Chapter 12: Mendel and Heredity Study Guide
Section 1 – Origins of Hereditary Science
A. Vocabulary – Use your textbook to define the following terms.
1. Cross breeding – to breed or mate 2 individuals.
2. Characters (Characteristics) – physical features that are inherited.
Ex: hair color, shape of nose.
3. Trait – several possible forms of a character/characteristic.
Ex: Hair Color - brown, blonde, red hair color
4. Self-pollinate – occurs when a plant fertilizes itself; pollen + egg from
the same plant.
5. Cross-pollinate – when pollen is transferred to another plant of the same
species by insects (bees, butterflies, moths), birds (hummingbird), wind
(can transfer pollen), water.
6. Monohybrid Cross – cross involving one pair of traits. Ex: flower color in
peas – purple vs. white flowers.
7. Dihybrid Cross – cross involving two pairs of traits. Ex: flower color
(purple vs. white flowers) & plant height (tall vs. short) in peas.
8. Generation – group of offspring from a given group of parents.
Ex: Our grandparents represent the 1st generation, parents are the 2nd
generation & we are the 3rd generation.
9. Parental/P Generation – the first 2 individuals that mate in a genetic
cross. Ex: Grandparents
10. F1 Generation/First Filial Generation – the offspring from the P
generation. Ex: Our parents
11. F2 Generation/Second Filial Generation – the offspring from the F1
generation. Ex: Us
12. Allele – Each version of a gene. Ex: brown, blonde, red hair color
13. Dominant – the allele that is expressed in the phenotype (physical
appearance – what you can see). Ex: purple pea flowers (PP or Pp)
REMEMBER: the dominant trait masks/hides the recessive when the 2
are together. (T=tall t=short; Tt tall dominates over short)
14. Recessive – the allele that is not expressed/seen in the phenotype when
the dominant allele is present. Ex: white pea flowers (pp)
15. Phenotype – the outward physical appearance of the organism; what you
see when looking at the organism. NOTE: written as words; purple
flowers
16. Genotype – the set of alleles/genes an organism has for a particular trait.
Ex: PP or Pp both produce purple flowers in peas. NOTE: written as
LETTERS
17. Ratio – a comparison of several numbers. Ex: 50 : 1 (50 to 1 odds of
something happening). NOTE: written as NUMBERS
18. Homozygous – describes an organism that carries 2 identical/same alleles
for a particular trait. Ex: AA or aa
19. Heterozygous – describes an organism that carries 2 different alleles for
a particular trait. Ex: Aa
20. Punnett Square – graphic model used to predict the possible offspring
from a genetic cross.
21. Probability – the likelihood or chance that a specific event will occur,
expressed in mathematical terms. Ex: The probability of flipping heads
on a coin is 50:50
B. Mendel’s Breeding Experiments
1. Who was Gregor Mendel & when did he live?
Austrian monk who lived from 1822 - 1884. Studied math & science in
college. Father of Genetics.
2. What plants did he work with? Pea plants (from 1856 – 1863)
C. Features of Pea Plants
1. How many traits (features) did Mendel’s peas have? 7 traits
2. Why is the garden pea a good plant to study?
a. Easy to grow & take care of.
b. Reproduce quickly; can study many generations.
c. Have contrasting traits that were easy to distinguish.
d. Easy to breed; able to self-pollinate or cross-pollinate
e. Produce lots of offspring in one growing season.
f. Short life-span; easy to study several generations in a short time.
3. What is a hybrid? Offspring of parents with different traits.
Example: AA x aa  Aa is a hybrid of the parents.
D. Mendel’s First Experiments
1. When Mendel crossed a pure/homozygous purple-flowered plant with a
pure/homozygous white-flowered plant, what type of offspring did he
get? All of the offspring were purple in the first (F1) generation.
2. When Mendel allowed the offspring from the above cross to breed, what
color flowers did he get?
Offspring in F2 generation- some purple (3) & some white (1)
3.
