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Chapter 15: The Chromosomal Basis of Inheritance
Students
½ Learning log – due Tuesday (10/6)
Genetics Problems – both chapters – due 10/15
Cell phones in bin….off or muted…please & thank you
Figure 15.2 The chromosomal basis of Mendel’s laws
P Generation
Starting with two true-breeding pea plants,
we follow two genes through the F 1 and F2
generations. The two genes specify seed
color (allele Y for yellow and allele y for
Y
green) and seed shape (allele R for round
and allele r for wrinkled). These two genes are
on different chromosomes. (Peas have seven
chromosome pairs, but only two pairs are
illustrated here.)
Yellow-round
seeds (YYRR)
Green-wrinkled
seeds (yyrr)
Y
R
r
R
y
r
y
Meiosis
Fertilization
y
R Y
Gametes
r
All F1 plants produce
yellow-round seeds (YyRr)
R
R
y
F1 Generation
y
r
r
Y
Y
Meiosis
LAW OF SEGREGATION
r
R
Y
1 The R and r alleles segregate
R
at anaphase I, yielding
two types of daughter
cells for this locus.
Y
y
R
Y
y
LAW OF INDEPENDENT ASSORTMENT
r
Anaphase I
y
Y
r
1 Alleles at both loci segregate
in anaphase I, yielding four
types of daughter cells
R
depending on the chromosome
arrangement at metaphase I.
Compare the arrangement of
the R and r alleles in the cells
y
on the left and right
r
R
y
y
Metaphase II
Y
y
Y
Y
Y
Gametes
r
r
R
2 Each gamete
gets one long
chromosome
with either the
R or r allele.
Two equally
probable
arrangements
of chromosomes
at metaphase I
R
R
r
1
YR
4
Y
r
r
1
yr
4
F2 Generation
3 Fertilization
recombines the
R and r alleles
at random.
Y
Y
r
1
Yr
4
2 Each gamete gets
a long and a short
chromosome in
one of four allele
combinations.
y
y
R
R
1
yR
4
Fertilization among the F1 plants
9
:3
:3
:1
3 Fertilization results
in the 9:3:3:1
phenotypic ratio in
the F2 generation.
Let’s review
- Ch 13 – Meiosis makes gametes – sperm & egg
- Ch 14 – Mendel studied peas
- gametes pass on traits
- unknown what was in the gametes
Chapter 15 Essential Questions
LO 3.9 The student is able to construct an explanation, using visual
representations or narratives, as to how DNA in chromosomes
is transmitted to the next generation via mitosis, or meiosis
followed by fertilization.
LO 3.15 The student is able to explain deviations from Mendel’s model
of the inheritance of traits.
LO 3.16 The student is able to explain how the inheritance patterns of
many traits cannot be accounted for by Mendelian genetics.
LO 3.17 The student is able to describe representations of an
appropriate example of inheritance patterns that cannot be
explained by Mendel’s model of the inheritance of traits.
Chapter 15: The Chromosomal Basis of Inheritance
1. How was it determined that chromosomes carry genes?
- Thomas Hunt Morgan
- 1st to trace a specific gene to a specific chromosome
- Noticed a fly with white eyes (wild-type is red)
- Wild-type – phenotype most common in the natural population (+)
- Mutants – alternative trait to the wild-type
Figure 15.4 In a cross between a wild-type female fruit fly and a mutant
white-eyed male, what color eyes will the F1 and F2 offspring have?
P
Generation
X
F1
Generation
F2
Generation
Expected
3:1
Observed
3:1
Problem!!!!!!
Only males had white eyes…..hmmm.
Figure 15.4 In a cross between a wild-type female fruit fly and a mutant
white-eyed male, what color eyes will the F1 and F2 offspring have?
P
Generation
F1
Generation
X
P
Generation
W+
X
X
X
X
Y
W+
W+
W
W+
W
Ova
(eggs)
F1
Generation
Sperm
W+
W
F2
Generation
W+
Ova
(eggs)
F2
Generation
Sperm
W+
W
W+
W+
W+
W
W
W+
Chapter 15: The Chromosomal Basis of Inheritance
1. How was it determined that chromosomes carry genes?
2. Morgan’s next cross showed that linked genes are inherited together.
EXPERIMENT
Morgan first mated true-breeding wild-type flies with black, vestigial-winged flies to produce
heterozygous F1 dihybrids, all of which are wild-type in appearance. He then mated wild-type F1 dihybrid females with
black, vestigial-winged males, producing 2,300 F2 offspring, which he “scored” (classified according to phenotype).
