This document discusses the chromosomal basis of inheritance, including: 1) Chromosomes segregate during meiosis and assort independently, explaining Mendel's laws of segregation and independent assortment. 2) Thomas Hunt Morgan's work with fruit flies demonstrated that eye color in flies is sex-linked, carried on the X chromosome. 3) Sex-linked genes are located on the X chromosome and are more likely to be expressed in males who only have one X. 4) Genes located near each other on the same chromosome tend to be inherited together but can assort independently due to crossing over.

1. The Chromosomal Basis of
Inheritance
AP Biology
Chapter 12
http://genetics-education-partnership.mbt.washington.edu/cool/images/chromi_sm.jpg

2. Chromosome Theory of Inhertiance
• Locus: location of a
gene on a chromosome
• Allele: different form of
a gene
• Chromosome:
– Segregation
– Independent Assortment
http://www.nwcreation.net/articles/images/genelocus.JPG

3. The Chromosomal Basis of Mendel’s laws
Figure 15.2
Yellow-round
seeds (YYRR)
Green-wrinkled
seeds (yyrr)
Meiosis
Fertilization
Gametes
All F1 plants produce
yellow-round seeds (YyRr)
P Generation
F1 Generation
Meiosis
Two equally
probable
arrangements
of chromosomes
at metaphase I
LAW OF SEGREGATION LAW OF INDEPENDENT ASSORTMENT
Anaphase I
Metaphase II
Fertilization among the F1 plants
9 : 3 : 3 : 1
1
4
1
4
1
4
1
4
YR yr yr yR
Gametes
Y
R
R
Y
y
r
r
y
R Y y r
R
y
Y
r
R
y
Y
r
R
Y
r
y
r R
Y y
R
Y
r
y
R
Y
Y
R R
Y
r
y
r
y
R
y
r
Y
r
Y
r
Y
r
Y
R
y
R
y
R
y
r
Y
F2 Generation
Starting with two true-breeding pea plants,
we follow two genes through the F1 and F2
generations. The two genes specify seed
color (allele Y for yellow and allele y for
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.)
The R and r alleles segregate
at anaphase I, yielding
two types of daughter
cells for this locus.
1
Each gamete
gets one long
chromosome
with either the
R or r allele.
2
Fertilization
recombines the
R and r alleles
at random.
3
Alleles at both loci segregate
in anaphase I, yielding four
types of daughter cells
depending on the chromosome
arrangement at metaphase I.
Compare the arrangement of
the R and r alleles in the cells
on the left and right
1
Each gamete gets
a long and a short
chromosome in
one of four allele
combinations.
2
Fertilization results
in the 9:3:3:1
phenotypic ratio in
the F2 generation.
3

4. Thomas Hunt Morgan & Fruit Flies
• Drosophila melanogaster
– 2 week generations
– 4 Chromosomes (3 autosomes & one pair of sex
chromosomes)
– Easy to breed
• Wild Type, what exists in nature has a +
superscript
– W+ red eyes (wild type)
– W white eyes (mutant)
– Sex linked: on X chromosome
• in F2 generation, only males showed the white
eyes
• If not sex linked, ½ White eyes female & ½ white
eyes male
– If it weren’t sex linked, the F2 white eyed flies
would include females http://www.humansystemstherapeutics.com/fruitfly.gif
http://www.iq.poquoson.org/2001vasol/eocbio/e
ocbio0115_.png

5. CONCLUSION
Since all F1 offspring had red eyes, the mutant
white-eye trait (w) must be recessive to the wild-type red-eye trait (w+).
Since the recessive trait—white eyes—was expressed only in males in
the F2 generation, Morgan hypothesized that the eye-color gene is
located on the X chromosome and that there is no corresponding locus
on the Y chromosome, as diagrammed here.
P
Generation
F1
Generation
F2
Generation
Ova
(eggs)
Ova
(eggs)
Sperm
Sperm
X
X
X
X
Y
W
W+
W+
W
W+
W+ W+
W+
W+
W+
W+
W+
W
W+
W W
W

6. Sex Linked Genes
• Carried on the X chromosome
– Heterozygous females are carriers
they can pass allele to offspring
w/out showing symptoms
• Females get disease if they inherit
both recessive X alleles
– Men are more likely to have
diseases because they only have 1
X chromosome
• Duchenne Musculare dystrophy
• Color Blindness
• ALD
• Hemophilia

