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Concept Sheet
Heredity and Variation
LS. 13: The student will investigate and understand that organisms reproduce and transmit genetic information to
new generations.
1. The nucleus of each cell contains DNA, deoxyribonucleic
acid. This DNA is coiled up into rod-shaped structures called
chromosomes. The DNA contains coded instructions that sore
and pass on genetic information from one generation to the next.
Every cell in an organism contains the same DNA code to make
an exact copy of the organism. The DNA in the nucleus is
copied and passed on to the new cells during mitosis.
The Watson and Crick model of DNA is also called a double
helix. It is composed for two strands of sugar and phosphate
molecules that make up the sides of the “twisted ladder”
structure of DNA. The “rungs” are four different nitrogen bases:
guanine (G), cytosine (C), adenine (A) and thymine (T). The
changes to order of these four nitrogen bases creates all
Remember:
the variation we observe in the world around us.
In DNA: C always pairs
with G and A with T.
2. Chromosomes are the strands of tightly wound DNA
In RNA, thymine is
found in the nucleus of each cell. Humans have 46
replaced with uracil, so
chromosomes (or 23 pairs). Sections of the
the base pairs are C-G
chromosomes that code for a particular trait
and A-U.
(characteristic) are called genes. Genes can be made up
of one or more alleles, one or more forms of a gene.
These alleles can either be dominant or recessive. The combination of these alleles determines what traits will
show in an organism.
Sometimes during cellular replication, errors happen and the order or type of nitrogen base pairs. This is called a
mutation. Most mutations go unnoticed, but a few can cause problems. If these mutations happen in meiosis,
which forms sex cells, then the mutations can be inherited by the next generation.
3. When discussing traits there are two things to look at. The first is the genotype. This is the combination of
alleles, represented by letters. A dominant allele is represented with a capital letter and a recessive allele is
represented with the lowercase of the same letter. The second way to discuss traits is their phenotype, which
talks about the physical appearance of the trait.
For example, in pea plants smooth seeds is a dominate trait over wrinkled
seeds. Looking at the genotypes, you can see there are three combinations of
alleles: SS, Ss and ss. Then if you look at the phenotype, there are two
potential results: smooth seeds or wrinkled seeds.
4. Heredity is the passing of traits from one generation (parents) to the next generation (offspring). Traits are either
dominant or recessive. A dominant trait will always appear in the phenotype, regardless of the other allele. A
recessive trait will only appear in the phenotype when combined with another recessive trait.
If the genotype has two of the same alleles, it is called homozygous. If they are both dominant alleles it is called
homozygous dominant, whereas if they are both recessive alleles it is called homozygous recessive. If there is
one of each allele present, that is called heterozygous.
In our pea example earlier. The dominant allele is for smooth seeds. So genotypes SS and Ss appear as smooth.
The first is homozygous dominant, the second is heterozygous. The only time we see the wrinkled pea is when
there are two recessive alleles, homozygous recessive. This is why offspring may look different than the parent,
the recessive trait may be hidden by a dominant trait or the child may have inherited one recessive allele from
each parent.
5. Traits are either inherited or acquired. Inherited traits are created by the genes you get from your parents.
Inherited traits include things like eye color, height, natural hair color, shape of your earlobe, etc. There are also
acquired traits that come from influences and/or interactions with your environment. Acquired traits include
language, preferred foods, sports ability, music preference, school subjects you enjoy, piercings, length of your
hair, etc. While people say things like “being musical runs in our family”, they are not actually talking about
heredity. Musicality, being something that is important to the family, is encouraged in each family member,
which causes the skill to be practiced more than it might otherwise.
6. A Punnett square is a tool used to predict the outcome of specific traits using probability. Each box in the
Punnett square represent a potential combination of genes in an offspring, it does NOT represent number of
offspring. If the same genes were to recombine again and create a second offspring, it would have the same
probability of outcomes as the first offspring. The most basic Punnett square looks at the potential outcome of
one gene.
In mice, black fur is dominant over white fur. We’ll represent black fur
with a B and white fur with a b (it is important to choose the same letter
in cases of complete dominance). If we take two heterozygous black
furred mice (Bb) and mate them. What would the potential genotypes
and phenotypes of the resulting offspring.
