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Transcript
Chapter 14 Study Guide
Courtesy of Julia Beamsderfer (2005)
Gregor Mendel’s Discoveries- Mendel, a monk, discovered the basic principles of heredity by breeding
garden peas.
I.
Mendel’s experiment
A. He used garden peas because they are available with characters (a heritable feature
that varies among individuals) containing a variety of traits (each variant for each
character); also they were either-or characters
B. Experiments were started with true-breeding varieties, meaning that their offspring
were of the same variety
C. Hybridization- the mating or crossing of two varieties
1. Monohybrid Cross- a cross that tracks the inheritance of a single character
D. Generations of Breeding- Mendel’s experiment
1. P Generation- the true-breeding parents- 1 purple flower and 1 white flower
2. F1 Generation- the hybrid offspring of the P generation- all purple flowers
3. F2 Generation- self-pollinated offspring from F1 generation- 3:1 ratio of purple
to white
II.
Mendel’s Law of Segregation
A. Alternative versions of genes (different alleles located at the flower-color loci on
each homologous chromosome) account for variations in inherited characters
B. For each character, an organism inherits two alleles, one from each parent (or each
certain locus on a pair of homologous chromosomes)
C. If the two alleles differ, then the dominant allele is fully expressed in the organism’s
appearance; the recessive allele has no noticeable effects on the organism’s
appearance
D. The two alleles for each character segregate during gamete production- an ovum and
a sperm each get only one of the two alleles.
1. If an organism has identical alleles for a particular character (true-breeding), that
allele exists in a single copy in all gametes
2. If contrasting alleles are present (F1 hybrids), then 50% of the gametes receive
the dominant allele and 50% receive the recessive allele
E. Can account for 3:1 ratio observed in F2 generation, illustrated using a Punnett
square
III.
Genetic Vocabulary
A. Homozygote (-ous)- organism that has a pair of identical alleles for a character, truebreeding (produce gametes all with same allele)
B. Heterozygote (-ous)- organism having two different alleles for a gene (not true
breeding, produce gametes having one or the other of the different alleles)
C. Phenotype- the appearance of an organism; its physical traits
D. Genotype- an organism’s genetic makeup
IV.
Testcross- the breeding of a recessive homozygote with an organism of dominant
phenotype but unknown genotype
V.
Mendel’s Law of Independent Assortment- the independent segregation of each pair of
alleles during gamete formation
A. Dihybrid Cross- the mating of parental varieties differing in two characters
1. Mendel did this with two pea characteristics
a. Color- Yellow (Y) or Green (y)
b. Shape- Round (R) or Wrinkled (r)
2. The two pairs of alleles segregate independently of each other, with genes
packaged into gametes in all possible allelic combinations and each gamete
having 1 allele for a gene
3. Gametes separate into- ¼ YR, ¼ Yr, ¼ yR, ¼ yr
4. Four classes ova x Four classes of sperm = 16 probable ways alleles combine
in F2 generation
Combinations make up four phenotypic categories in ration of 9:3:3:1 (YR, Yr,
yR, yr)
VI.
Reflection of Probability on Mendelian Genetics
1
Chapter 14 Study Guide
Courtesy of Julia Beamsderfer (2005)
A. Probability is used often in genetics, with 1 being certain to occur and 0 certain not
to occur
B. Rule of Multiplication
1. To determine the chance that two or more independent events will occur
together in some specific combination, you must compute the probability for
each independent event then multiplying them together to obtain the probability
of the events occurring together
a. EX- finding the probability of an Pp F1 plant to produce a pp F2 plant
means that both sperm and egg must carry p allele (probability of ovum p is
½ x probability of sperm p is ½ = ¼)
2. Can also be used in dihybrid crosses
a. EX- probability of an YyRr F1 plant producing an F2 plant having the
genotype YYRR is ( ¼ probability of ovum to be YR ( ½ probability of Y x
½ probability of X = ¼ ) x ¼ probability of sperm to be YR (^) = 1/16)
C. Rule of Addition
1. Determines the probability of an event that can occur in two or more different
ways is found by adding the separate probabilities
a. EX- the probability that F2 from monohybrid cross is heterozygous is ( ¼
probability of one heterozygote + ¼ probability of other heterozygote)
D. Using Rules of Probability to Solve Genetics Problems
1. (see example in text pg. 247)
Extending Mendelian Genetics
I.
Dominance
A. Spectrum- Complete Dominance…Incomplete Dominance…Codominance
1. Complete Dominance- when the phenotypes of the heterozygote and dominant
homozygote are indistinguishable.
2. Incomplete Dominance- when the F1 hybrids have an appearance between the
phenotypes of the two parental varieties
a. EX- the crossing of red and white snapdragons to produce pink flowers with
a phenotypic ratio of F2 offspring as 1:2:1 (red: pink: white)
3. Codominance- both alleles are separately expressed in the phenotype
B. Dominant allele does not subdue a recessive allele, but they reflect the mechanisms
by which specific alleles are expressed in phenotype
C. Dominant alleles are not more common in a population
II.
Multiple Alleles- most genes actually exist in populations of more than two allelic forms
A. EX- blood types- A(IA,IA, IAi) , B (IB, IB, IBi), AB (IAIB), and O (ii) *IA and IB
are dominant towards i
III.
Pleiotropy- The ability of a gene to affect an organism in many ways and more than one
phenotypic character
A. EX- the hereditary disease sickle-cell disease causes multiple symptoms
IV.