E. Ratios in Mendel’s Results
1. Here are Mendel’s actual results of his experiments!
2. How is each of Mendel’s ratios similar? (HINT: Round each RATIO to the
nearest whole number) Every ratio is 3 : 1
Section 2 – Mendel’s Theory
Mendel came to three conclusions at the end of his experiments. These three conclusions
led to the development of a new science that studies heredity; GENETICS.
Explain each of the following:
1. Law of Dominance – The dominant gene/allele will always mask/hide the recessive
gene/allele when the 2 are together.
Ex: homozygous purple flower X homozygous white flower  100% purple flowers
PP x
pp 
Pp
2. Law of Segregation of Alleles – When 2 genes for the same trait are located on
different homologous chromosomes, they will separate during meiosis when gametes are
formed. Ex: Aa individual will produce half of their sex cells with the “A” allele/gene and
the other half will have the “a” allele/gene.
Aa
(organism)

A, a
(possible sex cells)
3. Law of Independent Assortment – Homologous pairs of chromosomes separate
during meiosis, when gametes/sex cells are formed. Ex: AaBb  AB, Ab, aB, ab
(organism)
(possible sex cells)
Section 3 – Modeling Mendel’s Laws
A. Using Punnett Squares
1. What does a Punnett square predict? Predicts the POSSIBLE offspring
that could be produced from the gametes of the 2 parents.
2. What does a Punnett square show (inside the boxes)?
POSSIBLE offspring
3. What is written along the top and sides of a Punnett square?
The sex cells that the male can produce (from meiosis) go along the
top and the sex cells that the female can produce (from meiosis) go
along the left side of the Punnett square.
4. What is probability?
The chance of something happening.
5. How can probability be expressed (think math)?
Expressed using numbers, percent, or fraction. Ex: 3:1 or 75%:25% or ¾ : ¼
B. Using a Pedigree
1. What is a pedigree and how is it used?
A pedigree is a chart used to show how a trait is passed from parent
to offspring over several generations.
Check out the pedigree of Queen Victoria’s family pedigree for hemophilia.
2. Define the term genetic disorder.
A disease or disorder that can be inherited. Ex.- cystic fibrosis
3. Why would a couple want to have a pedigree done?
To see whether or not they carry the gene for the genetic disorder;
lets them know whether or not their children could inherit the
disorder.
4. What does it mean when someone is a carrier?
The person is heterozygous (Aa) and “carries” the recessive gene in
her/his DNA. Parent may pass the recessive gene to their offspring.
5. Define the term albinism.
Disorder in which the person/organism does not have the gene that
enables them to produce the skin pigment called melanin.
6. What causes someone to have albinism?
The person inherited 2 recessive genes; does not have the gene which
would enable their cells to produce melanin.
7. Is albinism caused by a dominant or a recessive allele? Recessive allele.
8. What 3 aspects of inheritance can a pedigree help identify (what can a
pedigree tell you?
a. Whether or not the disorder is sex-linked (present either on the
X or Y sex chromosome, but not both)
b. Whether the inherited trait is dominant or recessive.
c. Whether the person is homozygous or heterozygous for the trait.
C. Sex-Linked Genes
1. Identify the 2 sex chromosomes.
X
and
Y
2. What is a sex-linked gene? A gene located on either the X or Y sex
chromosome, but not both. Most sex-linked genes are located on the
larger X sex chromosome.
3. Which sex chromosome holds fewer genes? Y Why? It is much
smaller than the X sex chromosome.
4. XNXn – represents a female with a dominant (N) allele on one of her X
sex chromosomes a recessive (n) allele on the other X sex
chromosome. This makes her a heterozygous carrier for the
recessive trait; she does NOT express the recessive allele in her
phenotype.
5. If a male (XnY) inherits the Xn chromosome from his mom & the
Y chromosome from his dad, the male will inherit the disorder; there
is no dominant allele on the Y sex chromosome to “mask” the recessive
allele.
6. Three sex-linked traits in humans are:
a. Colorblindness – X sex-linked genetic disorder in which a person
cannot distinguish between 2 colors such as red and green.
b. Hemophilia – X sex-linked genetic disorder in which a person’ blood
does NOT clot properly; a serious injury may cause them to bleed to
death.
c. Hairy Pinna – Y sex-linked trait where hair grows on the outside of
the ear. Only affects males; gene on Y sex chromosome.
