1. Meiosis, Sexual Life Cycles Mendelian Genetics Exceptions to the Rule
Chapter 13-15
Meiosis, Sexual Life Cycles
Mendelian Genetics
Exceptions to the Rule

2. Overview: Living Things Can Reproduce!!
Heredity
Is the transmission of traits from one generation to the next
Variation
Shows that offspring differ somewhat in appearance from parents and siblings
Genetics
Is the scientific study of heredity and hereditary variation

3. Asexual Reproduction In asexual reproduction
One parent produces genetically identical offspring by mitosis
Figure 13.2
Parent
Bud
0.5 mm

4. Sexual Reproduction In sexual reproduction
Two parents give rise to offspring that have unique combinations of genes inherited from the two parents

5. Life Cycle Fertilization & meiosis alternate in sexual life cycles
A life cycle
Is the generation-to-generation sequence of stages in the reproductive history of an organism

6. After Replication: identical sister chromatids
Figure 13.4
Key
Maternal set of
chromosomes (n = 3)
Unlike somatic cells
Gametes, sperm and egg cells , are haploid cells containing only one set of chromosomes
2n = 6
Paternal set of
chromosomes (n = 3)
Two sister chromatids
of one replicated
chromosome
Centromere
Two nonsister
chromatids in
a homologous pair
Pair of homologous
chromosomes
(one from each set)

7. The Human Life Cycle Key Figure 13.5 Haploid (n) Diploid (2n)
Haploid gametes (n = 23)
Ovum (n)
Sperm
Cell (n)
MEIOSIS
FERTILIZATION
Ovary
Testis
Diploid
zygote
(2n = 46)
Mitosis and
development
Multicellular diploid
adults (2n = 46)
At sexual maturity
The ovaries and testes produce haploid gametes by meiosis
During fertilization
These gametes, sperm and ovum, fuse, forming a diploid zygote
The zygote
Develops into an adult organism

8. Meiosis reduces chromosome from diploid to haploid
Figure 13.7
Interphase
Homologous pair
of chromosomes
in diploid parent cell
Chromosomes
replicate
Homologous pair of replicated chromosomes
Sister
chromatids
Diploid cell with
replicated
chromosomes 1 2
Homologous
separate
Haploid cells with
replicated chromosomes
Sister chromatids
Haploid cells with unreplicated chromosomes
Meiosis I
Meiosis II

9. Figure 13.8 Centrosomes (with centriole pairs) Sister chromatids
Chiasmata
Spindle
Tetrad
Nuclear
envelope
Chromatin
Centromere
(with kinetochore)
Microtubule
attached to
kinetochore
Tertads line up
Metaphase
plate
Homologous
chromosomes
separate
Sister chromatids
remain attached
Pairs of homologous
chromosomes split up
Chromosomes duplicate
Homologous chromosomes
(red and blue) pair and exchange
segments; 2n = 6 in this example
INTERPHASE
MEIOSIS I: Separates homologous chromosomes
PROPHASE I
METAPHASE I
ANAPHASE I
Synapsis and crossing over: physical connection and exchange genetic information during the tetrad stage
Separation of the homologous pairs
Figure 13.8

10. MEIOSIS II: Separates sister chromatids
TELOPHASE I AND
CYTOKINESIS
PROPHASE II
METAPHASE II
ANAPHASE II
TELOPHASE II AND
MEIOSIS II: Separates sister chromatids
Cleavage
furrow
Sister chromatids
separate
Haploid daughter cells
forming
During another round of cell division, the sister chromatids finally separate;
four haploid daughter cells result, containing single chromosomes
Two haploid cells
form; chromosomes
are still double
Figure 13.8
Sister chromatids splits

