humans
which is determined by the two alleles of the blood group gene
there's three possible alleles
there are four possible blood types (phenotypes)
B
has carbohydrate B
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AB
has carbohydrate AB
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A
has carbohydrate A
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O
no carbohydrates attached
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each letter refers as the carbohydrates attached
I^A
i
I^B
pleiotropy
is when genes have multiple phenotypic effects
and are responsible for multiple symptoms associated with hereditary diseases
extending medlian genetics for two or more genes
epistasis
phenotypic expression of a gene at one locus
which alters that of a gene at a second locus
in Labradors,
black fur (B) is dominant to brown (b)
genotype bb
will result in a phenotype of chocolate lab
it's second gene at the second locus
will have to be EE or Ee
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ee second alleles
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polygenic inheritance
quantitative characters
is where things vary on a gradient
the quantitative variation
indicates the polygenic inheritance
which is an additive effect of two or more genes on a single phenotypic character
examples
height
is affected by 700 genetic variations over 180 genes
skin
consists of separately inherited genes
three genes for dark-skin alleles
A, B, or C
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there are seven phenotypes that results from mating AaBbCc heterzygotes
environmental impact on phenotype
humans physical attributes can be altered
nutrients is an example of this
if a human received proper nutrients
their height changes; they grow taller
or become wider
sun exposure
causes the skin to become darker
excerise
helps people become leaner
as the genotype dictates the phenotype
such as
the blood type
however,
its blood count could be altered
by infectious agents
phenotypic range
is the broadest for polygenic characters
multifactorial
is where there's many factors, genetic and environmental
that influence phenotype
Laws of Inheritance
Mendel's Discovery
He bred garden peas to study inheritance
peas are easier subject to study
comes in a variety colors
that trait of variable colors is
the different variable is a
trait
known as character
short generation
large number of offspring
achieved cross-pollution of two plants
immature stamens were removed
then dusted the pollen onto an altered plant
which allowed for new parentage of peas
each of his experiments began with
true-breeding plants
where the offspring was consistent with the parent plant
for multiple generations
hybridization
the mating of two true-breeding plants
purple true-breed matched with a white true-breed
those parents are known as the
p generation
the first offspring,
F1 generation
, produced
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only the p generation are mated with other plants
the offsprings of the parents self-pollinate instead
laws of segregation
correct blending model of inheritance
purple and white-flowered pea plants
produces offspring,
F1 generation
, that are
pale purple flowers
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other blending model of inheritance
purple and white-flowered pea plants
produces the
F1 generation
that are purple or white
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whatever color is present
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color isn't present
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for example,
a true-breeding smooth, round pea paired with a wrinkled pea
the
F1 generation
came out to be smooth and round
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Mendel's Model
developed to explain the 3:1 ratio that was observed in the
F2 generation
alternative variations of genes
accounts for variations in inherited characters
which are known as
alleles
i.e. the gene for purple flower and white flower
alleles
are the
DNA sequence variations
possible at the nucleotides/locus on a plants' chromosomes
two copies of a gene are inherited
one from each parent
represented twice in a diploid cell
two differing alleles at a locus
one will be the
dominant
which will help establish the offspring's appearance
while the other one is
recessive
the hidden attribute
law of segregation
two heritable alleles segregate
during the gamete formation
and end in different gametes
one of the two alleles only goes to the egg or sperm
punnett square
terminology
homozygote and homozygous
homozygote
are the identical alleles for a gene encoding a character
homozygous
is what the organism with homozygote alleles is
breeds true
as all the gametes contain the same alleles
for example,
homozygous dominant (PP)
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heterozygote and heterozyous
heterozygote
two different alleles for a gene
heterozygous
is a gene that contains different alleles
