Download FUNDAMENTALS OF GENETICS

Earlobes
Widow’s Peak
MENDELIAN
GENETICS
Principles of Heredity
• There are two factors which determine
physical features and behavior of an
organism:
– HEREDITY- the genetic make-up
– ENVIRONMENT- conditions during
development
• Is it “NATURE” (Heredity) OR
“NURTURE” (Environment) that
determines the ultimate product?
– Studies on identical TWINS are inconclusive
Hereditary Terms
• Gene – sequence of DNA that codes
for a protein (hundreds to thousands of
genes per chromosome).
• Allele – different forms of a gene
(B= brown eyes or b= blue eyes)
• Phenotype- observable feature (eye
color, hair color, blood type)
• Genotype- arrangement of alleles (Aa,
tt, or BB )
• Dominant- allele that masks the
presence of the other allele (capital)
• Recessive- allele that is masked by
the dominant allele (lower case)
• Homozygous (pure)– when the 2
alleles of a gene are the same (tt or
TT)
• Heterozygous (hybrid or carrier)when the 2 alleles of a gene are
different (Tt)
• P generation (parental) - Pure (is
not a carrier of another allele form)
• F1 generation (first filial) - offspring
of P generation
• F2 generation (second filial) offspring from F1 cross
Why does every trait have 2
alleles?
• Because each chromosome of a
homologous pair carries 1 allele of a trait.
The “paired” alleles are in the same
position on homologous chromosomes.
B
b
Father of Genetics
• Gregor Mendel:
– Austrian Monk (1822-1884)
– Published the results of scientific research
on Garden Peas (Pisium sativum) in 1865.
Why study peas?
– Great natural variation- stem length, seed
color, pod shape, pod color, and flower
color.
– They are small, easy to grow, and have
both male and female reproductive parts
(easy to cross fertilize).
Floral Anatomy
Mendel’s Experiments
• Mendel started with pure (homozygous)
pea plants and selectively crossed them.
• These plants are the parental (P)
generation.
• He crossed a pure purple flowered plant
with a pure white flowered plant.
•
•
All offspring (F1) plants had purple
flowers!
What did this tell Mendel?
Mendel’s Experiments
• Mendel then took purple flowered plants
from the F1 generation and crossed
them.
•
•
¾ of the offspring (F2) had purple flowers
and ¼ of the offspring had white flowers.
What did this tell Mendel?
Mendel’s Laws of Inheritance
• From his experiments, Mendel came up with 2
laws of inheritance.
1. The Law of Segregation- Alleles separate
from each other when gametes are formed
during meiosis. Remember that homologous
chromosomes separate in Meiosis I.
2. The Law of Independent AssortmentGenes for different traits are sorted
independently of one another. For example,
brown haired people don’t always have
brown eyes!
Who was Punnett?
• REGINALD CRUNDALL
PUNNETT (1875-1967)
was among the first
English geneticists. He
created the “Punnett
Square” – a diagram to
work out the possible
allele combinations of the
offspring of two parents.
Monohybrid vs. Dihybrid
• Monohybrid cross- A genetic cross
between 2 individuals involving 1 trait;
4 squares.
• Dihybrid cross- A genetic cross between 2
individuals involving 2 traits; 16 squares
Setting up a Punnett Square
#1.
• Brown eyes (B) are
dominant to blue eyes (b)
• Cross a homozygous
brown-eyed person with
a blue-eyed person
•
B
B
b
Bb
Bb
b
Bb
Bb
BB vs. bb
• What are the genotypic
and phenotypic ratios of
the offspring?
Dihybrid Cross
• Step 1: Figure out the different gametes for each
parent
• Step 2: Set-up the cross and complete
• Step 3: Calculate the genotypic and phenotypic
ratios
Dihybrid Cross
#1. Cross a pea plant that is homozygous purple and
wrinkled with one that is homozygous white and
smooth.
P= purple
p= white
N= non-wrinkled (smooth) n= wrinkled
#2. Cross 2 pea plants that are both heterozygous
purple flowered and heterozygous for smooth
seeds.
P= purple
p= white
N= non-wrinkled (smooth) n= wrinkled
The Chi-Square Test
• An important question to answer in any
genetic experiment is “How can we decide
if our data fits the Mendelian ratios we
have discussed”? A statistical test that can
answer this is the Chi-Square or
“Goodness of Fit” test. The calculations
from this test determine if the data is
acceptable.
Chi-Square Formula
2
X=
Σ
(observed value - expected value)2
(expected value)
• Σ (sigma) means “the sum of”
• Degrees of freedom (df) = n-1 where n is the
number of classes (different phenotypes)
Let's test the following data to determine if it
“fits” the expected ratio.
A genetics engineer was attempting to cross a
tiger and a cheetah. She predicted a
phenotypic outcome of the traits to be:
4/16 stripes only: 3/16 spots only: 9/16 both
stripes and spots.
When the cross was performed she found 50
with stripes only, 41 with spots only and 85
with both. According to the Chi-square test,
does her data fit the expected outcome?
Complete the chart below:
Exp ratio
Obs #
stripes
4
50
spots
3
41
stripes/spots
9
85
Total
Exp #
O-E
(O-E)2 (O-E)2/E
What next?
• Number of classes (phenotypes) (n) =
• Degrees of freedom = n-1 =
• Chi-square value (look up on the table)
– By statistical convention, we use the 0.05
probability level as our critical value. If the
calculated chi-square value is less than the
0.05 value, we accept the hypothesis. If the
value is greater than the value, we reject the
hypothesis.
A Chi-Square Table
Probability
Degrees of
Freedom
0.9
0.5
0.1
0.05
0.01
1
0.02
0.46
2.71
3.84
6.64
2
0.21
1.39
4.61
5.99
9.21
3
0.58
2.37
6.25
7.82
11.35
4
1.06
3.36
7.78
9.49
13.28
5
1.61
4.35
9.24
11.07
15.09
Review Questions
•
•
•
•
RRTT
RrTt
RRtt
Rrtt
• List gametes possible from these parents
• (T) is the allele for tall humans and is dominant over
the allele (t) for short humans. The allele for
widows peak (W) is dominant over (w) non-widows
peak hairlines.
What are the phenotypes for the following parents?
TtWw - _________________
TTww - _________________
What are the possible gametes formed by the
parents listed above?
AND
• 1. A woman with free ear lobes marries a
man with attached ear lobes. Two of their
children have attached ear lobes and two
have free ear lobes. The alleles are (E) free (e) - attached
•
A. what is the genotypes of the
woman?
•
B. what is the genotype of the man?
•
C. What are the chances of the 5th
child having free ear lobes?
• (F) striped fur is dominant over (f) white fur in
Australian cats.
What is the result of a cross between a
homozygous recessive parent and a
heterozygous parent? Give phenotypic ratios
and genotypic ratios.
Genotypic Ratio:
Phenotypic Ratio: