Download Glorious Genetics with a Marvelous Monk Named Mendel

Glorious Genetics with a
Marvelous Monk Named Mendel
Chapter 14
Science as a Process
 1. Gregor Mendel
– monk turned scientist, worked with garden peas
to study inheritance
– father of modern genetics
Science as a Process
 2. Why Peas???
– Come in many varieties (i.e. purple/white flowers,
round/wrinkled)
– Easy to control parentage
 sex organs are in flowers and each flower has both male
(stamens) and female (carpals) parts
Science as a Process
 3. How did he do it?
– Mendel removed Stamens before plants could selffertilize. The plant now only has the female parts.
– He then he put the pollen from another plant onto the
now “female” flower and made offspring (seeds)
– focused on either/or characters (there were only two
varieties of each trait)
– started with true-breeding plants = purple flowered
plants that produced only purple flowered offspring
Terrific Terminology
 Character - heritable feature (i.e. flower
color)
 Trait - variant of a character (purple, white)
 Hybridization - mating or crossing of two
varieties
 True-breeding- after many generations of
self pollination, parent plant produces only
the same variety of offspring (i.e. purple
plant makes only purple offspring)
Terminology - cont
 Monohybrid cross - looks at only one trait at
a time
 P generation - parental (original cross)
 F1 generation - offspring from P generation
 F2 generation - results if the F1 plants are
allowed to self - fertilize
QuickTime™ and a
decompressor
are needed to see this picture.
Mendel’s Shocking results
 P Generation - Purple x White flower
Produced
 F1 Generation - ALL PURPLE (GASP!!!)
 Let F1 self fertilize
 F2 Generation - 705 Purple, 224 White
(WOW!)
So Who Cares???
 Most scientists thought that traits blended
together. So after the Parents bred, the F1
should have been pale purple, but Mendel’s
results disproved this. Even more surprising
was that the white flowers showed up again
in the F2
 Mendel continued his studies with 6 other
characters (round/wrinkled, tall/short etc)
and found the same results in all characters.
What does it all mean???
 Alternate versions of genes account for
variations in inherited characters
 ex. There is a gene for flower color in peas.
This gene exists in two forms, purple or
white.
 allele - alternate form of a gene
How does it work???
1. Alternative versions of genes account for
variations in inherited characteristics
 Different alleles are caused by slight changes
in nucleotide sequences for a gene on the
DNA
 This change results in a slightly different
protein (causing the difference in appearance)
 The gene is in the same place on the
chromosome, but the order of the nitrogen
bases (A, T, G, C) is different.
How does it work???
2. An organism inherits two alleles for each trait,
one from each parent (one copy in sperm/pollen,
the other copy from egg).
 The offspring can inherit two of the same alleles,
or two different alleles
3. If the two alleles differ, then the Dominant allele
determines the organisms appearance and the
Recessive allele is not seen in the organisms’
appearance
–
(Mendel’s F1 generation looked purple because
purple color is dominant over white color in peas)
Why only one trait???
4. Two alleles for each character separate
during meiosis and only one is passed on
to the next generation = Law of
Segregation
Terrific Terminology cont
 Homozygous - organism with two of the same alleles for a
character
 Homozygous Dominant - AA Homozygous recessive - aa
 Heterozygous = organism with two different alleles for a
character = Aa
 Genotype = genetic makeup (actual genes the organism
has)
 Phenotype = physical appearance
 Punnett Square - box used to show all possible offspring
resulting from the cross between two parents
Punnett Square review
 Purple flowers are dominant to white
flowers. Mendel crossed a true breeding
(homozygous) purple flower to a true
breeding (homozygous) white flower. What
are the possible genotype and phenotype
results of this cross?
The Answer
 A = purple a = white  Genotype results
 4/4 Aa
 P gen. = AA x aa
A
a
a
A
Aa
Aa
Aa
Aa
 Phenotype results
 4/4 Purple
More fun!!!
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Next, Mendel let the F1 generation self fertilize.
What are the possible results of this cross?
Cross Aa x Aa
Genotype results
A
a
AA Aa
1/4 AA 1/2 Aa 1/4 aa
A
Phenotype results
Aa aa
a
3/4 Purple, 1/4 White
Do the results above agree with Mendel’s
results?
How can you tell if the plant is
Homozygous or Heterozygous???
 Do a Test-cross
 Cross your unknown purple plant with a
white plant.
 If the offspring are all purple, you know your
original plant is homozygous (AA).
 If 50% of the offspring are purple and 50%
are white, your original plant is
heterozygous (Aa)
 Do the squares if you don’t believe me!!!
Dihybrid Crosses - look at two traits
at once
 Used to determine if traits assort
independently of one another
 Try these (remember the 16 squares!!)
 Purple flowers are dominant to white
flowers.
 Round seeds are dominant to wrinkled
seeds.
 Cross a homozygous purple round plant
with a homozygous white wrinkled plant.
How do you do it???
