MENDEL’S LAWS
Copyright © 2009 Pearson Education, Inc.
9.1 The science of genetics has ancient roots
 Pangenesis was an early explanation for inheritance
– It was proposed by H...
9.2 Experimental genetics began in an abbey
garden
 Gregor Mendel discovered principles of genetics
in experiments with t...
Petal
Stamen
Carpel
Transferred
pollen from stamens of white
flower to carpel of purple flower
Stamens
Carpel
Parents
(P)
Purple
2
White
Remov...
Transferred
pollen from stamens of white
flower to carpel of purple flower
Stamens
Carpel
Parents
(P)
Purple
2
White
Remov...
Transferred
pollen from stamens of white
flower to carpel of purple flower
Stamens
Carpel
Parents
(P)
Purple
2
White
Remov...
Flower color White
Axial
Purple
Flower position Terminal
YellowSeed color Green
RoundSeed shape Wrinkled
InflatedPod shape...
9.3 Mendel’s law of segregation describes the
inheritance of a single character
 Example of a monohybrid cross
– Parental...
P generation
(true-breeding
parents)
Purple flowers White flowers
P generation
(true-breeding
parents)
Purple flowers White flowers
F1 generation All plants have
purple flowers
P generation
(true-breeding
parents)
Purple flowers White flowers
F1 generation All plants have
purple flowers
F2 generati...
9.3 Mendel’s law of segregation describes the
inheritance of a single character
 Four Hypotheses
1. Genes are found in al...
9.3 Mendel’s law of segregation describes the
inheritance of a single character
 Four Hypotheses
3. If the alleles differ...
P plants
1–
2
1–
2
Genotypic ratio
1 PP : 2 Pp : 1 pp
Phenotypic ratio
3 purple : 1 white
F1 plants
(hybrids)
Gametes
Gene...
9.4 Homologous chromosomes bear the alleles
for each character
 For a pair of homologous chromosomes, alleles of
a gene r...
Gene loci
Homozygous
for the
dominant allele
Dominant
allele
Homozygous
for the
recessive allele
Heterozygous
Recessive
al...
9.5 The law of independent assortment is
revealed by tracking two characters at once
 Example of a dihybrid cross
– Paren...
9.5 The law of independent assortment is
revealed by tracking two characters at once
 Law of independent assortment
– Eac...
P
generation
1–
2
Hypothesis: Dependent assortment Hypothesis: Independent assortment
1–
2
1–
2
1–
2
1–
4
1–
4
1–
4
1–
4
1...
Phenotypes
Genotypes
Mating of heterozygotes
(black, normal vision)
Phenotypic ratio
of offspring
Black coat, normal visio...
9.6 Geneticists use the testcross to determine
unknown genotypes
 Testcross
– Mating between an individual of unknown gen...
B_
or
Two possibilities for the black dog:
Testcross:
Genotypes
Gametes
Offspring 1 black : 1 chocolateAll black
Bb
bb
BB
...
9.7 Mendel’s laws reflect the rules of probability
 The probability of a specific event is the number
of ways that event ...
F1 genotypes
1–
2
1–
2
1–
2
1–
2
1–
4
1–
4
1–
4
1–
4
Formation of eggs
Bb female
F2 genotypes
Formation of sperm
Bb male
B...
9.8 CONNECTION: Genetic traits in humans can
be tracked through family pedigrees
 A pedigree
– Shows the inheritance of a...
Freckles
Widow’s peak
Free earlobe
No freckles
Straight hairline
Attached earlobe
Dominant Traits Recessive Traits
Freckles No freckles
Widow’s peak Straight hairline
Free earlobe Attached earlobe
Ff
Female Male
Affected
Unaffected
First generation
(grandparents)
Second generation
(parents, aunts,
and uncles)
Third ge...
9.9 CONNECTION: Many inherited disorders in
humans are controlled by a single gene
 Inherited human disorders show
– Rece...
Parents Normal
Dd
Offspring
Sperm
Eggs
dd
Deafd
Dd
Normal
(carrier)
DD
NormalD
D d
Dd
Normal
(carrier)
Normal
Dd×
 Genetic testing of parents
 Fetal testing: biochemical and karyotype analyses
– Amniocentesis
– Chorionic villus sampli...
