Download Intro. to Genetics

GENETICS
Chapter 6.3 -6.4 and Chp. 7
What is genetics?
 Genetics- The study of heredity
 Heredity- the passing of traits from parents to
offspring.
 Traits- a specific characteristic Ex: eye color, height
How are DNA-Genes-Chromosomes
related?
•DNA – Deoxyribonucleic
C
H
R
O
M
O
S
O
M
E
gene
acid- the genetic code of life
•DNA wound tightly makes
up a gene
•Stacks of genes make up a
chromosome
•Gene-chemical factors that
determine traits
Gregor Mendel “Father of Genetics”
 Australian monk that worked with pea plants to
discover many details of genetics.
 Mendel's Experiments
 Mendel chose pea plants as his experimental
subjects, mainly because they were easy to cross
and showed a variety of contrasting traits (purple
vs white flowers, tall vs short stems, round vs
wrinkled seeds)
 Pea plants usually self-pollinate- when pollen fertilizes
the egg of the same plant
 Mendel would cross pollinate pea plants which he
used pollen from one plant to fertilize(pollinate) the
egg of another pea plant.
 Mendel chose true-breeding




lines of each plant/trait he studied
(true breeding lines always
produced offspring of the same
type)
He crossed a true breeding plant
with a plant of the opposite trait
(purple x white). He called this the
Parental (P) generation.
He recorded data on the offspring
of this cross (First Filial, F1)
He self pollinated the F1 offspring
He recorded data on the offspring
of the second generation, calling
it the Second Filial generation
(F2)
Mendel’s Analysis of his Experiments
 The F1 generation always displayed one trait (he
later called this the dominant trait)
 The F1 generation must have within it the trait
from the original parents - the white trait
 The F2 generation displayed the hidden trait, 1/4
of the F2 generation had it (he later called this
hidden trait the recessive trait)
 Each individual has two "factors" that determine
what external appearance the offspring will
have. (We now call these factors genes or alleles)
Mendel established three principles
(or Laws) from his research
 1. The Principle of Dominance and Recessiveness -
one trait is masked or covered up by another trait
 2. Principle of Segregation - the two factors
(alleles) for a trait separate during gamete
formation
 3. Principle of Independent Assortment - factors of
a trait separate independently of one another
during gamete formation; another way to look at
this is, whether a flower is purple has nothing to do
with the length of the plants stems - each trait is
independently inherited
Modern Genetics~ Punnett Squares
 There are 2 alleles(genes) that determine traits.
 Alleles are represented by letters. Ex: Bb,bb,BB
 Dominant allele- the stronger allele;
represented by a capital letter EX: Brown eyes =B
 Recessive allele- the weaker allele; represented
by a lower case letter Ex: Blues eyes=b
 When a Dominant and recessive are paired, the
Dominant allele will show in the appearance of
the organism the recessive will not.
 Example: Bb- the child will have Brown eyes
Genotype- genetic make up
**when asked for the genotype give the two
alleles(letters) Ex: Bb or BB or bb
Phenotype- physical characteristics
** When asked for the phenotype look at the
alleles(letters) then give what the child would look
like: Ex: Brown eye or Blue eyes
Homozygous (aka.Pure Bred)- having the same
alleles for a trait Ex: BB or bb
Heterozygous (aka. Hybrid)- having different
alleles for a trait. Ex: Bb
Problem: What are the
combinations if the
mother has BB and father
has bb
b
Mom’s alleles
B
Bb
B
?
b
Father’s Alleles
•The top alleles drop down into the boxes & side alleles go in
the boxes across.
•Write the capital letter first.
Genetics and Probability
 Probability- the likelihood that a particular
event will take place.
 Ex: when flipping a coin the likelihood that it
will land on head is ½. If flipping twice the
likely hood is 1/2X1/2=1/4
 Each event is an independent event.
Probability of a Punnet Square
 It can be expressed as a fraction, percent or ratio.
 Punnett Square that have 4 squares the
probability is out of 4. Ex: 1 out of 4 = ¼ or 25% ;
2 out of 4 = ½ or 50%
B=Brown eyes
b = blue eyes
B
B
b
b
Genotype: BB=25% or ¼
Bb=50% or 2/4
bb=25% or ¼
Ratio = 1:2:1
Phenotype:
Brown eyes =75% or ¾
Blue eyes = 25% or ¼
Ratio: 3:1
Punnett Square practice problem:
Fill in the square and give the probabilities
in percent and the ratios
T
t
Genotypes:
T
Heterozygous:
t
Homozygous:
Phenotypes:
T= Tongue roller
t = Non tongue roller
This cross is a monohybrid cross= crosses 1
trait both parents are hybrids. The Ratio is
always 1:2:1
2 Factor Crosses
These two factors come from the 4 possible allele pairs
of gametes according to the law of independent assortment
Mom=RrYy
Possible Gametes= RY, Ry, rY, ry
DAD=RrYy
Ratio:
9:3:3:1
This is a Dihybrid
cross- 2 traits are
crossed at one time
and both parents
were hybrid for
both traits.
Ratio is always
9:3:3:1
Incomplete Dominance
 Incomplete Dominance- When traits are
neither dominant nor recessive. Hybrids
produce mixed traits
 Ex: Red flower x White flower produces all pink
flowers
 Codominance- When Both alleles contribute
to the phenotype. Example: instead of a
mixture being produced both traits are
produced
 Ex: Brown hair cow x White hair cow produces
cows that have both brown & white hairs .
Multiple Allele Traits
 Multiple Allele Traits- Traits that are
controlled by more than two alleles. Blood
type in humans is controlled by three alleles:
A, B, and O
Phenotype
Genotype
A
AA or AO
B
BB or BO
AB
AB only
O
OO only
Sample of Multiple Allele Crosses
Sex-linked traits
 Traits that are on one of the sex
chromosomes ( X or Y ) but not the other.
 Example: red/green color blindness carried on the
X chromosome
 If the trait is on the X chromosome then it
may be represented by Xc.
 X Xc. = (Female)a carrier but not in the
phenotype
 Xc. Xc. = (Female)trait shows in phenotype
 Xc. Y= (male) trait shows in phenotype
Sex Linked Punnett Square
h- represents balding
Xh
X
Xh
Xh Xh
Xh X
Y
Xh Y
XY