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Genetics
th
8
Grade Science
DNA
A = Adenine
T = Thymine
C = Cytosine
G = Guanine
The structure of DNA
Watson & Crick
 Watson & Crick did NOT discover DNA
 DNA was discovered in the late 1860s by Swiss chemist
Friedrich Miescher
 Other scientists over the years notably, Phoebus Levene and
Erwin Chargaff--carried out a series of research efforts that
revealed additional details about the DNA molecule, including
its primary chemical components and the ways in which they
joined with one another.
 Chargaff realized that A = T and C = G
 Watson and Crick made a three-dimensional, doublehelix model for the structure of DNA.
How they did it
 Watson and Crick's discovery was possible because of
advances in model building based upon known
molecular distances and bond angles, a technique
advanced by American biochemist Linus Pauling.
 Watson and Crick were worried that they would be
"scooped" by Pauling, who proposed a different model
for DNA just months before they did.
 Pauling's prediction was incorrect.
 Using cardboard cutouts representing the four bases
Watson and Crick shifted molecules around on their
desktops, as though putting together a puzzle.
Gregor Mendel
 The basic laws of heredity
were first formed during the
mid-1800’s by an Austrian
botanist monk named Gregor
Mendel.
 Because his work laid the
foundation to the study of
heredity, Mendel is referred
to as “The Father of Genetics.”
Mendel’ Pea Plants
• Mendel based his laws on his studies of
garden pea plants.
• Mendel was able to observe differences
in multiple traits over many generations
because pea plants reproduce rapidly, and
have many visible traits such as:
Plant Height
Tall
Short
Pod Shape
Smooth Pinched
Pod color
Green Yellow
Seed Color
Green
Yellow
Seed Shape
Wrinkled Round
Mendel’s Experiments
Mendel noticed that some plants always produced offspring that had
a form of a trait exactly like the parent plant. He called these plants
“purebred” plants. For instance, purebred short plants always
produced short offspring and purebred tall plants always
produced tall offspring.
X
Purebred Short Parents
Short Offspring
X
Purebred Tall Parents
Tall Offspring
Mendel’s First Experiment
Mendel crossed purebred plants with opposite forms of a trait. He
called these plants the parental generation . For instance, purebred
tall plants were crossed with purebred short plants.
X
Parent Tall
Parent Short
Offspring Tall
Mendel observed that all of the offspring grew to be tall plants.
None resembled the short parent.
Mendel’s Second Experiment
Mendel then crossed two of the offspring tall plants produced
from his first experiment.
Parent Plants
Offspring
X
Tall
3⁄4 Tall & 1⁄4 Short
To his surprise, Mendel observed that this generation had a mix
of tall and short plants. This occurred even though none of the
parents were short.
Mendel’s Conclusions
Each plant must have two sets of
instructions (genes) for each trait
The two forms of a gene are called
alleles.
Genes can be dominant or
recessive.
How gender is determined.
 Gender is determined by sex chromosomes.
 Humans have 23 pairs of chromosomes
 22 pairs are “body” chromosomes called
autosomes
 1 pair are the “sex” chromosomes – X and
Y
 Females will have XX
 Males will have XY
Males = XY
Females = XX
TERMS TO KNOW
ALLELES
DIFFERENT
FORMS OF A
TRAIT THAT A
GENE MAY HAVE
HOMOZYGOUS
AN ORGANISM
WITH TWO
ALLELES THAT
ARE THE SAME
HETEROZYGOUS AN ORGANISM
WITH TWO
DIFFERENT
ALLELES FOR A
TRAIT
T,t
TT, tt
Tt, Gg
TERMS TO KNOW
HYBRID
SAME AS
HETEROZYGOUS
DOMINANT
A TRAIT THAT
DOMINATES OR
COVERS UP THE
OTHER FORM OF
THE TRAIT
THE TRAIT
BEING
DOMINATED OR
COVERED UP BY
THE DOMINATE
TRAIT
RECESSIVE
Tt, Gg
REPRESENTED BY AN
UPPERCASE LETTER
T G
OR
REPRESENTED BY A
LOWER CASE LETTER
t g
or
TERMS TO KNOW
PHENOTYPE
THE PHYSICAL
APPEARANCE OF
AN ORGANISM
(WHAT IT LOOKS
LIKE)
TALL, SHORT,
GREEN,
WRINKLED
GENOTYPE
THE GENE ORDER
OF AN ORGANISM
(WHAT ITS GENES
LOOK LIKE)
TT, GG, Tt, gg
Gg, tt
RATIO
THE RELATIONSHIP
IN NUMBERS
BETWEEN TWO OR
MORE THINGS
3:1, 2:2, 1:2:1
(You can also
express as a
percentage.)
