Transcript Slide 1

Essentials of
Biology
Sylvia S. Mader
Chapter 10
Lecture Outline
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1.10 Mendel’s Laws
•
Gregor Mendel
•
•
•
•
•
Austrian monk
Worked with pea plants in 1860
When he began his work, most acknowledged that
both sexes contributed equally to a new individual.
Unable to account for presence of variations among
members of a family over generations
Mendel’s model compatible with evolution
•
•
Various combinations of traits are tested by the
environment.
Combinations that lead to reproductive success are the
ones that are passed on.
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Figure 10.1 Mendel working in his
garden
Trait
a.
Dominant
Pod shape
Tall
Inflated
Short
Constricted
Recessive
Characteristics
Stem length
b.
© Bettmann/Corbis
• Mendel’s experimental procedure
 Used garden pea, Pisum sativa
• Easy to cultivate, short generation time
• Normally self-pollinate but can be cross-pollinated
by hand
 Chose true-breeding varieties – offspring
were like the parent plants and each other.
 Kept careful records of large number of
experiments
 His understanding of mathematical laws of
probability helped interpret results.
 Particulate theory of inheritance – based on
the existence of minute particles (genes)
Figure 10.2 Garden pea
anatomy and traits
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
stigma
anther
ovary
Pollen grains containing sperm are
produced in the anther . When pollen
grains are brushed onto the stigma,
sperm fertilizes eggs in the ovary .
Fertilized eggs are located in ovules,
which develop into seeds.
a. Flower structure
Figure 10.2 continued
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1 Cut away anthers.
2 Brush on pollen from
another plant.
3 The results of cross from a
parent that produces round,
yellow seeds × parent that
produces wrinkled
yellow seeds.
b. Cross pollination
Figure 10.3 One-trait cross
• One-trait inheritance
 Original parents
called P generation
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• First-generation
offspring F1 generation
• Second-generation
offspring F2 generation
 Crossed tall pea
plants with short pea
plants
• All F1 are tall
• Had shortness
disappeared?
P generation
P gametes
F1 generation
All plants are tall.
×
TT
tt
T
t
Tt
Figure 10.3 continued
 Shows all possible
combinations of egg
and sperm offspring
may inherit
• When F1 allowed to
self-pollinate, F2 were
3/4 tall and 1/4 short
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eggs
F1 gametes
T
t
TT
Tt
Tt
tt
T
F2 generation
sperm
• Punnett square
t
 F1 had passed on
shortness.
offspring
F2 Phenotypic Ratio
3 tall : 1 short
Key:
T = tall plant
t = short plant
• Mendel reasoned 3:1 ratio only possible if
 F1 parents contained 2 separate copies of each
heritable factor (1 dominant and 1 recessive).
 Factors separate when gametes form and each
gamete carries only 1 copy of each factor.
 Random fusion of all possible gametes occurred
at fertilization.
• One-trait testcross
 To see if the F1 carries a recessive factor,
Mendel crossed his F1 generation tall plants
with true-breeding, short plants.
• He reasoned that half the offspring would be tall
and half would be short.
• His hypothesis that factors segregate when
gametes are formed was supported..
 Testcross
• Used to determine whether or not an individual
with the dominant trait has two dominant factors
for a particular trait
• One-trait testcross
 If a parent with the dominant phenotype has
only one dominant factor, the results among
the offspring are 1:1.
 If a parent with the dominant phenotype has
two dominant factors, all offspring have the
dominant phenotype.
Figure 10.4 One-trait testcross
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Parents
Parents
×
Possible
genotypes
×
Tt
tt
Tt
tt
Phenotypes
Phenotypic Ratio
a.
TT
Possible
genotype
tt
Tt
Phenotype
1 tall : 1 short
All tall plants
b.
• Mendel’s first law of inheritance – law of
segregation
 Cornerstone of his particulate theory of
inheritance
• The law of segregation states the
following:
 Each individual has two factors for each trait.
 The factors segregate (separate) during the
formation of the gametes.
 Each gamete contains only one factor from
each pair of factors.