Needle inserted
through abdomen to
extract amniotic fluid
Suction tube inserted
through cervix to extract
tissue from chor...
VARIATIONS ON MENDEL’S
LAWS
Copyright © 2009 Pearson Education, Inc.
9.11 Incomplete dominance results in
intermediate phenotypes
 Incomplete dominance
– Neither allele is dominant over the ...
P generation
1–
2
1–
2
1–
2
1–
2
1–
2
1–
2
F1 generation
F2 generation
Red
RR
Gametes
Gametes
Eggs
Sperm
RR rR
Rr rr
R
r
R...
HH
Homozygous
for ability to make
LDL receptors
hh
Homozygous
for inability to make
LDL receptors
Hh
Heterozygous
LDL
rece...
9.12 Many genes have more than two alleles in
the population
 Multiple alleles
– More than two alleles are found in the p...
9.12 Many genes have more than two alleles in
the population
 Codominance
– Neither allele is dominant over the other
– E...
Blood
Group
(Phenotype) Genotypes
O
A
ii
IA
IA
or
IA
i
Red Blood Cells
Carbohydrate A
Antibodies
Present in
Blood
Anti-A
A...
Blood
Group
(Phenotype) Genotypes
O
A
ii
IA
IA
or
IA
i
Red Blood Cells
Carbohydrate A
B
IB
IB
or
IB
i
Carbohydrate B
AB IA...
Antibodies
Present in
Blood
Anti-A
Anti-B
Reaction When Blood from Groups Below Is Mixed
with Antibodies from Groups at Le...
9.13 A single gene may affect many phenotypic
characters
 *Pleiotropy
– One gene influencing many characteristics
– The g...
Clumping of cells
and clogging of
small blood vessels
Pneumonia
and other
infections
Accumulation of
sickled cells in sple...
9.14 A single character may be influenced by
many genes
 *Polygenic inheritance
– Many genes influence one trait
– Skin c...
P generation
1–
8
F1 generation
F2 generation
Fractionofpopulation
Skin color
Eggs
Sperm
1–
8
1–
8
1–
8
1–
8
1–
8
1–
8
1–
...
P generation
1–
8
F1 generation
F2 generation
Eggs
Sperm
1–
8
1–
8
1–
8
1–
8
1–
8
1–
8
1–
8
1–
8
1–
8
1–
8
1–
8
1–
8
1–
8
...
Fractionofpopulation
Skin color
1––
64
15––
64
6––
64
20––
64
9.15 The environment affects many characters
 Phenotypic variations are influenced by the
environment
– Skin color is aff...
THE CHROMOSOMAL BASIS
OF INHERITANCE
Copyright © 2009 Pearson Education, Inc.
9.16 Chromosome behavior accounts for
Mendel’s laws
 Mendel’s Laws correlate with chromosome
separation in meiosis
– The ...
F1 generation R
Metaphase I
of meiosis
(alternative
arrangements)
r
Y
y
Rr
Y y
R r
Y y
All round yellow seeds
(RrYy)
F1 generation R
Metaphase I
of meiosis
(alternative
arrangements)
r
Y
y
Rr
Y y
R r
Y y
All round yellow seeds
(RrYy)
Anaph...
F1 generation R
Metaphase I
of meiosis
(alternative
arrangements)
r
Y
y
Rr
Y y
R r
Y y
All round yellow seeds
(RrYy)
Anaph...
9.17 Genes on the same chromosome tend to be
inherited together
 Linked Genes
– Are located close together on the same ch...
Purple long
Purple round
Red long
Red round
Explanation: linked genes
Parental
diploid cell
PpLl
Experiment
Purple flower
...
Purple long
Purple round
Red long
Red round
Experiment
Purple flower
PpLl Long pollenPpLl
Prediction
(9:3:3:1)
Observed
of...
Explanation: linked genes
Parental
diploid cell
PpLl
Most
gametes
Meiosis
PL
pl
PL
PL pl
pl
Fertilization
Sperm
Most
offsp...
