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Genetics
DNA
(Deoxyribonucleic Acid)
• DNA: stores and passes on genetic
information from one generation to the
next
• James Watson and Francis crick
discovered the structure of DNA
Chromosomes
(made up of)
↓
DNA
(contains)
↓
Genes
Structure of DNA
• DNA looks like a twisted ladder, or
spiral staircase called a double helix
• The “steps” are formed by pairs of
substances called nitrogen bases
• Nitrogen bases contain the element
nitrogen
Structure of DNA
• The four types of nitrogen bases
1. Adenine (A)
2. Thymine (T)
3. Cytosine (C)
4. Guanine (G)
• In the DNA ladder, A always pairs with T
C always pairs with G
Structure of DNA
• The genetic code is actually the
sequence of nitrogen bases on the
DNA molecule
• Any change in the order of bases can
result in a new gene that determines a
completely different trait.
DNA Replication
• Cells divide into two identical cells in
the process called mitosis
• Before the cell can divide, the DNA
must be replicated so that each cell
gets the same amount of DNA as the
parent cell.
• Replication= the process in which DNA molecules
form exact duplicates.
The Steps of Replication:
1. The DNA ladder separates, along the bases
2. Free nitrogen bases that are floating in the
cytoplasm begin to pair up with the bases on
each half of the DNA.
3. Two new DNA molecules form
GENETICS
• Genetics = The study of how traits are
inherited through interactions of alleles
• Heredity = the passing of traits from
parents to offspring
• Traits = characteristic of an organism
controlled by genes
Question: What are some examples of
traits?
• Purebred = an organism that always
produces offspring with the same form
of the trait as the parent
• Hybrid = an organism that has two
different alleles for a trait
GREGOR MENDEL’S PEA
EXPERIMENT
Mendel’s Results of Experiment
1. Factors that control each trait exist in pairs.
One parent contributes one factor, while
the other parent contributes the other
factor.
2. One factor can hide or masked by the other
factor
Examples:
• Dominant trait = a trait that always shows up
in the organism when the allele is present.
• Recessive Trait = a trait that is masked or
hidden, whenever the dominant allele is
present. (A recessive allele will only show
up if the organism does not have the
dominant allele.)
• Gene = a piece of hereditary
information on a chromosome
Example
• Alleles = different forms of a trait that a
gene may have
Example
USING SYMBOLS IN GENETICS
• A dominant allele is represented by a
capital letter
Example: Tall plants = T
• A recessive allele is represented by a
lowercase letter
Example: Short plants = t
• Homozygous Dominant = when a plant
inherits two dominant alleles = TT
• Homozygous Recessive = when a
plant inherits two recessive alleles = tt
• Heterozygous = when a plant inherits
one dominant allele and one recessive
allele = Tt
• Genotype = an organism’s genetic make-up
or allele combinations
Example: Genotype for plant height = Tt
• Phenotype = an organism’s physical
appearance, or visible traits
Example: Phenotype for Tt = tall plant
PUNNETT SQUARES
• Probability = the likelihood that a
particular event will happen
• Gregor Mendel was the first scientists
to realize that the principles of
probability can be used to predict the
results of genetic crosses
• Punnett Square = a chart that shows
the possible combinations of
genotypes that can result from a
genetic cross
• Scientists use a punnett square to
figure out all possible genotypes that
can result when crossing to organisms.
Types of Crosses
• Monohybrid Cross compares only one trait
• Dihybrid cross compares two traits
B= Brown Feathers
b= Yellow Feathers
Results =
• Ratio of Genotypes:
How often would expect to see brown
chicks?
Convert this fraction to a percentage:
Lemmings: Crossing Aa x Aa
Pedigree Charts
• Pedigree charts trace a genetic trait in
a family tree.
• It shows the presence or absence of a
particular trait in each generation.
GENETICS ODDS AND ENDS
2 Important Laws of Genetics
• Organisms inherit genes in pairs, one gene
from each parent
1. Meiosis= gene pairs separate during the
sex cell formation (egg or sperm cell)
2. Independent Assortment = each gene pair
for a trait is inherited independently of the
gene pairs for all other traits
Incomplete Dominance
• Incomplete dominance = neither gene
pairs mask the other
• Each gene is co-dominant with the
other
Mutations (a change in a gene or
chromosome) can be…
• Harmful
For example: sickle cell anemia
• Beneficial
For example: Naval oranges
• Single mutation occurred in 1820
• Seedless and bigger than regular
oranges
Sex-Linked Traits
• XX = female
XY = male
• X chromosomes: carry genes for traits other than
sex
• Y chromosomes: carry little to no traits, other than
those for maleness
• Therefore any gene- even a recessive genecarried on the X chromosome will produce a trait
in a male who inherits the gene. This is because
there is one gene to express it
• Question: Are sex-linked traits carried on the X or
Y chromosome?
Examples of Sex-Linked Traits
• Colorblindness- cannot see the
difference between certain colors,
such as red and green
• Loss of function of cone system