N o r t h E a st I n d ep en d e nt S c h o o l D i st ric t
8th Grade Science
Unit 3
Heredity & the Environment Spring 2006
HEREDITY & THE ENVIRONMENT
Heredity
Heredity is the passing of traits from parents to offspring. The study of heredity
is called genetics. The father of genetics is Gregor Mendel. He was an Austrian
monk that studied garden pea plants during the 1800’s. His work has contributed
to our modern understanding of one of the most pervasive areas of study in
Biology.
In 6th grade students learned that most cells have a nucleus that contains
chromosomes. They were also exposed to the fact that chromosomes contain the
genetic material passed from parents to offspring. Students were first
introduced to the concept of dominant and recessive traits in 7th grade. The
following statements regarding what students need to know about DNA are from
the 2005 TAKS information booklet: “Students need to understand that DNA
contains all genetic information but do not need to know details of DNA’s molecular
structure, which is taught in high school biology. Students should understand that
the nucleus of the cell contains the chromosomes, which are composed of DNA, but
that DNA can be inherited only through the sex cells (sperm and egg). “
This year the focus will be on alleles and making predictions based on different
allele combinations. One allele comes from the female organism and one allele
comes from the male organism. This is why children do not look exactly like either
parent. Some alleles are dominant while other alleles are recessive. The dominant
allele is the one that is observed. The recessive allele is the one that is covered
up. A trait that is controlled by being recessive can only be seen if both parents
contribute recessive alleles to the offspring.
An organism’s appearance is called its phenotype, and its genetic make-up is known
as the genotype. The terms homozygous and purebred are used to refer to an
individual having two of the same alleles for a genotype. The terms heterozygous
and hybrid are used to refer to an individual having two different alleles for a
genotype.
Clarifying statements are intended to deepen teachers’ understanding of science concepts and serve as a guide for
instructional design. They are not intended to serve as student instructional materials.
This publication is the property of North East Independent School District. Duplication in whole or in part, outside of NEISD, is
prohibited without express written permission from NEISD.
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N o r t h E a st I n d ep en d e nt S c h o o l D i st ric t
8th Grade Science
Unit 3
Heredity & the Environment Spring 2006
Using capital letters to represent dominant alleles and small letters to represent
recessive alleles, predictions can be made about possible outcomes of certain
crosses using a chart known as a Punnett square. For example, in Mendel’s garden
pea plant, the trait for purple flowers (P) is dominant over the trait for white
flowers (p). The letter P is used since it corresponds to purple, the dominant trait.
PP would be a homozygous purple pea plant whereas pp would be a homozygous
white pea plant. Pp would be a heterozygous purple pea plant.
In 6th grade students talked about selective breeding. This would be a good time
to make the connection between prior learning and the process of predicting
genetic outcomes. The 2005 TAKS information booklet states that students will
be held accountable for single-trait Punnett squares. Dihybrid crosses will be
encountered in high school biology. The following is an example of a problem
students should be able to complete at this level:
 Alleles:
 Cross: Pp X pp
 Punnett square:
Clarifying statements are intended to deepen teachers’ understanding of science concepts and serve as a guide for
instructional design. They are not intended to serve as student instructional materials.
This publication is the property of North East Independent School District. Duplication in whole or in part, outside of NEISD, is
prohibited without express written permission from NEISD.
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N o r t h E a st I n d ep en d e nt S c h o o l D i st ric t
8th Grade Science
Unit 3
Heredity & the Environment Spring 2006
 Phenotype Results: 50% purple and 50% white or 1:1 purple/white
 Genotype Results: 0% homozygous purple, 50% heterozygous purple, 25%
homozygous recessive or white
Given the number of individuals, students should calculate to apply the percentage
to a given number of offspring for any genotype. For example, students should be
able to calculate the following problem:
 What is the probability of obtaining heterozygous offspring from the cross
above (Pp X pp) if there are 8 offspring? = 4 out of 8 or 50%.
 If there are 16 offspring? = 8 out of 16 or 50%.
