UNIT VII – MEIOSIS & INTRODUCTION TO GENETICS
Although the resemblance between generations of organisms had been noted for thousands
of years, it wasn’t until the 1800s that scientific studies were carried out to develop an
explanation for this. Today we know that we resemble our parents because of _heredity__,
which is the set of characteristics we receive from _our parents___. The study of heredity is
known as _genetics____.
I. SEXUAL REPRODUCTION & MEIOSIS
(pp. 270-276)
In sexual reproduction, an egg and sperm cell fuse together to create a fertilized
egg or _zygote_. Egg and sperm cells are known as gametes. Gametes are the
only cells in the body that are not produced by mitosis. Instead gametes are
created through a special process of cell division called _meiosis_ which halves
the chromosome number. Meiosis only occurs in the _ovaries_ of females and
the _testes_ of males.
A. Chromosome Number
1. Human _somatic_ cells, or body cells, contain _46_ chromosomes. Somatic
cells are _diploid_ or 2n_ because these cells contain a _double_ set of
chromosomes – half (23) from _mom_ and half (23) from _dad_. These
“matching” chromosomes are known as _homologous pairs__. A
homologous pair is a pair of chromosomes – one from each parent with the
same _type of information____ or _genes_.
2. Human gametes (_egg_ and _sperm_) contain _23_ chromosomes. They
are _haploid_ or _n_. These cells contain _1/2__ the total number of
chromosomes, a _single___ set of chromosomes. When the gametes fuse
together in _fertilization_, the resulting _zygote_ has _46__ chromosomes.
B. Meiosis occurs in two stages:
1. Meiosis I - Prior to meiosis I, the DNA is replicated in _S_ of _interphase_.
When the chromosomes, each consisting of 2 _sister chromatids_ line up in
the middle of the cell in _metaphase_ I, they line up in _homologous_ pairs,
called a tetrad. The sister chromatids are held together at their centromeres,
and the homologous pairs are held together at chiasmata. In anaphase I and
telophase I, the homologous pairs _separate_, but the sister chromatids and
_centromeres_ are still intact. Two cells are formed, each with _23_ pairs of
_sister chromatids_ making up _23_ chromosomes, but there are no longer
any _homologous pairs_ present so the two cells are _haploid___ or _n___.
2. Meiosis II - The process continues with the two cells formed moving directly
into prophase II without any further _replication of DNA_. After the
chromosomes align in the middle of the cell in metaphase II, this time the
sister chromatids are pulled apart in _anaphase_II. Two new cells are
formed from each of the two cells formed in Meiosis I, resulting in a total of
_four_ new cells, each with _1/2___the original number of chromosomes. The
cells produced are called _gametes_____.
C. Crossing Over
Crossing over occurs during _prophase I_ when _homologous_ pairs of
chromosomes come together. A portion of one _sister chromatid_ may be broken
off and exchanged with the corresponding portion of a sister chromatid of the
homologous chromosome. Crossing over is very common and _increases_ the
genetic variability in offspring.
II. HISTORY OF GENETICS
(pp. 277-279)
A. Gregor Mendel – Known as the “Father of _Genetics__”, Mendel is famous
for his experiments with _pea__ plants.
He used true-breeding pea plants, which means _purebred, pure-line and
characteristics always show. This generation of true-breeding plants is
known as _P_ generation. Mendel studied seven _traits_, including plant height,
seed color, flower color, etc.
A trait is an _inherited characteristic__. Pea plants cross-pollinate,
meaning pollen from one plant fertilizes an egg from another, but they can
also self-pollinate, meaning pollen can fertilize egg from _same_ plant.
Mendel controlled the fertilization process of the pea plants by preventing
_self-pollination_ and controlling _cross-pollina tion_.
B. Mendel’s Results
1. P generation – Crossed _true-breeding__ plants with one trait with _true
-breeding_ plants with the other. For example, __tall plants x short plants
2. F1 generation – Offspring produced from _P x P_. In F1, one trait
appeared to _disappear_. For example, tall plants X short plants = _all tall
plants________.
3. F2 generation – Offspring produced from _F1 x F1_. In F2, trait that
disappeared in F1 reappeared in _1/4_ of the offspring; the other ¾
showed _the trait seen in F1__.
