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Chapter 14: Mendel and the Gene Idea (Concept 14.1 Mendel used the…
Chapter 14: Mendel and the Gene Idea
Concept 14.1 Mendel used the scientific approach to identify two laws of inheritance
Mendel discovered the basic principles of heredity by breeding garden pease in carefully planned experiments.
Mendel's Experimental, Quantitative Approach
Mendel's fresh approach allowed him to deduce principles that had remained elusive to others.
1 reason why Mendel worked with peas is due to its varieties.
For Ex, one had purple flowers, while the other had white flowers.
A heritable feat. that varies among ind. such as flower color is know as a
character
.
A
trait
is a variant for a character such as purple or white color for flowers.
Another advantage of using peas are their short generation time & large #s of offspring from each mating.
Mendel could control mating b/w plants.
Ea. pea flower has both pollen-producing organs (stamens) & an egg-bearing organ (carpel).
Pea plants usually self-fertilize: Pollen grains from the stamens land on the carpel of same flower, & sperm released from pollen grains fertilize eggs in the carpel.
Removed Stamens from purple flower
Transferred sperm bearing pollen from stamens of white flower to egg bearing carpel of purple flower.
Waited for pollinated carpel to mature into pod.
Planted seeds from pod.
Examined offspring: all purple flowers.
To achieve cross-pollination, Mendel removed immature stamens of a plant before they produced pollen & then dusted pollen from another plant onto the altered flowers.
Each resulting zygote then developed into a plant embryo encased in a seed (pea).
His method allowed him to always be sure of the parentage of new seeds.
He chose to track only characters that were in distinct form, such as purple or white.
True-Breeding
made sure his experiments started with varieties.
generations of self-pollination
only the same variety as the parent plant.
Ex: a plant w/purple flowers is true-breeding if the seeds produced by self-pollination in successive generations all give rise to plants that also have purple flowers.
hybridization
is the mating, or
crossing
of two true-breeding varieties.
P generation
refers to true-breeding parents
F1 generation
is their hybrid offspring.
F2 generation
are produced by F1 hybrids that self-pollinate.
The Law of Segregation
P generation purple and white flower, the white flower reappeared in F2 generation; appeared as 3:1 ratio
In Mendel's term, purple flower color is a
dominant
trait, and white flower color is a
recessive
trait.
White flowers were hidden in the presence of the purple-flower factor
Mendel's Model
Developed a model to explain 3:1 inheritance pattern that was observed among the F2 offspring in pea experiments.
Four related concepts (4th is the law of segregation)
Alternative versions of genes account for variations in inherited characters
The gene for flower color in pea plants.
These alternative versions of a gene are called
alleles
.
related to chromosomes and DNA.
DNA at the locus can vary slightly in nucleotide sequence; can affect function of protein & inherited character of organism.
The purple flower allele sequence allows synthesis of purple pigment, & white-flower allele sequence does not.
For each character, an organism inherits two copies (two alleles) of a gene, one from each parent.
genetic locus is actually represented twice in a diploid cell, on each homolog of a specific pair of chromosomes.
two alleles at a particular locus may be identical, as in P generation, or alleles may differ, as in F1 hybrids.
if the two alleles at a locus differ, then one, the
dominant allele
, determines the organisms appearance; the
recessive allele
has no noticeable effect on the organism's appearance
Mendel's F1 plants had purple flowers bc the allele for that trait is dominant and the allele for white is recessive.
the
law of segregation
states that
the two alleles for a heritable character segregate during gamete formation and end up in different gametes
An egg and sperm gets only one of the two alleles that are present in the somatic cells of the organism making the gamete.
in terms of chromosomes, segregation corresponds to distribution of copies of the 2 members of a pair of homologous chromosomes to different gametes in meiosis.
Offspring always looks like their parents; explains true-breeding.
If different alleles are present, for example F1 hybrids, then 50% of gametes receive dominant allele & 50% receive the recessive allele.
Punnett Square
1/2 of F2 offspring inherited purple flower allele, and one white-flower allele; and will also have purple flowers, dominant trait.
1/4 of F2 plants have inherited two white-flower alleles & express recessive trait.
Mendel's model accounts for the 3:1 ratio of traits observed in F2 generation.