P Generation
(homozygous)
b b vg vg
F1 dihybrid
(wild type)
(gray body,
normal wings)
b+
b
vg+
Double mutant
(black body,
vestigial wings)
x
Wild type
(gray body,
normal wings)
b+ b+ vg+ vg+
Double mutant
(black body,
vestigial wings)
TESTCROSS
x
b b vg vg
vg
RESULTS
b+vg+
b vg
965
944
Wild type
Black(gray-normal) vestigial
b+ vg
b vg+
206
Grayvestigial
185
Blacknormal
b vg
Sperm
b+ b vg+ vg b b vg vg b+ b vg vgb b vg+ vg
Parental-type
offspring
Recombinant (nonparental-type)
offspring
- Noticed a disproportionately
large number with same
phenotype as parents
- Deduced 2 genes must be
on the same chromosome
- Crossing over accounts for
the recombinant phenotypes
Chapter 15: The Chromosomal Basis of Inheritance
1. How was it determined that chromosomes carry genes?
2. Morgan’s next cross showed that linked genes are inherited together.
3. What if the genes were unlinked…meaning independent assortment?
P generation:
YyRr x
yyrr
Gametes from yellow-round
heterozygous parent (YyRr)
YR
Gametes from greenwrinkled homozygous
recessive parent (yyrr)
yr
Yr
yR
Yyrr
yyRr
yr
YyRr
Parentaltype
offspring
50%
yyrr
Recombinant
offspring
50%
Chapter 15: The Chromosomal Basis of Inheritance
1.
2.
3.
4.
How was it determined that chromosomes carry genes?
Morgan’s next cross showed that linked genes are inherited together.
What if the genes were unlinked…meaning independent assortment?
How often will recombination occur…frequency??
Figure 15.6 Chromosomal basis for recombination of linked genes
Testcross
parents
b+ vg+
Gray body,
normal wings
b vg
(F1 dihybrid)
Replication of
chromosomes
b+
Meiosis I: Crossing
over between b and vg
loci produces new allele
combinations.
Black body,
vestigial wings
b vg (double mutant)
Replication of
chromosomes

vg
b vg
b+ vg+
vg
b
b vg
vg
b
b vg
b vg
Meiosis II: Segregation
of chromatids produces
recombinant gametes
with the new allele
combinations.
Gametes
b vg
Meiosis I and II:
Even if crossing over
occurs, no new allele
combinations are
produced.
Recombinant
chromosome
Ova
Sperm
b+vg+
b vg
b+
vg
b
vg+
b vg
b+ vg+
Testcross
offspring
Sperm
b vg
965
Wild type
(gray-normal)
b vg
944
Blackvestigial
b+ vg
b vg+
206
Grayvestigial
185
Blacknormal
Ova
b+ vg+
b vg
b+ vg
Recombination
391 recombinants
=
b vg+ frequency
2,300 total offspring
b vg
b vg
b vg
b vg
Parental-type offspring
 100 = 17%
Recombinant offspring
Sturtevant – developed a genetic linkage map from recombination frequencies
Chapter 15: The Chromosomal Basis of Inheritance
1.
2.
3.
4.
5.
How was it determined that chromosomes carry genes?
Morgan’s next cross showed that linked genes are inherited together.
What if the genes were unlinked…meaning independent assortment?
How often will recombination occur…frequency??
How can a genetic map be created from recombination frequencies?
bcn 9%
cnvg 9.5%
bvg 17%
Recombination
frequencies
9.5%
9%
17%
Chromosome b
cn
vg
-1% RF = 1 map unit (m.u.)
-Some linked genes are so far apart that crossovers occur very often.
-50% RF is MAX
-Recall 50% is seen with unlinked genes
Chapter 15: The Chromosomal Basis of Inheritance
1. How was it determined that chromosomes carry genes?
2. Morgan’s next cross showed that linked genes are inherited together.
3. What if the genes were unlinked…meaning independent assortment?
P generation:
YyRr x
yyrr
Gametes from yellow-round
heterozygous parent (YyRr)
YR
Gametes from greenwrinkled homozygous
recessive parent (yyrr)
yr
Yr
yR
Yyrr
yyRr
yr
YyRr
Parentaltype
offspring
50%
yyrr
Recombinant
offspring
50%
Figure 15.8 A partial genetic (linkage) map of a Drosophila chromosome
I
Y
II
X
IV
III
Mutant phenotypes
Short
aristae
Black
body
0
Long aristae
(appendages
on head)
Cinnabar
eyes
Vestigial
wings
48.5 57.5 67.0
Gray
body
Red
eyes
Normal
wings
Wild-type phenotypes
Brown
eyes
104.5
Red
eyes
Students
-Get a folder
-From student table closest to the door
-Cross out current name
-Add your name
-Learning logs mid-point check is tomorrow (Ch 13 & 14)
-FCA – Career Center
-Meets every other Wednesday – Rm 112 7:25 am
-Fields of Faith
-Reagan Stadium
-7PM 10/14
-Phones in bin…muted off…please & thank you
Chapter 15: The Chromosomal Basis of Inheritance
1.