9.18 Crossing over produces new combinations
of alleles
 Linked alleles can be separated by crossing over
– Recombinant c...
Gametes
Tetrad Crossing over
Ba baa b
A B
A B A b
Experiment
Parental
phenotypes
Recombination frequency =
Black vestigial
Black body,
vestigial wings
GgLl
Offspring
Female...
Experiment
Parental
phenotypes
Recombination frequency =
Black vestigial
Black body,
vestigial wings
GgLl
Offspring
Female...
Explanation
G L
g l g l
g l
GgLl
(female)
ggll
(male)
G L g l g L
g l
g l
g l g l
g l
g l
G L
SpermEggs
Offspring
g L
G l
...
9.19 Geneticists use crossover data to map genes
 Genetic maps
– Show the order of genes on chromosomes
– Arrange genes i...
Chromosome
9.5%
Recombination
frequencies
9%
17%
g c l
Mutant phenotypes
Short
aristae
Black
body
(g)
Cinnabar
eyes
(c)
Vestigial
wings
(l)
Brown
eyes
Long aristae
(appendages
o...
SEX CHROMOSOMES AND
SEX-LINKED GENES
Copyright © 2009 Pearson Education, Inc.
9.20 Chromosomes determine sex in many
species
 X-Y system in mammals, fruit flies
– XX = female; XY = male
 X-O system ...
X
Y
(male)
Sperm
(female)
44
+
XY
Parents’
diploid
cells
44
+
XX
22
+
X
22
+
Y
22
+
X
44
+
XY
44
+
XX
Egg
Offspring
(diploid)
22
+
X
22
+
XX
76
+
ZZ
76
+
ZW
1632
 Sex-linked genes are located on either of the
sex chromosomes
– Reciprocal crosses show different results
– White-eyed f...
Female Male
XR
XR Xr
Y
XR
YXR
Xr
YXr
XR
Sperm
Eggs
R = red-eye allele
r = white-eye allele
Female Male
XR
Xr
XR
Y
XR
YXR
XR
YXR
XR
Sperm
Eggs
Xr
XR Xr
YXr
Female Male
XR
Xr Xr
Y
XR
YXR
XR
YXr
XR
Sperm
Eggs
Xr
Xr
Xr
YXr
9.22 CONNECTION: Sex-linked disorders affect
mostly males
 Males express X-linked disorders such as the
following when re...
Queen
Victoria
Albert
Alice Louis
Alexandra Czar
Nicholas II
of Russia
Alexis
9.23 EVOLUTION CONNECTION: The Y
chromosome provides clues about human
male evolution
 Similarities in Y chromosome seque...
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09 lecture presentation

  1. 1. MENDEL’S LAWS Copyright © 2009 Pearson Education, Inc.
  2. 2. 9.1 The science of genetics has ancient roots  Pangenesis was an early explanation for inheritance – It was proposed by Hippocrates – Particles called pangenes came from all parts of the organism to be incorporated into eggs or sperm – Characteristics acquired during the parents’ lifetime could be transferred to the offspring – Aristotle rejected pangenesis and argued that instead of particles, the potential to produce the traits was inherited  Blending was another idea, based on plant breeding – Hereditary material from parents mixes together to form an intermediate trait, like mixing paint Copyright © 2009 Pearson Education, Inc.
  3. 3. 9.2 Experimental genetics began in an abbey garden  Gregor Mendel discovered principles of genetics in experiments with the garden pea – Mendel showed that parents pass heritable factors to offspring (heritable factors are now called genes) – Advantages of using pea plants – Controlled matings – Self-fertilization or cross-fertilization – Observable characteristics with two distinct forms – True-breeding strains Copyright © 2009 Pearson Education, Inc.
