GRADE-8 SCIENCE
UNIT 3:
The Evolution of Life on Earth
*Course chapters #6-8
Chapter 6:
Chapter 7:
Chapter 8:
Heredity and Genetics
DNA and Reproduction
Evolution
Introduction
Every organism alive today comes from a long line of ancestors who reproduced
successfully in every generation. Reproduction is the transfer of genetic information
from one generation to the next. Heredity is the passing of traits from parents to
offspring. It can occur with mixing of genes from two individuals (sexual reproduction)
or it can occur with the transfer of genes from one individual to the next generation
(asexual reproduction). The ability to reproduce defines living things.
The process of reproduction drives changes in life through both the mixing and mutations
of genetic information from one generation to the next. Through the process of natural
selection, new traits that are helpful to survival within a particular environment are
passed on while new traits that are harmful to survival within a particular environment
tend to die out. Over long periods of time, this process changes the “face of life” on our
planet and the evolution of new species from old.
Course “chapter 6” correlates to part of chapter 5 from Holt Life Science textbook.
Ch.6 Study Guide
Topic 1:
The Mystery of Heredity
Read pages 114-116 (Holt Life Science)
1. The passing of traits from parents to offspring is known as
_______________________. A trait is a physical characteristic of an organism
that is largely determined by genetic information, such as eye or hair color. The
first scientists credited with major discoveries in the understanding of heredity
was __________________________ _____________________________.
2. He decided to limit his investigations to a single organism. He used
_________________ _______________________ because he was familiar with them
and they grow quickly. They are also able to both ______________-pollinate and
________________-pollinate.
3. True-breeding results from _____________ - pollination. In this case, the
offspring will always have the ________________ traits as the parent.
4. Cross-pollination results when pollen is carried by _____________________ or
_____________________ from one flower to another. This results in a mixing of
traits and produces offspring that are not identical to their parents.
5. A ___________________________ is a feature that has different forms in a
population. The different forms themselves are known as __________________.
Three characteristics of pea plants studied by Mendel were
_______________________, ________________________, and
__________________________.
tier 2&3 CROs:
CRO 1: Why are the patterns of inheritance not always clear?
CRO 2: (Research) Other than hair and eye color, identify three other traits that are
passed down through generations.
CRO3: Why were pea plants a good organism to use when investigating heredity?
CRO4: What does it mean to say that a plant is “self-pollinating”? Compare (and
contrast) this with “cross-pollination”.
Topic 2:
Mendel’s Experiments
Read pages 117-120
1. Mendel “crossed” plants with different characteristics, meaning he ___________pollinated them. The plants produced by this are called ________________________________________ plants. These offspring all had the same
________________ for each characteristic, even through the plants he crossed
had different ones.
2. The trait that appears in all first-generation offspring is called the
____________________ _________________. In the case of the pea-plant color,
it is the color ________________________. The trait that seems to disappear in
the first generation of offspring is called the __________________________
_______________.
3. In Mendel’s second set of experiments, he allowed the first-generation of
offspring to _______________ - pollinate. When he did this, he found that
_______ out of every 4 plants displayed the ________________________ trait.
4. The results of Mendel’s experiments can only be explained if each plant have
_______________ sets of instructions for each characteristic. Each parent would
then donate _________ set(s).
5. These sets of instructions are now known as ____________________. As a result
of cross-pollination in plants (or sexual reproduction in animals), offspring have
two forms of the same gene for every characteristic (one from each parent). The
different forms of a gene are known as ____________________.
Tier 2&3 CROs:
1. What is the difference between a trait and a characteristic? Give one example of
each.
2. Describe Mendel’s first set of experiments (procedure and outcome).
3. Describe Mendel’s second set of experiments (procedure and outcome).
4. Answer Critical Thinking question #9 on page 119.
5. Research: Identify three dominant traits (dominant alleles) and three recessive
traits (recessive alleles) in humans.
Topic 3:
Genetics
Read pages 120-125
1. An organism’s appearance (as based on its genes) is known as its
_________________________________. This depends on how the alleles are
expressed in an individual. Both alleles (for each trait) together form an
organism’s __________________________________. This can be thought of as
the entire genetic make-up of an organism.
2. The phenotype depends on the combination of alleles in the genotype AND the
probability of each allele being expressed. If a genotype consists of two dominant
alleles or two recessive alleles is said to be _________________________. The
phenotype of the organism has only one possibility (that of the dominant or
recessive allele respectively). This is what occurred in Mendel’s first experiment
since ___________________ is the dominant color and dominant alleles are
always expressed over recessive alleles in a genotype. It is only when both alleles
in a genotype are recessive that they are expressed (become the phenotype).
3. If a genotype consists of a combination of dominant and recessive alleles it is said
to be __________________________________. In this case, four combinations
of alleles are possible (dominant-dominant, dominant-recessive, recessivedominant, and recessive-recessive) but only _________ phenotypes are possible.
*This is what occurred in Mendel’s second experiment since each plant had both
alleles from cross-pollination in the first experiment. This explains why roughly
________ in four plants were the color ________________ (since the allele for this
color is recessive and both alleles in a genotype must be recessive to be expressed as
the phenotype).