produces hybrid gametes with different alleles
so it's not true breeding
genotype and phenotype
genotype
genetic makeup
testcross
where an organism is bred with an unknown genotype that has a recessive trait
phenotype
observable traits
law of independent assortment
monohybrids
the offspring,
F1 generation,
that was produced from true-breeding parents
which means they are heterozygous (two different alleles)
for one particular character being followed in the cross
the monohybrid cross is between heterozygotes
mendel followed two characters
color and shape
single character cross resulted in the discovery of
yellow seeds having a dominant (Y)
green seeds have recessive (y)
round has dominant (R) and wrinkle is recessive (r)
only one difference between true-breeding parents
dihybrids
are heterozygous organisms
as two characters being followed in a cross
the
F1 generation
of true-breeding parents with two differing characters
for example,
one parent with yellow-round seeds (YYRR) mates with another that has green-wrinkled seeds (yyrr)
F1 plants
that has the genotype YyRr
will exhibit both dominant phenotypes
which is the yellow, round seeds
when
F1 generation
reproduces by self-pollination
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dihybrid cross
a cross between F1 dihybrids
assortments
dependent
predicts a phenotype of F2 generation ratio to be 3:1
3/4 of yellow, round seeds
1/4 are green, wrinkled seeds
independent
phenotype ratio of 9:3:3:1
comes from when genes are packaged into gametes in all possible combinations
F1 plant will produce four class of gametes in equal quantities
YR, Yr, yR, yr
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9 out of 16 yellow, round seeds
3 out of 16 yellow, wrinkled seeds
3 out of 16 green, round seeds
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two or more genes assort independently
during gamete formation
each pair of alleles segregate independently of any other pair
only applies to genes (allele pairs) located on different chromosomes
Probability Laws
multiplication rule of monohybrid crosses
used to determine the chance of two coins tossed at the same time will both land upwards
this is done by
multiplying the probability of one event (one coin coming up heads)
by the probability of the other event (the other coin landing upwards)
which will 1/2 * 1/2 = 1/4
could be applied to monohybrid crosses
segregation in heterozygous plants
has 1/2 chance of carrying a dominant allele (R)
and 1/2 chance of recessive allele (r)
which applies to each sperm produced
for the F2 plants to have wrinkled seeds
both the egg and sperm must carry a recessive allele (r)
the probability of the r allele being present in both gametes
is found by multiplying
1/2 (the probability the egg will have a recessive allele)
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addition rule
helps figure out if the offspring will be heterozygous
the probability that any one of the two or more events that will occur
can by found by
adding their individual probabilities
the probability of individual probabilities found by the multiplication rule
will be used in the addition equation
1/4 (dominant allele from egg and recessive from sperm)
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dominant allele could come from either the egg or sperm
only one can be passed down
there's two sides of a coin
so the probability is
1/2 of getting heads
and 1/2 of tails
the probabilities of all possible outcomes must add up to one
complex genetic problems
could be solved with the rules of probability
each allelic pair segregates independently during gamete formation
thus,
a dihybrid or other multi character cross
is equivalent to two or more independently monohybrids
dihybrid cross between YyRr heterozygotes
monohybrid cross of Yy plants
use the punnett square to determine the probabilities of offspring genotypes
which turns out to be
1/4 YY, 1/2 Yy, 1/4 yy
monohybrid cross for Rr plants
punnett square
1/4 RR, 1/2 Rr, 1/4 rr
use the outcomes from both monohybrids crosses for possible genotypes
possible F2 genotypes
YYRR
probability
1/4 (probability of YY) * 1/4 (probability of RR)
equals 1/16
YyRR
probablilty
1/2 (probability of Yy) * 1/4 (probability of RR)
equals 1/8
combined probability rules
cross of two pea varieties with three characters
making it a trihybrid
trihybrid with purple flowers and yellow, round seeds (heterozygous for all three genes)
paired with
trihybrid with purple flowers and green, wrinkled seeds ((heterozygous for color but homozygous recessive for the other two genes)
the cross will be PpYyRr * Ppyyrr
list all the possible genotypes first
then calculate the probability for each of the genotypes
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ppyyRr, ppYyrrm Ppyyrr, PPyyrr and ppyyrr
mendelian patterns of inheritance
pedigree analysis
is where genetics analyze the matings that occurred already
this is done by collecting information about a family's history about a particular trait
and is assembled into a family tree
