Teacher Background – 7.14AC Heredity
A resource document which gives teachers relevant and essential background knowledge on
the science concept being addressed.
Objective(s):
7.14A Define heredity as the passage of genetic instructions from one generation to the
next generation.
7.14C Recognize that inherited traits of individuals are governed in the genetic material
found in the genes within the chromosomes in the nucleus.
Foundation:
Students have previously been introduced to the basic concepts of heredity by examining and
being aware of observable traits, such as eye color in humans or shapes of leaves in plants.
Such shared characteristics are different from learned behaviors, such as table manners or
learning a language. Students have likely also explored the basic concept of a cell and that it
contains a nucleus. They may even be aware that each human cell has 46 chromosomes, with
all of a person’s DNA organized into two sets of 23 chromosomes.
During this grade level, students will begin to get more in-depth in their understanding that
constructs called chromosomes contain the DNA for these traits and that traits, such as eye
color, are passed from one generation to the next by each parent contributing a set of
chromosomes to an offspring. This is why children look similar to their parents. Furthermore,
which set of chromosomes gets inherited from each parent is random. This is why siblings born
from separate pregnancies look similar but not identical, and why identical twins are just that,
because they actually do both carry the same inherited sets of chromosomes. Essentially, the
DNA provides the instructions or recipe for “building” an offspring, using the blueprint provided
by the combination of the two individual parents.
Heredity is not merely observed within single species, however. Mapping the human genome,
as well as that of other species, has provided insight into how different species are related to
each other. Not only have mammals inherited traits such as mammary glands and hair from a
common ancestor, for example, but also about 75% of known human disease genes have a
recognizable match in the genome of fruit flies. This infers that humans and fruit flies also share
some common ancestry.
Students will also learn the difference between genotype and phenotype. A genotype is the
genetic makeup of an organism, while the phenotype is a description of how that genotype is
expressed in the organism’s morphology and physiology. Furthermore, a genotype for a trait
often includes two variations that are referred to as a dominant allele and a recessive allele.
When both a dominant allele and a recessive allele are present for a trait, the dominant allele
will mask the recessive allele’s expression of the trait. Only when two copies of the recessive
allele are present – one from each parent – is the recessive form expressed. This concept is
especially easy to understand when examining phenotypic traits that are controlled by single
genes. The ability to roll your tongue or the presence of a Widow’s Peak hairline are examples
of dominant expression of traits that scientists believe are controlled by a single gene. If a
person’s DNA that controls hairline shape contains both the dominant allele (Widow’s peak
hairline) and the recessive allele (straight hairline), or a heterozygous state, then the person’s
phenotype will show a Widow’s Peak hairline. Individuals who have two recessive alleles, or a
recessive homozygous state, for the trait will have a straight hairline.
However, not all traits are controlled by single genes. Most inherited traits are controlled by a
combination of multiple genes. This fact makes genetic research especially complicated when
trying to figure out how defects and risks for disease are configured into a person’s genotype.
Surveying generational data, ongoing work on mapping genomes, and other studies continue to
further our understanding of how heredity works and how medical professionals can predict, and
possibly curb, health risks.
Wild animal and plant populations, of course, also demonstrate how traits are inherited. There
are eight genetic lineages of felines, for example. Lions, leopards, panthers, servals, cheetahs,
pumas, and mountain lions, all share genetic traits inherited from a common ancestor. Genetic
similarities can easily be observed between cat species, including teeth, nose, hair, feet, and tail
characteristics. Wild populations can suffer, however, when their numbers are reduced.
Inbreeding can occur, which results in low genetic variation and often causes what are typically
recessive, deleterious traits to show up in the phenotypes in successive generations of
offspring. Such growing homozygosity in recessive traits is observable in the Florida Panther,
for example. Abnormal phenotypic traits include kinked tails and severe birth defects.
The principles of inheritance are also studied and applied in domestication of wild species.
Artificial selection, or selective breeding, has produced a variety of livestock breeds and plant
types that boost human population survival and growth, while pet breeds provide comfort and
companionship. For hundreds of generations, humans have bred together two individuals with
desirable traits in order to enhance those desirable traits in their offspring. Unfortunately, also
due to the principles of inheritance and the nature of chromosomes and their contained DNA,
not all of the desirable traits can be teased from non-desirable traits. A hybrid plant that may
produce a high seed yield may also have a higher vulnerability to disease, for example.
Desirable traits in a dog breed may also be accompanied by a higher risk of hip dysplasia.
The educational groundwork done at this grade level is crucial for understanding the more
complex concepts in higher grade levels. More importantly, perhaps, is that these basic
concepts of heredity are applicable to multiple real-world fields and applications, such as in our
everyday casual observations and conversations, medicine, wildlife conservation, and food
supplies.