 Fertilization gives each new individual two
factors for each trait.
• The modern genetics view
 Scientists note parallel between Mendel’s
particulate factors and chromosomes.
 Chromosomal theory of inheritance
• Chromosomes are carriers of genetic information.
 Traits are controlled by discrete genes that
occur on homologous pairs of chromosomes at
a gene locus.
• Each homologue holds one copy of each gene pair.
 Meiosis explains Mendel’s law of segregation
and why only one gene for each trait is in a
gamete.
• When fertilization occurs, the resulting offspring
again have two genes for each trait, one from each
parent.
• Alleles – alternative forms of a gene
• Dominant allele masks the expression of the
recessive allele.
• For the most part, an individual’s traits are
determined by the alleles inherited.
• Alleles occur on homologous chromosomes
at a particular location called the gene locus.
Figure 10.5 Homologous chromosomes
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sister
chromatids
G
g
G
G
g
R
R
r
R
S
s
t
T
S
alleles of a
gene at a
gene locus
a. Various alleles are
located at specific loci.
t
S
t
s
T
g
s
T
b. Duplicated chromosomes
show that sister chromatids
have identical alleles.
• Genotype versus phenotype
 Genotype – alleles individual receives at fertilization
• Homozygous – 2 identical alleles
 Homozygous dominant
 Homozygous recessive
• Heterozygous – 2 different alleles
 Phenotype – physical appearance of individual
• Mostly determined by genotype
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Allele A
Allele B
Allele C
Additive effect of dominant
alleles on phenotype
• Two-trait inheritance
 Mendel crossed tall plants with green pods
(TTGG) with short plants with yellow pods
(ttgg).
 F1 plants showed both dominant
characteristics – tall and green pods
 2 possible results for F2
• If the dominant factors always go into gametes
together, F2 will have only 2 phenotypes.
 Tall plants with green pods
 Short plants with yellow pods
• If four factors segregate into gametes
independently, 4 phenotypes would result.
Figure 10.6 Two-trait cross done by Mendel
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×
P generation
P gametes
F1 generation
All plants are tall
with green pods.
TTGG
ttgg
TG
tg
TtGg
Figure 10.6 continued
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
eggs
F1 gametes
TG
Tg
tG
tg
TTGG
TTGg
TtGG
TtGg
TTGg
TTgg
TtGg
Ttgg
TtGG
TtGg
ttGG
ttGg
TtGg
Ttgg
ttGg
ttgg
TG
F2 generation
sperm
Tg
tG
tg
offspring
F2 Phenotypic Ratio
9 tall plant, green pod
3 tall plant, yellow pod
3 short plant, green pod
1 short plant, yellow pod
Key:
T = tall plant
t = short plant
G = green pod
g = yellow pod
• Based on the results, Mendel formulated
his second law of heredity.
• Law of independent assortment
 Each pair of factors segregates (assorts)
independently of the other pairs.
 All possible combinations of factors can occur
in the gametes.
• When all possible sperm have an
opportunity to fertilize all possible eggs,
the expected phenotypic results of a twotrait cross are always 9:3:3:1.
• Two-trait testcross
 Fruit fly Drosophila melanogaster
• Used in genetics research
 Wild-type fly has long wings and gray body.
• Some mutants have vestigial wings and ebony
bodies.
• L=long, l=short, G= gray, g=ebony
 Can’t determine genotype of long-winged
gray-bodies fly (L_G_)
• Cross with short-winged ebony-bodied fly (llgg)
Figure 10.7 Two-trait testcross
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P generation
LlGg
llgg
eggs
lg
LG
LlGg
Lg
F1 generation
sperm
• In this example,
1:1:1:1 ratio of
offspring indicates
L_G_ fly was LlGg
(dihybrid).
Llgg
lG
llGg
lg
llgg
offspring
F1 Phenotypic Ratio
1
1
1
1
long wings, gray body
long wings, black body
short wings, gray body
short wings, black body
Key:
L = long wings
l = short wings
G = gray body
g = black body
• Mendel’s laws and probability
 Punnet square assumes
• Each gamete contains one allele for each trait.