Use Genetic Vocabulary
homozygote
has a pair of identical alleles for a gene encoding a character
Homozygous
Ex: P generation (purple flowered pea plant) os homozygous for the dominant allele (PP), white is homozygous for recessive allele (pp).
true-breeders; same alleles
Heterozygote
: an organism that has two different alleles for a gene.
Produce zygotes w/different alleles; not true breeders.
Ex: P-& p containing gametes are both produce by our F1 hybrids. Self-pollination of the F1 hybrids of F1 hybrids thus produces both purple and white flower offspring.
Phenotype
An organism's appearance of observable traits.
Genotype
Genetic Makeup
Figure 14.5
PP & Pp plants have the same phenotype but different genotype.
The Test Cross
In order to determine whether a purple plant is homozygous (PP) or heterozygous (Pp), we can cross this plant with a white plant (
pp
), which may make only gametes w/ the recessive allele (p).
Breeding an organism of unknown genotype w/a recessive homozygote is called a
test cross
b/c it can reveal the genetic makeup of that organism.
The Law of Independent Assortment
Law of Segregation - Mendel derived this from experiments where he followed one character (one of the flower).
All F1 progeny produced in his crosses of true breeding parents were
monohybrid
, being heterozygous for one particular character in the cross.
A cross b/w heterozygotes is a
monohybrid cross
Mendel followed seed color & seed shape.
Seeds may be yellow or green; round or wrinkled
Mendel's allele for yellow seeds is dominant (
Y
)
Allele for green seeds is recessive (
y
).
Seed Shape allele for round is dominant (
R
) & allele for wrinkled is recessive (
r
).
Ex: A cross b/w a plant w/yellow-round seeds (
YYRR
) & a plant w/green wrinkled seed (
yyrr*
)
The F1 plants will be
dihybrids
, individuals heterozygous for two characters in the cross (
YyRr
).
Results of Mendel's experiment are the basis of
the law of independent assortment
, which states that two or more genes assort independently - each pair of alleles segregates independently of any other pair of alleles- during gamete formation.
4 classes = 16 equally probably ways
Concept 14.4 Many Human Traits follow Mendelian Patterns of Inheritance*
Pedigree Analysis
A family
pedigree
- a way of collecting info about a family's history for a certain trait & assembling it as a family tree describing the trait of parents & children across generations.
pedigrees are a more serious matter when the alleles in question cause disabling or deadly disease instead of innocuous human variations like hair line or inability to taste.
Recessively Inherited Disorders
1000s of gene disorder are inherited as simple recessive traits.
can range from mild (albinism) to severe (cystic fibrosis)
The Behavior of Recessive Alleles
Heterozygotes (
Aa
) typically have the normal phenotype b/c one copy of the normal allele (A) produces a sufficient amt. of the specific protein.
A recessive inherited disorder shows up only in the homozygous individuals (aa) who inherit a recessive allele from each parent.
Heterozygotes may transmit the recessive allele to their offspring and are called
carriers
Disease causing recessive allele is rare - it is unlikely that 2 carriers of the same harmful allele will meet & mate.
Ex: The probability of passing on recessive traits increases greatly, with close relatives.
Consanguineous ("same blood") matings are likely to produce offspring homozygous for recessive traits - including harmful.
Cystic Fibrosis
Most Common Lethal Genetic Disease in U.S.
one of 25 European descent are carriers of the cystic fibrosis allele.
The normal allele for gene codes for a membrane protein that functions in the transport of Cl- ions b/w certain cells & the extracellular fluid.
Cl- transports are absent in the plasma membranes of children who inherit two recessive alleles for CF.
Result: High concentration of intracellular Cl- causes an uptake of h2o due to osmosis.
Mucus builds up becoming thicker in the pancreas, lungs, digestive tract, etc. leading to multiple (
pleiotropic
) effects, causing poor absorption of nutrients from intestines, bronchitis, & bacterial infections.
Treat w/ abx, gentle pounding on chest, & other therapies to prolong life.
Sickle-Cell Disease
affects one of 400 African Americans.
caused by the substitution of a single AA in the hemoglobin protein in RBCs; in homozygous people, hemoglobin is of the sickle-cell variety.
