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Meiosis and Sexual Life Cycles

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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 BbCC
9 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

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