Download LE - 7 - Mendel and Genetics

Genetics
&
The Work of Mendel
Gregor Mendel
• Modern genetics began in the
mid-1800s in an abbey garden,
where a monk named Gregor
Mendel documented
inheritance in peas
– used good experimental design
– used mathematical analysis
• collected data & counted them
– excellent example of scientific
method
Mendel’s work
Pollen transferred from white
flower to stigma of purple flower
• Bred pea plants
– cross-pollinate
true breeding
parents
– raised seed &
then
observed traits
– allowed offspring
to self-pollinate
& observed next
generation
all purple flowers result
self-pollinate
?
Mendel collected data for 7 pea traits
Looking closer at Mendel’s work
Parents
1st
true-breeding
true-breeding
X
purple-flower peas
white-flower peas
100%
purple-flower peas
generation
(hybrids)
100%
self-pollinate
2nd
generation
75%
purple-flower peas
25%
white-flower peas
3:1
What did Mendel’s findings mean?
• Some traits mask others
– purple & white flower colors are separate
traits that do not blend
I’ll speak for
• purple x white ≠ light purple
• purple masked white
both of us!
– dominant allele
• functional protein
– affects characteristic
• masks other alleles
– recessive allele
• no noticeable effect
• allele makes a
non-functioning protein
allele producing
functional protein
mutant allele
malfunctioning
protein
homologous
chromosomes
Genotype vs. phenotype
• Difference between how an organism
“looks” & its genetics
– phenotype
• description of an organism’s trait
– genotype
• description of an organism’s genetic makeup
X
P
Explain Mendel’s results using
…dominant & recessive
…phenotype & genotype
purple
white
F1
all purple
Making crosses
• Can represent alleles as letters
– flower color alleles  P or p
– true-breeding purple-flower peas  PP
– true-breeding white-flower peas  pp
X
P
purple
PP x pp
white
Pp
F1
all purple
Punnett squares
Aaaaah,
phenotype & genotype
can have different
ratios
Pp x Pp
1st
generation
(hybrids)
%
genotype
male / sperm
female / eggs
P
P
PP
PP
p
Pp
Pp
%
phenotype
25%
75%
50%
Pp
p
Pp
pp
pp
25% 25%
1:2:1
3:1
Any Questions??
Beyond Mendel’s Laws
of Inheritance
2007-2008
Extending Mendelian genetics
• Mendel worked with a simple system
– peas are genetically simple
– most traits are controlled by single gene
– each gene has only 2 version
• 1 completely dominant (A)
• 1 recessive (a)
• But its usually not that simple!
Incomplete dominance
• Hybrids have “in-between” appearance
– RR = red flowers
– rr = white flowers
– Rr = pink flowers
RR
WW
RW
• make 50% less color
RR
Rr
rr
Incomplete dominance
P
X
true-breeding
red flowers
true-breeding
white flowers
100% pink flowers
1st
100%
generation
(hybrids)
self-pollinate
25%
Red
2nd
generation
50%
Pink
25%
White
1:2:1
Incomplete dominance
RW x RW
male / sperm
female / eggs
R
RR
W
RW
R
%
genotype
RR
RW
%
phenotype
25% 25%
50% 50%
RW
WW
W
RW
WW
25% 25%
1:2:1
1:2:1
Codominance
• Equal dominance
– human ABO blood groups
– 3 versions
• A, B, i
• A & B alleles are codominant
• both A & B alleles are dominant
over i allele
– the genes code for different
sugars on the surface of red
blood cells
• “name tag” of red blood cell
Blood donation
clotting clotting
clotting
clotting
clotting
clotting
clotting
One gene: many effects
• The genes that we have covered so far affect only
one trait
• But most genes are affect many traits
– 1 gene affects more than 1 trait
• dwarfism (achondroplasia)
• gigantism (acromegaly)
Acromegaly: André the Giant
Inheritance pattern of
Achondroplasia
Aa
a
x aa
a
Aa
A
x Aa
a
A
Aa
Aa
A
AA
Aa
a
aa
aa
a
Aa
aa
50% dwarf:50% normal or 1:1
67% dwarf:33% normal or 2:1
Many genes: one trait
• Polygenic inheritance
– additive effects of many genes
– humans
•
•
•
•
•
•
skin color
height
weight
eye color
intelligence
behaviors
Human skin color
• AaBbCc x AaBbCc
– can produce a wide
range of shades
– most children =
intermediate skin
color
– some can be very
light & very dark
Albinism
melanin = universal brown color
Johnny & Edgar Winter
Coat color in other animals
• 2 genes: E,e and B,b
– color (E) or no color (e)
– how dark color will be: black (B) or brown (b)
eebb
eeB–
E–bb
E–B–
Environment effect on genes
• Phenotype is controlled by
both environment & genes
Color of Hydrangea flowers
is influenced by soil pH
Coat color in arctic
fox influenced by
heat sensitive alleles
Human skin color is
influenced by both genetics
& environmental conditions
Genetics of sex
• Women & men are very different, but just a
few genes create that difference
• In mammals = 2 sex chromosomes
–X&Y
– 2 X chromosomes = female: XX
– X & Y chromosome = male: XY
X
X
X
Y
Sex chromosomes
Sex-linked traits
• Sex chromosomes have other genes on
them, too
– especially the X chromosome
– hemophilia in humans
• blood doesn’t clot
– Duchenne muscular dystrophy in humans X
X
• loss of muscle control
– red-green color blindness
• see green & red as shades of grey
X
Y
Dominant ≠ most common allele
• Because an allele is dominant
does not mean…
– it is better, or
– it is more common
Polydactyly
dominant allele
Polydactyly
individuals are born with
extra fingers or toes
the allele for >5 fingers/toes
is DOMINANT & the allele for
5 digits is recessive
recessive allele far more
common than dominant
 only 1 individual out of 500
has more than 5 fingers/toes
 so 499 out of 500 people are
homozygous recessive (aa)
Hound Dog Taylor