What is Heredity?
• Heredity is the passing of genetic material from
parents to offspring
• Gregor Mendel – an Austrian monk who began to
study the inheritance traits of pea plants in the
monestary’s garden
• He studied seven characteristics of pea plants
• A characteristic is a feature that has different forms
• A trait is the different form of the characteristic
• Mendel studied true – breeding plants – plants that
will always produce offspring with a certain trait if
allowed to self pollinate
Mendel’s Findings
• All the pea pods produced from the first generation
were green
• Mendel called the green pea pod trait the dominant
trait
• Mendel called the yellow pea pod trait the recessive
trait
• Mendel allowed this first generation of plants to self
– pollinate
• The second generation of pea pods produced ¾
green peas pods and ¼ yellow
• Mendel showed that plants could still pass on the
yellow trait even when it didn’t show up
How Are Traits Inherited?
• Genes are segments of DNA found in chromosomes
that give instructions for producing a certain
characteristic
• All offspring have two versions of the same gene
• The different versions are called alleles
• These genes are represented by letter symbols
• The capital letter means the allele is the dominant
allele
• The lower case letter means the allele is the recessive
allele
• An organism with two dominant or recessive alleles is
homozygous for that trait
• An organism with one dominant and one recessive
alleles is heterozygous for that trait
Genes Influence Traits
• The combination of alleles that you inherited from
your parents is your genotype
• Your observable traits make up your phenotype
• Some traits are dominant over others
• The dominant allele contributes to the phenotype if
one or two copies are present in the genotype
• The recessive allele contributes to the phenotype
only when two copies of it are present
• If one chromosome in the pair has a dominant allele
and the other contains a recessive allele, the
phenotype is determined by the dominant allele
• This shows complete dominance – one trait is
completely dominant over another
Many Genes/One Trait
Many Traits/One Gene
• Some characteristics such as eye color are the result
of several genes acting together
• Different combinations of alleles can result in
different shades of eye color
• Many traits do not have simple patterns of
inheritance
• Sometimes one gene influences more than one trait
• A single gene causes the tiger to have white fur
• The same gene influences the blue eye color
• Many genetic disorders are linked to a single gene
but affect many traits
Environment Influences Traits
• Environment can influence an organism’s
phenotype
• The gene for fur color in the arctic fox is affected by
daylight
• In winter, the daylight is short and fur turns white
• In summer daylight is longer, fur color turns brown
• Environment influences our growth
• You may carry the gene for tallness, but without a
healthy diet, you may not reach your full height
• Traits that are learned are not inherited – bike
riding
Bending the Rules
• Some human traits follow the pattern for
complete dominance others do not
• In incomplete dominance, each allele in a
heterozygous individual influences the
phenotype
• The result is a blend of the phenotypes of the
parents
• Both alleles of the gene have some influence
• Hair texture is an example of incomplete
dominance -one straight hair allele and one
curly hair allele will produce wavy hair
Codominance
• A trait that shows codominance has both of the
alleles in a heterozygous individual contribute to
the phenotype
• Heterozygous individuals have both of the traits
associated with the their two alleles
• An example in humans is blood type
• There are three alleles for blood type: A, B, and O
• The A and B alleles are codominant
• The blood type is AB
Punnett Squares
• One tool for understanding the patterns of
heredity is a diagram called a Punnett square
• A Punnett square is a graphic used to predict
possible genotypes of offspring
• Each parent has two alleles for a particular
gene
• An offspring receives one allele from each
parent for a particular trait
• A Punnett square shows all the possible allele
combinations in the offspring
• A Punnett square does not tell you what the exact
results of a certain cross will be
• It only helps you find the probability that a certain
genotype will occur
• Probability is the mathematical chance of a specific
outcome in relation to the total number of possible
outcomes
• Probability can be expressed as a ratio
• a ratio is written as 1:4 or ¼ and is read as “one to
four”
• Another way of expressing probability is percentage
• A percentage states a certain outcome out of 100
Pedigree Charts
• A pedigree is another tool used to
study patterns of inheritance
• It traces the occurrence of a trait
through generations of a family
• Pedigrees are useful in tracing a class
of inherited disorders known as sex –
linked disorders
• Examples of sex – linked disorders are
hemophilia and colorblindness
Sex – Linked Disorders
• Sex – linked disorders are associated with an allele
on a sex chromosome
• Many sex – linked disorders are caused by an allele
on the X chromosome
• Women have two X chromosomes so a woman can
have one allele for a sex – linked disorder without
having the disorder
• A woman who is heterozygous for the trait is a
carrier
• Men have just one X chromosome
• This single chromosome determines if the trait is
present
Cracking the
Code
• The genetic material in cells is contained in a
molecule called DNA
• DNA carries the information for cells to grow, divide
and function
• DNA is described as a code
• A code is a set of rules and symbols used to carry
information
• Computers use a code of ones and zeros that is
translated to letters, numbers and graphics
• Many scientists over the world have contributed to
our current understanding of DNA
Structure of
DNA
• The structure of DNA is a twisted ladder shape
called a double helix
• The two sides of the ladder, called the
backbone, are made of alternating sugars and
phosphate groups
• The rungs of the ladder are made of a pair of
bases, each attached to one of the sugars
• A base, a sugar and a phosphate group make
up a building block of DNA known as a
nucleotide
Nucleotides
• There are four different nucleotides in DNA
• They are identified by their bases: adenine (A),
thymine (T), Cytosine (C), and guanine (G)
• Adenine always pairs with thymine (A – T)
• Cytosine always pairs with guanine (C – G)
• The order of the nucleotides in DNA is a code that
carries information
• Genes are segments of DNA that describe a
different trait
• Each gene reads a part of the code - this code stores
information about which protein it should build –
the protein built determines your trait
Replication
• Cells are able to make copies of DNA molecules
through a process known as replication
• Two strands of DNA separate
• The bases on each side of the molecule are used as
a pattern for a new strand
• As the bases on the original are exposed,
complementary nucleotides are added
• When replication is complete, there are two
identical DNA molecules
• Replication occurs before cell divides so that each
new daughter cell will have a complete set of
instructions
Mutations
• Changes in the number, type, or order of bases on a
piece of DNA are known as mutations
• If a base is left out, it is known as a deletion
• If a base is added, it is known as an insertion
• The most common mutation happens when one
base replaces another – substitution
• Occurs due to random errors but can be influenced
by chemical agents known as mutagens – ultraviolet
light and cigarettes are examples
• Mutations may be beneficial, harmful or neutral
• A genetic disorder results from mutations that harm
the normal function of a cell
Protein Factory
• When the cell uses DNA to build proteins, it only
needs some of the information stored in the DNA
molecule
• Some of the information in the DNA is copied to a
separate molecule called ribonucleic acid, or RNA
• The copy is used to build the protein
• RNA has a similar structure to DNA, but instead of
thymine (T), RNA contains the base uracil (U)
• Three types of RNA: messenger RNA, ribosomal
RNA and transfer RNA
• Each type has a special role