2.
3.
4.
5.
6.
How was it determined that chromosomes carry genes?
Morgan’s next cross showed that linked genes are inherited together.
What if the genes were unlinked…meaning independent assortment?
How often will recombination occur…frequency??
How can a genetic map be created from recombination frequencies?
What determines male or female in utero?
- SRY – sex-determining region of Y
- w/ SRY – gonads develop into testes
- w/o SRY – gonads develop into ovaries
- X – has genes not associated w/ sex characteristics
- Sex-linked is usually X-linked
- Fathers pass X-linked alleles to daughters (XX)
- Moms pass X-linked alleles to sons or daughters
- If X-linked allele is recessive
- ♀ shows phenotype when homozygous
- ♂ shows phenotype when hemizygous – more males affected
Chapter 15: The Chromosomal Basis of Inheritance
1.
2.
3.
4.
5.
6.
7.
How was it determined that chromosomes carry genes?
Morgan’s next cross showed that linked genes are inherited together.
What if the genes were unlinked…meaning independent assortment?
How often will recombination occur…frequency??
How can a genetic map be created from recombination frequencies?
What determines male or female in utero?
How are sex-linked alleles transmitted?
Figure 15.10 The transmission of sex-linked recessive traits
XAXA
(a) A father with the disorder will transmit the mutant
allele to all daughters but to no sons. When the
mother is a dominant homozygote, the daughters
will have the normal phenotype but will be carriers
of the mutation.
Ova
Xa
(c) If a carrier mates with a male who has the
disorder, there is a 50% chance that each
child born to them will have the disorder,
regardless of sex. Daughters who do not have
the disorder will be carriers, where as males
without the disorder will be completely free of
the recessive allele.
Y
XAXa XAY
XA
XAxa XAY

XA
Ova
XaY
XA
XAXa
(b) If a carrier mates with a male of normal
phenotype, there is a 50% chance that each
daughter will be a carrier like her mother, and
a 50% chance that each son will have the
disorder.

XAY
Y
XA
XAXA XAY
Xa
a
XaxA X Y
XAXa 
Sperm
Sperm
XaY
Sperm
Xa
Ova
Y
XA
XAXa XAY
Xa
Xaxa XaY
Students
-Get test folder
-Grades posted – highlights are UNattractive
-Pull out Learning logs for check
-Phones in the bin…muted or off…please & thank you
Chapter 15: The Chromosomal Basis of Inheritance
1.
2.
3.
4.
5.
6.
7.
8.
How was it determined that chromosomes carry genes?
Morgan’s next cross showed that linked genes are inherited together.
What if the genes were unlinked…meaning independent assortment?
How often will recombination occur…frequency??
How can a genetic map be created from recombination frequencies?
What determines male or female in utero?
How are sex-linked alleles transmitted?
What are some sex-linked alleles in humans?
- Sex-linked recessive
- Duchenne’s muscular dystrophy
- dystrophin – key muscle protein is absent
- Progressive weakening of muscles & loss of coordination
- 1 in 3500 ♂ - rarely live past early 20s
- Hemophilia
- Protein needed for blood clotting
- Color blindness
Chapter 15: The Chromosomal Basis of Inheritance
1.
2.
3.
4.
5.
6.
7.
8.
9.
How was it determined that chromosomes carry genes?
Morgan’s next cross showed that linked genes are inherited together.
What if the genes were unlinked…meaning independent assortment?
How often will recombination occur…frequency??
How can a genetic map be created from recombination frequencies?
What determines male or female in utero?
How are sex-linked alleles transmitted?
What are some sex-linked alleles in humans?
What are Barr bodies?
- 1 of the 2 Xs (females) becomes almost completely inactive during
embryonic development in mammals
- Inactive X in each ♀ cell condenses into a Barr body
- Most genes on the Barr body are not expressed
- Barr body chromosomes are reactivated in ovary cells that
give rise to ova
- Calico cats
Figure 15.11 X inactivation and the tortoiseshell (calico) cat
Two cell populations
in adult cat:
Active X
Early embryo:
X chromosomes
Cell division
Inactive X
and X
chromosome Inactive X
inactivation
Allele for
orange fur Allele for
black fur
Orange
fur
Black
fur
Active X
Students
-New seats tomorrow
-Genetics Problems
-Write allele info: B - brown, b – black
-Write parents gentoype for crosses
-Bozeman videos
-28 – Meiosis
-29 & 30 – Genetics
-33 & 34 – Genotypes & Phenotypes, Variation
-Have a great day!!
