Genetics
1. Gregor Mendel
a. Known as “The father of genetics” because of his studies of heredity – the passing of traits from
parents to offspring
i. Genetics is the study of heredity--of how traits are passed from parents to offspring
b. Conducted years of controlled experiments on pea plants
i. First Mendel allowed plants to pollinate naturally—he let bees spread pollen they way they
always have. These plants that exist this way are called “true breeding” plants. Mendel used
these plants in his experiments. He called these plants his “First Generation” plants.
ii. Mendel controlled which of these plants pollinated other plants and documented the
characteristics (flower color, seed color, seed shape) of the offspring plants.
1. This is called “Cross pollination”
a. To do this, Mendel removed stamens --the male/sperm/pollen producing parts
of the flowers of each plant. This ensured that no bees or butterflies would help
spread pollen.
b. Mendel used a paint brush to place pollen on the pistils-- female/egg/seed
producing parts of his flowers
i. The offspring from cross pollinating true breeding plants on purpose are
called hybrids. He called these hybrids his “Second Generation” plants.
ii. Mendel cross-pollinated hybrids and was able to predict the
characteristics of the offspring.
1. He used ratios to predict the frequency a trait would show up.
c. Mendel’s Conclusions
i. Two genetic factors control each inherited trait—these are called alleles.
1. One factor for each trait is contributed by the mother. These are on the genetic
material of the egg.
2. One factor for each trait is contributed by the father. These are on the genetic material
of the sperm.
ii. Dominant and recessive traits—depend upon the alleles for each trait.
1. Usually one genetic factor (allele) over-powers the other one (allele) even though both
are present.
a. Dominant Traits—block the effect of recessive traits. The recessive factor is still
present, but doesn’t show through. This means there are 2 dominant alleles or
1 dominant allele and 1 recessive allele present.
b. Recessive Traits—are blocked or hidden if a dominant factor is present.
i. Recessive traits only show up if there are no dominant traits to block
them. This means there are two recessive alleles present.
2. Understanding Inheritance
a. Traits in organisms are controlled by genetic factors called alleles.
i. One is contributed by the father. One is contributed by the mother. These are found in the
chromosomes contributed by the egg and the sperm.
1. The chromosomes combine in an offspring and pair up as homologous chromosomes.
a. Genes—a section on a chromosome that has genetic information for one trait.
b. Alleles control how that trait is expressed.
Skin color genes
Height genes
b. Alleles control how a trait is expressed. Represented by letters:
i.
Upper case letters represent dominant alleles and dominant traits.
R R
Lower case letters represent recessive alleles and recessive traits.
r r
ii. Genotype- The combination of 2 alleles that control a trait
RR
-- Homozygous dominant –Two of the same type of alleles for a trait
R r --Heterozygous—Alleles for a trait are different.
rr
-- Homozygous dominant –Two of the same type of alleles for a trait
iii. Phenotype- The way a trait is expressed. (The way it looks or shows up)
R R
Each parent contributed a dominant allele—therefore the offspring will show the
dominant trait.
The offspring can only pass down the dominant trait to its offspring.
R r
r r
One parent contributed a dominant allele and one parent contributed a recessive allele
—therefore the offspring will show the dominant trait.
The offspring can pass down either the dominant trait or the recessive trait to its
offspring.
Each parent contributed a recessive allele—therefore the offspring will show the
recessive trait.
The offspring can only pass down either the recessive trait to its offspring.
3.
Modeling and Predicting Inheritance
a. Punnett Squares
4.
Complex Patterns of Inheritance
a. Incomplete Dominance- The offspring’s phenotype is a combination of their parents’ phenotypes.
Parents
Offspring
b. Co dominance- When both alleles can be observed in an offspring’s phenotype
c. Multiple Alleles- Some genes are controlled by more than 2 alleles.