11. (before chromosome replication)
Figure 13.9
MITOSIS
MEIOSIS
Prophase
Duplicated chromosome
(two sister chromatids)
Chromosome
replication
Parent cell
(before chromosome replication)
Chiasma (site of
crossing over)
MEIOSIS I
Prophase I
Tetrad formed by
synapsis of homologous
chromosomes
Metaphase
Chromosomes
positioned at the
metaphase plate
Tetrads
Metaphase I
Anaphase I
Telophase I
Haploid
n = 3
MEIOSIS II
Daughter
cells of
meiosis I
Homologues
separate
during
anaphase I;
sister
chromatids
remain together
Daughter cells of meiosis II n
Sister chromatids separate during anaphase II
Anaphase
Telophase
Sister chromatids
separate during
anaphase 2n
Daughter cells
of mitosis
2n = 6

12. Genetic Variations contributes to Evolution
Reshuffling of genetic material in meiosis
Produces genetic variation due to the chromosome behaviors
Homologous pairs orient randomly
Independent assortment
Crossing over
Random fertilization
mutations

13. Words to know Allele for purple flowers Pair of homologous chromosomes
Figure 14.4
Words to know
Allele for purple flowers
Pair of homologous chromosomes
Locus for flower-color gene
2 alleles per trait, dominant, recessie
Allele for white flowers

14. Mendel and Genetics Law of Segregation: Law of Independent Assortment
Two alleles for a trait separate during gamate formation.
Law of Independent Assortment
Each pair of alleles separates independently from other pairs.

15. Inheritance Patterns Dominance /recessive Codominance
Incomplete dominance
Levels: ex: Tay-Sachs Disease
Frequencies of dominant allele
Multiple alleles
Tay-Sachs disease is fatal; a dysfunctional enzyme causes an accumulation of lipids in the brain
At the organismal level, the allele is recessive
At the biochemical level, the phenotype (i.e., the enzyme activity level) is incompletely dominant
At the molecular level, the alleles are codominant

16. Pleiotropy
Most genes have multiple phenotypic effects, a property called pleiotropy
For example, pleiotropic alleles are responsible for the multiple symptoms of certain hereditary diseases, such as cystic fibrosis and sickle-cell disease
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17. Epistasis: BbEe BbEe Sperm 1/4 1/4 1/4 1/4 Eggs 1/4 1/4 1/4 1/4 9 : 3
In epistasis, a gene at one locus alters the phenotypic expression of a gene at a second locus bE
BbEE
bbEE
BbEe
bbEe
1/4 Be
BBEe
BbEe
BBee
Bbee
1/4 be
BbEe
bbEe
Bbee
bbee 9
: 3
: 4

18. Polygenetic: AaBbCc AaBbCc Sperm 1/8 1/8 1/8 1/8 1/8 1/8 1/8 1/8 1/8
Figure A simplified model for polygenic inheritance of skin color.
1/8
Eggs
1/8
1/8
1/8
1/8
Phenotypes:
1/64
6/64
15/64
20/64
15/64
6/64
1/64
Number of dark-skin alleles: 1 2 3 4 5 6

19. Pedigrees

20. Nature and Nurture: The Environmental Impact on Phenotype
Another departure from Mendelian genetics arises when the phenotype for a character depends on environment as well as genotype
The norm of reaction is the phenotypic range of a genotype influenced by the environment
For example, hydrangea flowers of the same genotype range from blue-violet to pink, depending on soil acidity
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21. Correlating Behavior of a Gene’s Alleles with Behavior of a Chromosome Pair
In one experiment, Morgan mated male flies with white eyes (mutant) with female flies with red eyes (wild type)
The F1 generation all had red eyes
The F2 generation showed the 3:1 red:white eye ratio, but only males had white eyes
Morgan determined that the white-eyed mutant allele must be located on the X chromosome
Morgan’s finding supported the chromosome theory of
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22. The Chromosomal Basis of Sex
In humans and other mammals:
X vs. Y
Y is tiny!!
The SRY gene on the Y chromosome
Some disorders caused by recessive alleles on the X:
Color blindness (mostly X-linked)
Duchenne muscular dystrophy
Hemophilia
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23. X Inactivation in Female Mammals
Barr body
Females are mosaic
In mammalian females, one of the two X chromosomes in each cell is randomly inactivated during embryonic development
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24. Cell division and X chromosome inactivation
Figure 15.8
X chromosomes
Allele for orange fur
Early embryo:
Allele for black fur
Cell division and X chromosome inactivation
Two cell populations in adult cat:
Active X
Figure 15.8 X inactivation and the tortoiseshell cat.
Inactive X
Active X
Black fur
Orange fur