-Phones in bin….muted or off…please & thank you!
Chapter 15: The Chromosomal Basis of Inheritance
1. How was it determined that chromosomes carry genes?
2. Morgan’s next cross showed that linked genes are inherited together.
3. What if the genes were unlinked…meaning independent assortment?
4. How often will recombination occur…frequency??
5. How can a genetic map be created from recombination frequencies?
6. What determines male or female in utero?
7. How are sex-linked alleles transmitted?
8. What are some sex-linked alleles in humans?
9. What are Barr bodies?
10. What are some chromosomal errors & exceptions?
- Nondisjunction
- Homologous chromosomes fail to separate during meiosis
- Chromosomal rearrangements
Figure 15.12 Meiotic nondisjunction
Meiosis I
Nondisjunction
Meiosis II
Nondisjunction
Gametes
n+1
n+1
n1
n+1
n –1
n–1
Number of chromosomes
(a) Nondisjunction of homologous
chromosomes in meiosis I
n
n
(b) Nondisjunction of sister
chromatids in meiosis II
Aneuploidy – an offspring that has an abnormal # of chromosomes (formed from a
nondisjunction gamete)
Trisomic – 2n + 1, Monosomic – 2n – 1
Polyploidy – more than 2 complete chromosome SETS: 3n – triploid, 4n - tetraploid
Crossing over is NOT exact (Figure 19.18)
This leads to deletions & duplications.
Figure 15.14 Alterations of chromosome structure
(a) A deletion removes a chromosomal
segment.
(b) A duplication repeats a segment.
(c) An inversion reverses a segment within
a chromosome.
(d) A translocation moves a segment from
one chromosome to another,
nonhomologous one. In a reciprocal
translocation, the most common type,
nonhomologous chromosomes exchange
fragments. Nonreciprocal translocations
also occur, in which a chromosome
transfers a fragment without receiving a
fragment in return.
A B C D E
F G H
A B C D E
F G H
A B C D E
F G H
A B C D E
F G H
Deletion
Duplication
Inversion
A B C E
F G H
A B C B C D E
A D C B E
F G H
M N O C D E
Reciprocal
translocation
M N O P Q
R
A B P
Q
F G H
R
F G H
Chapter 15: The Chromosomal Basis of Inheritance
1. How was it determined that chromosomes carry genes?
2. Morgan’s next cross showed that linked genes are inherited together.
3. What if the genes were unlinked…meaning independent assortment?
4. How often will recombination occur…frequency??
5. How can a genetic map be created from recombination frequencies?
6. What determines male or female in utero?
7. How are sex-linked alleles transmitted?
8. What are some sex-linked alleles in humans?
9. What are Barr bodies?
10. What are some chromosomal errors & exceptions?
11. What are some human disorders due to chromosomal alterations?
- Down syndrome
- Trisomy 21 aka nondisjunction of 21st chromosome
- Each cell has 47 chromosomes
Figure 15.15 Down syndrome
Chapter 15: The Chromosomal Basis of Inheritance
1. How was it determined that chromosomes carry genes?
2. Morgan’s next cross showed that linked genes are inherited together.
3. What if the genes were unlinked…meaning independent assortment?
4. How often will recombination occur…frequency??
5. How can a genetic map be created from recombination frequencies?
6. What determines male or female in utero?
7. How are sex-linked alleles transmitted?
8. What are some sex-linked alleles in humans?
9. What are Barr bodies?
10. What are some chromosomal errors & exceptions?
11. What are some human disorders due to chromosomal alterations?
- Down syndrome
- Nondisjunction of sex chromosomes
- Klinefelter syndrome – XXY – 1 in 2000 ♂
- XYY syndrome – 1 in 1000 ♂
- XXX - 1 in 1000 ♀
- Turner syndrome - XO – monosomy X – 1 in 5000 ♀
Chapter 15: The Chromosomal Basis of Inheritance
1. How was it determined that chromosomes carry genes?
2. Morgan’s next cross showed that linked genes are inherited together.
3. What if the genes were unlinked…meaning independent assortment?
4. How often will recombination occur…frequency??
5. How can a genetic map be created from recombination frequencies?
6. What determines male or female in utero?
7. How are sex-linked alleles transmitted?
8. What are some sex-linked alleles in humans?
9. What are Barr bodies?
10. What are some chromosomal errors & exceptions?
11. What are some human disorders due to chromosomal alterations?
12. How are extra-chromosomal genes inherited?
- Maternally – from egg – only sperm nucleus enters egg during fertilization
- All organelles including mitochondria