Ex. Human Blood Types
Blood Type A – Genotypes are AA-AO (IAIA – IA i)
Blood Type B— Genotypes are BB-BO (IBIB – IB i)
Blood Type AB- Genotype is AB
(IAIB)
Blood Type O- Genotype is OO
(ii)
d. Polygenic inheritance- When multiple genes determine the phenotype of a trait even though each gene
only has two alleles. There is a lot of variation in the phenotype because there is a lot of variation
among the chances for the type of allele combinations.
Ex. Eye color, height, weight, skin color in humans
DNA and Genetics
1. Structure of DNA—Model constructed by James Watson and Francis Crick who based
their work on the work of other scientists who used X-Rays to determine the
shape of DNA.
a. Double Helix—“twisted zipper” composed of groups of molecules called
i. Nucleotides—made of combinations of a sugar molecule, a phosphate molecule and nitrogen
bases.
The sugar and phosphate group make up the sides of the “zipper”.
The nitrogen bases bond to make the “teeth of the zipper”.
There are 4 nitrogen bases that pair off together:
A- Adenine
Only partner with each other
T- Thymine
G- Guanine
Only partner with each other
C- Cytosine
DNA is a macromolecule -- DNA stands for Deoxyribonucleic Acid
2. Replication of DNA
a. The process of copying a DNA molecule to make an identical DNA molecule.
i. Necessary for meiosis and mitosis
1. First the DNA zipper straightens out and then begins to “unzip” separating nitrogen
bases from their partners. New nucleotides latch on to the exposed bases. The result
of this is two identical DNA strands.
ii. Necessary for making proteins within our cells.
1. Proteins are extremely important. They are the instructions for all the activity and
functions of our cells. Without the ability to make proteins our cells could not function
and then we would die.
2. The instructions for making every type of protein are found on our DNA.
3. The DNA is used as a code to make RNA—
4. RNA is Ribonucleic Acid.
a. RNA is single stranded
b. RNA is made of nucleotides with some differences:
1. RNA has nitrogen base pairs but uracil replaces thymine.
Adenine pairs with uracil. Guanine still pairs with cytosine.
2. Ribose (sugar) replaces deoxyribose (sugar)
c. RNA is a type of nucleic acid that carries the code for making proteins from the
nucleus to the cytoplasm
A- There are 3 types of RNA that do different jobs in making proteins.
1. mRNA- messenger RNA
2. tRNA- transfer RNA
3. rRNa-ribosomal RNA
5. Protein Synthesis is a process that makes proteins in several steps:
a. Transcription—the process of making mRNA from DNA.
A- Occurs in the nucleus
B- DNA unzips and bases separate from their partners. Using the
exposed bases as a template, new mRNA nucleotides line up. This
copies the code for making proteins from DNA.
C- Complete strands of mRNA leave the nucleus and move into the
cytoplasm.
b. Translation—takes the copied code for making proteins from the mRNA and
makes sense of it. Proteins are made of smaller parts called amino acids.
A- Occurs in ribosomes
B- tRNA tries to match up new bases to the exposed bases of the mRNA.
This “cracks the code”. As tRNA matches up groups of 3 bases called
codons
1. Some codons signal for translation to begin. Others code for
translation to stop. Most code for amino acids.
C- Codons are linked together into an amino acid chain by rRNA. Chains
of amino acids build proteins.
Mutations- a change in the nucleotide sequence of a gene occurs if there is a problem with transcription or
translation. This changes the amino acid sequence so a different protein is made.
Mutations happen in 3 ways:
1. Substitution—Different base pairs are substituted for the correct ones.
2. Insertion—Extra bases are inserted into the sequence
3. Deletion—Base pairs are deleted from the sequence. Other base pairs move up to take
their place, changing the sequence of the codons.
Effects of mutations:
Negative - Cause genetic disorders such as Cystic Fibrosis or PKU
Neither negative nor positive –Albinism—no pigmentation changes the way we look, but not our
mental or physical abilities.
Positive- Some mutations can actually help an organism—such as a mutation that might allow
an animal to see better at night.