25. Recombination frequencies
Figure 15.11
Linked genes tend to be inherited together because they are located near each other on the same chromosome
Recombination frequencies 9%
9.5%
Chromosome
Figure Research Method: Constructing a Linkage Map
17% b cn vg

26. Long aristae (appendages on head) Gray body Red eyes Normal wings
Figure 15.12
Mutant phenotypes
Short aristae
Black body
Cinnabar eyes
Vestigial wings
Brown eyes
48.5
57.5
67.0
104.5
Figure A partial genetic (linkage) map of a Drosophila chromosome.
Long aristae (appendages on head)
Gray body
Red eyes
Normal wings
Red eyes
Wild-type phenotypes

27. Mutations Nondisjunction  Aneuploidy Breakage of a chromosome:
(a) Deletion
Mutations A B C D E F G H
A deletion removes a chromosomal segment. A B C E F G H
Nondisjunction  Aneuploidy
Breakage of a chromosome:
Deletion
Duplication
Inversion
Translocation
(b) Duplication A B C D E F G H
A duplication repeats a segment. A B C D E F G H
, pairs of homologous chromosomes do not separate normally during meiosis
Offspring with this condition have an abnormal number of a particular chromosome
(c) Inversion A B C D E F G H
An inversion reverses a segment within a chromosome. A D C B E F G H
(d) Translocation A B C D E F G H M N O P Q R
A translocation moves a segment from one chromosome to a nonhomologous chromosome. M N O C H F E D G A B P Q R
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28. Down Syndrome (Trisomy 21)
Figure 15.15
Down Syndrome (Trisomy 21)
Figure Down syndrome.
Down syndrome is an aneuploid condition that results from three copies of chromosome 21

29. Aneuploidy of Sex Chromosomes
Nondisjunction of sex chromosomes:
XXX
Klinefelter syndrome (XXY)
Monosomy X, called Turner syndrome, (X0) females, who are sterile
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30. Genomic Imprinting Mutant Igf2 allele inherited from mother
Mutant Igf2 allele inherited from father
Normal-sized mouse (wild type)
Dwarf mouse (mutant)
Normal Igf2 allele is expressed.
Mutant Igf2 allele is expressed.
Figure Genomic imprinting of the mouse Igf2 gene.
It appears that imprinting is the result of the methylation (addition of —CH3) of cysteine nucleotides
Genomic imprinting is thought to affect only a small fraction of mammalian genes
Most imprinted genes are critical for embryonic development
Mutant Igf2 allele is not expressed.
Normal Igf2 allele is not expressed.
(b) Heterozygotes

31. Inheritance of Organelle Genes
Mitochondria, chloroplasts, and
inherited maternally
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32. Ch 15: Chromosomes!
The location of a particular gene can be seen by tagging isolated chromosomes with a fluorescent dye that highlights the gene

33. Figure 15.2 The chromosomal basis of Mendel’s laws.
P Generation
Yellow-round seeds (YYRR)
Green-wrinkled seeds (yyrr) Y r y  Y R R r y
Meiosis
Fertilization R Y y r
Gametes
All F1 plants produce yellow-round seeds (YyRr).
F1 Generation R R y y r r Y Y
Meiosis
LAW OF SEGREGATION The two alleles for each gene separate during gamete formation.
LAW OF INDEPENDENT ASSORTMENT Alleles of genes on nonhomologous chromosomes assort independently during gamete formation. R r r R
Metaphase I
Figure 15.2 The chromosomal basis of Mendel’s laws. Y y Y y 1 1 R r r R
Anaphase I Y y Y y R r r R 2 2 Y y
Metaphase II Y y y Y Y y Y Y y y
Gametes R R r r r r R R
1/4 YR
1/4 yr
1/4 Yr
1/4 yR
F2 Generation
An F1  F1 cross-fertilization 3
Fertilization recombines the R and r alleles at random. 3
Fertilization results in the 9:3:3:1 phenotypic ratio in the F2 generation. 9
: 3
: 3
: 1

