Transcript Mendelian Genetics

Mendelian Genetics
Genetics
• The study of
heredity:
Passing
genetic traits
from one
generation to
next.
Two basic questions about heredity:
1. What determines the traits that an
organism has?
Examples: hair color, behavior
2. How are traits transmitted from one
generation to
the next? (Why do we resemble our
parents?)
Answer: Genes (of course)
Gregor Mendel
• “Father of Genetics”
• Austrian Monk
• Studied heredity in
1850s
• Used garden peas
• Mendel studied peas because their
phenotypes were easy to identify.
• Phenotype - the outward, physical
appearance of a particular trait
Mendel's pea plants exhibited the
following phenotypes:
- round or wrinkled seeds
- yellow or green seeds
- red or white flowers
- tall or short plants
Mendel’s Peas
Mendel’s Experiment
• He cross pollinated a tall plant with a short
plant
Results
In the 1st filial (F1) generation all of the
offspring showed the same trait – all tall.
F1 generation
– children of
the parents.
Next, he let the F1 generation self-pollinate.
Results: ¾ of the F2 generation showed the
dominant trait & ¼ showed the recessive trait.
F2 generation – grandchildren of the parents.
P (parent) generation were “pure-bred” for
traits…they only carried the trait they
showed!
Mendel did not know about chromosomes,
genes or DNA!
He suggested that some “factors” were
passed from parents to offspring.
“Factors” that Mendel talked about were
actually ALLELES (versions of a gene)
• “Factors” (alleles) are inherited from your
parents.
• You get 1 allele for each trait from each
parent.
Ex – You have 2 genes for tongue rolling.
You can have 2 alleles for the ability to roll
the tongue, 2 alleles for inability to roll, or
1 of each allele.
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Genotype Tongue Rolling (Dominant) roll/not roll
Widow's Peak (D) - just like Eddie Munster yes/no
Wet ear wax (D) - stick your finger in to check wet/dry
L/R interlocking finger (D) - without thinking, clasp your hands together, is
the right thumb over the left, or vice versa? L/R or R/L
Attached earlobes (D) - ask a neighbor or check out the mirror yes/no
Hitchhiker Thumb (r) - does it bend back at a 90 angle yes/no
PTC tasting (D) - I'll have some test paper in class yes/no
Chin fissure (D) - like actor Michael Douglas yes/no
Darwin tubercle (D) - little bump on the inside of the ear yes/no
S-methylthioester detection (Recessive) - can you smell asparagus odor
in urine? yes/no
Pigmented iris (D) - any color but blue yes/no
Freckles (D) yes/no
Polydactyl (D) - more than five fingers and/or toes yes/no
Dimples (D) yes/no
Wooly hair (R) yes/no
Long eyelashes (> 1cm; D) yes/no
Short big toe (D) - the big toe is shorter than your second one yes/no
Mendel’s laws
1) Law of Dominance – some alleles show
and some alleles are “hidden”. The traits
that mask other traits are dominant traits.
EX: Tall plant X Short plant
all offspring are Tall
Tall is the dominant trait!
• Use capital letter to represent dominant
allele
• Use lower case letter to represent
recessive allele
EX: T = tall
t = short
• REMEMBER…
Each organism has a pair of alleles (genes) for
each trait (diploid)
TT = homozygous dominant (tall)
Tt = heterozygous (tall, but carries an allele for
short)
tt = homozygous recessive (short)
2) Law of Segregation – during meiosis, one
member of a gene pair separates into
different gametes.
Therefore, each gamete only carries one
member of the gene pair.
3) Law of Independent Assortment –
Gametes unite at random and irrespective
of the other gene pairs involved.
Mendel’s principles reflect the
rules of probability
• Inheritance follows
the rules of
probability
– The rule of
multiplication and the
rule of addition can
be used to determine
the probability of
certain events
occurring
F1 GENOTYPES
Bb female
Bb male
Formation of eggs
Formation of sperm
1/
B
1/
2
B
2
B
B
1/
b
1/
1/
2
b
B
b
1/
4
b
b
4
B
1/
2
4
b
F2 GENOTYPES
1/
4
Figure 9.7
• Using symbols we can depict the cross of
tall and short pea plants in the following
manner:
• Allele - one alternative form of a given
gene pair.
• Tall and dwarf are the alleles for the height
of a pea plant.
• More than two alleles can exist for any
specific gene, but only two of them will be
found within any individual.
P1 Monohybrid Cross
r
r
R
Rr
Rr
R
Rr
Rr
Genotype: Rr
Phenotype: Round
• phenotypic ratios?
Dihybrid Cross
RY
RY
Ry
rY
ry
Ry
rY
ry
SEX CHROMOSOMES AND
SEX-LINKED GENES
Chromosomes determine sex in
many species
• A human male has one X chromosome and
one Y chromosome
• A human female has two X chromosomes
• Whether a sperm cell has an X or Y
chromosome determines the sex of the
offspring
(male)
(female)
Parents’
diploid
cells
X
Y
Male
Sperm
Egg
Offspring
(diploid)
Figure 9.21A
Sex-linked genes exhibit a unique
pattern of inheritance
• All genes on the sex chromosomes are said
to be sex-linked
– In many organisms, the X chromosome carries
many genes unrelated to sex
– Fruit fly eye
color is a
sex-linked
characteristic
Figure 9.22A
–Their inheritance pattern
reflects the fact that males
have one X chromosome and
females have two
Sex-linked disorders affect mostly
males
• Most sex-linked human
disorders are due to
recessive alleles
– Examples: hemophilia,
red-green color blindness
– A male receives a single
Figure 9.23A
X-linked allele from his mother, and will have the
disorder
A female has to receive the allele from both
parents to be affected
• Hemophilia - sex linked trait
• A high incidence of hemophilia has plagued
the royal families of Europe
Queen
Victoria
Albert
Alice
Louis
Alexandra
Czar
Nicholas II
of Russia
Alexis
Figure 9.23B
Many genes have more than two
alleles in the population
• In a population, multiple alleles often exist
for a characteristic
– The three alleles for ABO blood type in humans
is an example
– The alleles for A and B blood types are
codominant, and both are expressed in the
phenotype
Blood
Group
(Phenotype)
Genotypes
Antibodies
Present in
Blood
Reaction When Blood from Groups Below Is Mixed with
Antibodies from Groups at Left
O
Figure 9.13
O
ii
Anti-A
Anti-B
A
IA IA
or
IA i
Anti-B
B
IB IB
or
IB i
Anti-A
AB
IA IB
A
B
AB
• ABO blood types
Figure 9.13x