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
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A = purple a = white
R = round r = wrinkled
AARR x
aarr
First find the gametes
Gametes
AR
ar
Dihybrid crosses (cont)
 Next – make the
square – This one’s
easy
AR
AaRr
ar
Dihybrid crosses (cont)
Genotype results
1/1 AaRr
Phenotype results
1/1 Purple, Round
Dihybrid crosses (cont)
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





Now let the plants self-fertilize
AaRr x AaRr
What will the gametes be?
AR
Ar
aR
ar
Yikes – 16 boxes!!!
AR
AR
Ar
aR
ar
Ar
aR
ar
AR
Ar
aR
ar
AR
AARR
AARr
AaRR
AaRr
Ar
AARr
AArr
AaRr
Aarr
AaRR
AaRr
aaRR
aaRr
AaRr
Aarr
aaRr
aarr
aR
ar
Now Count 
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Genotype results
1/16 AARR
2/16 AaRR
2/16 AARr
4/16 AaRr
1/16 AArr
2/16 Aarr
1/16 aaRR
2/16 aaRr
1/16 aarr
More counting




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Phenotype results
9/16 Purple, Round
3/16 Purple, wrinkled
3/16 white, Round
1/16 white, wrinkled
Law of Independent Assortment
 If you look at each of the characters
individually you get the correct ratios
 Purple 3: White: 1
 Round 3: Wrinkled: 1
 Law of Independent Assortment – Alleles
sort independently of one another during
meiosis ( traits are not linked together)
Laws of Probability (shortcuts we
never learned in Biology)
 Rule of Multiplication – used to determine
the chance that two or more independent
events will occur together in some specific
combination
 To figure this out – find the probability of one
event occurring and multiply it by the
probability of the other event(s) occurring
Try it
 You are crossing two plants that are
heterozygous (Aa) for purple color. What is
the chance of getting a white (aa) offspring?
 Chance of plant 1 giving a = ½
 Chance of plant 2 giving a = ½
 Chance of white offspring aa = ¼
 Does it work??? Do the square to confirm
Now try with a dihybrid
 You are crossing two plants that are
heterozygous Purple, Round (AaRr). What
is the chance of getting a plant that is aarr?
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Chance of plant 1 being ar = ¼
Chance of plant 2 being ar = ¼
Chance of aarr = 1/4 x 1/4 = 1/16
Check the results in the dihybrid problem
Rule of Addition
 The probability of an event that can occur in two or
more different ways is the sum of the separate
probabilities of those ways.
 Cross two plants that are heterozygous (Aa).
What is the chance that the offspring will be
heterozygous?
 Chance of Aa = 1/4 (if plant 1 gives A, plant 2
gives a)
 Chance of Aa = 1/4 (if plant 2 gives A, plant 1
gives a)
 Chance of heterozygous = 1/4 + 1/4 = 1/2
Now for a tough one
 Parents AaRrTt x Aarrtt
 Looking for the chance of at least two
recessives
 aarrTt =
 Aarrtt =
 aarrtt =
 aaRrtt =
 AArrtt =
Solution AaRrTt x Aarrtt
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aarrTt = 1/4 x 1/2 x 1/2 = 1/16
Aarrtt = 1/2 x 1/2 x 1/2 = 1/8 = 2/16
aarrtt = 1/4 x 1/2 x 1/2 = 1/16
aaRrtt = 1/4 x 1/2 x 1/2 = 1/16
AArrtt = 1/4 x 1/2 x 1/2 = 1/16
Total chance of at least 2 recessives = 6/16
or 3/8
Now use the rules of probability to
determine the following
 PKU is an inherited disease caused by a
recessive allele. If a woman and her
husband are both carriers (Aa x Aa), what is
the probability of each of the following:
 All three of her children will be of normal
phenotype
 3/4 x 3/4 x 3/4 = 27/64
Cont
 One or more of the three children will have the
disease
 64/64 – 27/64 = 37/64
 All three children will have the disease
 1/4 x 1/4 x 1/4 = 1/64
 At least one child will be phenotypically normal
 64/64 – 1/64 = 63/64
Exceptions to Mendel’s findings
 Mendel looked only at traits following simple
inheritance (complete dominance).
Mendel’s laws lay the foundation for modern
genetics and the basic principles are true,
but there are exceptions to his rules.
Incomplete Dominance
 Heterozygous organisms show a blending of two
other traits.
 Ex. – red flower x white flower = pink flower
 Try a few problems
 Red color is incompletely dominant to white.
Heterozygous flowers are pink.
 What would be the results of a cross between a
red and white flower?
 RR x WW
 Genotype = 4/4 RW
 Phenotype = 4/4 pink
R
 Try these two
 Pink x Pink
 Red x Pink
W
W
R
RW
RW
RW
RW
R
W
R
R
R
W
RR
RW
RR
RR
RW
RW
R
RW
WW
W
1/4 RR, 1/2 RW, 1/4 WW
25% red, 50% pink, 25% white
1/2 RR, 1/2 RW
50% red, 50% pink
Codominance
 One gene is codominant to another.
