TOC
8 Genetic Notes 1/22
 9 Lab Activity: Predicting Genetic Ratios 1/22
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the branch of biology that studies the transmission of hereditary
information from parents to offspring
GENETICS
GREGOR MENDEL AND HIS PEA PLANTS
EXPERIMENTS (1857-1865)
WHY MENDEL? WHY PEAS?

Mendel was a monk who was educated--he knew MATH!
He also had a lot of free time.
As for the peas? Well, they were:
* easy to grow
* inexpensive
* easy to pollinate (either self or cross)
* easy to study their TRAITS (characteristics) such as:
height, seed pod shape, seed color, seed pod color,
seed texture, flower position, and seed shape
Mendel kept very accurate records for
hundreds of individual plants.
 He used statistical analysis (MATH) to study the
traits of different generations.
 Through his findings, he proposed that the
traits were passed on by some kind of
hereditary factors (now know to us as DNA).

HERE'S BASICALLY WHAT HE DID
One trait he studied was plant height. Mendel pollinated all tall plants for many
generations to get a pure population of tall plants. He also pollinated all short plants to
get a pure population of small plants. See picture below...

X means to cross
(as in cross-pollinate)
tall plants X tall plants as well as short plants X short plants
He then cross-pollinated a member of the tall pure population with a member of the short
pure population...
P1 (pure parent generation)
P1 TALL PLANT X P1 SHORT PLANT
ALL TALL PLANTS
F1 (first filial [family] generation)
Where did the short trait go? Mendel then crossed two tall members of the F1 generation.
F1 TALL PLANT X F1 TALL PLANT
THIS IS CALLED A MONOHYBRID CROSS (hybrids known for one trait)
F2 (second filial generation)
For every 3 TALL PLANTS there was 1 SHORT PLANT
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So, the short trait did not disappear. It was
present in the F1 generation, but IT WAS NOT
EXPRESSED (this means it did not show up!)
Somehow, it was expressed (visible) in the F2
generation.
MENDEL'S PRINCIPAL OF DOMINANCE
because the tall trait showed up more than
short, Mendel call this trait DOMINANT
 the short trait, because it seemed 'weaker' than
the tall trait, was called RECESSIVE
After this discovery...well, um...NOBODY CARED!
Later on, when scientists studies meiosis in
Drosophila (genus name for fruit flies), they
linked together Mendel's factors with the
chromosomes in gametes
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Mendel's factors are now called GENES
 genes
are segments of chromosomes (DNA) that
code for a characteristic
 these characteristics can be:
a)
b)
physical traits (can be seen like eye color, hair
color, height, etc.)
chemicals produced in the body (like for
hormones or enzymes)
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genes are lined up on chromosomes in a certain order,
like beads on a string
homologous (similar) chromosomes have the same
order of genes...HOWEVER..
these chromosomes might have the different forms of a
gene
ex. trait=eye color can be blue, black, brown, hazel,
green, etc.
these different forms of a gene are called alleles
in simple patterns of inheritance, there are 2 different
forms of a gene (alleles), where one is dominant and
one is recessive
DOMINANT IS SHOWN BY CAPITAL LETTERS
recessive is shown by lowercase letters
ex. in Mendel's peas T = tall t = short

diploid (having 2 sets of chromosomes)
organisms have 2 copies of genes, one on each
chromosome they have
 the
2 copies can be the same--HOMOZYGOUS, like
TT (homozygous dominant) or tt (homozygous
recessive)
 the 2 copies can be different--HETEROZYGOUS Tt
(heterozygous)
NOTICE THAT WHAT THE ORGANISM LOOKS LIKE
AND THE KIND OF GENES IT HAS ARE TWO
SEPARATE WAYS TO CLASSIFY AN ORGANISM
phenotype: what the appearance of an organism is
(how it looks) ph = physical
 genotype: what the genetic makeup of an
organism is (what genes it has) gen = genes
genotype can be:
homozygous dominant----------> TT
heterozygous (a hybrid--a mix)---> Tt
homozygous recessive----------> tt
 recessive phenotypes can only be expressed (show
up) when there are 2 copies of a recessive gene
present at the same time...if a dominant gene was
there, then that would be expressed, hiding the
recessive gene!
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MENDEL'S PRINCIPLE OF SEGREGATION AND
RECOMBINATION

