1. Meiosis and Sexual Life Cycles
Chapter 13
Meiosis and Sexual Life Cycles

2. Overview: Hereditary Similarity and Variation
Living organisms are distinguished by their ability to reproduce their own kind
Heredity is the transmission of traits from one generation to the next
Variation shows that offspring differ in appearance from parents and siblings
Genetics is the scientific study of heredity and variation

4. Concept 13.1: Offspring acquire genes from parents by inheriting chromosomes
In a literal sense, children do not inherit particular physical traits from their parents
It is genes that are actually inherited

5. Inheritance of Genes Genes are the units of heredity
Genes are segments of DNA
Each gene has a specific locus on a certain chromosome
One set of chromosomes is inherited from each parent
Reproductive cells called gametes (sperm and eggs) unite, passing genes to the next generation

6. Comparison of Asexual and Sexual Reproduction
In asexual reproduction, one parent produces genetically identical offspring by mitosis
In sexual reproduction, two parents give rise to offspring that have unique combinations of genes inherited from the two parents
Video: Hydra Budding

7. LE 13-2
Parent
Bud
0.5 mm

8. Concept 13.2: Fertilization and meiosis alternate in sexual life cycles
A life cycle is the generation-to-generation sequence of stages in the reproductive history of an organism

9. Sets of Chromosomes in Human Cells
Each human somatic cell (any cell other than a gamete) has 46 chromosomes arranged in pairs
A karyotype is an ordered display of the pairs of chromosomes from a cell
The two chromosomes in each pair are called homologous chromosomes, or homologues
Both chromosomes in a pair carry genes controlling the same inherited characteristics

10. Pair of homologous 5 µm chromosomes Centromere Sister chromatids
LE 13-3
Pair of homologous
chromosomes
5 µm
Centromere
Sister
chromatids

11. The sex chromosomes are called X and Y
Human females have a homologous pair of X chromosomes (XX)
Human males have one X and one Y chromosome
The 22 pairs of chromosomes that do not determine sex are called autosomes

12. Each pair of homologous chromosomes includes one chromosome from each parent
The 46 chromosomes in a human somatic cell are two sets of 23: one from the mother and one from the father
The number of chromosomes in a single set is represented by n
A cell with two sets is called diploid (2n)
For humans, the diploid number is 46 (2n = 46)

13. Key Maternal set of chromosomes (n = 3) 2n = 6 Paternal set of
LE 13-4
Key
Maternal set of
chromosomes (n = 3)
2n = 6
Paternal set of
chromosomes (n = 3)
Two sister chromatids
of one replicated
chromosomes
Centromere
Two nonsister
chromatids in
a homologous pair
Pair of homologous
chromosomes
(one from each set)

14. Gametes are haploid cells, containing only one set of chromosomes
For humans, the haploid number is 23 (n = 23)
Each set of 23 consists of 22 autosomes and a single sex chromosome
In an unfertilized egg (ovum), the sex chromosome is X
In a sperm cell, the sex chromosome may be either X or Y

15. Behavior of Chromosome Sets in the Human Life Cycle
At sexual maturity, the ovaries and testes produce haploid gametes
Gametes are the only types of human cells produced by meiosis, rather than mitosis
Meiosis results in one set of chromosomes in each gamete
Fertilization, the fusing of gametes, restores the diploid condition, forming a zygote
The diploid zygote develops into an adult

16. LE 13-5 Key Haploid gametes (n = 23) Haploid (n) Ovum (n) Diploid (2n)
Sperm
cell (n)
MEIOSIS
FERTILIZATION
Ovary
Testis
Diploid
zygote
(2n = 46)
Mitosis and
development
Multicellular diploid
adults (2n = 46)

17. The Variety of Sexual Life Cycles
The alternation of meiosis and fertilization is common to all organisms that reproduce sexually
The three main types of sexual life cycles differ in the timing of meiosis and fertilization

18. In animals, meiosis produces gametes, which undergo no further cell division before fertilization
Gametes are the only haploid cells in animals
Gametes fuse to form a diploid zygote that divides by mitosis to develop into a multicellular organism

