Genes and Alleles

In this family portrait, are the grandparents the parents to the mother
or father? How can you tell?
• Heredity – The passing of characteristics from
parent to offspring.
Genetics – the study of heredity
Gregor Mendel (1822-1884)
The father of genetics.
The Role of Fertilization
During sexual reproduction, male and female reproductive cells join
in a process known as fertilization to produce a new cell.
In peas, this new cell develops into a tiny embryo encased within a
The Role of Fertilization
Self fertilization creates offspring with identical traits to
themselves. Mendel called these plants true breeding.
Who is the father of Genetics?
• Gregor Mendel
A trait is a specific characteristic of an individual, such as
seed color or plant height, and may vary from one individual
to another.
The Role of Fertilization
Mendel studied seven different traits of pea plants, each of which
had two contrasting characteristics, such as green seed color or
yellow seed color.
Mendel crossed plants with each of the seven contrasting
characteristics and then studied their offspring.
The offspring of crosses between parents with different traits are
called hybrids.
Genes and Alleles
When doing genetic crosses, we call the original pair of plants the P,
or parental, generation.
Their offspring are called the F1, or “first filial,” generation.
In each cross, the nature of the other parent, with regard to each
trait, seemed to have disappeared.
Genes and Alleles
From these results, Mendel drew two
conclusions. His first conclusion
formed the basis of our current
understanding of inheritance.
An individual’s characteristics are
determined by factors that are
passed from one parental generation
to the next.
Scientists call the factors that are
passed from parent to offspring
Genes and Alleles
Each of the traits Mendel studied was controlled by one gene that
occurred in two contrasting varieties.
These gene variations produced different expressions, or forms, of
each trait.
The different forms of a gene are called alleles.
What are the genes?
What are the alleles for each gene?
Dominant and Recessive Traits
Mendel’s second conclusion is called the principle of dominance.
This principle states that some alleles are dominant and others are
An organism with at least one dominant allele for a particular form
of a trait will exhibit that form of the trait.
An organism with a recessive allele for a particular form of a trait
will exhibit that form only when the dominant allele for the trait is not
Which allele is dominant for each of the genes?
Mendel found in the F2 generation that about ¼ of the plants displayed the
recessive allele for the trait.
Gene – pea color
Alleles – yellow Dominant (Y)
green Recessive (y)
The pea plants in Mendel’s F1 generation had both alleles
Mendel suggested that during the formation of sex cell’s, or gametes,
these alleles seperated, or segregated from each other
• Segregation – seperation
• Gametes – sex cells
• During gamete formation, the alleles for each
gene segregate from each other, so that each
gamete carries only one allele for each gene.
• Name one gene(trait) that you inherited from
your parents. What are the possible alleles for
this trait.
How do scientist express alleles?
• With letter abbreviations,
• Capital letter for dominant allele
• Lower case letter for recessive allele
Allele representation
• G – green (dominant)
• g – yellow (recessive)
• The alleles that are present in the organism
for the specific gene.(The genetic make up)
• GG or Gg or gg
• The trait that is expressed (what we see)
What is the phenotype?
What are the possible genotypes?
• When an organism has two identical alleles for
a particular gene
• GG
• gg
• When an organism has two different alleles
for the same gene.
• Gg
What are the 4 possible allele
combinations for the gametes of a
parent who has the following
• Bb
According to what we learned in the past week, what would you
predict the offspring to look like?
This cross is an exception to Mendel’s principle. It is displaying Incomplete Dominance
Incomplete Dominance – when some alleles are neither dominant or recessive. The
heterozygous phenotype expresses itself somewhere between the 2 homozygous
• When the cross produces an offspring in
which both alleles are clearly expressed.
Multiple Alleles
• A gene with more than 2 alleles.
An individual will only have 2 of the alleles,
however there are many different alleles within
the population
Polygenic traits
• Traits produced by the expression of several
Genes and the Environment
• Environmental conditions can affect gene
expression and influence genetically
determined traits.
What is the difference between a
monohybrid cross and a dihybrid
• Monohybrid is a cross that looks at only one
• Dihybrid is a cross that is looking at two traits.
Which is the genotype found in a regular
cell in parent?
Which is the genotype found in a gamete
produced by parent?
A. RrGg

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