THE CHROMOSOMAL BASIS OF INHERITANCE
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Review of 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
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F1 generationR
Metaphase Iof meiosis
(alternativearrangements)
r
Y
y
Rr
Y y
R r
Y y
All round yellow seeds(RrYy)
F1 generationR
Metaphase Iof meiosis
(alternativearrangements)
r
Y
y
Rr
Y y
R r
Y y
All round yellow seeds(RrYy)
Anaphase Iof meiosis
Metaphase IIof meiosis
R
y
r
Y
r
y
R
Y
R r
Y y
Rr
Y y
F1 generationR
Metaphase Iof meiosis
(alternativearrangements)
r
Y
y
Rr
Y y
R r
Y y
All round yellow seeds(RrYy)
Anaphase Iof meiosis
Metaphase IIof 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
YGametes
Fertilization among the F1 plants
:39 :3 :1F2 generation
r
y
Genes on the same chromosome tend to go 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
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Purple long
Purple round
Red long
Red round
Explanation: linked genes
Parentaldiploid cellPpLl
Experiment
Purple flower
PpLl Long pollenPpLl
Prediction(9:3:3:1)
ObservedoffspringPhenotypes
284
21
21
55
215
71
71
24
Mostgametes
Meiosis
PL
pl
PL
PL pl
pl
Fertilization
Sperm
Mostoffspring Eggs
3 purple long : 1 red roundNot accounted for: purple round and red long
PL PL
PL
PL
plPL
pl
pl
pl
pl
Purple longPurple roundRed longRed round
Experiment
Purple flower
PpLl Long pollenPpLl
Prediction(9:3:3:1)
ObservedoffspringPhenotypes
284212155
215717124
Explanation: linked genes
Parentaldiploid cellPpLl
Mostgametes
Meiosis
PL
pl
PL
PL pl
pl
Fertilization
Sperm
Mostoffspring Eggs
3 purple long : 1 red roundNot accounted for: purple round and red long
PL PL
PL
PL
plPL
pl
pl
pl
pl
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
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Gametes
Tetrad Crossing over
Ba baa b
A BA B A b
Experiment
Parentalphenotypes
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)
Recombinantphenotypes
391 recombinants2,300 total offspring
= 0.17 or 17%
Explanation
G L
g l g l
g lGgLl
(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
Chromosome
9.5%
Recombinationfrequencies
9%
17%
g c l
Genetic maps • Show the order of genes on chromosomes
• Arrange genes into linkage groups representing individual chromosomes
Crossover data to map genes
Mutant phenotypes
Shortaristae
Blackbody(g)
Cinnabareyes(c)
Vestigialwings(l)
Browneyes
Long aristae(appendageson head)
Graybody(G)
Redeyes(C)
Normalwings(L)
Redeyes
Wild-type phenotypes
SEX CHROMOSOMES
AND
SEX-LINKED GENES
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X X-Y system in
mammals,
fruit flies
XX = female
XY = male
Chromosomes determine sex
(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
Chromosomes determine sex
X-O system in grasshoppers and roaches
XX = female; XO = male
76+
ZZ
76+
ZW
Chromosomes determine sex
Z-W in system in birds, butterflies, and some fishes
ZW = female, ZZ = male
1632
Chromosomes determine sex
Chromosome number in ants and bees
Diploid = female; haploid = male
Sex-linked genes are located on either of the sex chromosomes
• 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
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Female Male
XR XR Xr Y
XR YXR Xr
YXr
XR
Sperm
Eggs
R = red-eye alleler = 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 recessive alleles are present in one copy
• Hemophilia
• Colorblindness
• Duchenne muscular dystrophy
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QueenVictoria
Albert
Alice Louis
Alexandra CzarNicholas IIof Russia
Alexis
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
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1. Explain and apply Mendel’s laws of segregation and independent assortment
2. Distinguish between terms in the following groups: allele—gene; dominant—recessive; genotype—phenotype; F1—F2; heterozygous—homozygous; incomplete dominance—codominance
3. Explain the meaning of the terms locus, multiple alleles, pedigree, pleiotropy, polygenic inheritance
You should now be able to
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4. Describe the difference in inheritance patterns for linked genes and explain how recombination can be used to estimate gene distances
5. Describe how sex is inherited in humans and identify the pattern of inheritance observed for sex-linked genes
6. Solve genetics problems involving monohybrid and dihybrid crosses for autosomal and sex-linked traits, with variations on Mendel’s laws
You should now be able to
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