19. Haploid multicellular organism (gametophyte) Haploid multicellular
LE 13-6
Key
Haploid
Diploid
Haploid multicellular
organism (gametophyte)
Haploid multicellular
organism
Gametes n n
Mitosis n
Mitosis
Mitosis n
Mitosis n n n n n n
Spores n
MEIOSIS
FERTILIZATION n
Gametes
Gametes n
MEIOSIS
FERTILIZATION
MEIOSIS
FERTILIZATION
Zygote 2n 2n 2n 2n
Diploid
multicellular
organism
(sporophyte)
Zygote 2n
Diploid
multicellular
organism
Mitosis
Mitosis
Zygote
Animals
Plants and some algae
Most fungi and some protists

20. Plants and some algae exhibit an alternation of generations
This life cycle includes two multicellular generations or stages: one diploid and one haploid
The diploid organism, the sporophyte, makes haploid spores by meiosis
Each spore grows by mitosis into a haploid organism called a gametophyte
A gametophyte makes haploid gametes by mitosis

21. Haploid multicellular organism (gametophyte)
LE 13-6b
Key
Haploid
Diploid
Haploid multicellular
organism (gametophyte)
Mitosis n
Mitosis n n n n
Spores
Gametes
MEIOSIS
FERTILIZATION 2n 2n
Diploid
multicellular
organism
(sporophyte)
Zygote
Mitosis
Plants and some algae

22. In most fungi and some protists, the only diploid stage is the single-celled zygote; there is no multicellular diploid stage
The zygote produces haploid cells by meiosis
Each haploid cell grows by mitosis into a haploid multicellular organism
The haploid adult produces gametes by mitosis

23. LE 13-8b MEIOSIS II: Separates sister chromatids TELOPHASE I AND
CYTOKINESIS
TELOPHASE II AND
CYTOKINESIS
PROPHASE II
METAPHASE II
ANAPHASE II
Cleavage
furrow
Haploid daughter cells
forming
Sister chromatids
separate
Two haploid cells
form; chromosomes
are still double
During another round of cell division, the sister chromatids finally separate;
four haploid daughter cells result, containing single chromosomes

24. (before chromosome replication)
LE 13-9
MITOSIS
MEIOSIS
Parent cell
(before chromosome replication)
Chiasma (site of
crossing over)
MEIOSIS I
Propase
Prophase I
Chromosome
replication
Chromosome
replication
Tetrad formed by
synapsis of homologous
chromosomes
Duplicated chromosome
(two sister chromatids)
2n = 6
Chromosomes
positioned at the
metaphase plate
Tetrads
positioned at the
metaphase plate
Metaphase
Metaphase I
Anaphase
Sister chromatids
separate during
anaphase
Homologues
separate
during
anaphase I;
sister
chromatids
remain together
Anaphase I
Telophase
Telophase I
Haploid
n = 3
Daughter
cells of
meiosis I 2n 2n
MEIOSIS II
Daughter cells
of mitosis n n n n
Daughter cells of meiosis II
Sister chromatids separate during anaphase II

25. Property
Mitosis
Meiosis
DNA replication
During interphase
Divisions
One
Two
Synapsis and crossing over
Do not occur
Form tetrads in prophase I
Daughter cells, genetic composition
Two diploid, identical to parent cell
Four haploid, different from parent cell and each other
Role in animal body
Produces cells for growth and tissue repair
Produces gametes

26. Origins of Genetic Variation Among Offspring
The behavior of chromosomes during meiosis and fertilization is responsible for most of the variation that arises in each generation
Three mechanisms contribute to genetic variation:
Independent assortment of chromosomes
Crossing over
Random fertilization

27. Animation: Genetic Variation
Crossing Over
Crossing over produces recombinant chromosomes, which combine genes inherited from each parent
Crossing over begins very early in prophase I, as homologous chromosomes pair up gene by gene
In crossing over, homologous portions of two nonsister chromatids trade places
Crossing over contributes to genetic variation by combining DNA from two parents into a single chromosome
Animation: Genetic Variation

28. LE 13-11 Prophase I Nonsister of meiosis chromatids Tetrad Chiasma,
site of
crossing
over
Metaphase I
Metaphase II
Daughter
cells
Recombinant
chromosomes

29. Random Fertilization
Random fertilization adds to genetic variation because any sperm can fuse with any ovum (unfertilized egg)
The fusion of gametes produces a zygote with any of about 64 trillion diploid combinations
Crossing over adds even more variation
Each zygote has a unique genetic identity

