+ Intro to Genetics and Mendel
Honors Biology
Ms. Kim
+
Transmission (passing
down) of Traits
How?
One
possible explanation of
heredity is a “blending”
hypothesis
genetic material contributed by
two parents mixes
+
Another Hypothesis
An
alternative to the blending
model is the hypothesis of
inheritance (genes)
Parents pass on discrete
heritable units (factors) called
genes

http://wps.aw.com/bc_campbell_biology_7/29/7523/1925929.cw/nav_and_content/index.html

The Novelty Gene Video
+
Gregor Johann Mendel (1843)

Austrian Monk- “Father of Modern Genetics”

Documented a mechanism of inheritance
through his experiments with garden peas
 The scientific study of heredity is called
GENETICS!
 Worked with pea plants in his monastery
 Correctly believed that heritable factors (genes)
retain their individuality from generation to
generation
 i.e. – marbles (no blending of colors!)
Figure
14.1
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Gregor Johann Mendel
Mendel
used the scientific method to
identify two laws of inheritance
Mendel
discovered the basic principles
of heredity
By breeding garden peas in carefully
planned (CONTROLLED) experiments
+ Mendel’s Experimental Method
Why did Mendel choose pea plants?
available in many varieties of traits
They have seven distinct & observable traits
easy to get
he could strictly control which plants
mated with which
Grow quickly
They reproduce quickly & have a short life cycle
They produce many offspring in one cross
Mendel’s Experimental Cross
Purebred white
and purple
flowers
A. All Purple
B. All White
C. Both White/Purple
Offspring were
allowed to self
pollinate
White flowers
reappear in some
offspring
What did Mendel notice? Did the trait for white flowers
disappear in F1 generation?
+
Some genetic vocabulary
Character:
a heritable
feature, such as flower color
Trait: a variant of a
character, such as purple or
white flowers
+

Mendel
observed the
same
pattern
 In many
other pea
plant
characters
+
Pea Plant Fertilization
Self
fertilization : mate with self
 produce identical offspring
TRUE or PURE breeds
Cross
fertilization : mate with
another  can produce different
offspring
HYBRIDS

http://wps.aw.com/bc_campbell_biology_7/29/7523/1925929.cw/nav_and_content/index.html

Colored Cotton Video
+
Mendel’s Experiment
Mendel
only looked at “either-or”
characters
 Ex: Purple
OR white flowers
Mendel
started his experiments with
“true-breeding”
Made
through self fertilization so plants
are “TRUE” for only 1 trait
 Known
as HOMOZYGOUS for trait
+
What was Mendel’s Procedure?
1. He made 14 “TRUE
BREEDS”
 1 for EACH trait he
looked at

These are the
original parents
 Are called the P
generation
+
What was Mendel’s Procedure?
2. He used cross fertilization to mate 2 true
breeds for same gene
 Ex: Purple vs white flower color
3. He collected the offspring (progeny)
The hybrid (mixed)
offspring of the P
generation
Are called the F1
generation
+
What was Mendel’s Procedure?
4. He crossed (using cross fertilization)
male and female from F1 progeny
When F1
individuals are
mated together
The F2
generation is
produced
+ What did Mendel Discover?
P  F1  F2

A 3:1 ratio, purple to white flowers, in the F2 generation
P Generation
Where did the
white color go?
(truebreeding
parents)
F1
Generation
(hybrids)
F2 Generation
Purple
flower
s
All plants
had
purple
flowers
White
flower
s
+
+

17
Genetics Vocab
Mendel worked with his pea plants until he was sure that all were truebreeding varieties (pure bred)
P generation  parental
generation have offspring
called the F1 generation
(hybrids)
Hybrid (F1) the offspring of
two true breeding varieties
If F1 generations selffertilize/cross, their offspring
are called the F2 generation
+
What are Mendel’s
factors?
Mendel’s “factors” are
alleles
Alternative
now called
version or form of a gene
Allele for purple flowers
F
Locus for flower-color gene
f
Figure 14.4
Homologous
pair of
chromosomes
Allele for white flowers
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19
Mendel’s Experiments
 After
studying pea plants, Mendel concluded that:
 Traits are passed from one generation to the next
through genes.
 Each trait is controlled by a different form of a gene
called an allele
 Some alleles are dominant to others called
recessive traits
 New
question: Have the recessive alleles disappeared
or are they still present in the parents?
+
What did Mendel
Conclude?
Mendel
reasoned that
In the F1 plants, only 1 factor (ex:
purple flower) was affecting
physical outcome color in hybrids
This factor was dominant and
the hidden factor was recessive
+
Recessive is…
Represented
by a lowercase letter (it is
NOT the letter itself, though)
an
allele that does NOT produce a
characteristic effect when present with a
dominant allele
only
expressed when present with another
(identical) recessive allele
This is known as the homozygous
condition
aa or hh
+
Dominant is…
Represented
by a uppercase letter
an
allele that produces the same trait
whether inherited with a another
dominant allele (homozygous) or with a
recessive allele (heterozygous)
Aa or AA
The
allele that is expressed if present
+
Frequency of Dominant
Alleles
Dominant
Not
alleles
necessarily better, stronger, etc.
than recessive alleles
Ex: Polydactyl
+
24
Mendel’s Experiments

Mendel crossed the first generation and saw that the recessive
trait showed up in about 1 of 4 plants.

