Name______Test Date______
UNIT VIII – MUTATIONS, MEIOSIS, & INTRODUCTION TO GENETICS
I. MUTATIONS
A mutation is a __change_ in the DNA sequence.
The mutation may be classified as a: __chromosome__or __gene__ mutation.
Some mutations are harmful, but most are beneficial.
A. Gene Mutation
Gene mutations affect only __one_ gene on an individual chromosome. Gene mutations that result
in a change to only one or a few nucleotides are known as __point__ mutations.
There are 2 types of point mutations.
1. Substitutions
· One nucleotide in a gene is _____ replaced ______with a different nucleotide.
· May have little or no effect, OR may have a drastic effect.
· Ex: DNA triplet ACA, which codes for codon __UGU__ is changed to ACG, which now codes
for codon __UGC__. What is the change to the amino acid? _ None, Cys is still coded for _.
The result is no change to the protein, so this mutation is known as a __silent___ mutation.
· Ex: DNA triplet ACA, which codes for codon __UGU_ is changed to ACC, which now codes for
codon _UGG_. What is the change to the amino acid? ___ Cys is not coded for… new
codon codes for Trp ____. Even though the codon still codes for an amino acid, it is not the
right amino acid, so it does not make the right sense. This leads to a ___ non-functional ____
protein. This type of mutation is known as a ___Missense____ mutation.
· Ex: DNA triplet ACA, which codes for codon __UGU_ is changed to ACT, which now codes for
codon _UGA_. What is the change to the amino acid? ___ Cys is not coded for… new
codon codes for STOP ____. This stops the translation of the rest of the mRNA, which leads
to a _ non-functional ___ protein. This type of mutation is known as a _Nonsense_ mutation.
2. Frameshift Mutations
Frameshift mutations involve the __deletion__ or __insertion__ of a __nucleotide__in DNA.
· An insertion or deletion of a nucleotide causes a ___shift___ in the ___reading__ of the
remainder of the codons; therefore, the ___translation_ of the remainder of the mRNA is
_altered__. This will usually result in __tremendous__changes in the polypeptide chain
and completed__protein___.
B. Chromosomal Mutation
Chromosomal mutation involves the __number__ or ___structure_ of the ___entire__
chromosome. (Affects all genes on that chromosome!)
1. These errors generally occur during meiosis or ___mitosis___.
2. May result in a ___number__ disorder (extra chromosome or deleted chromosome), or
__insertion, deletion, or translocation__of a piece of chromosome and the corresponding genetic information.
3. Examples: Inversion, Translocation, Nondisjunction, duplication
C. Causes of Mutations
Some mutations are chemical mishaps that arise spontaneously, while others are caused by
environmental factors.
1. Mutagens – Environmental agents such as x-rays__ and _____gamma___ rays that may cause
a mutation in the DNA sequence.
2. Carcinogens - __cancer__ causing agents such as asbestos, ___benzene___(found in
detergents, plastics, and paints), and other pollutants.
3. Practice:
Normal DNA: T A C T T C A A A C C G A T T
mRNA: ______
Amino acids: ______
Mutated DNA – Type 1: T A C T T C A A A C C A A T T
mRNA: ______
Amino acids: ______
Type of gene mutation: ______
Effect on amino acid sequence: ______
Mutated DNA – Type 2: T A C T T A A A C C G A T T
mRNA: ______
Amino acids: ______
Type of gene mutation: ______
Effect on amino acid sequence: ______
Mutated DNA – Type 3: T A C T T C A A T C C G A T T
mRNA: ______
Amino acids: ______
Type of gene mutation: ______
Effect on amino acid sequence: ______
II. SEXUAL REPRODUCTION & MEIOSIS (pp. 270-276)
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____.
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. A View of Meiosis:
III. 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
IV. 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.
V. MONOHYBRIDS – Crosses considering __one __ trait at a time.
**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: ______
VI. 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 height and seed color. In this cross, tall is dominant to short, and yellow seed color is dominant to green seed color.
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: T = tall, t = short; Y = yellow seed, y = green seed
Cross: ______
Gametes: ______
Genotype ratio: ______
______
Phenotype ratio: ______
______
C. 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…
D. 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: ______
______
E. GGRr X Ggrr What are the phenotypes of the parent fruit flies?
______
Key: G = gray body R = red eyes
g = black body r = black eyes
Cross: ______
Gametes: ______
Genotype ratio: ______
Phenotype ratio: ______
______
VII. 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?
Key: ______
Cross: ______
Genotype ratio: ______
Phenotype ratio: ______
Now, cross two pink snapdragons and see what the offspring look like.
Key: ______
Cross: ______
Genotype ratio: ______
Phenotype ratio: ______
B. Codominance – Both alleles _share____ dominance and are always
_expressed___ if present. For example, in andalusian 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?
Key: ______
Cross: ______
Genotype ratio: ______
Phenotype ratio: ______
Now, cross two black and white colored chickens and see what the offspring look
like.
Key: ______
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___