30. Evolutionary Significance of Genetic Variation Within Populations
Natural selection results in accumulation of genetic variations favored by the environment
Sexual reproduction contributes to the genetic variation in a population, which ultimately results from mutations

32. Concept 14.1: Mendel used the scientific approach to identify two laws of inheritance
Mendel discovered the basic principles of heredity by breeding garden peas in carefully planned experiments

33. Mendel’s Experimental, Quantitative Approach
Advantages of pea plants for genetic study:
There are many varieties with distinct heritable features, or characters (such as color); character variations are called traits
Mating of plants can be controlled
Each pea plant has sperm-producing organs (stamens) and egg-producing organs (carpels)
Cross-pollination (fertilization between different plants) can be achieved by dusting one plant with pollen from another

34. LE 14-2 Removed stamens from purple flower Transferred sperm-
bearing pollen from
stamens of white
flower to egg-
bearing carpel of
purple flower
Parental
generation
(P)
Stamens
Carpel
Pollinated carpel
matured into pod
Planted seeds
from pod
Examined
offspring:
all purple
flowers
First
generation
offspring
(F1)

35. Mendel chose to track only those characters that varied in an “either-or” manner
He also used varieties that were “true-breeding” (plants that produce offspring of the same variety when they self-pollinate)

36. In a typical experiment, Mendel mated two contrasting, true-breeding varieties, a process called hybridization
The true-breeding parents are the P generation
The hybrid offspring of the P generation are called the F1 generation
When F1 individuals self-pollinate, the F2 generation is produced

37. The Law of Segregation
When Mendel crossed contrasting, true-breeding white and purple flowered pea plants, all of the F1 hybrids were purple
When Mendel crossed the F1 hybrids, many of the F2 plants had purple flowers, but some had white
Mendel discovered a ratio of about three to one, purple to white flowers, in the F2 generation

38. (true-breeding parents) White flowers
LE 14-3
P Generation
(true-breeding
parents)
Purple
flowers
White
flowers
F1 Generation
(hybrids)
All plants had
purple flowers
F2 Generation

40. The first concept is that alternative versions of genes account for variations in inherited characters
For example, the gene for flower color in pea plants exists in two versions, one for purple flowers and the other for white flowers
These alternative versions of a gene are now called alleles
Each gene resides at a specific locus on a specific chromosome

41. Allele for purple flowers
Homologous
pair of
chromosomes
Locus for flower-color gene
Allele for white flowers

42. The second concept is that for each character an organism inherits two alleles, one from each parent
Mendel made this deduction without knowing about the role of chromosomes
The two alleles at a locus on a chromosome may be identical, as in the true-breeding plants of Mendel’s P generation
Alternatively, the two alleles at a locus may differ, as in the F1 hybrids

43. The third concept is that if the two alleles at a locus differ, then one (the dominant allele) determines the organism’s appearance, and the other (the recessive allele) has no noticeable effect on appearance
In the flower-color example, the F1 plants had purple flowers because the allele for that trait is dominant

44. The fourth concept, now known as the law of segregation, states that the two alleles for a heritable character separate (segregate) during gamete formation and end up in different gametes
Thus, an egg or a sperm gets only one of the two alleles that are present in the somatic cells of an organism
This segregation of alleles corresponds to the distribution of homologous chromosomes to different gametes in meiosis

45. Mendel’s segregation model accounts for the 3:1 ratio he observed in the F2 generation of his numerous crosses
The possible combinations of sperm and egg can be shown using a Punnett square, a diagram for predicting the results of a genetic cross between individuals of known genetic makeup
A capital letter represents a dominant allele, and a lowercase letter represents a recessive allele

46. LE 14-5_2 3 : 1 P Generation Appearance: Purple flowers PP White
Genetic makeup:
Gametes P p
F1 Generation
Appearance:
Genetic makeup:
Purple flowers Pp
Gametes: 1 2 P 1 2 p
F1 sperm P p
F2 Generation P PP Pp
F1 eggs p Pp pp 3
: 1

47. Useful Genetic Vocabulary
An organism with two identical alleles for a character is said to be homozygous for the gene controlling that character
An organism that has two different alleles for a gene is said to be heterozygous for the gene controlling that character
Unlike homozygotes, heterozygotes are not true-breeding

