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Chapter 10Sexual Reproduction
and Genetics10.1 Meiosis
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Chromosomes and Chromosome Number
Human body cells have 46 chromosomes DNA on the chromosomes are arranged in
sections that code for a trait; these sections are genes
Humans have approximately 23,000 genes
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Chromosomes and Chromosome Number
Of the 46 human chromosomes each parent contributes 23 chromosomes
These chromosomes are paired
Homologous chromosomes—one of two paired chromosomes, one from each parent
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Homologous Chromosomes Same centromere
position Same length Homologous
chromosomes contain the same genes; although they may contain different versions of the gene
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Chromosomes and Chromosome Number
Haploid cells contain only one of the homologous pair of chromosomes (half the number of chromosomes)
Diploid cell contain the chromosomes in pairs (di=two)
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Chromosomes and Chromosome Number
Gametes (sex cells) contain the haploid number of chromosomes (n)
Body cells contain the diploid number of chromosomes (2n)
Human gametes (sperm and egg) have 23 chromosomes and body cells have 46 chromosomes
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Meiosis Gametes are formed
during the process of meiosis
Meiosis reduces diploid cells to haploid cells
Fertilization restores the diploid number
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Meiosis Meiosis involves two consecutive sets of cell
divisions Meiosis only occurs in the reproductive structures
of organisms who reproduce sexually: Most animals Most plants Most fungi Most protists
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Meiosis I and Meiosis II Meiosis I is the reduction division; cells start
out diploid and end up haploid In Meiosis II sister chromatids are
separated (much like mitosis)
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Interphase
Chromosomes replicate.
Chromatin condenses.Interphase
Meiosis I
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Prophase I
Pairing of homologous chromosomes occurs.
Each chromosome consists of two chromatids.
The nuclear envelope breaks down.
Spindles form.
Prophase I
Meiosis I
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Prophase I
Crossing over produces exchange of genetic information.
Crossing over—chromosomal segments are exchanged between a pair of homologous chromosomes.
Meiosis I
Tetrads are groups of four sister chromatids
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Metaphase I
Chromosome’s centromere attach to spindle fibers.
Homologous chromosomes line up at the equator in tetrads
Metaphase I
Meiosis I
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Anaphase I
Anaphase I
Homologous chromosomes separate and moveto opposite poles of the cell.
Meiosis I
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Telophase I
The spindles break down.
Chromosomes uncoil and form two nuclei.
The cell divides.
Telophase I
Meiosis I
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Prophase II
A second set of phases beginsas the spindle apparatus forms and the chromosomes condense.
Prophase II
Meiosis II
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Metaphase II
A haploid number of chromosomesline up at the equator.
Metaphase II
Meiosis II
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Anaphase II
Anaphase II
The sister chromatids arepulled apart at the centromere by spindle fibers and move toward the opposite poles of the cell.
Meiosis II
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Telophase II
The chromosomes reach the poles, andthe nuclear membrane and nuclei reform.
Telophase II
Meiosis II
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Cytokinesis results in four haploid cells, each with n number of chromosomes. Cytokinesis
Meiosis II
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Meiosis consists of two sets of divisions
Produces four haploid daughter cells that are not identical
Results in genetic variation
Meiosis
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Depending on how the chromosomes line up at the equator, four gametes with four different combinations of chromosomes can result.
Genetic variation also is produced during crossing over and during fertilization, when gametes randomly combine.
Meiosis
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Meiosis and Variation Number of possible genetic variations in the
gametes equals:2n where n is the haploid number
In humans number of possible genetic combinations in gametes is 223
Add the genetic combinations that exist when crossing over exists (at 3 per meiosis) and get (223)3
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The possibility that (223)3 variations exists for each gamete
When fertilization occurs this number must be doubled 2 x (223)3
You are unique; no one else exists or ever has existed that is just like you (unless you have an identical twin).
Meiosis and Variation, cont
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Advantages of Asexual Reproduction
The organism inherits all of its chromosomes from a single parent.
The new individual is genetically identical to its parent.
Usually occurs more rapidly than sexual reproduction
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Advantages of Sexual Reproduction
Beneficial genes multiply faster over time. The organisms inherits genes from two
parents and is not genetically identical to either parent.