4. During reproduction, each parent passes on _________ of its ___________ alleles
to its offspring. If the parent has two different alleles for a gene, each type has
a(n) __________________ chance of being passed on. The genotype of the
offspring results from a combination of alleles from the parents.
5. A _______________________ ______________________ is used to organize all
the possible combinations of genotypes based on the combination of alleles
passed on from the parents. This consists of ___________ boxes inside of a
square.
6. (A) If both parents both pass on a dominant allele, the offspring has a ________
% probability of a dominant trait phenotype and a ________ % chance of a
recessive trait phenotype.
(B) If both parents pass on a recessive allele, the offspring has a ________ %
probability of a dominant trait phenotype and a ________ % chance of a recessive
trait phenotype.
(C) If one parent passes on a dominant allele and the other a recessive allele, the
offspring has a ________ % probability of a dominant trait phenotype and a
________ % chance of a recessive trait phenotype.
Tier 2&3 CROs:
1. Red hair in humans is a recessive trait and is rare because both parents
must pass on the allele for red hair to their offspring for them to have red
hair. Describe why both parents, rather than just one, must pass on this
allele (use a Punnet square to illustrate). Must both parents have red hair to
have an offspring with red hair? Explain.
2. As it applies to CRO #1, describe the probability of having an offspring
with red hair if:
i. Neither parent has red hair but carries the allele for it.
ii. One parent has red hair and the other does not but carries the allele
for it.
iii. Both parents have red hair as their phenotype.
3. On page 125 in textbook, answer #3. Draw a Punnet square to support
your answer.
4. On page 125 in textbook, answer #7. Draw a Punnet square to support
your answer.
5. On page 125 in textbook, answer #8&9.
ALL TIERS- extension assignment
Visit the website genetics.thetech.org and select “online exhibits”, then select “what
color eyes will your children have?” Survey your parents and find out their eye color and
the eye color of their parents (your grandparents) if known. Enter your dad’s eye color in
the box “You” and your mom’s color in the box “Your mate”. Enter your grandparent’s
eye color from your dad’s side in the boxes “Your Mother” and “Your Father” and your
grandparents on your mom’s side in the boxes “Mate’s Mother” and Mate’s Father”. See
if this produces a genotype that matches your eye color and the probability of your actual
eye color (phenotype). Print out this page and submit.
Your eye color may not come up as a possible genotype. This does not necessarily mean
that mom and dad are not your biological parents but rather that other genes that
influence eye color have had an affect.
You may wish to have some fun and re-work this scenario for your children given the eye
color of your future husband or wife (hypothetically, of course).
Topic 4: Exceptions to the Rule
Read pages 124-125
1. Since Mendel’s discoveries, researchers have found that sometimes one trait or
allele is not completely dominant over another. When these alleles combine, each
has its own degree of influence in the overall phenotype (expressed trait). This
situation is known as _____________________ _______________________.
As an example, in the snapdragon the red-color allele and the white-color-allele
“co-dominant” and produce a flower color that is _________________.
2. In many cases one gene influences more than one ___________________. Also,
some traits are the result of several ___________________ acting together. An
example of the latter is in the case of many different
________________________ shades in people. These traits are referred to as
being “polygenetic”.
3. In addition to the genotype of an organism, many physical traits (phenotypes) are
also affected by the ______________________________. For instance, how tall a
tree grows depends on climate and how tall a person grows depends in part on
diet.
*There is much research underway to study the affects on the environment on how
genotypes are expressed. There is some evidence that genes themselves may be
altered by diet, exercise, exposure to radiation and pollution, drug use, etc.
ALL TIERS- extension assignment
Visit the website genetics.thetech.org and select “online exhibits”, then select “DNA
roulette”.
 Press “click to begin”. On a sheet of notebook paper, identify the 10 genetic
characteristics listed on the “choose a game” page and for each, identify what
amount (%) of the characteristic is genetic (the remaining % is based on
environmental factors).

For each characteristic, play the genetic roulette game. For each characteristic, list
the general population odds for a person of Northern European descent (from
round 2). Then, list the odds for the genotype of your person (from round 3).
 From the general population odds (round two of betting), determine the dominant
and recessive alleles involved. Then, from the odds for the genotype of your test
subject (round three), determine whether both, one, or neither of the person’s
parents likely had that characteristic. An example is shown below bitter-taste
perception:
o 99 % genetic ; 1 % environmental factors
o general population (northern european)= 75:25 for bitter taste perception
o “Sheila” DNA = 99% chance for bitter taste perception
 Dominant = bitter-taste perception (can taste PTC)
 Recessive= can’t taste PTC
o Sheila’s genotype possibilities= TT, Tt, tt (25%); based on odds she will
likely express the TT or Tt genotype.
o Parents = Both have Tt phenotype and thus both had bitter taste perception
(most likely).
You will need to copy numbers and bet quickly, as there is a time limit for each bet. You
can analyze data after playing the round. Do not complete for traits involving three or
more possible characteristics such as eye color.
“A decade ago, reading off one single human genome was a billion-dollar effort. Today,
the price tag is around $8,000 and falling fast; it will soon be feasible to sequence each
person’s complete DNA as part of routine medicine” – Elaine Mardis, co-director of the
Genome Institute at Washington University (St.Louis) in DISCOVER magazine January
2014.