48. Because of the different effects of dominant and recessive alleles, an organism’s traits do not always reveal its genetic composition
Therefore, we distinguish between an organism’s phenotype, or physical appearance, and its genotype, or genetic makeup

49. The Testcross
How can we tell the genotype of an individual with the dominant phenotype?
Such an individual must have one dominant allele, but the individual could be either homozygous dominant or heterozygous
The answer is to carry out a testcross: breeding the mystery individual with a homozygous recessive individual
If any offspring display the recessive phenotype, the mystery parent must be heterozygous

50. LE 14-7 Dominant phenotype, unknown genotype: PP or Pp?
Recessive phenotype,
known genotype: pp
If PP,
then all offspring
purple:
If Pp,
then 1 2 offspring purple
and 1 2 offspring white: p p p p P P Pp Pp Pp Pp P P Pp Pp pp pp

51. LE 14-8 P Generation YYRR yyrr Gametes YR yr YyRr F1 Generation
Hypothesis of
dependent
assortment
Hypothesis of
independent
assortment
Sperm 1 YR 1 Yr yR yr
Sperm 4 4 1 4 1 4
Eggs 1 YR yr 2 1 2 1 YR
Eggs 4
YYRR
YYRr
YyRR
YyRr 1 YR
F2 Generation
(predicted
offspring) 2
YYRR
YyRr 1 Yr 4
YYRr
YYrr
YyRr
Yyrr 1 yr 2
YyRr
yyrr 1 yR 4
YyRR
YyRr
yyRR
yyRr 3 4 1 4 1 yr 4
Phenotypic ratio 3:1
YyRr
Yyrr
yyRr
yyrr 9 16 3 16 3 16 3 16
Phenotypic ratio 9:3:3:1

52. Concept 14.2: The laws of probability govern Mendelian inheritance
Mendel’s laws of segregation and independent assortment reflect the rules of probability
When tossing a coin, the outcome of one toss has no impact on the outcome of the next toss

53. The Multiplication and Addition Rules Applied to Monohybrid Crosses
Segregation in a heterozygous plant is like flipping a coin: Each gamete has a 1/2 chance of carrying the dominant allele and a 1/2 chance of carrying the recessive allele

54. LE 14-9 Rr Rr Segregation of alleles into eggs Segregation of
alleles into sperm
Sperm 1 R 1 r 2 2 R R R r 1 R 2 1 1 4 4
Eggs r r R r 1 r 2 1 1 4 4

55. The Spectrum of Dominance
Complete dominance occurs when phenotypes of the heterozygote and dominant homozygote are identical
In codominance, two dominant alleles affect the phenotype in separate, distinguishable ways
In incomplete dominance, the phenotype of F1 hybrids is somewhere between the phenotypes of the two parental varieties

56. LE 14-10 P Generation Red CRCR White CWCW Gametes CR CW Pink CRCW
F1 Generation
Gametes 1 1 2 CR 2 CW
Sperm 1 2 CR 1 2 CW
Eggs
F2 Generation 1 CR 2
CRCR
CRCW 1 2 CW
CRCW
CWCW

57. The Relation Between Dominance and Phenotype
A dominant allele does not subdue a recessive allele; alleles don’t interact
Alleles are simply variations in a gene’s nucleotide sequence

58. Frequency of Dominant Alleles
Dominant alleles are not necessarily more common in populations than recessive alleles
For example, one baby out of 400 in the United States is born with extra fingers or toes
The allele for this unusual trait is dominant to the allele for the more common trait of five digits per appendage
In this example, the recessive allele is far more prevalent than the dominant allele in the population

59. Multiple Alleles
Most genes exist in populations in more than two allelic forms
For example, the four phenotypes of the ABO blood group in humans are determined by three alleles for the enzyme (I) that attaches A or B carbohydrates to red blood cells: IA, IB, and i.
The enzyme encoded by the IA allele adds the A carbohydrate, whereas the enzyme encoded by the IB allele adds the B carbohydrate; the enzyme encoded by the i allele adds neither

