Life Science Assessment
Study Sheet – The Big Ideas & Essential Questions in Life Science
Part 2. Final Exam (from mid-term)
1. Introduction to Genetics – How are traits passed from parent to offspring?
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Gregor Mendel’s work was the foundation for understanding why offspring have traits
similar to those of their parents.
Mendel was the first scientist to interpret the data from his breeding experiments with pea
plants using the principles of probability. Probability is the likelihood that a particular
event will happen.
Traits are inherited physical characteristics that an organism has and can pass on to its offspring
through its genes.
A gene is a section of DNA that codes for the expression of a trait. (Actually, genes are like
“recipes” for proteins. The proteins are expressed as traits.) Mendel called them “factors.” We
now call these sections of DNA that control traits “genes.”
The order of DNA bases along a gene specifies the type of protein that will be produced.
The rod-shaped structures located in the nucleus of every cell (except red blood
cells) of an organism are the chromosomes. Chromosomes are made of twisted strands of
DNA – deoxyribonucleic acid. Chromosomes are made up of many genes joined together.
The names of scientists most closely associated with the discovery of the structure of DNA are
Watson, Crick, Franklin, and Wilkins.
An allele is a different form of a gene.
An allele whose trait always shows up in an organism when the allele is present is called a
dominant allele.
An allele whose trait is masked by a dominant gene is called a recessive allele.
An organism is homozygous for a trait if each allele for the trait is the same.
An organism is heterozygous for a trait if each allele for the trait is different.
A hybrid is the offspring of parents that have different alleles for a trait.
Dominant alleles are represented by upper case (capital) letters. For example, the allele for the trait
of tall height is represented by “T.”
Recessive alleles are represented by lower case letters. For example, the allele for the trait of short
height is represented by “t.”
The notation TT means two dominant alleles. The notation tt means two recessive alleles. A
pea plant that is homozygous for tall stems has the alleles TT. A pea plant that is
homozygous for short stems has the alleles tt.
The notation Tt means one dominant allele and one recessive allele. A pea plant that is
heterozygous of tall stems has the alleles Tt.
An organism's phenotype is its physical appearance. In the case of the organism whose alleles
are TT. The phenotype would be tall.
An organism’s genotype is its genetic make up. The above mentioned organism’s genotype
would be TT.
Crossing two individuals with identical or very similar sets of alleles is called inbreeding.
The goal of inbreeding is to produce offspring that are the same as the parents. (Every beagle
looks pretty much the same.)
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An organism that always produces offspring with the same form of a trait as the parent is
purebred.
Crossing two individuals with different sets of alleles is called hybridization. The goal of
hybridization is to get the best traits of the two parents in the offspring. (Mules combine the
best traits of a horse and donkey.)
A Punnett square is a chart that shows all of the possible combinations of alleles that can
result from a genetic cross.
When a homozygous dominant organism is crossed with a homozygous recessive organism,
all offspring will have the dominant trait. On the left is a Punnett square for a cross
between a homozygous black guinea pig (BB) and a homozygous white guinea pig (bb). On
the right is a Punnett square for a cross between two heterozygous (hybrid) tall pea plants
(Tt).
B
B
b
Bb
Bb
b
Bb
Bb
Four in four guinea pigs will be black.
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T
t
T
TT
Tt
t
Tt
tt
Three in four pea plants will be tall.
The chromosome theory of inheritance states that genes are carried from parents to
offspring on chromosomes.
Meiosis is the process that produces sex cells a.k.a. gametes (sperm and egg). During
meiosis, the number of chromosomes is reduced by half when chromosome pairs separate
and are distributed into new sex cells. During meiosis a cell undergoes two divisions to
reduce the number of chromosomes by half in the resulting sex cells.
During protein synthesis, the cell uses information from a gene on a chromosome to
produce a specific protein.
Protein synthesis takes place on the ribosomes in the cytoplasm of a cell.
Before protein synthesis can take place, a “messenger” must first carry the genetic code from
the DNA inside the nucleus into the cytoplasm. This genetic messenger is called ribonucleic
acid or RNA.
