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CHAPTER 10
SEXUAL
REPRODUCTION AND
GENETICS
• SECTION 10.1 – MEIOSIS
• MAIN IDEA – MEIOSIS PRODUCES HAPLOID
GAMETES.
– All of the differences you see in the room,
different hair color, eye color, ear shapes, is the
result of 2 sex cells combining during sexual
reproduction.
• QUESTION: How are the following cell parts
involved in mitosis?
– Chromosome, spindle fibers, nucleus, and
nucleolus
• CHROMOSOMES AND CHROMOSOME NUMBER
• Everyone has characteristics passed on to them
from their parents.
• Each characteristic is called a trait.
– Ex: hair color, height or eye color
• The instructions for each trait are located on the
chromosomes, which are found in the nucleus of
cells.
• The DNA on the chromosomes is arranged in
segments called genes that control the production
of proteins.
– Each chromosome has 100’s of genes.
• HOMOLOGOUS CHROMOSOMES
• Human body cells (somatic cells)
have 46 chromosomes.
– Each parent gave you 23 chromosomes creating 23
PAIRS of chromosomes or 46 chromosomes.
• The chromosomes that make up a pair (the one from
mom & the one from dad) are called homologous
chromosomes
– Homologous chromosomes in body cells have the
same length, same centromere position, and carry
genes that control the same inherited traits.
• These genes each code for the same trait (ex:
earlobe) but maybe not the same type of trait
(earlobe)
• HAPLOID AND DIPLOID CELLS
• Gametes are sex cells that have half the number of
chromosomes.
– In humans the number of chromosomes in a gamete is 23.
• Each species has a different # of chromosomes
– The symbol n can be used to represent the number of
chromosomes in a gamete.
• Cells with n number of chromosomes is called a haploid
cell.
– Haploos means single.
• Fertilization is the process by which one haploid gamete (egg)
combines with another haploid gamete (sperm)
– Fertilization creates cells that are 2n (one n chromosome
from mom & one n chromosome from dad)
– 2n cells are called diploid cells
• MEIOSIS I
• Gametes are formed during a process called meiosis.
– Meiosis is a type of cell division that reduces the number
of chromosomes
– Meiosis is a reduction division.
• Mitosis maintains the chromosome #, meiosis
reduces the chromosome # by half through the
separation of homologous chromosomes.
• Cells begin as 2n, but create gametes with n number
of chromosomes
– Meiosis occurs in the reproductive structures of
organisms that reproduce sexually.
• Meiosis involves 2 consecutive cell divisions called meiosis I
& meiosis II
• INTERPHASE
• Just like with mitosis, cells that
undergo meiosis also go through
interphase as part of their cell cycle.
–While in interphase cells carry out
various metabolic processes,
including the replication of DNA
and the synthesis of proteins
• PROPHASE I
• Cells entering prophase I, the replicated (copied)
chromosomes become visible.
– Creating sister chromatids
• Homologous chromosomes (mom’s sister chromatid
+ dad’s sister chromatid) form during synapsis and
are held tightly together. Can be called a tetrad.
– Crossing over occurs this time.
• Crossing over is a process when segments of
chromosomes are exchanged between a pair of
homologous chromosomes.
• Prophase I continues with the centrioles moving to
the cell’s opposite poles, spindle fibers forming and
binding to the sister chromatids at the centromere.
• PROPHASE I
• METAPHASE I
• 2nd phase of meiosis
• Homologous chromosomes line up at the
equator of the cell.
• Spindle fibers are attached to the
centromere of each homologous
chromosome
• ANAPHASE I
• During anaphase I homologous chromosomes
separate.
–Sister chromatids are pulled to opposite
poles of the cell
–Chromosome number is reduced from 2n to
n when the homologous chromosomes
separate.
• TELOPHASE I
• Homologous chromosomes (sister chromatids)
reach the cell’s opposite poles.
– Sister chromatids are still joined at the
centromere
– Sister chromatids might not be identical from
when it started because of crossing over.
– During telophase I, cytokinesis usually occurs
pinching in the animal cell or forming cell plate in
plant cells.
– May or may not go into interphase. If cell goes
into interphase, DNA is NOT duplicated again.
• Telophase I
• MEIOSIS II
• Meiosis is only half done after meiosis I.
• Prophase II the sister chromatids condense and
spindle apparatus forms
• Metaphase II the sister chromatids line up at the
equator by the spindle fibers.
• Anaphase II the sister chromatids are pulled apart
at the centromere by the spindle fibers and the
chromosomes move toward the opposite poles of
the cell.
• Telophase II the chromosomes reach the poles and
the nuclear membrane and nuclei reform.
• At the end of meiosis II, cytokinesis occurs creating
4 haploid cells each with n number of chromosomes
• MEIOSIS II
• MEIOSIS PROVIDES VARIATION
• Genetic variation is produced during crossing over and
during fertilization when gametes randomly combine.
• Depending on how chromosomes line up at the equator, 4
gametes with 4 different combinations of chromosomes
can result.
• SEXUAL REPRODUCTION VS. ASEXUAL REPRODUCTION
• Asexual reproduction the organism inherits all of its
chromosomes from a single parent and are genetically
identical to the parent.
– Ex: bacteria
• Sexual reproduction allows variation and beneficial
mutations to accumulate faster in the population.
• SECTION 10.2 – MENDELIAN GENETICS
• MAIN IDEA – Mendel explained how a
dominant allele can mask the presence of a
recessive allele.
• QUESTION: Do all dogs look alike?
• What types of features indicate a particular
breed?
• Are these features inherited?