Ensures genetic variation
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Chapter 10Sexual Reproduction
and Genetics10.2 Mendelian Genetics
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Gregor Mendel Lived in Europe in
what is now Czech Republic near the Austrian border.
Father of Genetics Monk, entered
monastery 1843
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Gregor Mendel Failed teacher’s exam When to U of Vienna
Studied with physicist Doppler- science through experiment, applied math to science
Studied with botanist Unger- interest in causes of variation in plants
Passed teacher’s exam and taught at monastery’s school; also responsible for school’s garden
Published 1866, mathematics and plant breeding
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Mendel Studied Peas Available in many varieties Self pollinating (can manipulate pollination) “either-or” inherited traits Had “true breeder” for parental generation
(P) due to flower structure
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Mendel Studied Peas The petals enclose the
stamen (with pollen) so that cross pollination does not occur
Cross pollination is easily accomplished by peeling back the petals and moving pollen with a paint brush
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Inheritance of Traits
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The second filial (F2) generation is the offspring from the F1 cross.
The offspring of this P cross are called the first filial (F1) generation.
Inheritance of Traits
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Pea Traits Studied by Mendel
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Mendel studied thousand of pea plants for the seven traits.
He concluded that: Genes are in pairs Different versions of genes (alleles) account for
variation in inherited characteristics Alleles can be dominant or recessive
Inheritance of Traits
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Dominant and Recessive Alleles can be dominant or recessive. An allele is dominant if it appears in the F1
generation when true breeder parents are crossed.
An allele is recessive if it is masked in the F1 generation.
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Symbols To help make genetics easier symbols are used Capital letters are used for dominant alleles Lower case letters are used for recessive alleles The letter to use is based on the dominant trait Example: purple is dominant to white, P would be
the dominant allele and p the recessive allele
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Homozygous and Heterozygous Dominant traits can be homozygous or
heterozygous Homo= same; the alleles would be the
same, PP Hetero=different; the alleles would be
different, Pp For the recessive trait to be expressed both
alleles would be recessive, pp
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Genotype and Phenotype Genotype is the organism's gene pairs: PP,
Pp or pp Phenotype is the outward physical
appearance or expression of the genotype: purple or white
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Genotype and Phenotype If the phenotype
displays the recessive trait (white) then you know the genotype; pp
If the phenotype displays the dominant trait (purple) then the genotype could be homozygous dominant (PP) or heterozygous (Pp)
Genotype is ppGenotype is PP or Pp
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Punnett Squares Mathematical device for
predicting the results of genetic cross
Male gametes are written across the top
Female gametes are written along the side
Genetic possibilities of the offspring are in the boxes
Expect 3:1 phenotypic ratio
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Monohybrid Cross Mono= one One trait is studied at
a time This one is seed color Monohybrid crosses
provided evidence for the Law of Segregation
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Mendel’s Law of Segregation
Two alleles for each trait separate during meiosis
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Dihybrid Cross Di= two Two traits are studied
at a time This one is seed color
(yellow or green) and seed shape (wrinkled or round)
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Dihybrid Cross Four types of alleles from
the male gametes and four types of alleles from the female gametes can be produced.
The resulting phenotypic ratio is 9:3:3:1 which gave evidence for the Law of Independent Assortment
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Mendel’s Law of Independent Assortment
Random distribution of alleles occurs during gamete formation
Genes on separate chromosomes sort independently during meiosis.
Each allele combination is equally likely to occur.
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Mendel’s Law of Independent Assortment
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Probability Genetic crosses predict what to expect in
the phenotypes and genotypes of the offspring.
Observed results are what you actually see with the organisms.
The larger the number of offspring the closer the expected and observed results usually are.
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Chapter 10Sexual Reproduction
and Genetics10.3 Gene Linkage and Polyploidy
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Gene Linkage The linkage of genes on a chromosome results in
an exception to Mendel’s law of independent assortment because linked genes usually do not segregate independently.
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Polyploidy Polyploidy is the
occurrence of one or more extra sets of all chromosomes in an organism.
Approximately 30% of flowers are polyploidy
Strawberries are octoploidy (8n)
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Polyploidy Horticultural important
plants are forced to polyploidy to increase the size and flavor of flowers and fruits and overall vigor of the plants.
Polyploidy is uncommon in animals.