A GOOD TOOL TO KNOW
PARENT’S GENES
PARENT’S GENES
A PUNNET SQUARE
IS A TOOL USED TO
PREDICT THE
POSSIBLE
GENOTYPES FOR
THE OFFSPRING OF
TWO KNOWN
PARENTS.
HOW TO USE A PUNNETT SQUARE
Parents = BB X bb
THE PARENTS’
ALLELES GO ON
THE OUTSIDE OF
THE SQUARE
B
b
b
B
HOW TO USE A PUNNETT SQUARE
THE PARENTS’
ALLELES GO ON THE
OUTSIDE OF THE
SQUARE
b
THE ORDER DOES
NOT MATTER IN THE
BOXES, BUT
UPPERCASE FIRST IS
A GOOD RULE
b
B
B
Bb
Bb
Bb
Bb
DROP THE
LETTERS ON
THE TOP,
INTO EACH
SQUARE
MOVE EACH
LETTER ON
THE SIDE,
INTO EACH
SQUARE
HOW TO USE A PUNNETT SQUARE
LET’S LOOK
AT ANOTHER
PUNNETT
SQUARE AND
PREDICT THE
OUTCOME
T IS THE
DOMINANT
ALLELE FOR
TALLNESS
T
T
t
t IS THE RECESSIVE ALLELE
FOR SHORTNESS
TT
Tt
t
Tt
tt
WHAT ARE
THE RESULTS?
PHENOTYPE:
75% TALL
25% SHORT
3 TO 1 RATIO:
TALL TO SHORT
GENOTYPE:
1TT: 2Tt: 1tt
1:2:1 RATIO
25 %TT, 50%
Tt, 25% tt
Try it
1. IN DROSOPHILA
MELANOGASTER (FRUIT
FLIES), RED EYE COLOR
(R) IS DOMINANT OVER
BROWN EYE COLOR (r).
IF THE FLIES IN THE
PICTURE WERE CROSSED,
WHAT PERCENT OF
THEIR OFFSPRING
WOULD BE EXPECTED TO
HAVE BROWN EYES?
ANSWER: 1. 50%
Try it
2. WHICH OF THE
FOLLOWING HAS THE hh
GENOTYPE?
A. 1 & 3
B. 2
C. 4
D. NONE
3. WHICH OF THE FOLLOWING IS A TRUE
STATEMENT?
A. INDIVIDUAL 4 IS RECESSIVE
B. INDIVIDUALS 1 & 3 ARE
HETEROZYGOUS
C. INDIVIDUAL 2 IS DOMINANT
D. ALL INDIVIDUALS ARE FEMALE
H
H 4
h 3
ANSWER:
2. B
3. B
h
1
2
Try it
4. IF B IS THE ALLELE FOR
BLACK FUR AND b IS THE
ALLELE FOR WHITE FUR,
WHAT PERCENT WOULD
BE BLACK?
A. 25%
B. 50%
C. 100%
D. 75%
B
b
B BB Bb
b Bb bb
5. WHAT FRACTION IS HOMOZYGOUS DOMINANT IN
THE ABOVE CROSS?
ANSWER:
A. 1/2
B. 1/3
C.1/4
D. 3/4
4. D. 75%
5. 1/4
Try it
6. IN THIS CROSS, WHAT IS
THE RATIO OF BB TO Bb?
A. 3 : 1
B. 4 : 1
C. 2 : 2
D. 0 : 4
ANSWER:
6. C. 2:2
B
B
B BB BB
b Bb Bb