Download Freeman 1e: How we got there

A Rap Tribute to Mendel
• Check this out:
Movie: http://www.youtube.com/watch?v=EvR_Sdm1orU
http://history.nih.gov/exhibits/nirenberg/images/photos/01_mendel_pu.jpg
Alleles Alter Phenotypes in
Different Ways
• Alternative forms of a gene are called alleles.
• Mutation is the source of alleles.
• The wild-type allele is the one that occurs most
frequently in nature and is usually, but not always,
dominant.
• A mutation that results in a loss of function is
called a null allele.
• Phenotypic traits may be influenced by more than
one gene (and associated alleles).
Geneticists Use a Variety of
Symbols for Alleles
If wildtype is dominant:
• Wildtype (dominant)  e+ /e+ or e+/e (gray body)
• Recessive (mutant)  e (ebony body = mutant)
If wildtype is recessive:
• Mutant (dominant)  Wr/Wr or Wr/Wr+ (both have
wrinkled wings)
• Wildtype (recessive)  Wr+ / Wr+ (normal straight wings)
• If no dominance exists, italic uppercase letters and
superscripts are used to denote alternative alleles (R1, R2;
CW, CR).
Incomplete Dominance
• In incomplete dominance, neither trait is dominant
• Offspring from a cross with contrasting traits
showing intermediate phenotype.
•R1 = red allele
R2 = white allele
•Red snapdragons x white pink
–R1R1 = red
R2R2 = white
R1R2 = pink
•What are the genotypes of the F2 generation (so F1
pink) x F1 pink?
Tay-Sachs Disease
Incomplete Dominance
• Double recessive offspring die w/in 3 years
• Heterozygotes are phenotypically normal
• Why is this an example of inc. dominance?
A. Heterozygotes only produce about half of the
normal enzymes coded by the dominant allele
B. The mutant allele does not show up in the
heterozygote individual
C. The dominant allele masks any traits connected to
the recessive allele
Correct answer: A
Tay-Sachs Disease
Incomplete Dominance
• Having one “good” allele insures that just
enough gene product is produced to thwart off
the genetic disorder.
•The Threshold Effect for this disease is just
under 50% of gene product production.
Codominance: Both Alleles
Influence Phenotype
• In the heterozygote, gene products of both alleles can be
detected.
• One example is the MN blood group Two possible
glycoproteins (antigens) attach to surface of red blood
cells in humans.
• If the 2 alleles are represented by LM and LN, what
genotypes and phenotypes are possible?
• If a man with this blood group, LMLN, had a child with a
MLN, what are the odds that
woman of this blood group,
L
Answer:
their child
would be
of the
M, LMLN, and LNLN
Genotypes
could
be:MN
LMLphenotype?
A. 25% B. 50% C. 75% D. 100%
Phenotypes could be: M, MN, and N
Multiple Alleles of a Gene May
Exist in a Population
• Multiple alleles (>2) can be studied only in
populations, because any individual will have at most
+ same gene.
two alleles of the
cn or wcn
•3 or more alleles of the same gene
•The ABO blood groups are an example of multiple
alleles.
w
se or wse
•Alleles: IA, IB, IO (or i)
•Phenotypes: A, B, AB, or O
3 alleles for the ABO blood group
• IA and IB alleles are codominant to each other
• IA and IB alleles are dominant to the IO allele
• The A antigen is made, the B antigen is made, both
are made or neither is made.
– Antigens are merely carbohydrate receptor sites on RBCs
“A” carbohydrate (AcGalNH) “B” carbohydrate (galactose)
The “H substance”
Multiple Alleles: ABO Groups
•With the ABO blood groups, how many
genotypes are possible? Explain…
A. 3
B. 4
C. 6
D. 9
Correct answer: C
Solution: IAIO, IBIO, IAIA, IBIB, IAIB,& IOIO
Multiple Alleles: ABO Groups
• A baby was born with the O phenotype and
the mother was type B. The husband was
suspicious of the pregnancy and decided to
have his blood group determined (IAIO).
Should he be reassured by this information?
Solution: The baby’s genotype has to be IOIO. The mother has to
be IBIO . And so the baby is possibly his. Blood testing never
proves it however…
Calculating phenotypes with the
blood groups
• If a “B” phenotype male has kids with an “AB”
phenotype female, what are the probable
phenotypes for the kids?