Organismal
level - normal allele is incompletely dominant sickle-cell allele.
Molecular
level - two alleles are codominant; normal & abnormal hemoglobins are made in heterozygotes (carriers) to have sickle-cell traits.
Multifactorial Disorders
Multifactorial basis - genetic + environment are most people susceptible to diseases.
Ex: Heart disease, diabetes, cancer, alcoholism & certain mental illnesses like schizophrenia (considered polygenic).
Dominant Inherited Disorders
a number of human disorders are to dominant alleles
Ex: Achondroplasia, a form of dwarfism that occurs in one of every 25,000 people.
A lethal allele is only lethal when recessive; can be passed to heterozygous carriers b/c they have normal phenotypes
Ex:
Huntington's Disease
: a degenerative disease of nervous systems caused by lethal dominant allele that has no phenotype effect until 35-45 yrs of age.
Genetic Testing & Counseling
Fetal & Newborn testing can also reveal genetic disorder.
Counseling Based on Mendelian Genetics & Probability Rules
1AA:2Aa:1aa for offspring of Aa x Aa cross, John & Carol each have 2/3 chance of being a carrier (Aa).
There is a 1/4 chance that any child can get this disease
Test for Identifying Carrier
To assess genetic risk for a particular disease, you need to find out whether the prospective parents are heterozygous carriers of the recessive allele.
Today, tests can identify carriers of alleles for certain diseases.
Genetic Information Discrimination Act - signed in 2008 to prohibit discrimination in employment or insurance.
Fetal Testing/ Compare&Contrast
amniocentesis
.
A physician inserts needle into uterus & extracts 10 mL of amniotic fluid.
other tests are performed on DNA of cells.
is a test that can be done to determine whether a developing fetus has Tay-Sachs disease (performed 15th week of pregnancy)
Chorionic Villus Sampling (CVS)
narrow tubing through the cervix into uterus & suctions a tiny sample of
tissue
from the placenta (transmits nutrients & fetal wastes b/w mom and fetus).
cells of chorionic villi are from the fetus & have the same genotype & DNA.
Performed 10th week of pregnancy
Isolating fetal cells that have traveled through mom's blood.
Newborn Screening
One common screening is for phenylketonuria (PKU).
1 out of 10,000-15,000 births
children cannot properly metabolize the amino acid phenylalanine.
causes mental retardation
if detected, a special diet may allow normal development.
All depends on the Mendelian model of Inheritance.
Concept of heritable factors is transmitted through quantitative experiments of Gregor Mendel.
Concept 14.3 Inheritance Patterns are Often More Complex Than Predicted by Simple Mendelian Genetics
Extending Mendelian Genetics for a Single Gene
The inheritance of characters determined by a single gene deviates from simple Mendelian patterns when alleles are not completely dominant or recessive, when a gene has more than two alleles, or when a gene produces multiple phenotypes.
Degree of Dominance
Allele shows various degrees of dominance and recessiveness in relation w/each other.
The F1 offspring always looks like one of the two parental varieties bc 1 allele in a pair showed
complete dominance
over the other.
Incomplete Dominance
F1 hybrids have a phenotype somewhere b/w the two parental varieties.
Is seen when Red shaped dragons are crossed w/white snapdragons: all F1 hybrids have pink flowers.
codominance
the two alleles each affect the phenotype in separate, distinguishable ways.
Ex: The human MN blood group is determined by codominant alleles for two specific molecules located on the surface of RBCs, the M &N molecules.
The Relationship between Dominance and Phenotype
It's important to know an allele is dominant b/c it is seen in its phenotype, not b/c is subdues a recessive allele.
Round v. Wrinkled pea seed shape.
The Dominant Allele (round) codes for an enzyme that helps convert an unbranched form of starch to a branched form in the seed.
The Recessive Allele (wrinkled) codes for a defective form of the enzyme, leading to accumulation of unbranched starch, which causes excess H2O to enter seed by osmosis.
If dominant allele is present, no excess h2o will enter & won't wrinkle when it dries.
One dominant allele in the enzyme synthesize adequate amts. of branched starch; homozygotes & heterozygotes have the same phenotype: round seeds.
Tay-Sachs Disease
is an inherited disorder in humans.