61. 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

62. Epistasis
In epistasis, a gene at one locus alters the phenotypic expression of a gene at a second locus
For example, in mice and many other mammals, coat color depends on two genes
One gene determines the pigment color (with alleles B for black and b for brown)
The other gene (with alleles C for pigment color and c for no pigment color ) determines whether the pigment will be deposited in the hair

63. LE 14-11 BbCc BbCc Sperm BC bC Bc bc BC BBCC BbCC BBCc BbCc bC BbCC9 3 4 16 16 16

64. Polygenic Inheritance
Quantitative characters are those that vary in the population along a continuum
Quantitative variation usually indicates polygenic inheritance, an additive effect of two or more genes on a single phenotype
Skin color in humans is an example of polygenic inheritance

65. LE 14-12 20/64 15/64 6/64 1/64 AaBbCc AaBbCc aabbcc Aabbcc AaBbcc
Fraction of progeny
6/64
1/64

66. 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

68. Norms of reaction are generally broadest for polygenic characters
Such characters are called multifactorial because genetic and environmental factors collectively influence phenotype

69. Pedigree Analysis
A pedigree is a family tree that describes the interrelationships of parents and children across generations
Inheritance patterns of particular traits can be traced and described using pedigrees

70. Dominant trait (widow’s peak)
LE 14-14a
First generation
(grandparents) Ww ww ww Ww
Second generation
(parents plus aunts
and uncles) Ww ww ww Ww Ww ww
Third
generation
(two sisters) WW ww or Ww
Widow’s peak
No widow’s peak
Dominant trait (widow’s peak)

71. Recessive trait (attached earlobe)
LE 14-14b
First generation
(grandparents) Ff Ff ff Ff
Second generation
(parents plus aunts
and uncles)
FF or Ff ff ff Ff Ff ff
Third
generation
(two sisters) ff FF or Ff
Attached earlobe
Free earlobe
Recessive trait (attached earlobe)

72. Cystic Fibrosis
Cystic fibrosis is the most common lethal genetic disease in the United States,striking one out of every 2,500 people of European descent
The cystic fibrosis allele results in defective or absent chloride transport channels in plasma membranes
Symptoms include mucus buildup in some internal organs and abnormal absorption of nutrients in the small intestine

73. Sickle-Cell Disease
Sickle-cell disease affects one out of 400 African-Americans
The disease is caused by the substitution of a single amino acid in the hemoglobin protein in red blood cells
Symptoms include physical weakness, pain, organ damage, and even paralysis

74. Dominantly Inherited Disorders
Some human disorders are due to dominant alleles
One example is achondroplasia, a form of dwarfism that is lethal when homozygous for the dominant allele

76. Huntington’s disease is a degenerative disease of the nervous system
The disease has no obvious phenotypic effects until about 35 to 40 years of age

77. Multifactorial Disorders
Many diseases, such as heart disease and cancer, have both genetic and environment components
Little is understood about the genetic contribution to most multifactorial diseases

78. Video: Ultrasound of Human Fetus I
Fetal Testing
In amniocentesis, the liquid that bathes the fetus is removed and tested
In chorionic villus sampling (CVS), a sample of the placenta is removed and tested
Other techniques, ultrasound and fetoscopy, allow fetal health to be assessed visually in utero
Video: Ultrasound of Human Fetus I

79. LE 14-17a Amniocentesis Amniotic fluid withdrawn A sample of
amniotic fluid can
be taken starting at
the 14th to 16th
week of pregnancy.
Fetus
Centrifugation
Placenta
Uterus
Cervix
Fluid
Fetal
cells
Biochemical tests can be
performed immediately on
the amniotic fluid or later
on the cultured cells.
Biochemical
tests
Several
weeks
Fetal cells must be cultured
for several weeks to obtain
sufficient numbers for
karyotyping.
Karyotyping

80. LE 14-17b Chorionic villus sampling (CVS) A sample of chorionic villus
tissue can be taken as early
as the 8th to 10th week of
pregnancy.
Fetus
Suction tube
inserted through
cervix
Placenta
Chorionic villi
Fetal
cells
Biochemical
tests
Karyotyping and biochemical
tests can be performed on
the fetal cells immediately,
providing results within a day
or so.
Several
hours
Karyotyping

81. Newborn Screening
Some genetic disorders can be detected at birth by simple tests that are now routinely performed in most hospitals in the United States