Mendel’s Findings:
C. Mendel’s Principles – After analyzing his results carefully, Mendel formed
conclusions that increased understanding of inheritance and opened the door
for the study of genetics.
1. Individual units called _genes_ determine inheritable characteristics.
A gene is a portion of DNA_ that codes for a specific _trait_. Different
forms or possibilities for a gene are called _alleles__. For example, the
alleles for the gene for plant height are _tall_ and _short_.
2. For each gene, an organism inherits two alleles, one from each
_parent___.
a. If the two alleles are the same, the organism is said to be homozygous
for that trait and the allele will be expressed.
b. If the two alleles differ, the organism is said to be _heterozygous_ for
that trait and only one allele will be expressed. The expressed allele is
the _dominant_ allele. It is designated by an _upper__- case letter.
The allele that is not expressed in a heterozygous trait is _recessive_
and it is designated by a _lower__-case letter. A recessive allele is
only expressed when an organism is _homozygous_ for that allele.
3. In meiosis, the two alleles for a trait segregate (_separate_). Each
egg or sperm cell receives a copy of one of the two alleles present in the
somatic cells of the organism. Due to the random separation of
chromosomes in meiosis, there is a _50%_ chance that a copy of that
allele will end up in the gamete produced. This is known as the principle
of segregation.
D. Genetics Terminology
1. Phenotype - _Physical ___ description of trait; for example, _tall, short_
2. Genotype – Genetic make-up of an organism or set of alleles; for
example, _TT, Tt, tt_
3. Application of Terminology - If round pea seeds are dominant to
wrinkled pea seeds, round is designated _R___ and wrinkled is
designated _r____.
a. Homozygous dominant for pea seed shape is written _RR___.
Genotype = _RR___; Phenotype = _Round__
b. Heterozygous for pea seed shape is written _Rr____.
Genotype = __Rr_; Phenotype = __Round_
c. Homozygous recessive for pea seed shape is written _rr___.
Seed shape? Wrinkled
III. ANALYZING INHERITANCE
(pp.280 - 282)
A. Probability
Due to the law of segregation, if you know the genotype of the parents, you
can predict the likelihood of a trait occurring in the offspring. Probability can
be written 3 ways. The probability of a coin coming up heads after being
flipped is (fraction) _1/2____, (ratio) __1:2____, or (percent) _50%____.
B. Punnett Squares
A Punnett square is a tool used to predict the possible outcomes of
_meiosis_and _fertilization_; in other words, a Punnett square is used to
determine the probability of certain traits appearing in offspring.
IV. PUNNETT PRACTICE
**Please note: To earn full credit, you must include a key and cross with each
problem!**
A. Construct a Punnett square to determine the probability of white flowers if a
heterozygous purple (Pp) flower is crossed with a homozygous white
(pp) flower.
Key: _________________________________________
Cross: ________________________________________
Probability of white flowers = ___________________
B. Construct a Punnett square to determine the probability of short pea plants if
a homozygous tall (TT) plant is crossed with a heterozygous tall (Tt)
plant.
Key: _________________________________________
Cross: ________________________________________
Probability of short pea plants = _________________
Probability of tall pea plants = ___________________
C. If round peas are dominant over wrinkled peas, make a Punnett square to
determine the genotype and phenotype ratios of the offspring if a
heterozygous plant is crossed with a homozygous recessive plant.
Key: _________________________________________
Cross: ________________________________________
Genotype ratio: _________________________________________
Phenotype ratio: ________________________________________
D. Use a Punnett square to determine the genotype and phenotype ratios of the
offspring from a cross between a homozygous dominant yellow pea plant
and a homozygous green pea plant.
Key: _________________________________________
Cross: ________________________________________
Genotype ratio: _________________________________________
Phenotype ratio: ________________________________________
V. DIHYBRID CROSSES (pp. 280 - 282)
The Punnett squares we have been doing are known as _monohybrid crosses_,
meaning that only one trait has been considered at a time. In a dihybrid cross,
_2_ different _traits_ on 2 different _chromosomes_ are analyzed. Mendel performed
dihybrid crosses in plants that were tru-breeding for two traits.
