Heredity Cheat Sheet
Alleles: Alternative forms of a gene
Autosomal chromosome: A nonsex chromosome
Chromosome: A linear or circular strand composed of DNA
that contains genes
Diploid: An organism with two copies of each chromosome
DNA: Deoxyribonucleic acid; the molecule that carries
genetic information
Dominant: A phenotype or allele that completely masks
the presence of the other, recessive allele in the
heterozygote
Gene: The fundamental unit of heredity; a specific
section of DNA within a chromosome
Genotype: The genetic makeup of an individual; the
allele(s) possessed at a given locus
Heterozygote: An individual with two different alleles
of a given gene or locus
Homozygote: An individual with two identical alleles of
a given gene or locus
Locus: A specific location on a chromosome
Phenotype: The physical characteristics of an
individual
Recessive: A phenotype or allele exhibited only when
homozygous
The Structure of DNA
DNA is made up of long chains of nucleotides. To make a
complete DNA molecule, single nucleotides join to make
chains that come together as matched pairs and form long
double strands. Each nucleotide is comprised of the
following:
A five-sided (pentose) sugar called deoxyribose
A phosphate
One of four nitrogen-rich bases: adenine,
guanine,cytosine, or thymine
Nucleotides are joined together by phosphodiester bonds.
Nucleotide chains are antiparallel and complementary.
Unifying theory:
DNA->mRNA->amino acids->proteins
Replication (copying of DNA)
Transcription (message is copied into mRNA)
Translation (message in mRNA is converted to an amino
acid sequence at the ribosomes [rRNA] using tRNA)
Mendel's Laws of Inheritance
Genetic inheritance boils down to three simple concepts
put forth by Gregor Mendel, a humble monk and part-time
scientist who founded the entire discipline of genetics:
Segregation: In diploid organisms, chromosome pairs
(and their alleles) are separated into
individual gametes (eggs or sperm) to transmit genetic
information to offspring.
A dominant allele completely masks the effects of a
recessive allele. A dominant allele produces the same
phenotype in heterozygotes and in homozygotes.
Independent assortment: Alleles on different
chromosomes are distributed randomly to individual
gametes.
Genetics Cheat Sheet A = dominant a = recessive
Genotypes: Actual letters representing alleles on genes
AA = homozygous dominant
Aa = heterozygous
aa = homozygous recessive
Phenotypes: outer appearance
AA or Aa = dominant trait is expressed
aa = recessive trait is expressed
Pedigree: family tree
Square: boy
Circle: girl
── : mating
│
: offspring Colored: have trait
Punnett Square: show all possible outcomes of a mating
Sex-Linked: examples: hemophilia, colorblindness
h
Male with trait: X Y
H
Male without trait: X Y
h
h
Female with trait: X X
H
h
Female carrier: X X
H
H
Female without trait: X X
Incomplete Dominance: the heterozygous form is a
mixture of the dominant and recessive trait
AA: dominant trait (red)
Aa: mixture (pink)
aa: recessive trait (white)
Co-Dominance: the heterozygous is a combination of both
dominant traits; no recessive trait
st
RR: 1 dominant trait (red)
RR’ : Both dominant trait (roan-both red/white)
nd
R’R’ : 2 dominant trait (white)
Blood Types: an example of codominance with a recessive
A B
A A
A
I I : AB blood
I I or I i : A blood (AO or AA)
B B
B
ii: O blood (OO
I I or I i : B blood (BO or BB)
Typical Crosses
Homozygous dominant (AA) x homozygous recessive (aa):
100% of offspring are heterozygous (Aa)
Heterozygous dominant (Aa) x Heterozygous dominant (Aa)
25% AA/50% Aa/25% aa or a 1:2:1 ratio
Heterozygous dominant (Aa)x homozygous recessive (aa):
50% Aa/ 50% aa (half show recessive trait) or 1:1
Dihybrid crosses: AaBb x AaBb
9:3:3:1 (9/16 will have show both dominant traits, 1/16
will have both recessive traits)
Mitosis/Meiosis “Cheat” Sheet
Biological Reproduction and Cell Division
Biology is all about life. And life is really all about
continuation, and in order for life to continue,
reproduction has to occur at the cellular level so that
genetic information (DNA) is replicated. The following
concepts (mitosis and meiosis) are key to understanding
how cells reproduce:
Cells do mitosis when they are going to make an exact
copy of themselves for asexual reproduction, growth, or
tissue repair. DNA replication occurs once, followed by
a single division. The parent and daughter cells are
both diploid, which means they have a double set of
chromosomes.Cells do meiosis in order to produce
gametes (eggs and sperm) for sexual reproduction. DNA
replication occurs once, followed by two divisions. The
parent cell is diploid, but the daughter cells are
haploid, which means they have half the number of
chromosomes as their parent cells.
Mitosis: Mitosis is used to duplicate cells or for the
repair of damaged cells and growth of the organism.
Mitosis occurs most in the cells in organs (such as the
skin, stomach) that get the most wear.
It produces 2 genetically identical cells that have 2
full sets of chromosomes and are therefore diploid.
One set of chromosomes came from each parent – this is
the number of chromosomes in all somatic cells (body
cells).
The cell goes through 1 cycle of the 5 phases of mitosis
Prophase – where DNA condenses for form visible
chromosomes.
Metaphase – where the chromosomes line up in the center
of the cell
Anaphase- where the chromosomes start to separate
Telophase – where the two cells begin to separate
Cytokinesis - where the two cells complete their
separation and begin to grow
During S phase of the cell cycle (but not a part of
mitosis) the DNA is replicated
During interphase (also, not part of mitosis) the cell
is performing its function for the organs – Cells spend
most of their time in interphase
Meiosis
For production of gametes, the sex cells, sperm and egg,
meiosis produces 4 cells that are genetically different.
Each gamete has only 1 set of chromosomes (1N). The
gametes are haploid. During fertilization, the egg and
sperm join and create a zygote that has the full 2 sets
of chromosomes.
In meiosis, the cell goes through 2 cycles of the 5
phases of mitosis.
In Prophase I small pieces of DNA may cross over from
one chromosome to the other – this is called crossingover. During crossing-over, there is increased genetic
recombination.
Throughout the first cycle (Prophase I – Cytokinesis I)
the duplicated chromosomes remain connected at the
centromere. At this point, the cells are haploid or 1N.
Prohpase II through Cytokinesis II are very similar to
these phases of mitosis, except there is no duplication
of the DNA –the duplicated chromosomes are separated
and the cell divides.