when gametes are formed during meiosis
(DIPLOID to HAPLOID) the homologous
chromosomes (which contain the many
different genes) separate randomly
 this
is called SEGREGATION
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if the organism has a homozygous (the same) genotype for a
given trait (TT or tt), then all gametes will also have that trait
TT <----diploid----> tt
meiosis
T or T <----haploid----> t or t
if the organism has a heterozygous (not the same) genotype
for a given trait (Tt), then half of the gametes will have one
trait, and the other half will have the other trait
Tt <----diploid
meiosis
T or t <----haploid
when the haploid gametes fuse during fertilization, the
diploid number of chromosomes is restored
this can result in a new combination of genes
this is called RECOMBINATION
LET'S LOOK BACK AT MENDEL'S FIRST FEW PEA
PLANT CROSSES

P1 (pure parent generation) cross was: TT X tt

F1 (first filial generation) cross was Tt X Tt

F2 (second filial generation) offspring had
genotypes of TT, Tt, tT, and tt
NOW, COMPARE THE DIFFERENT PHENOTYPES
(PHYSICAL APPEARANCE) AND GENOTYPES
(GENETIC MAKEUP).

For the F2 generation what is the PHENOTYPIC
RATIO?
3 tall : 1 short
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For the F2 generation, what is the GENOTYPIC
RATIO?
1 homozygous dominant : 2 heterozygous : 1 homozygous recessive
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PHENOTYPE DOES NOT ALWAYS EQUAL
GENOTYPE!
TO SHOW SEGREGATION (SEPARATION OF
GENES/CHROMOSOMES) AND RECOMBINATION
(DURING FERTILIZATION), WE USE PUNNETT
SQUARES
PUNNETT SQUARE METHOD
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Problem: A homozygous tall pea plant is
crossed with a short pea plant. (Remember, tall
is dominant over short for pea plants!) What
would the expected genotypes (the genotypic
ratio) and phenotypes (the phenotypic ratio) of
their offspring?
STEP 1: SET UP THE KEY THAT WILL SHOW HOW
EACH ALLELE (FORM OF A GENE) WILL BE
SHOWN
T = tall
t = short
STEP 2: SET UP THE PARENTS THAT WILL BE
CROSSED; USE THE INFORMATION FROM THE
WORD PROBLEM ITSELF
TT x tt
tall homozygous parent & short parent
* remember, to be short, it has to have both
short alleles!
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STEP 3: do the Punnett square; that is, put one
parent on the top and the other parent on the
left of the square...then from a blank box, take
the allele on the top and the allele from the left
and fill them in
STEP 4: SHOW THE RESULTS (RATIOS) OF THE
CROSS
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GENOTYPIC RATIO: Tt, Tt, Tt, Tt
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PHENOTYPIC RATIO: 4 Tall
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Homework: Complete handout on Monohybrid
crosses and online questions. Review notes
and be prepared for quiz.
TRY THESE PUNNETT SQUARE PROBLEMS
1)
2)
3)
Use a Punnett square to show the offspring of a
cross between two pea plants that are
heterozygous for height (Tt). Give the phenotype
and genotypes of the offspring. (That is, what is
the phenotypic and genotypic ratios!)
Use a Punnett square to show the offspring of a
cross between a pea plant that is homozygous
tall and one that is heterozygous. Give the
phenotypes and genotypes of the offspring.
Use a Punnett square to show the offspring of a
cross between a pea plant that is heterozygous
tall and one that is homozygous recessive for
height. Give the phenotypes and genotypes of the
offspring.