• What does this tell you about the inheritance
of these features?
• HOW GENETICS BEGAN
• In 1866 Gregor Mendel, an Austrian monk,
published a book on the method of inheritance in
garden pea plants.
• Inheritance or heredity is the passing of traits from
one generation to the next.
• Mendel chose pea plants to study because they are
true-breeding, meaning that they consistently
produce offspring with only one form of a trait.
– EX: always purple flowers
• Pea plants self-pollinate, but Mendel began to
cross-pollinate the pea plants.
• THE INHERITANCE OF TRAITS
• Mendel noticed that some varieties of peas
produced a specific form of a trait generation
after generation.
–EX: always green seeds or yellow seeds
• Mendel cross-pollinated (male gamete of one
plant & combined with female gamete of the
other plant)
–Mendel referred to the male gamete &
female gametes used in the crosspollination as the P generation or the
parent generation.
• F1 AND F2 GENERATION
• Mendel grew the seeds from the cross from the parent
generation (P generation), one green seed and one yellow
seed and the results were all the offspring had yellow
seeds.
– The offspring from the parent generation (P generation)
are called the F1 generation or first filial generation.
• Mendel questioned if the green seed was gone or being
hidden or masked.
• Mendel planted the F1 generation and allowed them to
self-pollinate and then examined the seeds from this cross.
– The offspring from the F1 generation is called the F2
generation or second filial generation.
– Mendel’s results were a 3:1 ratio of 6022 yellow seeds to
2001 green seeds
• GENES IN PAIRS
• Mendel concluded there must be 2 forms of the
seed trait in pea plants – yellow and green seeds.
• Allele is an alternative form of a single gene passed
from generation to generation.
• Gene for yellow seeds and the gene for green seeds
are each different forms (alleles) of a single gene.
• In Mendel’s experiments the yellow seed color was
dominant over the recessive green seed color.
– Reason why the F1 generation was yellow and
explaining why the F2 generation had the 3:1
ratio of yellow to green
• DOMINANCE
• When the dominant allele is present it will be the one that is shown.
– Ex: Yellow seeds are dominant over the green seeds, results
shown in the F1 generation cross that Mendel made
• When modeling inheritance, the dominant allele is represented by a
capital letter.
– The letter used is the beginning letter of the dominant trait.
• Ex: Yellow seeds used the capital Y
• Ex: Round seeds dominant use capital letter R
• When modeling inheritance, the recessive allele is represented by a
lower case letter.
• Homozygous for a particular trait means that the organism has the
same 2 alleles.
– Ex: homozygous (dominant) yellow seeds = YY
– EX: homozygous (recessive) green seeds = yy
• DOMINANCE – CONTINUED
• Heterozygous for a particular trait means
that the organism has 2 different alleles.
–EX: one yellow allele and one green
allele = Yy
• The dominant allele will always be
expressed when the organism is
heterozygous
• GENOTYPE AND PHENOTYPE
• Genotype is the organisms allele pairs.
–Ex: YY is the genotype for homozygous
dominant yellow seeds
–Ex: Yy is the genotype for heterozygous
yellow seeds
–EX: yy is the genotype for homozygous
recessive green seeds
• Phenotype is the observable characteristic or
outward expression of an allele pair.
–Ex: yellow seeds or green seeds
• MENDEL’S LAW OF SEGREGATION
• Mendel’s Law of Segregation states that the 2
alleles for each trait separate during meiosis.
–During fertilization the 2 alleles for that trait
unite.
• Hybrids are
heterozygous
organisms.
• LAW OF INDEPENDENT ASSORTMENT
• Law of independent assortment states that a
random distribution of alleles occurs during gamete
formation. Genes on separate chromosomes sort
independently during meiosis.
– Ex: an organism heterozygous for seed color (Yy)
crossed with another heterozygous organism (Yy)
means that the Y allele and the y alleles from
both organisms can recombine in any
combination.
• Either YY, Yy or yy
• PUNNETT SQUARES
• Punnett squares are used to predict the possible
offspring of a cross between 2 known genotypes.
• PUNNETT SQUARE – MONOHYBRID CROSS
• The number of squares is determined by the number of
types of alleles produced by each parent.
• Male gametes are written across the horizontal side.
• Female gametes are written on the vertical side of the
Punnett square.
• Possible combinations of each male and female gamete
are written on the inside of each corresponding square.
– Dominant allele is written first, whether it comes
from the male or female gamete.
• PUNNETT SQUARE
• Cross between heterozygous male (Bb) and a
heterozygous female (B)
• PUNNETT SQUARE – DIHYBRID CROSS
• Dihybrid crosses is when we look at the inheritance
of 2 traits at the same time.
– Ex: pea plants with yellow seeds (1trait) and round
seeds (1 trait)
– If Mendel started with homozygous yellow round seeds
plant and green wrinkled seeds the F1 generation would
be represented by YyRr
– Crossing the F1 generation yields a 9:3:3:1 ratio
• PUNNETT SQUARE – DIHYBRID CROSS
• PROBABILITY
• The inheritance of genes can be
compared to the probability of flipping a
coin.
–Probability of the coin landing on heads
is 50% or half the time.
–Large number of results makes it more
likely that the results will match the
predicted results of the Punnett square
• SECTION 10.3 – GENE LINKAGE & POLYPLOIDY
• READ & TAKE NOTES OVER PAGES 283, 284, &
285.
• BE PREPARED TO SHARE YOUR NOTES WITH
THE CLASS
• BE PREPARED TO TAKE A SHORT QUIZ OVER
THESE PAGES