Heterozygous organisms show both traits.
 Ex – Normal blood x sickle blood = both
normal and sickle cells
 Try some problems – Normal red blood cells
are codominant to sickle cells.
Heterozygous individuals (carriers) have
both normal and sickle cells (also resistant
to malaria – Cool!!)
 Normal x Sickle
 Sickle x Carrier
 Carrier x Carrier
N
N
NS
NS
NS
NS
S
S
N
4/4 NS = 100% carriers
S
N
S
NS
NS
SS
SS
S
NN
NS
NS
SS
N
S
S
2/4 = NS, 2/4 SS
50% carrier, 50% sickle
1/4 NN, 2/4, NS, 1/4 SS
25% normal, 50% carrier, 25% sickle
Multiple Alleles
 More than two alleles exist for a trait
 Ex – blood types – A, B, AB, O
Phenotype
A
B
AB
O
Genotype
IAIA or IAi
IBIB or IBi
IAIB
ii
Antibodies
Anti – B
Anti – A
None
Anti-A,Anti - B
Try some probs
 Homozygous A x O
 AB x O
 Hetero A x Hetero B
IA
i
i
IA
I Ai
IAi
I Ai
IAi
IA
i
4/4 IAi
100% Type A
i
IB
IA
i
IAIB
IBi
IAi
ii
i
1/4 IAIB, 1/4 IBi, 1/4 IAi, 1/4 ii
25% AB, 25% B, 25% A, 25% O
IB
IAi
IBi
IAi
IBi
2/4 IAi, 2/4 IBi
50% type A, 50% type B
Pleiotropy
 One gene can affect an organism in many
ways
 Ex – sickle cell disease causes many
different symptoms in the patient
Epistasis
 A gene at one locus changes the expression
of another gene
 Ex – one gene causes pigment, the other
causes the actual color
Sample problems
 One dominant gene in mice causes pigment
to appear (D). If the mouse is homozygous
recessive (dd), no pigment will appear.
 Another gene (B) causes black color, the
recessive form (bb) causes brown color.
 Cross DDbb x Ddbb – How many black mice
will result?
 Chance of pigment 1/1 x chance of black 0
= no black mice
Try another
 DdBb x DdBb – How many brown mice will
result
 Chance of pigment 3/4 x chance of brown
1/4 = 3/16
QuickTime™ and a
decompressor
are needed to see this picture.
Polygenic
 More than one gene controls a trait
 Ex – hair color, skin color in humans
 Result is a range of trait in offspring - i.e not
just black or brown hair, but many shades in
between
Nature vs Nurture
 Some phenotypes are affected by
environmental factors
 Ex – amount of red blood cells depends not
only on genes, but on altitude, nutrition,
physical activity
Pedigree
 Used to trace traits in organisms that don’t
produce many offspring or have long
generation span (like humans)
 Square = male
 Circle = female
 Shaded = trait being traced – can be
dominant or recessive
Recessively inherited Disorders
 Caused only when organism inherits two
recessive alleles (aa)
Cystic Fibrosis
 1/2500 whites of European descent.
 Normal people can transport chloride into
cells, channels are defective in disease.
 Causes excessive mucous build up in many
organs
 Untreated = death by 15, treated can live
longer
. Tay-Sachs
 Dysfunction in enzyme that doesn’t break
down brain lipids.
 Causes seizures, blindness, decreased
motor performance
 Usually death within a few years.
 High incidence in Jewish people
Sickle-cell disease
 Most common in Africans.
 Caused by substitution of one amino acid in hemoglobin.
 Red Blood cells abnormally shaped, can’t carry oxygen
affectively.
 Treated with blood transfusions, but no cure.
 Heterozygous individuals have both normal/sickle blood
cells but show a resistance to Malaria (common in Africa).
 Heterozygotes have an advantage in these areas which
may be why the gene has remained in the population.
Dominantly inherited disorders
 Occur when at least one dominant gene is
present in an organism.
Huntington’s disease
 Degenerative disease of nervous system.
 Gene has remained in population because
illness doesn’t occur until 35-40 yrs of age.
 Many people with disease have children and
pass disease to them before they even
know they have it.
Achondroplasia
 Dwarfism - Aa = dwarf (.01%),
 aa = normal size (99.99%)
 AA = lethal (death before birth)
 Many other dominant disease are lethal and
kill organisms before birth
_s-xclick
Genetic testing
 Prenatal - amniocentesis - chromosome
analysis of amniotic fluid (weeks)
– carrier recognition - determining whether
parents are carriers
 CVS - chorionic villus sampling - fetal tissue
from placenta is used to do chromosome
analysis (24 hrs)
 Ultrasound - looks for developmental
problems in organs
Newborn screening
 many tests are done before babies leave
hospital to indicate genetic disorders
 PKU - tests for phenylketonuria - if detected,
parents can alter diet of children to prevent
symptoms (no phenylalanine)