• IBIB x IAIB  50% “type A”, 50% “B”
• IBIO x IAIB> 25% “A”, 50% “B”, 25% “AB”
X-Linked Genes
• Genes present on the X chromosome.
• Males have only one X chromosome and therefore only one
copy of the gene
– Express whichever allele they get
• Drosophila eye color was one of the first examples of Xlinkage.
• If an X-linked allele is recessive and LETHAL before
reproduction can occur, the disorder will occur only in
males
• Ex. Duchene muscular dystrophy & Lesch-Nyhan
Syndrome
No gene!
Fly Cross #1
• How would
you express the
ratios of the F2
phenotypes?
2 red –eyed female:
1 red-eyed male:
1 white-eyed male
Fly Cross #2
• How would you
express the ratios
of the F2
phenotypes?
• Do the cross…
Solution: 1 red eyed female: 1 white-eyed female:
1 red-eyed male: 1 white-eyed male
Fly Cross #2
Solution
• Question: Can a
male inherit the
gene for whiteeyes from
his father?
Answer: No, of course not!
He has to inherit the Y
chromosome from his dad.
Color-blindness is X-linked
• Review: Anthony’s Color-blindness.ppt
• Check out: Ishihara Test for Color B.doc
http://www.toledobend.com/colorblind/Ishihara.html
• Check out: Seeing Color.doc
http://askabiologist.asu.edu/research/seecolor/colortest.html
• Look at: X-linked_traits.ppt for review
Sex-limited Inheritance
•Expression of a specific phenotype is absolutely
limited to one sex.
–Autosomal genes that are affected by sex hormones
•In chickens, an autosomal gene controls feathering:
Genotype
HH
Hh
hh
F phenotype
Hen-like
Hen-like
Hen-like
M phenotype
Hen-like
Hen-like
Roster-like
•Presence of F or M sex hormones makes the difference.
Sex-influenced Inheritance
• Sex of an individual influences the expression of a
phenotype, but not limited to one sex or the other.
• Male pattern baldness can happen in women, but
its affect is usually less and happens later in life.
Genotype
BB
Bb
bb
F phenotype
Bald
Not bald
Not bald
M phenotype
Bald
Bald
Not bald
Can Hair Loss be Treated? See a short movie…
Male Pattern Baldness
Can Hair Loss be Treated? See a short movie…
Lethal Alleles
• A loss of function mutation can sometimes be tolerated in
the heterozygous state but may behave as a recessive lethal
allele in the homozygous state.
• Homozygous recessive individuals will not survive.
• Heterozygotes may show a mutation in phenotype and it
may be dominant over the wild type.
– These alleles are generally very rare.
• Ex. Coat color in mice (agouti is wildtype = A allele)
Cross #1: agouti x agouti  all agouti
Cross #2: yellow x yellow  2/3 yellow; 1/3 agouti
Cross #3: agouti x yellow  1/2 yellow; 1/2 agouti
EXPLAIN???
Dominant Lethal Alleles
•Sometimes mutation can be a dominant lethal allele.
–Even a heterozygote will not survive.
–One copy of the normal allele isn’t sufficient to reach the
threshold level of the protein product
• For dominant lethal alleles to persist in a population,
individuals MUST be able to reproduce before dying.
–Ex. Huntington’s Disease
–H = dominant lethal allele; affected individuals are Hh
•Disease does not become apparent until approx. age 40
•Famous folk singer Woody Guthrie died of this had this genetic
disease and died at age 39
Combinations of Two Gene Pairs
• Mendel’s independent assortment also governs 2 separate
genes and respective alleles.
• Consider this cross: Aa IAIB x Aa IAIB where allele “a” is a
recessive trait for albinism. IAIB are the blood group alleles.
• If these parents are crossed what is the most effective way
for you to determine the possible phenotypes of the
offspring?
Solution: Must use the forked-line method and consider the
outcomes of each gene separately…
Combinations of Two Gene Pairs
•
Which of these statements is the right way to
start?
A. For the Aa x Aa cross, 3/4 of the offspring will be the
normal, pigmented phenotype and ¼ will be albino.
