UNIT 5.1: SEXUAL REPRODUCTION & BASIC GENETICS
Topic 1 Intro to Sexual Reproduction & Heredity
These are the chromosomes found in a skin
cell taken from an organism.
Is this a diploid cell or haploid cell? Provide
TWO reasons for how you know.
Draw how the chromosomes would appear
in a sperm cell isolated from this organism.
Is this a diploid cell or haploid cell?
Draw a pair of unduplicated homologous
chromosomes with genes X, Y, and Z
Draw the two previous chromosomes after
they duplicate during S phase in interphase.
Label the sister chromatids.
Explain the difference between sister
chromatids and homologous
chromosomes.
HEREDITY
Diploid vs. Haploid Cells
• Most multicellular organism reproduce sexually. Sexual
reproduction involves two parents contributing DNA to produce
offspring.
• Many sexually reproducing organisms have somatic cells that
are diploid. Somatic cells are all the body cells except the
reproductive cells (sperm and eggs).
o Diploid cells contain two complete sets of chromosomes,
one set from each parent.
• Reproductive cells, called gametes (sperm and eggs), are
haploid.
o Haploid cells contain a single set of chromosomes (half as
many chromosomes as somatic cells)
• Every eukaryotic species has a characteristic number of
chromosomes in each cell’s nucleus.
o The variable “2n” represents the diploid number and the
variable “n” represents the haploid number.
• Humans have a diploid number of 46 (2n = 46). This means
human somatic cells contain 46 chromosomes, made up of two
sets of 23.
• Humans have a haploid number of 23 (n = 23). This means that
human gametes (sperm and eggs) contain 23 chromosomes.
Intro to Sexual Reproduction
• During sexual reproduction, a gamete cell from each parent (egg
and sperm) fuse together to create a diploid (2n) cell that
contains a combination of DNA from both parents.
o This process is called fertilization.
• In order to create gametes, dedicated cells in an organisms must
undergo meiosis, a special cell division process that produces
haploid daughter cells from diploid parent cells.
• A fertilized egg (the new diploid cell) is called a zygote, which
will then divide through mitosis and develop into the offspring.
• Meiosis = cell division that creates haploid cells from diploid
cells.
• Mitosis = cell division that creates diploid cells from other
diploid cells.
Intro to Heredity & Genes
• During sexual reproduction, an offspring will inherit genes from
two parents.
• Genes are the units of heredity.
• Heredity is the transmission of traits from one generation to the
next.
• Genes are sequences of DNA that have the instructions to
produce proteins or functional pieces of RNA.
o The products of genes are responsible for an organism's
traits
• Genes are located at a specific location on a chromosome, called
a gene locus (loci; plural).
Homologous Chromosomes
• Somatic diploid cells have two sets of chromosomes, meaning
that there are two of each type of chromosome.
• The two chromosomes in a pair (one inherited from each
parent) are called homologous chromosomes, or homologues.
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UNIT 5.1: SEXUAL REPRODUCTION & BASIC GENETICS
What is the difference between autosomes
and sex chromosomes?
Is the karyotype shown in the slides from a
male or female? How do you know?
o Chromosomes in a homologous pair are the same size and
carry genes controlling the same inherited characters.
• NOTE: Homologous chromosomes are not identical! They do not
have the exact same DNA.
o Ex: Your mom and dad both gave you a chromosome
containing the gene for eye color. The DNA in the
chromosomes from your mom could code for brown eyes,
whereas the DNA in the chromosomes from your dad could
code for green eyes.
• Chromosomes are duplicated before cell division (mitosis or
meiosis).
• Duplicated chromosomes are made up of two sister chromatids,
which are NOT homologous chromosomes.
o Sister chromatids are exact copies of the same
chromosomes held together at the centromere.
o Homologous chromosomes are two distinct chromosomes
that carry the same genes, but have different DNA.
HEREDITY
Sex Chromosomes vs. Autosomes
• A karyotype is an ordered display of the pairs of chromosomes
from a cell.
• The sex chromosomes (X and Y) are not true homologous
chromosomes.
o The Y chromosome contains very few genes, and is thus a lot
smaller than the X chromosome.
• Human females have a homologous pair of X chromosomes (XX),
while human males have one X and one Y chromosome (XY).
