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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 .

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mendelian genetics

Mendelian Genetics

How pea plants and humans mix it up

early ideas about heredity
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
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
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
The Garden Pea Plant
  • Self-pollinating
  • True breeding (different alleles not normally introduced)
  • Can be experimentally cross-pollinated
mendel s monohybrid cross results
Mendel’s Monohybrid Cross Results

5,474 round

1,850 wrinkled

6,022 yellow

2,001 green

882 inflated

299 wrinkled

428 green

152 yellow

F2 plants showed dominant-to-recessive ratio that averaged 3:1

705 purple

224 white

651 long stem

207 at tip

787 tall

277 dwarf

mendel s theory of segregation
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
punnett square of a monohybrid cross

Female gametes

A a

A

AA

Aa

Male

gametes

a

Aa

aa

Punnett Square of a Monohybrid Cross

Dominant

phenotype can

arise 3 ways,

recessive only

one

test cross
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
slide14

Homozygous

recessive

Homozygous

recessive

a a

a a

A

A

Aa

Aa

Aa

Aa

a

A

aa

Aa

aa

Aa

Punnett Squares of Test Crosses

Two phenotypes

All dominant phenotype

a dihybrid cross f 1 results
A Dihybrid Cross - F1 Results

purple

flowers, tall

white

flowers,

dwarf

TRUE-

BREEDING

PARENTS:

AABB

x

aabb

GAMETES:

AB

AB

ab

ab

AaBb

F1 HYBRID

OFFSPRING:

All purple-flowered, tall

16 allele combinations in f 2

ab

ab

aB

AB

AB

Ab

Ab

aB

16 Allele Combinations in F2

1/4

1/4

1/4

1/4

1/4

1/16

1/16

1/16

1/16

AABB

AABb

AaBB

AaBb

1/4

1/16

1/16

1/16

1/16

AABb

AAbb

AaBb

Aabb

1/4

1/16

1/16

1/16

1/16

AaBB

AaBb

aaBB

aaBb

1/4

1/16

1/16

1/16

1/16

AaBb

Aabb

aaBb

aabb

explanation of mendel s dihybrid results

ab

ab

aB

AB

AB

Ab

Ab

aB

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

1/4

1/4

1/4

1/4

1/16

1/16

1/16

1/16

AABB

AABb

AaBB

AaBb

1/4

1/16

1/16

1/16

1/16

AABb

AAbb

AaBb

Aabb

1/4

1/16

1/16

1/16

1/16

AaBB

AaBb

aaBB

aaBb

1/4

1/16

1/16

1/16

1/16

AaBb

Aabb

aaBb

aabb

phenotypic ratios in f 2
Phenotypic Ratios in F2

Four Phenotypes:

  • Tall, purple-flowered (9/16)
  • Tall, white-flowered (3/16)
  • Dwarf, purple-flowered (3/16)
  • Dwarf, white-flowered (1/16)

AaBbX

AaBb

independent assortment
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
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