During protein synthesis, messenger RNA copies the coded message from the DNA in the
nucleus and carries it into the cytoplasm.
One the code has been carried into the cytoplasm, transfer RNA brings amino acids to the
ribosome to be assembled into a specific protein.
A mutation is any change in a gene or chromosome. Mutations can cause a cell to produce
an incorrect protein during protein synthesis. Mutations occur when one or more nitrogen
bases is substituted for another.
Some mutations are harmful; some are helpful; others are neither helpful nor harmful.
Albinism is an example of a mutation.
2. Modern Genetics: What have scientists have learned to do with genes?
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A variation is any difference between individuals of the same species. Human beings
show a great deal of variation.
Some human traits such blood type are controlled by a single gene that has more than two
alleles. The three alleles of the single gene that controls blood type are said to be multiple
alleles. Three alleles control the inheritance of blood type: A, B, and O. There are four main
blood types: A, B, AB, and O. The allele for A type blood is codominant with the allele for
blood type B. The allele for blood type O is recessive. For example, blood type A can be
produced by two alleles for A and also by one A allele and an O allele. So if both parents of a
child have type A blood, their child could possibly have A type blood or O type. This would
depend on whether the parents were AA or AO. If both parents were AO then a child could
possibly be OO –with one O allele inherited from each parent.
Some human traits show a large number of phenotypes because the traits are controlled by
many genes. The genes act together as a group to produce a single trait. Skin color, eye
color, and height are human traits controlled by many genes. Skin color is controlled by
three different genes; four genes control height. Various combinations of these genes control
the traits.
The effects of genes are often altered by the environment. For example, people’s diets can
affect their height.
The sex chromosomes determine if a person is male or female. The combination of two X
chromosomes (XX) results in a female offspring. The combination of XY results in a male
offspring.
Sex cells are called gametes. Male sex cells are called sperm. Female sex cells are called
eggs. Sex cells are haploid. They contain only half the number of chromosomes that a
body cell (autosome) contains. The full number of chromosomes is called the diploid
number.)
The process by which a sperm cell fuses with an egg cell is called fertilization.
A fertilized egg cell is called a zygote.
A karyotype is a photograph of an individual’s chromosomes. A doctor can look at the
chromosomes in the karyotype to determine the presence of genetic abnormalities.
An abnormal condition a person inherits through genes or chromosomes is a genetic
disorder. Genetic disorders are caused by DNA mutations during meiosis or changes in
chromosomes that are present in a parent’s sex cells.
A mutation in a sex cell can be passed from parent to offspring.
Genes on the X or Y chromosomes are often called sex-linked genes because their alleles
are passed from parent to child on a sex chromosome. Traits controlled by sex-linked
genes are called sex-linked traits.
Sex-linked traits are more common in males than females. Because males have only one X
chromosomes, males are more likely than females to have a sex-linked trait that is controlled
by a recessive allele. A recessive allele on the X chromosome will always produce the
trait in a male.
A carrier is a person who has one recessive allele for a trait and one dominant allele,
but does not have the trait.
People with an extra copy of chromosome 21 have a disorder called Down Syndrome.
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Amniocentisis is a technique by which a small amount of the fluid that surrounds a
developing baby is removed, and the cells found in the fluid are analyzed for genetic
disorders. A picture of the chromosomes from a cell is taken and scientists count and study
them.
A chart that tracks which members of a family have a particular trait is a pedigree.
Pedigrees are used to trace the inheritance of traits in humans.
All of the DNA in one cell of an organism is called a genome.
The goal of the Human Genome Project was to identify the DNA sequence of every of
every gene in the human genome. This project was completed in 2003.
Except for identical twins, all people have different DNA.
The transfer of a gene(s) from one organism to another is called genetic engineering.
Researchers are using genetic engineering to try to correct some genetic disorders. This
process is called gene therapy. Gene therapy involves inserting working copies of a gene
directly into the cells of a person with a genetic disorder.