B. Take the Aa gene pair and combine it with the IAIB
blood group to make a zygote.
C. Realize that the A combines with the IA and the “a”
combines with the IB .
Correct answer: A.
Epistasis
• Many traits are actually affected by more than one gene.
• One gene may mask the effect of another gene.
or…
• Two gene pairs may complement each other.
–At least one dominant allele at each loci is needed or desirable
product won’t be made.
• Di-hybrid crosses typically yield the F2 phenotypic ratio of
9:3:3:1. If a cross yields some other ratio, epistasis is
probably to blame.
Epistasis and the Bombay
Phenotype
• Individuals appear to be type O blood group
• Have rare recessive mutation: can’t make complete
“H” substance (molecule on red blood cells).
• Have working genes to make antigens A and/or B,
but antigens can’t attach to “H”.
–Phenotypically  “O” blood; Genotypically  IAIB
• Conclusion: homozygous recessive condition (H
mutation) masks gene expression at second locus (A
and/or B antigen production).
•H-substance gene is epistatic to the ABO antigen gene
–Work out the following cross… IAIB Hh x IAIB Hh
The H Substance
• Normal
Mutation
Antigens: A and/or B
Cell
Normal “H” substance
Cell
Mutant “H” substance
Conclusion? Antigens can’t attach. Appears as “no antigens”, type O
Determine the Resulting Phenotypes
• In mice, wildtype coloring is agouti (A). This is
dominant to black (a). When homozygous (bb), a
recessive mutation at a different loci eliminates
color altogether resulting in albino mice.
– It doesn’t matter what the genotype at the A loci is!
Cross:
Ratio
9/16
Genotype Phenotype
A_B_
AaBb x
AaBb
3/16
A_bb
3/16
aaB_
1/16
aabb
Five Cases of Epistasis
Recessive epistasis (case 1), dominant epistasis (case 2), and
complementary gene interaction (case 3).
Figure 4-8
Copyright © 2006 Pearson Prentice Hall, Inc.
Pleiotropy
• Expression of a single gene has multiple phenotypic
effects
– This is quite common
• Marfan syndrome  dominant mutation resulting in
defective fibrillin a common connective tissue protein
• Can affect: eye lens, blood vessel lining, & bone
formation.
• Porphyria variegata  don’t break down part of
hemoglobin in red blood cells
• Symptoms  deep red urine, abdominal pain, headaches,
insomnia, vision problems etc.
Environmental Influences
• Phenotypic expression of a trait may be influenced
by environment as well as by genotype.
• Temperature sensitive genes produce different
phenotypes if the developmental temperature of tissue
was is cool vs. warm.
–Siamese cats: body parts such as tail, ears, and paws are all
extremities and are “cooler” than core body parts.
–In warm body parts, black pigment enzyme doesn’t function
properly and no pigment (white) is produced.
–The evening primrose is red when grown at 23oC & white at
18oC.
Position Effect
• Physical location of a gene may influence its
expression due to a position effect.
• Translocation or inversion of genes on a
chromosome may affect position, and
therefore expression of gene.
Genes along a chromosome  normal (top)
translocation  below
Proximity to yellow gene may change blue gene expression
Fruit fly Eye Mutation
• Normal dominant
phenotype showing
brick-red eye color.
• Variegated color
caused by
rearrangement of the
“white” gene to
another location in the
genome.
Figure 4-17
Copy right © 2006 Pearson Prentice Hall, Inc.
Nutritional Mutations
• May prevent the phenotype from reflecting the
genotype.
• Key nutrients are not made or can’t be broken down.
• Supplements can overcome detrimental effect.
•Ex. Phenylketonuria (PKU)
–Mutation that prevents a person from metabolizing the
amino acid phenylalanine.
–Must eliminate that amino acid from their diet or
complications, including brain damage result.
–Babies are routinely “pricked” at birth to screen for this
disorder.
Lactose Intolerance
• After maturing, the ability to produce an enzyme,
lactase (which breaks down the milk sugar, lactose)
often shuts down.
• Milk intolerant people often have cramps, diarrhea,
and bloating.
• Yogurt, cheeses, butter  low lactose alternatives
• Lactase can also be purchased to alleviate
symptoms.