 Law of segregation
• Collectively the gametes have all possible combinations of
alleles.
 Law of independent assortment
• Male and female gametes combine at random.
 Use rules of probability to calculate expected
phenotype ratios.
 Rule of multiplication - chance of two (or more)
independent events occurring together is the product
of their chances of occurring separately.
• Coin flips – odd of getting tail is 1/2, odds of getting tails
when you flip 2 coins 1/2 x 1/2= 1/4.
• Mendel’s laws and
meiosis
Figure 10.8 Mendel’s laws and
meiosis
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Dominant
 Gene for earlobes
and hairline on
different
chromosomes
 Gametes have all Unattached earlobes: EE or Ee
possible
combination of
alleles.
Recessive
Attached earlobes: ee
Widow’s peak: WW or Ww Straight hairline: ww
(earlobes, both): © The McGraw-Hill Companies, Inc./John Thoeming, photographer; (widow's peak): ©
SuperStock; (straight): © Michal Grecco/Stock Boston
Figure 10.8 continued (potential gametes produced by a
person who is EeWw)
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Key:
W = widow’s peak
w = straight hairline
E = unattached earlobes
e = attached earlobes
Parent cell has two pairs
of homologues.
E
one
pair
one
pair
w
e
W
either
or
Meiosis I
E
E
e
e
E
E
e
e
W
W w
w
w
w W
W
Homologues can
align either way
during metaphase I.
Meiosis II
E
E
e
e
E
E
e
e
W
W
w
w
w
w
W
W
E
e
E
W
W
EW
e
w
w
ew
E
e
E
w
w
Ew
e
W
W
eW
All possible combinations
of chromosomes and
alleles result.
10.2 Beyond Mendel’s Laws
• Incomplete dominance
 Heterozygote has
intermediate
phenotype.
 Four-o’clock flowers
• Red, pink and white
• NOT blending
inheritance – pink
flowers can have red,
white or pink offspring.
 Human wavy hair is
intermediate between
curly and straight hair.
Figure 10.9 Incomplete
dominance in four-o’clocks
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required for reproduction or display.
C RC R
C RC W
CWCW
• Multiple-allele traits
 ABO blood group inheritance has 3 alleles.
• IA = A antigen on red blood cells
• IB = B antigen on red blood cells
• i = neither A or B antigen on red blood cells
 Each person has only 2 of the 3 alleles.
 Both IA and IB are dominant to i
 IA and IB are codominant – both will be
expressed equally in the heterozygote.
Figure 10.10 Inheritance of ABO blood type
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Type A = IAIA, IAi
Type B = IBIB, IBi
Type AB = IAIB
Type O = ii
Parents
IBi
IAi
eggs
IA
i
IB
IAlB
IBi
i
IAi
ii
sperm
•
•
•
•
offspring
Phenotypic Ratio
1 :1 :1 :1
Key:
Blood type A
Blood type B
Blood type AB
Blood type O
• Polygenic inheritance
 Trait is governed by 2 or more sets of alleles.
 Each dominant allele has a quantitative effect
on phenotype and effects are additive.
 Result in continuous variation – bell-shaped
curve
 Multifactorial traits – polygenic traits subject to
environmental effects
• Cleft lip, diabetes, schizophrenia, allergies, cancer
• Due to combined action of many genes plus
environmental influences
Figure 10.11 Height in humans, a polygenic trait
Number of Men
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most
are
this
height
few
62
short
64
few
66
68
70
Height in Inches
72
74
tall
Courtesy University of Connecticut, Peter Morenus, photographer
• Environment and the phenotype
 Relative importance of each can vary.
 Temperature can effect coat color.
• Rabbits homozygous for ch have black fur where
the skin temperature is low.
• Enzyme encoded by gene is active only at low
temperatures.
Figure 10.12 Coat color in Himalayan rabbits
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• Pleiotropy
 Single genes have more than one effect.
 Marfan syndrome is due to production of
abnormal connective tissue.