7. Human males inherit the recessive allele for colorblindness and
hemophilia from their mother, who gives them their X sex
chromosome. Females don’t usually inherit these diseases because
they inherit two X sex chromosomes (one from mom and one from
dad); as the dominant allele on one of the X sex chromosomes
“masks” the recessive disease-causing allele. (See #4 above.)
8. Sex-Linked Trait Punnett Squares
a. Cross a female carrier for the hemophilia gene( XHXh )with a male
who has normal blood clotting (XHY); see Punnett square below.
What is the probability of having a male child with hemophilia?
50% chance of a male having hemophilia.
What is the probability of having a female child with hemophilia?
0% chance of a female having hemophilia; one daughter will be a
carrier for the gene (XHXh) .
b. Cross a female carrier for the hemophilia gene( XHXh )with a male
who has hemophilia (XhY); see Punnett square below.
What is the probability of having a male child with hemophilia?
50% chance of a male having hemophilia.
What is the probability of having a female child with hemophilia?
50% chance of a female having hemophilia.
Section 4 – Beyond Mendelian Heredity
A. Many Genes, Many Alleles – not all characteristics follow Mendel’s
pattern of inheritance known as complete dominance; other patterns of
inheritance include polygenic inheritance, incomplete dominance,
multiple alleles, and codominance.
1. Polygenic inheritance – occurs when several genes work together to
produce a particular trait. Examples: eye, skin, hair color and height
in humans.
2. Incomplete dominance – pattern of inheritance where both alleles
produce a third phenotype. Example: Cross a red snapdragon flower
(rr) with a white snapdragon flower (ww) produces a pink snapdragon
flower (rw), a third phenotype. If you cross 2 pink snapdragon
flowers rw x rw will produce some
¼ red flowers (rr), ½ pink flowers (rw), ¼ white flowers (ww).
3. Multiple alleles - genes that have 3 or more possible alleles, but the
individual only carries 2 of the alleles. Example in humans is blood
type. Identify the 3 genes for the ABO blood groups: A,B,O or
IA,IB,i
a. How many blood types are there? Four
b. Identify the blood types based on their genotypes:
* IAIA (AA) or IAi (AO) = blood type
* IBIB (BB) or IBi (BO) = blood type
* IAIB (AB) = blood type
* ii (OO) = blood type
A
B
AB
O
4. Codominance - occurs when both alleles for the trait are expressed in the
phenotype. Example: The AB blood type in humans; the IA allele and the IB
allele work are each expressed to give the AB blood type. In animals,
codominance produces Andalusian chickens; cross a black rooster and a white
hen and the offspring have black & white feathers.
5. Explain how codominance & incomplete dominance differ from one
another. With codominance the offspring has both alleles expressed
in the phenotype; with incomplete dominance the 2 alleles give a
blended phenotype.
B. Genes Affected by the Environment
1. Name 2 environmental conditions that can affect the phenotype of an
organism. Nutrients and Temperature
2. Explain, in detail, how temperature affects fur color of the Arctic
fox. How does this benefit the fox? During the summer, the warmer
temperatures activate genes in the fox’s skin cells to produce a dark
pigment causing the new fur to grow in dark. Allows fox to blend in
with grass or woods and hide from prey. Colder autumn and winter
temperatures cause the genes to stop producing pigments and the new
fur grows in white (no color). Enables fox to blend in with the snow.
3. Explain, in detail, how human height may be affected by their
environment. Child may inherit genes for tall height, but if they don’t
receive proper nutrients then cell division can’t happen as often as it
should, resulting in shorter height.
C. Genes Linked Within Chromosomes – the chromosomes of organisms may
carry several hundred to several thousand genes. The closer genes are to
one another on the same chromosome the less likely they are to be
separated by crossing over than genes that are farther apart. Genes,
and the traits they code for, that are close together on the same
chromosome are said to be linked.