DNA from samples of hair, skin, saliva, and blood can be used to identify a person. In DNA
fingerprinting, enzymes are used to cut a DNA sample into fragments. An electric current is
used to separate the fragments of DNA by size to create a pattern of bands that is unique
to every person. This pattern of bands is called a DNA fingerprint.
Cloning involves producing an organism that is identical to the organism from which it
is produced. Cloning results in two organisms that are genetically identical. (Cloning uses
the diploid nucleus of a body, cell which is inserted in an egg cell of a similar organism from
which the haploid nucleus has been removed.)
During the process of evolution, change in a species over time, only traits that are
controlled by genes can be acted upon by natural selection. Natural selection is the
process by which individuals that are better adapted to their environments are more
likely to survive and reproduce than other members of the same species.
3. Evolution: How have living things have changed over time?
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A species is a group of closely related organisms that can mate and produce fertile offspring.
(An organism is considered fertile if it can produce offspring.)
Any difference between individuals of the same species is called a variation.
An inherited characteristic that improves an organism’s chances to survive and reproduce
is called an adaptation.
Evolution occurs by means of a process called natural selection. Natural selection is the
process by which individuals that are better adapted to their environments are more
likely to survive and reproduce than other members of the same species.
Natural selection is affected by variation. The only traits that can be acted upon by
natural selection are those that are controlled by genes.
Evolution is the change in a species over time.
The theory of evolution that proposes that evolution occurs steadily in tiny changes over
long periods of time is called gradualism.
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The theory of evolution that proposes that species evolve during short periods of rapid
change is called punctuated equilibria.
The scientist whose name is most associated with the theory of evolution through natural
selection is Charles Darwin.
Only traits that are inherited (a.k.a controlled by genes) can be acted upon by natural
selection.
Living things are grouped based on their cell structure - depending on whether on not they
have a nucleus in their cells.
Other ways of grouping organisms include whether or not they can make their own food
(autotrophs vs. heterotrophs) and whether they are made up of one cell (unicellular/singlecelled) or multicellular).
The needs of living things include: energy, water, living space, and a stable internal
environment.
Scientists use fossil evidence, embryo development, homologous structures, and chemical
(protein sequences) evidence to show how and when evolution occurred.
Fossils are the preserved remains or traces of organisms that lived in the past.
Most fossils form when a dead organism becomes buried in sediment.
A fossil is the preserved remains or traces of an organism that lived in the past.
In sedimentary rock, the oldest fossils are usually found in the deepest layers.
Some fossils form when organisms become trapped in amber or tar or are frozen.
There are two ways of determining the age of fossils. Relative dating provides scientists
with a approximate age of a fossil. Relative dating is used to determine which of two fossils
is older. The most important evidence when using relative dating is the position of the fossil
in sedimentary rock layers.
Absolute dating is used to determine the exact actual age of a fossil. Absolute dating
involves the use of radioactive elements, which have a known half life. The half life of a
radioactive element is the time it takes for half of the atoms in a sample to decay. Scientists
can compare the amount of the radioactive element in a sample to the amount of the element
into which it breaks down.
The millions of fossils that scientists have collected are called the fossil record.
Almost all of the species preserved as fossils are now extinct. A species is extinct if no
members of that species are still alive.
The Geologic Time Scale is a kind of “calendar’ of Earth’s history that spans more than 4.6
billion years. It is divided into smaller units called eras and periods.
The largest span of time in the Geologic Time Scale is Precambrian Time. It covers the first
4 billion years of Earth’s history.
In addition to examining fossils, scientists use similar body structures, development before
birth, DNA sequences and protein structures to determine the evolutionary relationships
among species.
The more similar the DNA sequences of two species are, the more closely related they are.
Homologous structures are body parts that are similar in design but used for different
purposes. (Ex. horse leg, human arm, bat wing, whale flipper) Homologous structures show
that related species have inherited these structures from a common ancestor.
If two organisms look very similar during their early stages of development, this is evidence
that the organism evolved from a common ancestor.
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Branching trees are charts that show evolutionary relationships among animals. On a
branching tree, a species is more closely related to another species on the same branch than
to another species on a separate branch.