To set up our Punnett square, we draw our 2x2 box. Each parent can give one of
their alleles, so we place one of the father’s alleles over each small box and one
of the mother’s alleles next to each small box. Then we fill in the potential
combinations. This allows us to see that there is a 75% chance the offspring will
have black fur (25% chance for it to be homozygous black furred and 50%
chance for it to be heterozygous black furred) and only a 25% chance that it will
be white furred.
7. Creating organisms with desirable traits is something humans have been doing for thousands of years. The oldest
form of this is known as selective breeding. The process of selective breeding is how we developed most of our
modern fruits and vegetables, as well as our domestic dogs and cats. Humans looked for desirable traits and bred
together individuals that exhibited those traits until we got the desired result.
For example, broccoli is the flower head of a plant. In the wild, the flower head is relatively small and not very
compact. To increase volume of edible parts, people selected broccoli
plants with larger, denser flower heads and bred them together. They
continued this for many generations until we got to the recognizable
broccoli we buy from the store.
Another way to create an organism with desirable traits is through genetic engineering. This technique was
developed shortly after we became to understand DNA and how it works. Instead of breeding together
individuals with desired traits and hoping the trait we want will become more prominent, scientists insert/replace
a segment of DNA from one organism into the DNA of another organism. In this way, a cell of an organism can
be made to produce a desired trait it doesn’t normally have or to eliminate an undesirable one. This is used to
help improve taste, color, texture, nutritional value, plant yield, or to make organisms more resistant to drought,
disease and other environmental hazards.\
We also create organisms with desirable traits through cloning. Cloning is the process of making a second
organism that is genetically identical to the original organism. Cloning does happen naturally in nature with
many types of asexual reproduction and in the case of identical twins, however in the lab humans have never
been attempted to be cloned. One Potential application of cloning include creating genetically identical organs
and tissues for victims of trauma. Having the replacement organs/tissues be the genetic clone of the persons own
organs/tissues with significantly reduce the risk of the body rejecting the replacements. Other potential
applications relate to medicine development and disease prevention
8. As our understanding of heredity grew, so did our desire to be able to better understand human genetics. In
October 1990, the Human Genome Project (HGP) began. Its goal was to map and sequence human DNA as
well as study the ethical, legal and social issues that relate to a better understanding of
human’s genetic makeup. Completed in 2003, two years early, the HGP mapped all of the
30,000 genes on the 23 pairs of human chromosomes. This information is helping scientists
and doctors identify, prevent and treat inherited disorders such as cystic fibrosis, muscular
dystrophy and cancer. The Human Genome Project has been compared to the Apollo moonlanding project as an outstanding scientific achievement of humankind.
Watch any crime show and you’ll see how we already use genetic information to assist us with
a crime solving. DNA fingerprinting is the process of matching the genetic information of a
piece of evidence, like a hair or blood sample, to an actual person. Just like our fingerprints are
unique to us, our DNA is unique to each individual (except with identical twins). The use of
DNA fingerprinting has helped our judicial system more effectively determine who is guilty
and who is innocent. Some wrongly accused individuals from past cases have later been proven
innocent through the review of evidence using DNA fingerprinting.
With our increased understanding of genetics also comes concerns about potential discrimination due to a
person’s genetics. Scientists and lawmakers have worked together to pass laws to help protect individuals from
genetic discrimination from work places and insurance companies through things like the federal Genetic
Information Nondiscrimination Act. For example, people who have inherited a harmful mutation to their BRCA1
and BRCA2 genes are at a much higher risk of developing breast and/or ovarian cancer. There are tests a person
can take to see if they have this gene, however, these laws protect them from having to share information about
their genetics with their jobs and most insurances, so it cannot be used against them in decision making
situations.
9. Gregor Mendel, the Father of Genetics, was an Austrian monk born in 1822. Using pea
plants, he was the first to study and explain how traits were passed from one generation to
the next.
Rosalind Franklin was the first scientist to research the complex structure of the DNA
molecule. Her x-rays of the DNA molecule were used by James Watson and Francis Crick to develop the
“double helix” model of DNA we use today.
(There is a great deal of controversy
over how much of who actually
discovered the structure of DNA first,
due to conflict between Franklin and
Maurice Wilkins, a peer researcher in
the lab Franklin worked. Wilkins is
who showed Franklin’s x-rays to Watson. Upon seeing the
x-rays, the structure of DNA became clear to him. Watson
and Crick beat Franklin to publication and later, with
Wilkins, received a Nobel Prize in 1962 for the double helix model of DNA).