  4. 4. Petal Stamen Carpel
  5. 5. Transferred pollen from stamens of white flower to carpel of purple flower Stamens Carpel Parents (P) Purple 2 White Removed stamens from purple flower 1
  6. 6. Transferred pollen from stamens of white flower to carpel of purple flower Stamens Carpel Parents (P) Purple 2 White Removed stamens from purple flower 1 Pollinated carpel matured into pod 3
  7. 7. Transferred pollen from stamens of white flower to carpel of purple flower Stamens Carpel Parents (P) Purple 2 White Removed stamens from purple flower 1 Pollinated carpel matured into pod 3 Offspring (F1) Planted seeds from pod 4
  8. 8. Flower color White Axial Purple Flower position Terminal YellowSeed color Green RoundSeed shape Wrinkled InflatedPod shape Constricted GreenPod color Yellow TallStem length Dwarf
  9. 9. 9.3 Mendel’s law of segregation describes the inheritance of a single character  Example of a monohybrid cross – Parental generation: purple flowers × white flowers – F1 generation: all plants with purple flowers – F2 generation: of plants with purple flowers of plants with white flowers  Mendel needed to explain – Why one trait seemed to disappear in the F1 generation – Why that trait reappeared in one quarter of the F2 offspring Copyright © 2009 Pearson Education, Inc. 3/4 1/4
  10. 10. P generation (true-breeding parents) Purple flowers White flowers
  11. 11. P generation (true-breeding parents) Purple flowers White flowers F1 generation All plants have purple flowers
  12. 12. P generation (true-breeding parents) Purple flowers White flowers F1 generation All plants have purple flowers F2 generation Fertilization among F1 plants (F1 × F1) of plants have purple flowers 3– 4 of plants have white flowers 1– 4
  13. 13. 9.3 Mendel’s law of segregation describes the inheritance of a single character  Four Hypotheses 1. Genes are found in alternative versions called alleles; a genotype is the listing of alleles an individual carries for a specific gene 2. For each characteristic, an organism inherits two alleles, one from each parent; the alleles can be the same or different – A homozygous genotype has identical alleles – A heterozygous genotype has two different alleles Copyright © 2009 Pearson Education, Inc.
  14. 14. 9.3 Mendel’s law of segregation describes the inheritance of a single character  Four Hypotheses 3. If the alleles differ, the dominant allele determines the organism’s appearance, and the recessive allele has no noticeable effect – The phenotype is the appearance or expression of a trait – The same phenotype may be determined by more than one genotype 4. Law of segregation: Allele pairs separate (segregate) from each other during the production of gametes so that a sperm or egg carries only one allele for each gene Copyright © 2009 Pearson Education, Inc.
  15. 15. P plants 1– 2 1– 2 Genotypic ratio 1 PP : 2 Pp : 1 pp Phenotypic ratio 3 purple : 1 white F1 plants (hybrids) Gametes Genetic makeup (alleles) All All Pp Sperm Eggs PP p ppPp Pp P pP pP P p PP pp All Gametes F2 plants
  16. 16. 9.4 Homologous chromosomes bear the alleles for each character  For a pair of homologous chromosomes, alleles of a gene reside at the same locus – Homozygous individuals have the same allele on both homologues – Heterozygous individuals have a different allele on each homologue Copyright © 2009 Pearson Education, Inc.
  17. 17. Gene loci Homozygous for the dominant allele Dominant allele Homozygous for the recessive allele Heterozygous Recessive allele Genotype: P Ba P PP a aa b Bb
  18. 18. 9.5 The law of independent assortment is revealed by tracking two characters at once  Example of a dihybrid cross – Parental generation: round yellow seeds × wrinkled green seeds – F1 generation: all plants with round yellow seeds – F2 generation: of plants with round yellow seeds of plants with round green seeds of plants with wrinkled yellow seeds of plants with wrinkled green seeds  Mendel needed to explain – Why nonparental combinations were observed – Why a 9:3:3:1 ratio was observed among the F2 offspring Copyright © 2009 Pearson Education, Inc. 9/16 3/16 3/16 1/16
  19. 19. 9.5 The law of independent assortment is revealed by tracking two characters at once  Law of independent assortment – Each pair of alleles segregates independently of the other pairs of alleles during gamete formation – For genotype RrYy, four gamete types are possible: RY, Ry, rY, and ry Copyright © 2009 Pearson Education, Inc.