Given the genotypes of the offspring of a cross, students should also be able to
predict the genotypes of the parents. The following problem provides an example:
Students do not have to know which is the dominant trait in a situation. The
dominant and recessive forms of the trait have to be given. Students should
understand the results given in a Punnett square do not mean that the results
would be exactly that way. It does, however, tell what the probability is of having
that outcome. On TAKS, students may be asked to record these percentages as
griddable items.
Mendel discovered that not all organisms have traits that are either dominant or
recessive. For example if a red flower is crossed with a white flower and the
result is a pink flower, then Mendel called this phenomenon incomplete dominance.
Clarifying statements are intended to deepen teachers’ understanding of science concepts and serve as a guide for
instructional design. They are not intended to serve as student instructional materials.
This publication is the property of North East Independent School District. Duplication in whole or in part, outside of NEISD, is
prohibited without express written permission from NEISD.
3
N o r t h E a st I n d ep en d e nt S c h o o l D i st ric t
8th Grade Science
Unit 3
Heredity & the Environment Spring 2006
This means that each allele from each parent has equal influence. There are also
other traits that are determined by multiple alleles. This is important for
students to understand when questions regarding traits such as height, hair and
skin color arise.
The following chart contains some examples of traits in humans that are controlled
by a single pair of alleles:
DOMINANT TRAITS
RECESSIVE TRAITS
unattached earlobes
attached earlobes
straight thumb
hitchhiker’s thumb
Source: http://extension.usu.edu/aitc/teachers/pdf/heredity/comparing_traits.pdf
Source: http://extension.usu.edu/aitc/teachers/pdf/heredity/comparing_traits.pdf
can roll tongue
cannot roll tongue
freckles
no freckles
Source: http://extension.usu.edu/aitc/teachers/pdf/heredity/comparing_traits.pdf
Source: http://extension.usu.edu/aitc/teachers/pdf/heredity/comparing_traits.pdf
naturally rests left thumb on top when
hands are folded
naturally rests right thumb on top when
hands are folded
dimples
no dimples
Clarifying statements are intended to deepen teachers’ understanding of science concepts and serve as a guide for
instructional design. They are not intended to serve as student instructional materials.
This publication is the property of North East Independent School District. Duplication in whole or in part, outside of NEISD, is
prohibited without express written permission from NEISD.
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N o r t h E a st I n d ep en d e nt S c h o o l D i st ric t
8th Grade Science
DOMINANT TRAITS
Unit 3
Heredity & the Environment Spring 2006
RECESSIVE TRAITS
middigital hair
no middigital hair
widow’s peak
no widow’s peak
The Effect of Environmental Conditions on Traits
Environment may also influence characteristics. A person who is exposed to more
learning experiences may be more successful than a person that has not had those
experiences. Two people may have differing weights due to diet and exercise and
may not because of genetic inheritance.
Some organisms are affected by temperature. For example, the arctic fox’s fur is
brown in summer because pigments are made when the temperature is higher.
When the temperature drops, pigments to darken the fur are not made and the
arctic fox’s fur is white in winter. This is also clearly an adaptation that has
evolved to help the arctic fox survive by providing protective coloration for
different seasons. Any trait that improves the chances for survival is known as an
adaptation. The 7th grade student studied adaptations and received the first
introduction to the theory of natural selection.
Natural Selection
Species that have the most favorable traits are the ones that survive and
reproduce. This is known as the theory of natural selection. Charles Darwin
proposed the theory of natural selection in an attempt to explain why some species
have survived while others have not been successful. The theory has four main
Clarifying statements are intended to deepen teachers’ understanding of science concepts and serve as a guide for
instructional design. They are not intended to serve as student instructional materials.
This publication is the property of North East Independent School District. Duplication in whole or in part, outside of NEISD, is
prohibited without express written permission from NEISD.
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N o r t h E a st I n d ep en d e nt S c h o o l D i st ric t
8th Grade Science
Unit 3
Heredity & the Environment Spring 2006
points: overproduction, genetic variation, struggle to survive, and successful
reproduction.