Epistasis- when a gene of one locus alters the phenotypic expression of a gene at a second
locus
A. EX- mice have an allele that determines color and another that determines whether or
not it has color or pigment, therefore affecting the first allele
V.
Polygenic Inheritance- an additive effect of two or more genes on a single phenotypic
character (often quantitative character- one that varies along a continuum as opposed to
either-or)
A. ES- skin color is controlled by at least three genes of incomplete dominance so that
each dark skin unit (A, B, C) contributes to skin color (from 0 – 6)
VI.
Environmental Impact on Phenotype- phenotype does not depend only on genes but on
the environment as well, such as skin color is affected by sun. The norm of reaction for a
genotype is the range of a phenotype due to environmental influence, broadest for
polygenic characters (called multifactorial)
Mendelian Inheritance in Humans
2
Chapter 14 Study Guide
Courtesy of Julia Beamsderfer (2005)
I.
II.
Pedigree Analysis- geneticists often analyze family pedigrees (a family tree describing
history for a particular trait) to help understand ancestors genotypes as well as predict the
genotypes of offspring
Genetic Disorders- Thousands of genetic disorders are inherited as simple alleles which
either cause production of malfunctional proteins or no proteins at all.
A. Recessively Inherited Disorders- In disorders classified as recessive, heterozygotes
are normal in phenotype but are carriers of the disorder, whereas only homozygotes
with one recessive allele from each parent inherit the disorder (aa).
1. Most people with recessive disorders are born to parents with normal phenotype
who are both characters, for they have a ¼ chance of having a child with
disorder. Few children born with disorders are born from parents that have
them, because the parents usually die or are sterile
2. Examples
a. Cystic Fibrosis- most common lethal genetic disease in the US
- The normal allele for this gene causes synthesis for a membrane protein
that transports chloride ions between certain cells and their extracellular
fluid.
- In those with CF, cells do not have these proteins or they are defective,
causing abnormally high concentration of extracellular chloride,
favoring bacterial infections and disabling a natural antibiotic.
- Untreated, most kids die before their fifth birthdays, but with gentle
pounding on the chest, daily doses of antibiotics, and other preventive
measures, life can be prolonged.
b. Tay-Sachs Disease- A recessive allele for this gene causes dysfunctional enzymes that
break down brain lipids of a certain class, causing seizures, blindness,
and degeneration of motor and mental performance in infants soon after
birth.
- Usually children with Tay-Sachs disease do not live more than a few
years
- The disease is most common in Ashkenazic Jews than in any other
population
c. Sickle-Cell Disease
- Sickle Cell disease affects 1/400 African Americans
- It is caused by the substitution of a single amino acid in the hemoglobin
protein of red blood cells. When oxygen of person’s blood is low, the
sickle-cell hemoglobin deforms the red cells, leading to other
symptoms
- Regular blood transfusions and new drugs can help treat problems, but
there is no cure
- It is said that those who are carriers, having the sickle-cell trait, assists
them in warding off malaria.
3. Consanguineous (same blood mating)- when members of a family mate, they are
more likely to produce offspring with a harmful allele because it is likely that
they are both carriers of a recessive trait.
B. Dominantly Inherited Disorders- many human disorders are due to dominant alleles
1. Dominantly Inherited Disorders are rarer than recessive ones because the effects
of a lethal dominant allele are not masked, and if individuals with the disease do
not live to reproductive maturity, they will not pass on the mutation.
2. Examples
a. Achondroplasia- a form of dwarfism occurs in people who are
heterozygotes of the allele resulting in a dwarf phenotype. The majority of
the population is homozygous for the recessive allele.
b. Huntington’s Disease- a degenerative disease caused by a lethal dominant
allele not visible until the individual is 35-45 years old.
3
Chapter 14 Study Guide
Courtesy of Julia Beamsderfer (2005)
It is now able to tell whether or not one has Huntington’s disease for it
can be detected on a locus near the tip of chromosome 4.
C. Multifactorial Disorders- disorders caused by a combination of genetic components
(often polygenic) plus environmental influence
1. Include heart disease, diabetes, cancer, alcoholism, and certain mental illnesses
like schizophrenia and manic depression.
2. Little is known about the genetic contribution to most multifactorial diseases
Technology in Genetic Testing and Counseling
A. Often the rules of probability are used for couples to assess the risk that a particular
genetic disorder will occur in their offspring by predicting their genotypes, etc.
B. Carrier Recognition- certain tests can be used to identify people as carriers of certain
diseases to then assess the risk of them producing offspring with the disease
C. Fetal Testing
1. Amniocentesis- At 14-16 weeks old, a fetus can be tested for a disorder by
having a physician insert a needle into the uterus, extract some amniotic fluid,
and culture some cells which can be used for karyotyping to identify
chromosomal defects
2. Chorionic Villus Sampling (CVS)- at 8-10 weeks old, a physician is able to
suction off tissue from the placenta, which are immediately used in karyotyping
giving results in 24 hours
3. Ultrasound- enables physician to examine a fetus for major abnormalities using
sound waves to produce an image of it
4. Fetoscopy- a needle-thin tube containing a viewing scope and fiber optics can
enable physician to examine the fetus
D. Newborn Screening- some genetic disorders can be detected at birth by simple tests
that are now routinely performed in most hospitals in the US
1. There is a screening program for recessively inherited disorder called
phenylketonuria, where kids cannot breakdown the amino acid phenylalanine,
causing mental retardation. Detected early, retardation can be prevented by
altering diets, low in phenylalanine.
-
III.
4