  20. 20. P generation 1– 2 Hypothesis: Dependent assortment Hypothesis: Independent assortment 1– 2 1– 2 1– 2 1– 4 1– 4 1– 4 1– 4 1– 4 1– 4 1– 4 1– 4 9–– 16 3–– 16 3–– 16 1–– 16 RRYY Gametes Eggs F1 generation SpermSperm F2 generation Eggs Gametes rryy RrYy ryRY ryRY ry RY Hypothesized (not actually seen) Actual results (support hypothesis) RRYY rryy RrYy ryRY RRYY rryy RrYy ry RY RrYy RrYy RrYy rrYYRrYY RRYyRrYY RRYy rrYy rrYy Rryy Rryy RRyy rY Ry ry Yellow round Green round Green wrinkled Yellow wrinkled RY rY Ry
  21. 21. Phenotypes Genotypes Mating of heterozygotes (black, normal vision) Phenotypic ratio of offspring Black coat, normal vision B_N_ 9 black coat, normal vision Black coat, blind (PRA) B_nn 3 black coat, blind (PRA) Chocolate coat, normal vision bbN_ 3 chocolate coat, normal vision Chocolate coat, blind (PRA) bbnn 1 chocolate coat, blind (PRA) Blind Blind BbNn BbNn
  22. 22. 9.6 Geneticists use the testcross to determine unknown genotypes  Testcross – Mating between an individual of unknown genotype and a homozygous recessive individual – Will show whether the unknown genotype includes a recessive allele – Used by Mendel to confirm true-breeding genotypes Copyright © 2009 Pearson Education, Inc.
  23. 23. B_ or Two possibilities for the black dog: Testcross: Genotypes Gametes Offspring 1 black : 1 chocolateAll black Bb bb BB Bb bb B b Bb b bB
  24. 24. 9.7 Mendel’s laws reflect the rules of probability  The probability of a specific event is the number of ways that event can occur out of the total possible outcomes.  Rule of multiplication – Multiply the probabilities of events that must occur together  Rule of addition – Add probabilities of events that can happen in alternate ways Copyright © 2009 Pearson Education, Inc.
  25. 25. F1 genotypes 1– 2 1– 2 1– 2 1– 2 1– 4 1– 4 1– 4 1– 4 Formation of eggs Bb female F2 genotypes Formation of sperm Bb male B B B B B B b b bbb b
  26. 26. 9.8 CONNECTION: Genetic traits in humans can be tracked through family pedigrees  A pedigree – Shows the inheritance of a trait in a family through multiple generations – Demonstrates dominant or recessive inheritance – Can also be used to deduce genotypes of family members Copyright © 2009 Pearson Education, Inc.
  27. 27. Freckles Widow’s peak Free earlobe No freckles Straight hairline Attached earlobe Dominant Traits Recessive Traits
  28. 28. Freckles No freckles
  29. 29. Widow’s peak Straight hairline
  30. 30. Free earlobe Attached earlobe
  31. 31. Ff Female Male Affected Unaffected First generation (grandparents) Second generation (parents, aunts, and uncles) Third generation (two sisters) Ff Ff Ff Ff Ff Ff ff ff ff ff ff FF FF or or
  32. 32. 9.9 CONNECTION: Many inherited disorders in humans are controlled by a single gene  Inherited human disorders show – Recessive inheritance – Two recessive alleles are needed to show disease – Heterozygous parents are carriers of the disease-causing allele – Probability of inheritance increases with inbreeding, mating between close relatives – Dominant inheritance – One dominant allele is needed to show disease – Dominant lethal alleles are usually eliminated from the population Copyright © 2009 Pearson Education, Inc.
  33. 33. Parents Normal Dd Offspring Sperm Eggs dd Deafd Dd Normal (carrier) DD NormalD D d Dd Normal (carrier) Normal Dd×
  34. 34.  Genetic testing of parents  Fetal testing: biochemical and karyotype analyses – Amniocentesis – Chorionic villus sampling  Maternal blood test  Fetal imaging – Ultrasound – Fetoscopy  Newborn screening 9.10 CONNECTION: New technologies can provide insight into one’s genetic legacy Copyright © 2009 Pearson Education, Inc. Video: Ultrasound of Human Fetus
  35. 35. Needle inserted through abdomen to extract amniotic fluid Suction tube inserted through cervix to extract tissue from chorionic villi Ultrasound monitor Fetus Placenta Chorionic villi Uterus Cervix Amniocentesis Chorionic villus sampling (CVS) Ultrasound monitor Fetus Placenta Uterus Cervix Centrifugation Fetal cells Amniotic fluid Several weeks Biochemical tests Karyotyping Fetal cells Several hours
  36. 36. VARIATIONS ON MENDEL’S LAWS Copyright © 2009 Pearson Education, Inc.
