Name _____________________________________________________
Test Date _Tues, 2/1____
UNIT 10 - 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. 275-278)
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 characteristics___ 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. Formation of Gametes
Gametes are formed in meiosis – a special type of cell division that only occurs in the _ovaries___
of females and _testes__ of males. In meiosis, DNA is replicated once in _S__ of _interphase___, but
the cell goes through two cell divisions, resulting in _4__ cells with _1/2___ the original chromosome
number.
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. 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 _4_____
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. 263-270)
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, pureline____ 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-pollination_______.
B. Mendel’s Results
1. P generation – Crossed _pureline____ plants with one trait with _pureline___ 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 _trait seen in F1______.
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____.
2. For each gene, an organism inherits two alleles, one from each _parent______. Alleles are
different forms or _possibilities______ of a _gene_____.
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___. Seed shape? _round___
Genotype = _RR_____; Phenotype = _round____
b. Heterozygous for pea seed shape is written _Rr__. Seed shape? _round_____
Genotype = _Rr____; Phenotype = _round____
c. Homozygous recessive for pea seed shape is written _rr___. Seed shape? _wrinkled___
Genotype = _rr___; Phenotype = _wrinkled_______
III. ANALYZING INHERITANCE
(pp.135-137)
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 pea plant and a homozygous recessive green pea pea
plant.
Key: _________________________________________
Cross: ________________________________________
Genotype ratio: _________________________________________
Phenotype ratio: ________________________________________
V. DIHYBRID CROSSES
(pp. 270-271)
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.
A. 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: ___________________________________________________________
B.
Key: G = gray body
g = black body
R = red eyes
r = black eyes
Cross: GGRr X Ggrr

What are the phenotypes of the parent fruit flies? _________________________________
Cross: __________________________________________
Genotype ratio: _________________________________________________________________________
_____________________________________________________________________________________
Phenotype ratio: ________________________________________________________________________
______________________________________________________________________________________
VI. A CLOSER LOOK AT HEREDITY
(pp. 272, 273)
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, ____________________________________________________
Cross: ________________________________________
Genotype ratio: _________________________________________
Phenotype ratio: ________________________________________
Cross: ________________________________________
Genotype ratio: _________________________________________
Phenotype ratio: ________________________________________
B. Codominance – Both alleles _share___ dominance and are always _expressed___ if
present.
For example, ________________________________________________________________________
___________________________________________________________________________________
Cross: ________________________________________
Genotype ratio: _________________________________________
Phenotype ratio: ________________________________________
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 _two___ 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.
Suggested Study Questions
p. 283 (1-5, 11, 12, 17-20)