The brain cells inside a child with disorder cannot metabolize certain lipids bc a crucial enzyme doesn't work properly.
A child begins to suffer certain seizures, blindness, & degeneration go motor & mental performance, & death.
Only children who inherit
two copies of Tay-Sachs (homozygotes) have the disease.
Molecular
level , the normal allele & the Tay Sachs allele are codominant.
Organismal
level, Tay Sachs is recessive
Biochemical
level, characteristic of incomplete dominance of either allele
Frequency of Dominant Alleles
Not as common as it seems
Ex: One baby out of 400 in the U.S. are born w/an extra finger or toes, known as a rare dominant allele -
polydactyl
.
Some are caused by the presence of dominant alleles.
Multiple Alleles
ABO Blood groups in humans are determined by that person's two alleles of the blood group gene; 3 possible alleles: I
A
, I
B
, and
i
.
Pleiotropy
Most genes have multiple phenotypic effects, known as
pleiotropy
In humans for
ex
, pleiotropic alleles are responsible for multiple symptoms associated w/ certain hereditary diseases, like cystic fibrosis & sickle-cell disease.
Garden Pea- the gene that determines flower color, also affects color of the coating on outer surface of the seed. can either be great or white.
Extending Mendelian Genetics for Two or More Genes
First Case: Epistasis - one gene affects the phenotype of another b/c two gene products interact.
Second Case: Polygenic inheritance - multiple genes independently affect a single trait.
Epistasis
"standing upon"
phenotypic expression of a gene at one locus alters that of a gene at a second locus.
Ex: The gene for pigment deposition (
E/e
) is said to be epistatic to the gene that codes for black or brown pigment (
B/b
).
Polygenic Inheritance
Quantitative Characters
vary in the population in gradation along a continuum; usually indicates
polygenic inheritance
- an additive effect of 2 or more genes on a single phenotypic character.
Ex: Height; many variations were in or near genes involved in biochemical pathways affecting growth of the skeleton, but other genes not related to growth.
Nature v. Nature: The Environmental Impact on Phenotype
simple Mendelian genetics - arises when the phenotype for a character depends on environment as well as genotype.
Whether human characters are more influenced by genes or the environment - nature v. nurture.
Genotype is not associated w/a rigidly defined phenotype, but w/a range of phenotypic possibilities due to environmental influences.
Multifactorial meaning that many factors, both genetic & environmental, collectively influence phenotype.
A Mendelian View of Heredity & Variation
Phenotypic refers to specific characters, but also to an organism in its entirety -
all
aspects of it physical appearance, internal anatomy, physiology, & behavior.
Genotype can refer to an organism's entire genetic makeup, not just its alleles for a single genetic locus.
In heredity & variation, an organism's phenotype reflects its overall genotype & unique environment history.
Concept 14.2 Probability Laws Govern Mendelian Inheritance
Mendel's law & independent assortment reflects the same rules of probability as tossing coins or rolling a dice.
An event w/ a probability of 1 is certain to happen while 0 wouldn't happen
Ex: Tossing a coin that has heads on both sides; probability is 1 & tails is 0.
A normal coin: the probability of both is 1/2 & 1/2
Each toss is independent of every other toss.
The Multiplication & Addition Rules Applied to Monohybrid Crosses
Multiplication Rules
to determine this probability, multiple the probability of one event (a coin coming up heads) by the probability of the other event (other coins coming up heads).
1/2 x 1/2 = 1/4
Addition Rules
The probability that anyone of two or more mutually exclusive events will occur is calculated by adding their individual properties.
The multiplication rule gives us the individual probability that will be added together.
Ex: The probability of the recessive allele from the egg & the dominant allele from the sperm is 1/4. We can calculate the probability of an F2 heterozygote as 1/4 + 1/4 = 1/2
Solving Complex Genetic Problems with the Rules of Probability
Recall that each allele segregates independently during gamete formation.
For a monohybrid cross of
Yy
plants, we can determine the offspring genotypes are 1/4
YY
, 1/2 for
Yy
& 1/4 for
yy
Ex: Probability of YYRR = 1/4 (YY) x 1/4 (RR) = 1/16. Probability of YyRR = 1/2 (Yy) x 1/4 (RR) = 1/8