  37. 37. 9.11 Incomplete dominance results in intermediate phenotypes  Incomplete dominance – Neither allele is dominant over the other – Expression of both alleles is observed as an intermediate phenotype in the heterozygous individual Copyright © 2009 Pearson Education, Inc.
  38. 38. P generation 1– 2 1– 2 1– 2 1– 2 1– 2 1– 2 F1 generation F2 generation Red RR Gametes Gametes Eggs Sperm RR rR Rr rr R r R r R r Pink Rr R r White rr
  39. 39. HH Homozygous for ability to make LDL receptors hh Homozygous for inability to make LDL receptors Hh Heterozygous LDL receptor LDL Cell Normal Mild disease Severe disease Genotypes: Phenotypes:
  40. 40. 9.12 Many genes have more than two alleles in the population  Multiple alleles – More than two alleles are found in the population – A diploid individual can carry any two of these alleles – The ABO blood group has three alleles, leading to four phenotypes: type A, type B, type AB, and type O blood Copyright © 2009 Pearson Education, Inc.
  41. 41. 9.12 Many genes have more than two alleles in the population  Codominance – Neither allele is dominant over the other – Expression of both alleles is observed as a distinct phenotype in the heterozygous individual – Observed for type AB blood Copyright © 2009 Pearson Education, Inc.
  42. 42. Blood Group (Phenotype) Genotypes O A ii IA IA or IA i Red Blood Cells Carbohydrate A Antibodies Present in Blood Anti-A Anti-B Reaction When Blood from Groups Below Is Mixed with Antibodies from Groups at Left Anti-B O A B AB B IB IB or IB i Carbohydrate B AB IA IB — Anti-A
  43. 43. Blood Group (Phenotype) Genotypes O A ii IA IA or IA i Red Blood Cells Carbohydrate A B IB IB or IB i Carbohydrate B AB IA IB
  44. 44. Antibodies Present in Blood Anti-A Anti-B Reaction When Blood from Groups Below Is Mixed with Antibodies from Groups at Left Anti-B O A B AB — Anti-A Blood Group (Phenotype) O A B AB
  45. 45. 9.13 A single gene may affect many phenotypic characters  *Pleiotropy – One gene influencing many characteristics – The gene for sickle cell disease – Affects the type of hemoglobin produced – Affects the shape of red blood cells – Causes anemia – Causes organ damage – Is related to susceptibility to malaria Copyright © 2009 Pearson Education, Inc.
  46. 46. Clumping of cells and clogging of small blood vessels Pneumonia and other infections Accumulation of sickled cells in spleen Pain and fever Rheumatism Heart failure Damage to other organs Brain damage Spleen damage Kidney failure Anemia Paralysis Impaired mental function Physical weakness Breakdown of red blood cells Individual homozygous for sickle-cell allele Sickle cells Sickle-cell (abnormal) hemoglobin Abnormal hemoglobin crystallizes, causing red blood cells to become sickle-shaped
  47. 47. 9.14 A single character may be influenced by many genes  *Polygenic inheritance – Many genes influence one trait – Skin color is affected by at least three genes (maybe more) – Opposite of pleiotropy. Copyright © 2009 Pearson Education, Inc.