overproduction: In an effort to allow a population to survive most organisms
will produce more offspring than is needed. In general, the more parental
care needed, the fewer the offspring. In some cases, more offspring are
produced to increase the odds of survival for species that are not well
protected by parents.

genetic variation: Among the many individuals of a single population of
species there exists variation. A great example of how this can help a
species survive is the example of the peppered moths of England. Within
the population some of the moths were black, some were white, and others
were gray. Before the Industrial Revolution, most of the tree barks were
white and the white moth was camouflaged and protected from extensive
predation by birds. However, after the Industrial Revolution began, coal
soot made the bark of the trees very dark. Had all the moth’s been white
then the species might have become extinct. The grey and black moths were
now camouflaged, protected from extensive predation, and the moths lived
on. To illustrate this concept, many teachers conduct an activity utilizing
moths cut out of newspaper and black construction paper. The moths are
laid down on a background of newspaper and a student is given a short
amount of time to pick up as many moths as possible. When the moths are
counted, more black moths should have been “preyed upon” than the
camouflaged newspaper moths.

struggle to survive: All species compete for food, water, air, and habitat (a
place to live). Those species that have the adaptations best suited to the
environment are the ones most likely to survive.

successful reproduction: Individuals do not have to reproduce in order to
survive, however, some individuals in a population have to reproduce or the
species will die out. The organisms that are best adapted are going to be
Clarifying statements are intended to deepen teachers’ understanding of science concepts and serve as a guide for
instructional design. They are not intended to serve as student instructional materials.
This publication is the property of North East Independent School District. Duplication in whole or in part, outside of NEISD, is
prohibited without express written permission from NEISD.
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N o r t h E a st I n d ep en d e nt S c h o o l D i st ric t
8th Grade Science
Unit 3
Heredity & the Environment Spring 2006
the ones who reproduce and this allows the species to continue its survival in
nature.
Natural selection is the process that may lead to evolution and speciation.
Evolution is the process of change in a species over time. Adaptations are
examples of evolution on a small scale. Genetic variation may lead to differences in
one population of a species. If this population becomes different enough,
speciation may occur. Speciation is the formation of a new species which is now
reproductively isolated from those they could have once mated with. The following
is an example of how this occurred in a squirrel population:
Factors that Contribute to Extinction
Extinction of a species happens when all members of that species die out. The
fossil record shows that over time there have been many mass extinctions.
Probably the most well known is the death of the dinosaurs at the end of the
Clarifying statements are intended to deepen teachers’ understanding of science concepts and serve as a guide for
instructional design. They are not intended to serve as student instructional materials.
This publication is the property of North East Independent School District. Duplication in whole or in part, outside of NEISD, is
prohibited without express written permission from NEISD.
7
N o r t h E a st I n d ep en d e nt S c h o o l D i st ric t
8th Grade Science
Unit 3
Heredity & the Environment Spring 2006
Mesozoic Era. Scientists estimate that there are about 5 million species on earth.
Extinction is a natural process that has occurred since the beginning of life.
Usually, this process occurs very slowly and only affects a small number of species.
However, today plants and animal species are dying out at an alarming rate. The
world is a healthier place when there is a balance among species. Biodiversity or a
variety of species leads to a more stable environment.
The following are examples of how human events may contribute to extinction:





Habitat loss and destruction - probably the #1 reason on earth today
Pollution - any addition to the environment that hurts the life of a species,
including smog, noise, pesticide usage etc.
Over-consumption and/or unregulated hunting - when man over hunts or
fishes an area and this action depletes the area of its species
Foreign animal trade – capturing and/or killing species for profit
Introduced species (invasive, non-native, alien) - bringing in an unknown
species to a new environment and then the new species begins to take over
the food and habitats or niche of the endangered species
The following examples are a result of extinctions due to natural occurrences:




Climate change such as an ice age
Geologic changes such as volcanic eruptions
Large asteroid strikes Earth and triggers world–wide habitat changes
Catastrophic events such as hurricanes
Students should also understand that negative impacts on the environment from
human activity does not necessarily lead to extinction. According to the TAKS
information booklet, students should understand “that there may be degrees of
consequences.” They should know “there may be a more gradual process that allows
for recovery, as with the American bald eagle.”
Clarifying statements are intended to deepen teachers’ understanding of science concepts and serve as a guide for
instructional design. They are not intended to serve as student instructional materials.
This publication is the property of North East Independent School District. Duplication in whole or in part, outside of NEISD, is
prohibited without express written permission from NEISD.
8
N o r t h E a st I n d ep en d e nt S c h o o l D i st ric t
8th Grade Science
Unit 3
Heredity & the Environment Spring 2006
Interactions within Ecosystems
The 8th grade student should be able to distinguish between inherited and learned
traits. In addition, they need to gain an understanding that some traits are
influenced by both heredity and the environment. A good example of this would be
an animal whose fur changes color in winter. The 8th grade student also looks at
interactions within the environment through very specific relationships. The
concepts of limiting factors and carrying capacity are studied for the first time.
An ecosystem represents all of the populations that live in an area and the abiotic
factors that influence these populations. Each species needs food, habitat, and
water. Those resources that are needed by a population are called limiting factors.
Limiting factors determine how large a population can be. The term carrying
capacity refers to the largest numbers of individuals in a population that an
ecosystem can support. If the population gets too large, then lack of food, water,
or habitat becomes an issue and usually some of the population dies out. Organisms
compete for earth’s resources. The ones who get them survive and the ones who
don’t get them die out. The following chart shows the normal ebb and flow in
population size once the carrying capacity is reached:
Clarifying statements are intended to deepen teachers’ understanding of science concepts and serve as a guide for
instructional design. They are not intended to serve as student instructional materials.
This publication is the property of North East Independent School District. Duplication in whole or in part, outside of NEISD, is
prohibited without express written permission from NEISD.
9
N o r t h E a st I n d ep en d e nt S c h o o l D i st ric t
8th Grade Science
Unit 3
Heredity & the Environment Spring 2006
Interactions between organisms within an ecosystem can be illustrated to show
energy transfer in the form of a food chain or food web. Students were first
introduced to food chains in 2nd grade and have revisited this concept in the years
since. This would be an appropriate time, however, to insure students understand
the food chain and web are models of energy transfer from one organism to the
next. They should also understand that all food chains must begin with a producer
which is reliant on radiant energy. Food webs at this level should include
decomposers. Students should be able to identify the consumers as primary,
secondary, etc. and whether they are herbivores, carnivores or omnivores.
According to the 2005 TAKS information booklet, food chains and webs should
include “examples from Texas or the southwestern United States in addition to
global examples.”
The following food chain is from an aquatic ecosystem. Students can expand on
their understanding of ecology by being exposed to examples of chains and webs
from diverse ecosystems.
The following food web is an example from the grasslands of Africa. When
students look at food webs such as this, they should be reminded that diagrams
are examples of models. Whenever models are addressed in the classroom,
Clarifying statements are intended to deepen teachers’ understanding of science concepts and serve as a guide for
instructional design. They are not intended to serve as student instructional materials.
This publication is the property of North East Independent School District. Duplication in whole or in part, outside of NEISD, is
prohibited without express written permission from NEISD.
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N o r t h E a st I n d ep en d e nt S c h o o l D i st ric t
8th Grade Science
Unit 3
Heredity & the Environment Spring 2006
students should be asked to identify limitations of that model (per TEKS 8.3C). A
hint at a limitation can easily be noted in this example through the title which
names the web as “partial,” meaning that not all organisms in the ecosystem are
represented. Another example of a limitation would be that this web is not
representative of all African grasslands.
Clarifying statements are intended to deepen teachers’ understanding of science concepts and serve as a guide for
instructional design. They are not intended to serve as student instructional materials.
This publication is the property of North East Independent School District. Duplication in whole or in part, outside of NEISD, is
prohibited without express written permission from NEISD.
11
N o r t h E a st I n d ep en d e nt S c h o o l D i st ric t
8th Grade Science
Unit 3
Heredity & the Environment Spring 2006
Predation: When one animal eats another animal for its food then that animal is
known as a predator. The animal that gets eaten is called the prey. There has to
be a balance between the predator/prey relationship for the environment to be
stable. The following graph shows the normal changes in population sizes that
occur as a result of predator – prey interaction:
Predators may have speed, good eyesight, sharp claws, sharp teeth just to name a
few adaptations that help predators capture their prey. The animals that are
considered prey also have adaptations that help them to escape being eaten. Some
of these adaptations might be camouflage or coloring that helps the prey blend
into the environment, bright colors which usually tells the predator to stay away
because the prey is poisonous. Some populations live in great numbers to protect
each other. Anything that helps the prey to escape being eaten would be
considered an adaptation.
Symbiosis: When two organisms live in close proximity and establish a relationship
by which at least one of them benefits, this is known as symbiosis. There are
three types of symbiosis, including mutualism, commensalism, and parasitism.
Clarifying statements are intended to deepen teachers’ understanding of science concepts and serve as a guide for
instructional design. They are not intended to serve as student instructional materials.
This publication is the property of North East Independent School District. Duplication in whole or in part, outside of NEISD, is
prohibited without express written permission from NEISD.
12
N o r t h E a st I n d ep en d e nt S c h o o l D i st ric t
8th Grade Science
Unit 3
Heredity & the Environment Spring 2006

