Download Genetic Origin and Theory

Genetics Origin and
Theory
Gregor Mendel
Gregor Mendel
Pea Plant Characters and Traits
Wrinkled
Character
Trait
Short
Mendel’s Experiments

Began with monohybrid crosses


Crosses that involve organisms
that are different in only one trait
Ex- tall pea plant with a short
plant (P1)
3 steps…
1st step…
• Allow self-pollination for
several generations
• Result: true-breeding P
generation (parent
generation)
• Purple flowers plants
will only produce
purple flowered
offspring
2nd step….
• Cross-pollination of
P generation
• Result: F1
generation (first
filial generation)
• All same trait
…3rd step
• Self-pollination of
F1 generation
• Result: F2
generation (second
filial generation)
• Some purple,
• Some white
His data
Mendel’s Hypotheses
For each inherited character, an individual has two
copies of the gene-one from each parent.
2. There are alternative versions of genes: alleles
3. When two different alleles occur together, one of
them may be completely expressed (dominant )
while the other may have no observable effect on
the organism’s appearance (recessive)
4. When gametes are formed, the alleles for each
gene separate independently of the other (law of
independent assortment
1.
Monohybrid cross- plant height
• Two traits (alleles)  tall and
short
• Dominant trait: tall
• Recessive trait: short
In genetics problems we generally assign a
letter to each trait with the upper case
being the dominant and lower being the
recessive allele.
Ex. Height T = Tall & t = short
Monohybrid cross
 Example of a
Monohybrid Cross
with seed shape
 Create a visual of
Mendel’s
Experiment with
plant height.
F2 Generation
 From this cross his 2nd generation were.
 2nd
generation 75% tall and 25% short
(F2)
 Why was the short gene apparent in the
2nd and not in the 1st Generation of
offspring?
Tt
Fertilization
T
T
t
t
Back to his hypotheses

From this experiment Mendel concluded that for
each gene we carry there are two factors called
Alleles.


One allele from each homologous chromosome
One from Mom and one from Dad
Mendel Cont.

This brought up the terms recessive and
dominant


For a recessive trait to show both alleles must
be recessive.
For a dominant trait to show only one
dominant allele needs to be present
 Can be a carrier if you have one
dominant and one recessive (carrier for
recessive) – The recessive trait does not
show.
Passing on the Trait

When gametes (sex cells) are formed only one of
the alleles is passed on from each parent
The allele combination that the child
receives is called the genotype. (Tt)

Genotype= genomic composition
The genotype may be homozygous or
heterozygous





homozygous have the same alleles (TT, tt)
Heterozygous have different alleles (Tt)
The trait that physically appears is the
phenotype (tall or short)
Write out the genotype for pea height ex for….
Homozygous dominant, homozygous recessive,
and heterozygous
Punnett squares
 The Punnett square is a diagram that is used to
predict an outcome of a particular cross or
breeding experiment.
 Named after Regi Punnett
 Is a summary of every possible combination
Draw the punnett squares for the
height example
Mendel’s Laws
 Law of Segregation: Gene pairs
separate when gametes form.
 Alleles separate during meiosis

Alleles are found on the homologous
chromosomes so when they
separate the Gene Pairs separate.
Mendel’s Law
 Law of Independent Assortment: Gene
pairs segregate into gametes randomly and
independently of each other regardless of
how they are connected in the organism.


Note: they have to be on different
chromosomes.
These allele pairs are then randomly united
at fertilization.
Y = Yellow P = Purple
y = green p = White
YyPp
Independent Assortment
YP
Yp yP yp
Review
 What is an allele?
 What is a phenotype?
 What is a genotype?
 Define dominant and recessive based upon
allele presence.
 Explain Law of Segregation
 Explain Law of Independent Assortment.