• The 22 pairs of chromosomes that do not determine sex are
called autosomes.
Topic 2 Meiosis & Gamete Formation
Describe what happens during crossing
over.
Write the phase of meiosis that matches
each description below:
_______________ paired homologous
chromosomes line up in the middle of the
cell
_______________ duplicated
chromosomes line up individually in the
middle of the cell
_______________ homologous
chromosomes pair up and crossing over
occurs
Overview of Meiosis
• Like mitosis, replication of chromosomes takes place during
interphase, before meiosis.
• Meiosis takes place in two sets of cell divisions, called meiosis I and
meiosis II
• The two cell divisions result in four daughter cells, rather than the two
daughter cells in mitosis
• Each daughter cell produced is haploid, containing half as many
chromosomes as the parent cell.
Meiosis I
• In the first cell division, meiosis I, homologous chromosomes separate.
• Meiosis I starts with a diploid parent cell and ends with two haploid
daughter cells.
o For this reason, meiosis I is called a reduction division.
• Meiosis I consists of four phases:
o Prophase I, Metaphase I, Anaphase I, and Telophase I (with
cytokinesis)
Prophase I
• Duplicated chromosomes condense and loosely pair up with their
homologue, aligning gene by gene.
o This forms a tetrad, a group of four chromatids.
• Nonsister chromatids on adjacent homologous chromosomes
exchange segments of DNA in a process called crossing over.
o This takes place at chiasmata, x-shaped regions where homologous
pairs make direct contact.
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UNIT 5.1: SEXUAL REPRODUCTION & BASIC GENETICS
_______________ sister chromatids are
pulled apart
_______________ homologous
chromosomes are pulled apart
Fill in the blanks in the statements below to
summarize what occurs in meiosis I and II.
During meiosis I, ____________________
_____________________ are separated.
During meiosis II, ____________________
_____________________ are separated.
Circle the correct words in the statements
below:
MEIOSIS I starts with a [ diploid | haploid ]
cell that has [ duplicated | unduplicated ]
chromosomes and produces two
[ diploid | haploid ] daughter cells that
have [ duplicated | unduplicated ]
chromosomes.
MEIOSIS II starts with two [ diploid |
haploid ] cell that have [ duplicated |
unduplicated ] chromosomes and produces
a total of four [ diploid | haploid ] daughter
cells that have [ duplicated | unduplicated]
chromosomes.
Determine if the statements below apply to
mitosis (MIT), meiosis (MEI), or BOTH.
__________ DNA replicates during
interphase
__________ nucleus divides once
__________ nucleus divides twice
__________ homologous chromosomes
pair up.
__________ crossing over occurs
__________ used to produce gametes for
sexual reproduction
HEREDITY
o The regions that are exchanged between homologous
chromosomes during crossing over are different each time meiosis
occurs, creating chromatids with unique combinations DNA.
Metaphase I
• Tetrads line up at the metaphase plate, with one chromosome facing
each pole.
• Microtubules from one pole are attached to the kinetochore of one
chromosome of each tetrad
• Microtubules from the other pole are attached to the kinetochore of
the other chromosome
Anaphase I
• Pairs of homologous chromosomes separate
• One chromosome moves toward each pole, guided by the spindle
apparatus
• Sister chromatids remain attached at the centromere and move as one
unit toward the pole
Telophase I & Cytokinesis
• In the beginning of telophase I, each half of the cell has a haploid set of
chromosomes and each chromosome still consists of two sister
chromatids.
• Cytokinesis usually occurs simultaneously, forming two haploid
daughter cells
• In animal cells, a cleavage furrow forms; in plant cells, a cell plate
forms.
• No chromosome replication occurs between the end of meiosis I and
the beginning of meiosis II because the chromosomes are already
replicated.
Meiosis II
• In the second cell division, meiosis II, sister chromatids separate.
• Meiosis II starts with the two haploid cells produced in meiosis I (each
still containing replicated chromosomes) and results in four haploid
daughter cells with unduplicated chromosomes.
• Meiosis II consists of four phases:
o Prophase II, Metaphase II, Anaphase II, and Telophase II (with
cytokinesis)
• In prophase II, a new spindle apparatus forms and begins moving the
chromosomes toward the metaphase plate.
