Transcript Mendelian Genetics

Mendelian Genetics
Vocabulary
Heredity
Genetics
Trait
Gamete
Pollination
Fertilization
Law of segregation
Law of independent
assortment
Dominant
Recessive
Hybrid
Phenotype
Genotype
Homozygous
Heterozygous
Allele
Genetics
• Scientific study of heredity
• Heredity-passing of traits from
parents to offspring
• Trait -characteristic that is
inherited
• Many had realized family
resemblance but didn’t know
how
Gregor Mendel
•Austrian monk
•1822-1884
•Came up with
the laws of
heredity
•Importance of
work not
realized until
1900
Mendel’s Work
• First to succeed in predicting
how traits would be
transferred from one
generation to the next
• Used garden peas
(reproduced sexually via
gametes)
• Pollination -transfer of male
pollen grains to the pistil of
flower (female)
Pollination
• Results in fertilization (fusion of
male and female gametes)
• Pea plants normally went
through self-pollination
(gametes from self)
• Mendel needed to cross
pollinate
• Taking pollen from one plant
and using it to fertilize another
plant
Monohybrid Crosses
• Crosses where the parents
only differed by one trait
(mono)
• Hybrid -offspring formed by
parents having different forms
of a trait
• First trait he looked at was
height
First Generation
• Six-foot-tall pea plant that
came from a population of pea
plants, all of which were over
six feet tall
• Short pea plant that was less
than two feet tall and which
came from a population of pea
plants that were all short
• Cross-pollinated the two
plants
Result
• All of the offspring grew to be
as tall as the taller parent
• It was as if the shorter parent
had never existed
• Ratio of 4:0
Second Generation
• Allowed the tall plants in this first
generation to self-pollinate
• Counted more than 1000 plants in
this second generation
• ¾ of the plants were tall
• ¼ of the plants were short
• Ratio of approximately three tall
plants to one short plant (3:1)
Generations
• Original parents, the truebreeding tall and short plants,
are known as the P1
generation (P- parent)
• Cross of P1 are known as the
F1 generation (F- filial or
son/daughter)
• Cross two F1 plants with each
other equals F2 generation
Further Experimentation
• Did similar monohybrid
crosses with a total of seven
pairs of traits
• Studying one pair of traits at a
time
• In every case, he found that
one trait of a pair seemed to
disappear in the F1
generation, only to reappear
unchanged in one-fourth of
the F2 plants
Genes
• Located on chromosomes,
units of heredity
• The rule of unit factors
• Each organism has two
factors that control each of its
traits
• Genes exists in alternative
forms
Alleles
• Alternative forms of a gene for
each variation of a trait of an
organism
• Could have 2 alleles for
tallness
• 2 alleles for shortness
• 1 allele for tallness and 1 for
shortness
• Receive 1 allele from each
parent
The Rule of Dominance
• One gene in a pair masks
another
• Dominant -observed trait of an
organism that masks the
recessive form of a trait
• Recessive -trait of an organism
that can be masked by the
dominant form of a trait
How to Write the Alleles
• Each gene is expressed as a
letter
• Dominant allele is expressed
as a uppercase letter
• Recessive allele is expressed
as a lower case letter
• Previous example: T for tall, t
for short
Law of Segregation
• 1. There are alternative forms for
genes.
• 2. For each characteristic or trait
organisms inherit two alternative
forms of that gene, one from
each parent. These alternative
forms of a gene are called
alleles.
Law of Segregation
• 3. When gametes (sex cells) are
produced, allele pairs separate
or segregate leaving them with a
single allele for each trait.
• 4. When the two alleles of a pair
are different, one is dominant
and the other is recessive.
Phenotypes and Genotypes
• What was the difference
between the tall plants in P1
and the tall plants in F1?
• Two organisms, therefore, can
look alike but have different
underlying gene combinations
Phenotype
• Outward appearance of an
organism, regardless of its
genes
• How the organism looks
• A tall TT plant and a tall Tt
plant look what?
• Their phenotype would be tall
Genotype
• Combination of genes in an
organism
• Does the tall TT plant and the
tall Tt plant have the same
genotype?
• Can’t always tell an
organism’s genotype by looks
Homozygous vs. Heterozygous
• Homozygous- when there are
two identical alleles for a trait
• TT and tt
• Heterozygous- when there are
two different alleles for a trait
• Tt
Law of Independent Assortment
• Genes for different traits are
inherited independently of
each other
• For example, seed shape and
seed color
Punnett Squares
• Shorthand way of finding the
expected proportions of
possible genotypes in the
offspring of a cross
How to Work Punnett Square
• Step #1: Determine the
genotypes of the parent
organisms
• Step #2: Write down your "cross"
(mating). Write the genotypes of
the parents in the form of letters
(ex: Tt x tt)
Step #3: Draw a p-square
T is the allele for being tall and t is the allele for
being short, cross one plant that is homozygous
recessive (tt) with another that is heterozygous tall
(Tt).
Step #4: "Split" the letters of the
genotype for each parent & put them
"outside" the p-square
t
T
t
t
Step #5: Determine the possible
genotypes of the offspring by filling
in the p-square