Transcript Mendelian Genetics

Mendelian Genetics
How pea plants and humans mix
it up
Early Ideas About Heredity



People learned that it took two
parents to make a baby
Many early beliefs about how
characteristics are transmitted.
Blending theory
Early Research



Mendel was not the first to perform experiments with pea
plants
British farmers (stem height)
 Performed almost the same experiments
 Obtained the same results
 Over 200 years before
T A Knight (flower color)



Same experiments
Same results
1790’s
Gregor Mendel

Born in 1822 in the
Chech republic

Joined an Augustinian
order in 1843


Flunked out of college,
but made some great
friends who showed him
the value of good data
Using pea plants, found
indirect but observable
evidence of how parents
transmit genes to
offspring
The Garden Pea Plant



Self-pollinating
True breeding (different alleles
not normally introduced)
Can be experimentally crosspollinated
F1 Results of One
Monohybrid Cross
F2 Results of
Monohybrid Cross
Mendel’s
Monohybrid
Cross Results
F2 plants showed
dominant-torecessive ratio that
averaged 3:1
5,474 round
1,850 wrinkled
6,022 yellow
2,001 green
882 inflated
299 wrinkled
428 green
152 yellow
705 purple
224 white
651 long stem
207 at tip
787 tall
277 dwarf
Mendel’s Theory
of Segregation


An individual inherits a unit of
information (allele) about a
characteristic from each parent
During gamete formation, the alleles
segregate from each other
Now we know about Alleles, alternative forms of a gene
Punnett Square of a
Monohybrid Cross
Female gametes
A
Male
gametes
a
A
AA
Aa
a
Aa
aa
Dominant
phenotype can
arise 3 ways,
recessive only
one
Test Cross


Individual that shows dominant
phenotype is crossed with individual
with recessive phenotype
Examining offspring allows you to
determine the genotype of the dominant
individual
Punnett Squares of
Test Crosses
Homozygous
recessive
a
a
Homozygous
recessive
a
a
A
Aa
Aa
A
Aa
Aa
a
aa
aa
A
Aa
Aa
Two phenotypes
All dominant phenotype
A Dihybrid Cross - F1
Results
purple
flowers,
tall
TRUEBREEDING
PARENTS:
AABB
GAMETES:
AB
x
AB
white
flowers,
dwarf
aabb
ab
ab
AaBb
F1 HYBRID
OFFSPRING:
All purple-flowered, tall
16 Allele
Combinations in F2
1/4
AB
1/4
Ab
1/4
aB
1/4
ab
1/4
AB
1/4
Ab
1/4
aB
1/4
ab
1/16
1/16
1/16
1/16
AABB AABb AaBB AaBb
1/16
1/16
1/16
1/16
AABb AAbb AaBb Aabb
1/16
1/16
1/16
1/16
AaBB AaBb aaBB aaBb
1/16
1/16
1/16
1/16
AaBb Aabb aaBb aabb
Explanation of Mendel’s
Dihybrid Results
If the two
traits are
coded for by
genes
on separate
chromosomes,
sixteen gamete
combinations
are possible
1/4
AB
1/4
Ab
1/4
aB
1/4
ab
1/4
AB
1/4
Ab
1/4
aB
1/4
ab
1/16
1/16
1/16
1/16
AABB AABb AaBB AaBb
1/16
1/16
1/16
1/16
AABb AAbb AaBb Aabb
1/16
1/16
1/16
1/16
AaBB AaBb aaBB aaBb
1/16
1/16
1/16
1/16
AaBb Aabb aaBb aabb
Phenotypic Ratios in F2
AaBb
X
AaBb
Four Phenotypes:

Tall, purple-flowered
(9/16)

Tall, white-flowered
(3/16)

Dwarf, purple-flowered
(3/16)

Dwarf, white-flowered
(1/16)
Independent Assortment


Mendel concluded that the two “units”
for the first trait were to be assorted
into gametes independently of the two
“units” for the other trait
Members of each pair of homologous
chromosomes are sorted into gametes
at random during meiosis