A. Let’s perform Mendel’s original cross between 2 true-breeding parents for pod color and
seed color. In this cross, the traits for green pod color (GG) and yellow seed color (YY)
are dominant. Yellow pod color (gg) and green seed color (yy) are recessive
Key: _________________________________________
_________________________________________
Cross: ________________________________________
Gametes: _____________________________________
_____________________________________
Genotype ratio: ________________________________
_______________________________
Phenotype ratio: _______________________________
_______________________________
B. After observing the results of the dihybrid cross, Mendel allowed all of the F1 plants to
self-pollinate. He referred to these offspring as the F2 generation. Let’s see what his
results were…
Key: R = round, r = wrinkled; Y = yellow, y = green
Cross: ________________________
Gametes: _____________________
Genotype ratio: _______________
_______________________________
Phenotype ratio: ________________
______________________________
 Mendel's Law of Independent Assortment
Mendel performed similar experiments focusing on several other traits like seed color
and seed shape, pod color and pod shape, and flower position and stem length. He
noticed the same ratios in each case. From these experiments Mendel formulated
what is now known as Mendel's law of independent assortment. This law states that
__allele pairs__ separate independently during the formation of gametes in the
process of __meiosis___. Therefore, traits are transmitted to offspring independently
of one another.
More Dihybrid Practice…
C. If a pea plant with genotype RRYy (round, yellow peas) is crossed with a pea
plant with genotype rrYy (wrinkled, yellow peas), what would the results be?
Key: R = round, r = wrinkled; Y = yellow, y = green
Cross: ________________________________________
Genotype ratio: __________________________________________________
Phenotype ratio: ________________________________________________
D. GGRr X Ggrr
Key: G = gray body
g = black body
R = red eyes
r = black eyes
Cross: __________________________________________
Gametes: ________________________________________
Genotype ratio: _____________________________________________________
___________________________________________________
Phenotype ratio: _____________________________________________________
________________________________________________________
VI. A CLOSER LOOK AT HEREDITY (pp. 302 - 305)
A. Incomplete Dominance – Neither allele has “complete” dominance over the
other - heterozygous phenotype is a _blend of the 2 homozygous
phenotypes_ For example, in snapdragons, __Red flowers are incompletely
dominant over white flowers. Red flowers are designated RR, White flowers
are designated R’R’, and RR’ flowers are PINK!! . A farmer crosses a red
snapdragon with a white snapdragon. What do the offspring of these parents look
like?
Cross: ________________________________________
Genotype ratio: _________________________________________
Phenotype ratio: ________________________________________
Now, cross two pink snapdragons and see what the offspring look like.
Cross: ________________________________________
Genotype ratio: _________________________________________
Phenotype ratio: ________________________________________
B. Codominance – Both alleles _share____ dominance and are always
_expressed___ if present. For example, __In chickens, black feathers and white
feathers are both dominant and will therefore be expressed if allele is present.
BB = black feathers, WW = white feathers, and BW = black AND white feathers
If a farmer crosses a black feathered chicken with a white feathered chicken,
what would the offspring look like?
Cross: ________________________________________
Genotype ratio: _________________________________________
Phenotype ratio: ________________________________________
Now, cross two black and white colored chickens and see what the offspring look
like.
Cross: ________________________________________
Genotype ratio: _________________________________________
Phenotype ratio: ________________________________________
C. Polygenic Traits – “_Many genes____” Many traits are controlled by more
than one gene.
Examples include _hair color, eye color, skin tone___
D. Multiple Alleles – Many genes have more than _2_ alleles, although an
individual only has _2_ alleles for the gene, one from _mom__ and one from
_dad__. An example is _blood group_. There are _3_ possible alleles for this
gene, ___IA, IB, and i______. __IA__ and __IB__ are both co-dominant over the
allele, __i_. With this being said, there are 9 different genotype combinations (4
different phenotypes, when these alleles are passed on through sexual reproduction.
Two individuals, one with blood type A, and the other with blood type B decide to
have a child. What are all the possible genotypes
and phenotypes of their child?
Cross: ________________________________
Genotype ratio: __________________________
Phenotype ratio: _________________________
Two individuals, one with blood type O, and the other with blood type B decide to
have a child. What are all the possible genotypes and phenotypes of their child?
Cross: _______________________________
Genotype ratio: ________________________
Phenotype ratio: _______________________