2nd Marking period Study Guide
1. What do we know about the structure of DNA? Who were the key players in figuring out its structure,
and what, specifically, did they contribute?
James Watson and Francis Crick are credited with discovering that the DNA molecule is a supercoiled double
helix. They were able to do this partly because of the xray photographs of DNA crystals taken by Rosalind
Franklin and Maurice Wilkes.
2. T/F: Every cell spends at least part of its life going through cycles of cell division. False. Some cells
break out of the cell cycle and become highly specialized, thereby losing the ability to reproduce. They
spend their entire lives in G1, simply doing their jobs.
3. What is the relationship between a codon and an anticodon? A codon is a sequence of 3 nucleotides
located on an mRNA molecule that was transcribed from DNA. The anticodon is the 3 nucleotide
sequence on the tRNA molecule that is complementary to the mRNA strand and a specific amino acid.
4. What is metastasis? The spread of cancer from its site of origin.
5. Differentiate between tumor-suppressor genes and oncogenes. Oncogenes are mutant genes that lack
the ability to regulate cell division. Tumor-suppressor genes have the ability to regulate cell division, but
they can become mutated and become cancerous.
6. Differentiate between genotype and phenotype. Genotypes are the actual genes present in an
organism. In genetics problems, these are represented by capital and lower case letters. Phenotype is
the appearance of an organism based on genotype, and are described using adjectives (tall, purple, etc.)
7. Why is the genetic code said to be universal to all living things? All organisms ever discovered have
genetic material composed of the same nitrogen bases: adenine, guanine, thymine, cytosine and uracil.
8. Describe the most common kinds of gene mutations, and identify which type causes each of the
following conditions: Down syndrome, PKU, sickle-cell anemia, albinism, Turner’s syndrome,
Klinefelter’s syndrome, polyploidy
Down syndrome, Klinefelter’s syndrome and Turner’s syndrome are all chromosomal mutations (the
wrong number) because of nondisjunction during meiosis.
PKU is a point mutation to the PAH gene. An enzyme needed to break down the amino acid
phenylalanine is not able to be made, causing phenylalanine to build up in the body. If it gets too high, it
will be toxic and cause brain damage.
Sickle-cell anemia is also a point mutation that occurs on one or more of the HBB genes responsible for
producing hemoglobin. The hemoglobin in the red blood cells is distorted, causing the RBCs to become
sickle shaped, and inefficient oxygen carriers as a result.
Polyploidy is a condition in which there are multiple sets of homologous chromosomes present in a cell.
It is more common in plants, causing them to be larger in size. This is usually due to nondisjunction
during meiosis. In animals, the cause may be the mating of two organisms with different numbers of
chromosomes, and results in hybrid offspring which are sterile. The offspring will also have a different
number of chromosomes than either parent.
9. Differentiate between sex-linked and sex-influenced traits. Give an example of each. Sex-linked traits
are located on the X or Y chromosome (usually the X, as there are more genes on it), such as
colorblindness. Sex-linked traits create different patterns of inheritance due to the fact that men are
hemizygous for the X chromosome (have only one). Sex-influenced traits are not necessarily on the sex
chromosome, but are influenced by it somehow, such as with hormones. Pattern baldness is a sexinfluenced trait. Both genders may have it, but it is exhibited differently between men and women
10. Who was Gregor Mendel? A European monk/scientist who is considered the father of modern genetics.
11. Differentiate between each of the following:
Complete dominance also called Mendelian inheritance, as he discovered this type of heredity. In diploid
organisms (has 2 copies of each gene for a trait), one form of the trait (called an allele) completely masks the
other form (recessive) if it is present. An example of this would be the round seed shape in Mendel’s
experiments.
Incomplete dominance Inheritance in which neither allele is completely dominant over the other, resulting
in a heterozygote that has a different phenotype than either parent, and the phenotype is somewhere in
between both heterozygotes. An example would be in certain flowers, where one homozygote is red, the
other homozygote is white, and the heterozygote is pink.
Multiple alleles Also called codominance, where there may be several alleles for a trait (but an organism can
only have 2 of them), and both are fully expressed. An example would be human blood type, type AB. Both
the A gene and the B gene are fully expressed.
Sex-linked inheritance genes located on the X or Y chromosome, usually the X
12. State Mendel’s Laws of Heredity.
The Law of segregation states that “pairs of factors is separated, or segregated, during the formation of
gametes.”
The Law of independent assortment states that “factors for different characteristics are distributed to
gametes independently.” Simply put, even though certain traits are frequently seen together (blond
hair & blue eyes, for example), they are not inherited together.
13. What is a karyotype? What can it tell us? A “picture” of all the chromosomes in a cell. It can tell us the
gender of the individual, and if the correct number of chromosomes is present.
14. Differentiate between diploid and haploid cells. Use humans as an example. Diploid cells contain 2
copies of each autosome, and 2 sex chromosomes. Haploid cells contain only one copy of each
autosome, and 1 sex chromosome (in humans, this is the X or the Y…this is why fathers determine what
gender a baby is!)
15. How do large DNA molecules become small enough to fit in the nucleus of eukaryotic cells? The strands
are wrapped around histone and nonhistone proteins, then tightly coiled into a helix (known as the
double helix)
16. What is the purpose of a chromatid? A kinetochore? It carries one copy of the chromosome’s DNA after
replication. The 2 chromatids (sisters) are held together at the centromere. As mitosis progresses, the
spindle fibers, which are attached to the kinetochore (a protein disk on the centromere) separate the
two copies.
17. What is a gamete? A zygote? A gamete is the technical name for a sex cell (egg or sperm in mammals,
although things like pollen are also gametes). This haploid cells unite to form a single diploid cell called
a zygote, which will mature into an organism.
18. What is the difference between binary fission and mitosis? Binary fission is a form of asexual
reproduction that occurs in bacteria and some protists. It is similar to mitosis in that the DNA is copied,
then divided between the two new cells, but only superficially. In binary fission, specialized structures
such as the centrioles and spindle fibers are not present. The cell simply copies its DNA, doubles to
approximately its original size, the divides into equal cells. Mitosis usually involves more than one
chromosome, and requires an intricate series of stages to make sure the end result is two identical cells
PUNNETT SQUARES: You will need to be able to—
Identify the genotypes and/or phenotypes of parental crosses and offspring
Correctly recognize homozygous and heterozygous genotypes
Complete a cross, given the genotypes of the parents
Give correct ratios of phenotypic probabilities
For both monohybrid and dihybrid crosses. You need to be able to show how Mendel arrived at his expected
phenotypic ratios for both of these.
19. Show the monohybrid cross from Mendel’s F1 cross (and state the genotypic and phenotypic
probabilities). Where both parents are tall parents (T) who carry the recessive trait for shortness (t).
P generation
Tt x Tt
Gametes possible
T, t (for both parents)
Punnett square
You will need 4 boxes in your punnett square
Genotypic results
1 TT : 2Tt : 1tt
Phenotypic results
3 tall, 1 short
20. Show Mendel’s dihybrid cross for Tall (T) plants with purple (P) flowers. White is recessive to purple in
flower color. For dihybrid crosses, only phenotypic results are necessary.
P generation
Gametes possible
Punnett square
Phenotypic results
TtPp x
TtPp
TP, Tp, tP, tp (the same for both parents.
You will need 16 boxes in your punnett square
9 tall plants with purple flowers
3 tall plants with white flowers
3 short plants with purple flowers
1 short plant with white flowers
21. Show the gametes that could result from a parent with the genotype RrYyPp.
RYP
RYp
RyP
Ryp
rYP
rYp
ryP
ryp