34. P Generation F1 Generation F2 Generation
Figure 15.4b
CONCLUSION
P Generation w w X X X Y w w
Sperm
Eggs
F1 Generation w w w
Figure 15.4 Inquiry: 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?
Morgan’s experiment for X-linked gene w w
Sperm
Eggs w w
F2 Generation w w w w w w

35. X Y Figure 15.5 Figure 15.5 Human sex chromosomes.
In humans and other mammals, there are two varieties of sex chromosomes: a larger X chromosome and a smaller Y chromosome
Only the ends of the Y chromosome have regions that are homologous with corresponding regions of the X chromosome
The SRY gene on the Y chromosome codes for a protein that directs the development of male anatomical features Y

36. (d) The haplo-diploid system
Figure 15.6
44  XY
Parents
44  XX
22  X or
22  Y
22  X
Sperm
Egg
44  XX or
44  XY
Zygotes (offspring)
(a) The X-Y system
22  XX
22  X
Figure 15.6 Some chromosomal systems of sex determination.
(b) The X-0 system
76  ZW
76  ZZ
(c) The Z-W system
32 (Diploid)
16 (Haploid)
(d) The haplo-diploid system

37. Cell division and X chromosome inactivation
Figure 15.8
X chromosomes
Allele for orange fur
Early embryo:
Allele for black fur
Cell division and X chromosome inactivation
Two cell populations in adult cat:
Active X
Figure 15.8 X inactivation and the tortoiseshell cat.
barr bodies
Inactive X
Active X
Black fur
Orange fur

38. Figure 15.UN01
Linked Genes
b+ vg+
b vg
F1 dihybrid female and homozygous recessive male in testcross
b vg
b vg
Inherited together! Body color and wing size are inherted together
b+ vg+
b vg
Most offspring or
b vg
b vg

39. Genetic Recombination and Linkage
Offspring with a phenotype matching one of the parental phenotypes are called parental types
Offspring with nonparental phenotypes (new combinations of traits) are called recombinant types, or recombinants
A 50% frequency of recombination is observed for any two genes on different chromosomes

40. Gametes from yellow-round dihybrid parent (YyRr)
Figure 15.UN02
Gametes from yellow-round dihybrid parent (YyRr) YR yr Yr yR
Gametes from green- wrinkled homozygous recessive parent (yyrr)
Figure 15.UN02 In-text figure, p. 294 yr
YyRr
yyrr
Yyrr
yyRr
Parental- type offspring
Recombinant offspring

41. Long aristae (appendages on head) Gray body Red eyes Normal wings
Figure 15.12
Mutant phenotypes
Short aristae
Black body
Cinnabar eyes
Vestigial wings
Brown eyes
48.5
57.5
67.0
104.5
Figure A partial genetic (linkage) map of a Drosophila chromosome.
Long aristae (appendages on head)
Gray body
Red eyes
Normal wings
Red eyes
Wild-type phenotypes

42. A deletion removes a chromosomal segment.
Figure 15.14
(a) Deletion A B C D E F G H
A deletion removes a chromosomal segment. A B C E F G H
(b) Duplication A B C D E F G H
A duplication repeats a segment. A B C D E F G H
Figure Alterations of chromosome structure.
(c) Inversion A B C D E F G H
An inversion reverses a segment within a chromosome. A D C B E F G H
(d) Translocation A B C D E F G H M N O P Q R
A translocation moves a segment from one chromosome to a nonhomologous chromosome. M N O C H F E D G A B P Q R

43. Figure 15.15
Figure Down syndrome.
Down syndrome is an aneuploid condition that results from three copies of chromosome 21

44. Others Imprinting Organelle genes
Silencing of a gene during gamate formation (depends on which parent)
Organelle genes
Mitochondria, choloroplast
Maternally inherited
example, mitochondrial myopathy and Leber’s hereditary optic neuropathy