Figure 10.13 Marfan syndrome, multiple effects of a single
human gene
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Connective tissue defects
Skeleton
Chest wall deformities
Long, thin fingers, arms, legs
Scoliosis (curvature of the spine)
Flat feet
Long, narrow face
Loose joints
Heart and blood vessels
Mitral valve
prolapse
Enlargement
of aorta
Eyes
Lungs
Skin
Lens dislocation
Severe nearsightedness
Collapsed lungs*
Stretch marks in skin
Recurrent hernias
Dural ectasia: stretching
of the membrane that
holds spinal fluid
Aneurysm
Aortic wall tear*
(tissue): © Ed Reschke; (athlete): © AP/Wide World Photos
10.3 Sex-linked Inheritance
• Females are XX.
 All eggs contain 1 X.
• Males are XY.
 Sperm contain either an X or a Y.
• Y carries SRY gene – determines
maleness.
• X is much larger and carries more genes.
 X-linked – gene on X chromosome
Figure 10.14 Inheritance of gender in human beings
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44 autosomes
+ XX
44 autosomes
+ XY
egg
22 + X
sperm
22 + X
44 + XX
22 + X
44 + XY
© Ryan McVay/Getty RF
• X-linked alleles
 Fruit flies have same sex chromosome pattern
as humans.
 When red-eyed female mated white mutant
white-eyed male, all offspring were red-eyed.
 In the F2, the 3:1 ratio was found but all of the
white-eyed flies were males.
 Y chromosome does not carry alleles for Xlinked traits.
 Males always receive X from female parent, Y
from male parent.
 Carrier – female who carries an X-linked trait but
does not express it.
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Figure 10.15 x-linked
inheritance
P generation
XRXR
XrY
P gametes
Xr
XR
Y
F1 generation
XRY
XRXr
eggs
F1 gametes
XR
Xr
XRXR
XRXr
XRY
XrY
F2 generation
sperm
XR
Y
offspring
F2 Phenotypic Ratio
females: all red-eyed
males: 1 red-eyed
1
white-eyed
Key:
XR = red eyes
Xr = white eyes
10.4 Inheritance of Linked Genes
• Some fruit fly crosses violated the law of
independent assortment.
 Offspring simply resembled one of the
parents.
• 2 traits on same chromosome – gene
linkage
• 2 traits on same chromosome do NOT
segregate independently.
• Recombination between linked genes
 Linked alleles stay together –
heterozygote forms only 2 types of
gametes, produces offspring only with 2
phenotypes.
Figure 10.16 Linked alleles and crossing-over
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sister
chromatids
G
G g
g
G
G g
g
G G g g
R
R r
r
R
R r
r
R R r r
tetrad
alleles
are linked
a. Linked alleles usually stay together
resulting
daughter
chromosomes
• Occasionally crossing-over produces
new combinations.
 Nonsister chromatids exchange genes.
 Recombinant gametes have a new
combination of alleles.
Figure 10.16 continued
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nonsister
chromatids
G
G g
g
G
g
Gg G g
R
R r
r
R
r
RR r r
linked alleles
sometimes cross-over
resulting
tetrad
daughter
chromosomes
b. Crossing-over results in recombination of alleles
• Distance between genes
 The closer 2 genes are on a chromosome, the less
likely they are to cross-over.
 You can use the percentage of recombinant
phenotypes to determine the distance between
genes.
 1% crossing-over = 1 map unit.
 In a black-body and purple-eye cross, 6% of offspring
are recombinant = genes are 6 map units apart.
 Results can make a chromosome map.
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P generation
Offspring
GgRr
ggrr
Predicted
Observed
25%
47%
25%
47%
25%
3%
25%
3%
GgRr
ggrr
Ggrr
ggRr
F1 Phenotypic Ratio
1
1
1
1
gray body, red eyes
black body, purple eyes
gray body, purple eyes
black body, red eyes
Key:
G = gray body
g = black body
R = red eyes
r = purple eyes
Figure 10.17 Linked
alleles do not assort
independently
Figure 10.18 Mapping chromosomes
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black
body
purple
eyes
6 map units
vestigial
wings
12.5 map units
18.5 map units