• In metaphase II, the sister chromatids are arranged at the metaphase
plate
o Because of crossing over in meiosis I, the two sister chromatids of
each chromosome are no longer genetically identical
• In anaphase II, the sister chromatids separate
o The sister chromatids of each chromosome now move as two
newly individual chromosomes toward opposite poles.
• In telophase II, the chromosomes arrive at opposite poles, nuclei form,
and the chromosomes begin decondensing.
• Cytokinesis separates the cytoplasm and results in a total of four
daughter cells.
Comparing Mitosis & Meiosis
• Mitosis conserves the number of chromosome sets, producing cells
that are genetically identical to each other and the parent cell.
o Starts with a diploid cell and ends with two new diploid cells after
one round of cell division.
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UNIT 5.1: SEXUAL REPRODUCTION & BASIC GENETICS
__________ used for growth,
development, and replacing
damaged cells
__________ produces genetically identical
daughter cells
__________ produces genetically distinct
daughter cells
__________ starts with a diploid cell
__________ produces diploid cells
__________ produces haploid cells
HEREDITY
o Duplicate chromosomes line up individually in the middle of the cell
and sister chromatids are separated into two daughter cells.
 Homologous chromosomes DO NOT pair up in mitosis!
• Meiosis reduces the number of chromosome sets from two sets
(diploid) to one set (haploid).
o Starts with a diploid cell and ends with four new haploid cells after
two rounds of cell division.
o The resulting cells from meiosis differ genetically from each other
and from the parent cell.
• Three events unique to meiosis:
o In prophase I, homologous chromosomes physically connect and
exchange genetic information through crossing over.
o At the metaphase plate in metaphase I, there are paired
homologous chromosomes (tetrads), instead of individual
replicated chromosomes
o At anaphase I, it is homologous chromosomes, instead of sister
chromatids, that separate and are carried to opposite poles of the
cell
Cell Starting Meiosis:
Practice! To the right is a picture of a cell that has completed interphase and is ready to
start meiosis. It has a diploid number of four (2n=4). Draw the chromosomes as they would
appear in the phases listed below. Color code the chromosomes each time to keep track of
the paternal (father) DNA and the maternal (mother) DNA.
Prophase I (label a tetrad
and show crossing over)
Take both of the cells produced during Meiosis I
and draw what they look like during Metaphase II:
Metaphase I
Daughter cells produced after Meiosis I:
Draw the four final daughter cells that
are produced at the end of Meiosis II:
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UNIT 5.1: SEXUAL REPRODUCTION & BASIC GENETICS
Topic 3 Mendel’s Model of Inheritance
What is a true-breeding plant?
The diagram on the next page illustrates
what Mendel did in his experiments. Label
the P, F1, and F2 generation.
When Mendel crossed a true breeding
white flowered plant with a true-breeding
purple flowered plant, what did the
offspring (F1 generation) look like?
What did the offspring (F2 generation) look
like when he self-pollinated that F1 hybrid
offspring?
What is the gist of each concept in
Mendel’s model for inheritance?
Concept #1:
Concept #2:
Concept #3:
Concept #4:
HEREDITY
Gregor Mendel
• In the second half of the 1800s, Gregor Mendel, an Austrian monk,
discovered the fundamental laws of inheritance that built the
foundation for modern day genetics.
o For this reason he is known as the “father of genetics.”
• Mendel extensively studied the inheritance of various traits in pea
plants and found that many of the traits followed predictable patterns.
• Mendel had no molecular mechanism to explain the results of his
experiments; he had no knowledge of DNA or chromosomes.
• Instead, Mendel developed a model, with an included set of laws, to
explain his experimental results.
Mendel’s Experimental Setup
• Mendel chose to track specific characteristics that occurred in two
distinct, alternative forms.
o Ex: purple or white flower color
• He started his experiments with true-breeding plants, which means
that over many generations of self-pollination, they always produce
the same variety as the parent.
o Ex: purple flower plants that always produced purple flower
offspring when self-pollinated.
• Mendel then cross-pollinated two contrasting, true-breeding pea
varieties.
o Ex: purple-flowered plants with white-flowered plants
o The mating of two true-breeding varieties is called hybridization.
o The true breeding parents are called the P generation, and their
hybrid offspring are called F1 generation.