that describes the traits of parents and children for generations
which is known as a
family pedigree
could help determine what future children traits will be like
by using the laws of probability
such as
diseases
how to read a pedigree
goes by generations (1st, 2nd, 3rd)
colored is affected
circle is female
square is male
recessively inherited disorders
behavior of recessive alleles
genetic disorders
are caused by
an allele that codes for
a malfunctioning protein or no protein
are not evenly distributed
due to different genetic histories of people
who were isolated or restricted to a group
is rare
as most of the same recessive alleles
are mainly carried by relatives
and will only occur
with same-blood matings
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mating with others
will give more variety to genes
by building up immunity
and reducing the possibility of inheriting a disease
a recessive inherited disorder
will only occur when
an individual has a recessive homozygote alleles (aa)
inheriting one from each parent
the parent may not be affected
however, they carry the recessive gene for the disorder
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the mating of two carriers
produces a ratio for the offspring
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cystic fibrosis
is the most common lethal genetic disease
prevalent in european descendants
as 1 in 25 (4%) are carriers of the cystic fibrosis allele
two recessive cystic fibrosis alleles
codes chloride transport channels
that are defective or absent
in the plasma membranes of children who inherited the alleles
which results in an abnormally high concentration of intercellular chloride
which uptakes of water from osmosis
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sickle-cell disease
most common among those who are of african descent
affecting one out of 400 people
is caused by
the substitution of a single amino acid in the hemoglobin protein of red blood cells
in homozygous individuals
the hemoglobin is of sickle-cell (abnormal) variety
does not necessarily need to be homozygous
organismal level
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molecular level
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could still experience issues
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affected individual
oxygen content is low
the hemoglobin proteins aggregate into long fibers
that deform the red cells into a sickle shape
the sickle-cell
could clump and clog small blood vessels
that will lead to issues throughout the body
dominantly inherited disorders
achondroplasia
is a form of dwarfism
which occurs in one of every 25,00 people
they're mainly heterozygous individuals
for the dominant allele
cannot be passed on
and can kill people before they mature
unless the symptoms occur after reproductive age
the individual has already transmitted their allele
huntington''s disease
a degenerative disease of the nervous system
symptoms only occur around 35 to 45 years of age
deterioration of nervous system is irreversible and fatal
can be discovered through DNA testing
multifactorial disorders
people are more susceptible to disease
those that have a multifactorial basis,
which is a genetic component plus a significant environmental influence
some of the heredity components
are polygenic
examples
heart disease, diabetes, cancer, alcoholism, and some mental illnesses
genetic testing and counseling
genetic counseling
helps determine the risk of a child having a disease
family history is discussed
then probability is calculated
through multipication
test for identifying carriers
objective is to find out whether or not
the parents are heterozygous carriers of recessive alleles
there are tests that distinguish
homozygous dominant and heterozygous carriers
by providing family histories of genetic disorders
then decisions could be made
such as
getting pregnant or doing genetic testing
fetal testing
amniocentesis
a test to determine whether or not a fetus has Tay-Sach disease
done at 15 weeks of pregnancy
the procedure is done by
a physician inserting a needle into the uterus
and extracts about 10 mL of amniotic fluid
which is the liquid that surrounds the fetus
that liquid could help detect diseases from certain molecules present
chorionic villus sampling (CVS)
where a physician inserts a narrow tube through the cervix of the uterus
and suctions out a sample tissue from the placenta
the organ that transfers nutrients from the mother to the baby
since it's derived from the fetus
it'll show the same genotype and DNA sequence
the cells extracted
can proliferate rapidly
to allow karyotyping to carry out immediately
faster than amniocentesis
as it can be done at 10 weeks of pregnancy
newborn screening
phenylketonuria (PKU)
is what commonly screened for
as it is a recessive inherited disorder
that occurs in about one out of every 10,000-15,000 births
those born with the disorder
cannot properly metabolize the amino acid phenylalanine
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