Conclusion: Law of Segregation!

What is the Law of Segregation?!

Organisms inherit two copies of each gene
(one from each parent)

Organisms donate only one copy of each
gene in their gametes (sex cells)

Therefore, the two copies of each gene
segregate (separate) during gamete
formation (meiosis)
+
The Law of Segregation:
Mendel’s 1st Law
gamete ONLY gets 1
allele
Each
+

More about Alleles…
Each individual has 2 alleles for
the same gene
located on homologous chromosomes
Each
parent passes 1 allele for each gene to
his/her offspring
In sperm or egg
What
stage of meiosis are alleles segregated
into gametes?
Meiosis Anaphase I
+
a
A
+
28
Mendel’s Observations

Used pea plants to see patterns in the way various traits
were inherited

Using his data, he saw that different traits are inherited
separately
 Example: Green pea color isn’t always inherited with
wrinkled pea shape
 Green peas can be smooth and round too!
 This
is called the Law of Independent Assortment!
+
29
What is the Law of Independent
Assortment?

Allele pairs (traits) separate independently
of each other during gamete formation
(meiosis)
 Different traits are inherited separately
 Example – peas can be green and
wrinkled OR green and round
 This explains genetic diversity among
organisms
+
Law of Independent Assortment:
Mendel’s 2nd Law
genes are inherited
independently of other
genes
Says
Genes
are not linked unless on the
same chromosome!
Mendel
assumed traits occur on
different chromosomes!
Occurs
during Metaphase I
+

Useful Genetic Vocabulary
Homozygous
 A pair of IDENTICAL (same) alleles for that
gene
 Exhibits true-breeding
 aa = homozygous recessive (or just recessive)
 HH = homozygous dominant
 Heterozygous
 Pair
of alleles that are different for that gene
 Aa or Hh
+
More Genetic Vocabulary
organism’s genotype
Is its genetic (DNA) makeup
A.k.a.-the allele combination
(includes 2 alleles)
An
organism’s phenotype
Is its physical outcome of the
genotype
An
Ex:
blue eyes or AB blood type
Mendelian Genetics…
aka- COMPLETE DOMINANCE
+
If an organism is heterozygous (Hh),
The effect of the recessive allele is
HIDDEN
Heterozygous
and homozygous
dominant have SAME phenotype
The 1st allele is “completely
dominant” over the 2nd allele
Phenotype
+
Purple
3
Purple
Genotype
PP
(homozygous)
1
Pp
(heterozygous)
2
1
Figure 14.6
Purple
Pp
(heterozygous)
White
pp
(homozygous)
Ratio 3:1
Ratio 1:2:1
1
+ Why Did Mendel Keep Getting
the SAME results?
We
can answer this question using a
Punnett square
a diagram (box) used to predict
probabilities of possible outcomes
for offspring that will result from a
cross between 2 parents
SHOWS EXPECTED RESULTS (not
necessarily actual)
+
Practice
PURPLE X WHITE
 Which
A.
B.
C.
PURPLE
PURPLE & WHITE
D.
flower color is recessive?
White
Purple
Neither
Both
+
Practice
PURPLE X WHITE
 Which

flower color is recessive?
WHITE
 What
would the genotype be for the
recessive flower?
PURPLE
PURPLE & WHITE
A. PP homozygous dominant
B. pp homozygous recessive
C. Pp Heterozygous
+
Practice
PURPLE X WHITE
 Which

flower color is recessive?
WHITE
 What
would the genotype be for the
recessive flower?
PURPLE

pp homozygous recessive
 Which
A.
B.
PURPLE & WHITE
C.
D.
flower color is dominant?
White
Purple
Neither
Both
+
Practice

Which flower color is recessive?

PURPLE X WHITE

What would the genotype be for the
recessive flower?



pp homozygous recessive
Which flower color is dominant?

PURPLE
WHITE
PURPLE
What would the genotype be for the
dominant flower color?
A. PP homozygous dominant
B. Pp heterozygous
C. pp homozygous recessive
D. Both A & B
PURPLE & WHITE
+
40
Genetics Vocab (pt 2)

Monohybrid cross  cross where parents differ in only one
trait (Rr x rr)

Dihybrid cross  cross where parents differ in two traits
(RrHh x rrHH)

Punnett square – a diagram that shows the gene
combinations that might result from a genetic cross of two
parents