• Lastly, Mendel allowed the F1 hybrids to self-pollinated (or cross with
other F1 hybrids), which produced an F2 generation.
Mendel’s Experimental Results
• Mendel’s data showed that the F1 generation plants were all identical
to one of the true-breeding parents, but not the other parent.
o Ex: all the offspring's flowers were purple.
• In the F2 generation, however, the missing trait reappeared in some of
the offspring.
o Ex: most of the F2 generation had purple flowers, but some had
white flowers.
• His data further showed that many traits in the F2 generation
appeared in a 3:1 ratio (75%, 25%).
• Mendel used the data from his experiments to develop a model of
inheritance based on different key concepts.
• Each concept is explained by information we now know today.
Mendel’s Model of Inheritance: Alleles
• Concept #1: Alternative versions of genes account for variations in
inherited characteristics.
o Ex: the gene for flower color has two versions, white and purple.
o These alternative versions are called alleles.
o Today we know that each gene is a sequence of nucleotides at a
specific place, or locus, along a particular chromosome.
o The DNA at that locus, however, can vary slightly in its nucleotide
sequence, which can affect the function of the encoded protein
and thus the inherited characteristic of the organism.
o Each allele is a different DNA sequence variation!
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UNIT 5.1: SEXUAL REPRODUCTION & BASIC GENETICS
The following pair of chromosomes found
in a pea plant contain the gene for flower
color. Label the locus for the gene and an
allele.
If these chromosomes were taken form a
pea plant produced in the F1 generation,
what two alleles would be found on the
chromosomes?
Which allele is the dominant allele for
flower color? Explain how you know.
Label the flowers from the P and F1
generations in Mendel’s experiment with
their phenotype and genotype.
P generation
(true-breeding)
F1 generation
(hybrids)
Create a Punnett
square for the
cross pictured
above.
Create a Punnett square showing the
possible offspring created when a F1 hybrid
plant is self-pollinated (as shown in the
diagram below).
F1 generation
(hybrids)
Self-pollinated
F2 generation
What are the possible genotypes for the
purple flowered plants produced in the F2
generation?
HEREDITY
• Concept #2: For each characteristic, an organism inherits two copies
(two alleles) of a gene, one from each parent.
o Ex: pea plants have two alleles for flower color.
o Today we know that each somatic cell in a diploid organism
contains two sets of chromosomes, one set from each parent.
o Thus, there are two loci for each gene (one on each homologous
chromosome) which gives an individual two alleles for every gene.
• Concept #3: If the two alleles for a specific gene differ, then the
dominant allele, determines the organism’s appearance; the other, the
recessive allele, has no noticeable effect on the organism’s
appearance.
o This is called the law of dominance.
o Ex: if a pea plant has both a purple and white allele for flower color,
it will have purple flowers… purple is dominant.
o A capital letter is used to represent a dominant allele, and a
lowercase letter is used to represent a recessive allele.
o For a given trait, the same letter is usually used for both the
recessive and dominant allele. Typically, it is the letter that refers
to the dominant trait. Ex: P = purple allele; p = white allele
Genotype vs. Phenotype
• The genetic makeup, or two alleles, that an organism has is called its
genotype.
o An organism with two identical alleles for a characteristic is said to
be homozygous for the gene controlling that characteristic.
 Ex: having two purple alleles (PP)
 Ex: having two white alleles (pp)
 “True-breeding” plants are homozygous
o An organism that has two different alleles for a gene is said to be
heterozygous for the gene controlling that characteristic
 Ex: having a purple and white allele (Pp)
• The organism's physical appearance is called its phenotype.
o In the example of flower color in pea plants, PP and Pp plants have
the same phenotype (purple) but different genotypes.
Mendel’s Model of Inheritance: Law of Segregation
• Concept #4: The two alleles for a heritable characteristic segregate
(separate from each other) during gamete formation and end up in
different gametes; only one allele from each gene can be passed on to
an offspring.
o This is called the law of segregation.
o Ex: if a pea plant has an allele for both purple and white flowers,
half the gametes produced by the plant will have the purple allele
and the other half will have the white allele. The plant CANNOT
pass on both the white and purple allele to one of its offspring.
o The law of segregation can be explained today, knowing now that
the homologous chromosomes carrying each allele in a diploid cell
separate during meiosis into the resulting sex cells.