7. Recall: Crossing Over
• Causes recombination in
linked genes
• Occurs during prophase I
of meiosis between non-
sister chromatids of
homologous
chromosomes
– Recombinant frequency

8. Linked Genes
• Each chromosome
contains hundreds or
thousands of genes
• Genes on the same
chromosomes are
usually inherited
together – linked gene
– Not inherited together
if crossing over takes
place between the
genes

9. Parental Types vs. Recombinants
• Parental Type - The
offspring's phenotype
resembles the parents
• Recombinant – Offspring’s
Genotype is different from
parental & Offspring’s
genotype is new combination
of parental genes
• 50% each = non linked
– Caused by Independent
assortment – genetic
recombination
• Less than 50% recombinants
= linked genes

10. Linkage Mapping
• The further apart 2 genes
are on a chromosome the
greater the chance of
crossing over – greater
recombinant frequency
• Map unit = recombinant
frequency
• A frequency below 50%
indicates that 2 genes are
carried on the same
chromosome aka linked

11. Linkage Mapping
1% recombination frequency = 1 map unit
• Recombination Frequencies
• A/B = 19%
• B/C = 4%
• A/D = 12%
• B/D = 31%
• What is the correct order of the genes?
DACB or DABC

12. X Inactivation
• In mammals only one X
chromosome is expressed
in somatic cells
• Second X condenses to
become a barr body
• Barr bodies are
reactivated during
gamete formation
• Tortoiseshell Cats
– Female: orange fur where
one X chromosome is
expressed, black fur where
other X is expressed

13. Sex Chromosomes (Just FYI)
• X-Y system – mammals
– XX = female
– XY = male
• X-O system – some insects –
grasshoppers, cockroaches
– XX = female
– X = male (sperm contained no sex
chromosome
• Z-W system – birds, fish, some
insects
– ZW = female (determines sex)
– ZZ = male
• Haplo-diploid system – bees
and ants – no sex chromosomes
– Diploid – females
– Haploid – males (haploid,
parthenogenic development)
22 +
XX
22 +
X
76 +
ZZ
76 +
ZW
16
(Haploid)
16
(Diploid)
(b) The X–0 system
(c) The Z–W system
(d) The haplo-diploid system

14. Nondisjunction
• Homologous
chromosomes do not
separate in anaphase of
meiosis
• Result is aneuploidy
– Trisomy – 3 chromosomes
• 2n + 1
– Monosomy – 1
chromosome
• 2n – 1
– Polyploidy – 3 or more
chromosomes

15. Chromosomal Alterations
A B C D E F G H
Deletion
A B C E G H
F
A B C D E F G H
Duplication
A B C B D E
C F G H
A
A
M N O P Q R
B C D E F G H
B C D E F G H
Inversion
Reciprocal
translocation
A B P Q R
M N O C D E F G H
A D C B E F H
G

16. Chromosomes Alterations
Type Explanation Example
1. Deletion Removal of a
chromosome segment
ABCDE  ABDE
2. Duplication Repetition of a
chromosomal segment
ABCDE  ABBCDE
3. Inversion Reversal of a
chromosome segment
ABCDE  ABDCE
4. Translocation Movement of a
segment on one
chromosome to
another -
nonhomologous
ABCDE  FGCDE
FGHIJ  ABHIJ

17. Full Chromosomal Disorders
• Syndromes
– Down Syndrome –
Trisomy 21
– Kleinfelters – XXY
• Sterile
– Trisomy X – XXX
– Turner syndrome – XO
• Sterile

18. Altered Chromosomal Disorders
• Cri du Chat
– Deletion on
chromosome 5
• Leukemia – CML
– Reciprocal
translocation between
chromosome 9 and 22

19. Genomic Imprinting
• Genes on autosomal chromosomes that are expressed
depending on whether they come from the mother or
father
• Insulin growth factor – only the paternal gene is activated

20. Organelle Inheritance
• Chloroplasts are inherited through the cytoplasm from the
egg NOT the pollen
• Mitochondria are also passed in the cytoplasm of the egg
– Not carried in sperm cells