  48. 48. P generation 1– 8 F1 generation F2 generation Fractionofpopulation Skin color Eggs Sperm 1– 8 1– 8 1– 8 1– 8 1– 8 1– 8 1– 8 1– 8 1– 8 1– 8 1– 8 1– 8 1– 8 1– 8 1– 8 aabbcc (very light) AABBCC (very dark) AaBbCc AaBbCc 1–– 64 15–– 64 6–– 64 1–– 64 15–– 64 6–– 64 20–– 64 1–– 64 15–– 64 6–– 64 20–– 64
  49. 49. P generation 1– 8 F1 generation F2 generation Eggs Sperm 1– 8 1– 8 1– 8 1– 8 1– 8 1– 8 1– 8 1– 8 1– 8 1– 8 1– 8 1– 8 1– 8 1– 8 1– 8 aabbcc (very light) AABBCC (very dark) AaBbCc AaBbCc 1–– 64 15–– 64 6–– 64 1–– 64 15–– 64 6–– 64 20–– 64
  50. 50. Fractionofpopulation Skin color 1–– 64 15–– 64 6–– 64 20–– 64
  51. 51. 9.15 The environment affects many characters  Phenotypic variations are influenced by the environment – Skin color is affected by exposure to sunlight – Susceptibility to diseases, such as cancer, has hereditary and environmental components Copyright © 2009 Pearson Education, Inc.
  52. 52. THE CHROMOSOMAL BASIS OF INHERITANCE Copyright © 2009 Pearson Education, Inc.
  53. 53. 9.16 Chromosome behavior accounts for Mendel’s laws  Mendel’s Laws correlate with chromosome separation in meiosis – The law of segregation depends on separation of homologous chromosomes in anaphase I. – The law of independent assortment depends on alternative orientations of chromosomes in metaphase I Copyright © 2009 Pearson Education, Inc.
  54. 54. F1 generation R Metaphase I of meiosis (alternative arrangements) r Y y Rr Y y R r Y y All round yellow seeds (RrYy)
  55. 55. F1 generation R Metaphase I of meiosis (alternative arrangements) r Y y Rr Y y R r Y y All round yellow seeds (RrYy) Anaphase I of meiosis Metaphase II of meiosis R y r Y r y R Y R r Y y Rr Y y
  56. 56. F1 generation R Metaphase I of meiosis (alternative arrangements) r Y y Rr Y y R r Y y All round yellow seeds (RrYy) Anaphase I of meiosis Metaphase II of meiosis R y r Y r y R Y R r Y y Rr Y y 1– 4 R y Ry R y r Y 1– 4 rY r Y 1– 4 ry r y 1– 4 RY R Y R Y Gametes Fertilization among the F1 plants :39 :3 :1F2 generation r y
  57. 57. 9.17 Genes on the same chromosome tend to be inherited together  Linked Genes – Are located close together on the same chromosome – Tend to be inherited together  Example studied by Bateson and Punnett – Parental generation: plants with purple flowers, long pollen crossed to plants with red flowers, round pollen – The F2 generation did not show a 9:3:3:1 ratio – Most F2 individuals had purple flowers, long pollen or red flowers, round pollen Copyright © 2009 Pearson Education, Inc.
  58. 58. Purple long Purple round Red long Red round Explanation: linked genes Parental diploid cell PpLl Experiment Purple flower PpLl Long pollenPpLl Prediction (9:3:3:1) Observed offspringPhenotypes 284 21 21 55 215 71 71 24 Most gametes Meiosis PL pl PL PL pl pl Fertilization Sperm Most offspring Eggs 3 purple long : 1 red round Not accounted for: purple round and red long PL PL PL PL pl PL pl pl pl pl
  59. 59. Purple long Purple round Red long Red round Experiment Purple flower PpLl Long pollenPpLl Prediction (9:3:3:1) Observed offspringPhenotypes 284 21 21 55 215 71 71 24
  60. 60. Explanation: linked genes Parental diploid cell PpLl Most gametes Meiosis PL pl PL PL pl pl Fertilization Sperm Most offspring Eggs 3 purple long : 1 red round Not accounted for: purple round and red long PL PL PL PL plPL pl pl pl pl
  61. 61. 9.18 Crossing over produces new combinations of alleles  Linked alleles can be separated by crossing over – Recombinant chromosomes are formed – Thomas Hunt Morgan demonstrated this in early experiments – Geneticists measure genetic distance by recombination frequency Copyright © 2009 Pearson Education, Inc.