Punnett Squares
• 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.
• The alleles that can be found in the gametes from each parent are
written outside the Punnett square (one parent on the left and the
other parent on the top).
• The boxes inside the Punnett square represent the possible ways the
alleles could combine when gametes join from each parent.
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UNIT 5.1: SEXUAL REPRODUCTION & BASIC GENETICS
A testcross can be done to determine if one
of the purple flowers produced in the F2
generation is homozygous or heterozygous.
If the results of the testcross produced all
purple flowered plants, was the F2 plant
homozygous or heterozygous? Create a
Punnett square to support your answer.
What is a dihybrid? Give an example.
What was the phenotypic ratio that
resulted when Mendel crossed two
dihybrid plants for seed color (Yy) and seed
shape (Rr).
What is the Law of Independent
Assortment?
HEREDITY
• Punnett squares can also be used to determine the expected genotypic
and phenotypic ratios in the offspring.
• They can also be used to determine the probability of certain allele
combinations being inherited by the offspring.
• Ex: crossing two heterozygous purple flower pea plants (Pp x Pp)
results in offspring with these expected ratios:
Genotypic Ratio = 1 PP : 2 Pp : 1 pp
Phenotypic Ratio = 3 Purple : 1 White
The Testcross
• A testcross can be done to determine the genotype of an individual
with the dominant phenotype, who could be either homozygous or
heterozygous.
• A testcross is when the mystery individual is crossed with a
homozygous recessive individual.
• If any offspring display the recessive phenotype, the mystery parent
must be heterozygous.
Mendel’s Dihybrid Crosses
• Mendel also followed multiple characteristics at the same time by
creating dihybrids, plants that are heterozygous for two characteristics.
• He then crossed two dihybrids, called a dihybrid cross.
o Ex: crossing two dihybrid plants that are heterozygous for seed
color (Yy) and seed shape (Rr).
 Y = yellow; y = green
 R = round seeds; r = wrinkled seeds
• The results of his dihybrid cross experiments consistently produced a
9:3:3:1 phenotypic ratio in the offspring.
Mendel’s Model: Law of Independent Assortment
• The results proved that alleles from each gene separate independently
during gamete formation.
• Concept #5 Alleles from different genes are randomly combined during
gamete formation. The allele passed on for one gene has no effect on
the allele passed on for another gene. All combinations are equally
possible.
o This is called the law of independent assortment.
o Ex: If a pea plant is heterozygous for both seed color and seed
shape (YyRr), then it can produce gametes with the following allele
combinations (which will all be equally likely): YR, Yr, yR, and yr
o Mendel’s Law of Independent Assortment can be explained today
by the fact that during meiosis and gamete production,
homologous chromosomes line up and separate randomly and
independently from the way other homologous chromosomes line
up and separate.
Practice! Punnett Squares: Determine the expected genotypic and phenotypic ratios from the following crosses:
1. Yellow seeds (Y) are dominant over green seeds (y).
Cross homozygous recessive and homozygous dominant parents.
2. Green pod color (G) is dominant over yellow pod color (g).
Cross homozygous dominant and heterozygous parents.
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UNIT 5.1: SEXUAL REPRODUCTION & BASIC GENETICS
HEREDITY
3. Round seeds (R) are dominant over wrinkled seeds (r).
Cross homozygous recessive and heterozygous parents.
4. Inflated pod shape (I) is dominant over constricted pod shape (i).
Cross homozygous recessive and homozygous dominant parents.
5. Tall pea plants (T) are dominant over short pea plants (t).
Cross two heterozygous parents.
Practice! Law of Independent Assortment: Determine all the possible allele combinations that could be
found in the gamete cells of plant with the genotype:
6. YyRr
7. YYRr
8. yyRr
9. YYRR
Practice! Crossing Multiple Genes
10. Determine the genotypic and phenotypic ratios from crossing two dihybrid pea plants with the following genotypes: YyRr x YyRr
11. Determine the genotypic and phenotypic ratios from crossing two pea plants with the following genotypes: ttPp x TtPp
8