Mutualism: In a mutualistic relationship, both organisms help each other
without harm coming to either one. The clown fish and sea anemone provide
an excellent example of this type of relationship. The clown fish is coated
with a mucous that protects it from the stinging tentacles of the anemone.
When the clown fish is between these tentacles, it is protected from
predators. At the same time, the clown fish helps to keep the anemone
clean by eating debris off of it. Another example can be found when
examining the relationship between fruit-bearing plants and birds that eat
that fruit. As the bird consumes the fruit, the seeds contained within can
not be digested and are passed in fecal matter to lie and grow into fruit
trees. In this relationship, the bird benefits from the meal and the plant
benefits from the aid in reproduction.

Parasitism: In a parasitic relationship, one organism is helped and the other
organism is harmed. The organism that is harmed is called the host and the
organism that does the feeding is the parasite. Fleas are example of a
parasite that harms the host by biting, sucking blood, and causing the host
to itch. The fleas are provided with food and a warm environment, but the
host is made very uncomfortable. Tapeworms that live in the digestive
tracts of animals are also parasites. They deprive their host of necessary
nutrients by consuming some of the food they have taken into their
digestive tract. Parasites will usually harm their host, however they usually
do not cause death as this will in turn remove food and/or habitat from the
parasite itself.

Commensalism: In this relationship, one organism is helped but the other
organism is not harmed. The remora, or sucker fish, living on a shark and
feeding off of scraps from its meals without harming the shark is a classic
example of commensalism. Another example is the barnacle which attaches
to other organisms, such as whales. The barnacle is a filter feeder that will
benefit from the place to live in which water is moving past it. The whale is
presumably not bothered by the barnacle.
Clarifying statements are intended to deepen teachers’ understanding of science concepts and serve as a guide for
instructional design. They are not intended to serve as student instructional materials.
This publication is the property of North East Independent School District. Duplication in whole or in part, outside of NEISD, is
prohibited without express written permission from NEISD.
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