  62. 62. Gametes Tetrad Crossing over Ba baa b A B A B A b
  63. 63. Experiment Parental phenotypes Recombination frequency = Black vestigial Black body, vestigial wings GgLl Offspring Female Male Gray long 965 944 206 185 ggll Gray vestigial Black long Gray body, long wings (wild type) Recombinant phenotypes 391 recombinants 2,300 total offspring Explanation = 0.17 or 17% G L g l g l g l GgLl (female) ggll (male) G L g l g L g l g l g l g l g l g l G L SpermEggs Offspring g L G l G l
  64. 64. Experiment Parental phenotypes Recombination frequency = Black vestigial Black body, vestigial wings GgLl Offspring Female Male Gray long 965 944 206 185 ggll Gray vestigial Black long Gray body, long wings (wild type) Recombinant phenotypes 391 recombinants 2,300 total offspring = 0.17 or 17%
  65. 65. Explanation G L g l g l g l GgLl (female) ggll (male) G L g l g L g l g l g l g l g l g l G L SpermEggs Offspring g L G l G l
  66. 66. 9.19 Geneticists use crossover data to map genes  Genetic maps – Show the order of genes on chromosomes – Arrange genes into linkage groups representing individual chromosomes Copyright © 2009 Pearson Education, Inc.
  67. 67. Chromosome 9.5% Recombination frequencies 9% 17% g c l
  68. 68. Mutant phenotypes Short aristae Black body (g) Cinnabar eyes (c) Vestigial wings (l) Brown eyes Long aristae (appendages on head) Gray body (G) Red eyes (C) Normal wings (L) Red eyes Wild-type phenotypes
  69. 69. SEX CHROMOSOMES AND SEX-LINKED GENES Copyright © 2009 Pearson Education, Inc.
  70. 70. 9.20 Chromosomes determine sex in many species  X-Y system in mammals, fruit flies – XX = female; XY = male  X-O system in grasshoppers and roaches – XX = female; XO = male  Z-W in system in birds, butterflies, and some fishes – ZW = female, ZZ = male  Chromosome number in ants and bees – Diploid = female; haploid = male Copyright © 2009 Pearson Education, Inc.
  71. 71. X Y
  72. 72. (male) Sperm (female) 44 + XY Parents’ diploid cells 44 + XX 22 + X 22 + Y 22 + X 44 + XY 44 + XX Egg Offspring (diploid)
  73. 73. 22 + X 22 + XX
  74. 74. 76 + ZZ 76 + ZW
  75. 75. 1632
  76. 76.  Sex-linked genes are located on either of the sex chromosomes – Reciprocal crosses show different results – White-eyed female × red-eyed male red-eyed females and white-eyed males – Red-eyed female × white-eyed male red-eyed females and red-eyed males – X-linked genes are passed from mother to son and mother to daughter – X-linked genes are passed from father to daughter – Y-linked genes are passed from father to son 9.21 Sex-linked genes exhibit a unique pattern of inheritance Copyright © 2009 Pearson Education, Inc.
  77. 77. Female Male XR XR Xr Y XR YXR Xr YXr XR Sperm Eggs R = red-eye allele r = white-eye allele
  78. 78. Female Male XR Xr XR Y XR YXR XR YXR XR Sperm Eggs Xr XR Xr YXr
  79. 79. Female Male XR Xr Xr Y XR YXR XR YXr XR Sperm Eggs Xr Xr Xr YXr
  80. 80. 9.22 CONNECTION: Sex-linked disorders affect mostly males  Males express X-linked disorders such as the following when recessive alleles are present in one copy – Hemophilia – Colorblindness – Duchenne muscular dystrophy Copyright © 2009 Pearson Education, Inc.
  81. 81. Queen Victoria Albert Alice Louis Alexandra Czar Nicholas II of Russia Alexis
  82. 82. 9.23 EVOLUTION CONNECTION: The Y chromosome provides clues about human male evolution  Similarities in Y chromosome sequences – Show a significant percentage of men related to the same male parent – Demonstrate a connection between people living in distant locations Copyright © 2009 Pearson Education, Inc.

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