Download Introduction to Genetics

Introduction to Genetics
How are traits passed on?
S
Gregor Mendel
S Austrian monk who studied pea plants
S He chose these plants to study because they were:
S Easy to grow
S Mature quickly
S Have sharply contrasting traits
Traits of
Pea Plants
Pea Plants
S Naturally, pea plants self-pollinate.
S Mendel observed that some plants, when they were allowed
to self-pollinate, always “bred” true.
S This means that these plants were “true-breeding” (Ex. Tall
plants always produced tall plants.)
S Mendel cross-pollinated some plants to observe the results.
S When he crossed true-breeding tall plants and true breeding short
plants, he found that he always got tall plants
S Then, he crossed the resulting tall plants, and found that the short
plant reappeared.
S P = Parental Generation
S F1 = first filial generation
S F2 = Second filial generation
Why did Mendel get these
results?
S All living things have genes, or sections of chromosomes
that have instructions for making different characteristics
S There are always two versions of these genes, one from each
parent
S
Called Alleles
S
Represented by letters
Alleles
S Alleles, or the two forms of each gene, can be
S Dominant –
S
this trait will always show up if the dominant allele is present
S
Represented by capital letters
S Recessive –
S
this trait will only show up in the absence of the dominant allele
S
Represented by lowercase letters
S Examples of
S Dominant traits – tongue rollers, brown eyes, dark hair, left
thumb hand clasping
S Recessive traits – non-tongue rollers, light eyes, light hair, right
thumb hand clasping
Allele Combinations
S Depending on the two alleles present, an organism can be
considered homozygous or heterozygous for a trait
S Homozygous – two alleles are the same, either both dominant
or both recessive (TT or tt)
S Heterozygous/Hybrid – two alleles are different, one
dominant and one recessive (Tt)
S The different arrangements of alleles is called an
organism’s genotype
Phenotype
S An organism’s genotype will determine its phenotype, or
physical characteristic
S For example:
S A plant with the alleles TT is Homozygous Dominant & one
with the alleles Tt is heterozygous, and tt is read Homozygous
Recessive
S Both plants have the dominant gene for tallness (T) and will
therefore have a tall phenotype
Back to Mendel’s Plants
S Why did the trait for short plants disappear in the F1
generation?
S Why did it reappear in the F2 generation?
S Punnett squares can be used to demonstrate the different
possible combinations of alleles…..
Punnett Squares
t
T
T
Tt
Tt
T
T
t
TT
Tt
F1
results
F2 results
crossed

t
Tt
Tt
t
Tt
tt
Mendel’s Laws
S
Law of Dominance
S When there are two forms of a gene for one trait, one form
may be dominant over the other
Law of Segregation
S Two forms of each gene are segregated during the formation
of reproductive cells
Law of Independent
Assortment
S Genes for different traits may assort independently of one
another