Gregor Mendel – 1822-1884

Gregor Mendel – 1822-1884

Mendelian Genetics

Asexual Reproduction

• Bacteria can reproduce as often as every 12 minutes – and may go through 120 generations in one day

• Thus capable of producing 6 x 1035 offspring per day

• Bacteria often produce 1 mutation per 1000 replications of DNA

• So for fast-growing species, mutation is a good way to respond to a changing environment

JohnMaynardSmith

Why Sex?

Sexual reproduction leads to genetic variation via:

• Independent assortment during meiosis

• Crossing over during meiosis

• Random mixing of gametes (sperm and egg)

Independent Assortment

Prophase Iof meiosis

Nonsister chromatidsheld togetherduring synapsis

Pair of homologs

Chiasma

Centromere

TEM

Anaphase I

Anaphase II

Daughtercells

Recombinant chromosomes

• The random nature of fertilization adds to the genetic variation arising from meiosis.

• Any sperm can fuse with any egg.– A zygote produced by a mating of a woman and

man has a unique genetic identity.– An ovum is one of approximately 8,388,608

possible chromosome combinations (223).– The successful sperm represents one of 8,388,608

different possibilities (223).– The resulting zygote is composed of 1 in 70 trillion

(223 x 223) possible combinations of chromosomes.– Crossing over adds even more variation to this.

Gregor Mendel – 1822-1884

Mendelian Genetics

Two possible types of inheritance

• One possible explanation of heredity is a “blending” hypothesis– The idea that genetic material contributed by two

parents mixes in a manner analogous to the way blue and yellow paints blend to make green

• An alternative to the blending model is the “particulate” hypothesis of inheritance: the gene idea– Parents pass on discrete heritable units, later

known as genes

Mendel’s garden at Brunn (Brno) Monastery

Mendel’s time Today

Some genetic vocabulary

– Character: a heritable feature, such as flower color

– Trait: a variant of a character, such as purple or white flowers

Garden Pea

Flower Structure

Parentalgeneration(P) Stamens

Carpel

First filialgenerationoffspring(F1)

TECHNIQUE

RESULTS

3

2

1

4

5

In Mendel’s Experiments:

• Mendel chose to track– Only those characters that varied in an “either-or”

manner• Mendel also made sure that

– He started his experiments with varieties that were “true-breeding”

• In a typical breeding experiment– Mendel mated two contrasting, true-breeding

varieties, a process called hybridization

Breeding Terminology

• The true-breeding parents

– Are called the P (parental) generation

• The hybrid offspring of the P generation

– Are called the F1 (filial) generation

• When F1 individuals self-pollinate

– The F2 generation is produced

P Generation

EXPERIMENT

(true-breedingparents) Purple

flowersWhite

flowers

P Generation

EXPERIMENT

(true-breedingparents)

F1 Generation(hybrids)

Purpleflowers

Whiteflowers

All plants had purple flowers

Self- or cross-pollination

P Generation

EXPERIMENT

(true-breedingparents)

F1 Generation(hybrids)

F2 Generation

Purpleflowers

Whiteflowers

All plants had purple flowers

Self- or cross-pollination

705 purple-flowered

plants

224 whiteflowered

plants

Mendel developed a hypothesis to explain his results that consisted of four ideas

• Alternative versions of genes (different alleles) account for variations in inherited characters

• For each character, an organism inherits two alleles, one from each parent

• If two alleles differ, then one, the dominant allele, is fully expressed in the organism’s appearance. The other, recessive allele has no effect on a hybrid organism’s appearance

• The two alleles for each character segregate (separate) during gamete formation

Law of SegregationP Generation

Appearance:Genetic makeup:

Gametes:

Purple flowers White flowersPP pp

P p


F1 Generation


Gametes:


Gametes:

Purple flowers White flowers

Purple flowersPp

PP pp

P

P

p

p1/21/2


F1 Generation

F2 Generation


Gametes:


Gametes:

Purple flowers White flowers

Purple flowers

Sperm from F1 (Pp) plant

Pp

PP pp

P

P

P

P

p

p

p

p

Eggs from F1 (Pp) plant

PP

ppPp

Pp

1/21/2

3 : 1

Phenotype

Purple

Purple

Purple

White

3

1

1

1

2

Ratio 3:1 Ratio 1:2:1

Genotype

PP(homozygous)

Pp(heterozygous)

Pp(heterozygous)

pp(homozygous)

Test cross

Dominant phenotype,unknown genotype:

PP or Pp?

Recessive phenotype,known genotype:

pp

PredictionsIf purple-floweredparent is PP

If purple-floweredparent is Pp

or

Sperm Sperm

Eggs Eggs

or

All offspring purple 1/2 offspring purple and 1/2 offspring white

Pp Pp

Pp Pp

Pp Pp

pp pp

p p p p

P

P

P

p

TECHNIQUE

RESULTS

P Generation

F1 Generation

Predictions

Gametes

EXPERIMENT

RESULTS

YYRR yyrr

yrYR

YyRr

Hypothesis ofdependent assortment

Hypothesis ofindependent assortment

Predictedoffspring ofF2 generation

Sperm

Spermor

EggsEggs

Phenotypic ratio 3:1

Phenotypic ratio 9:3:3:1

Phenotypic ratio approximately 9:3:3:1315 108 101 32

1/21/2

1/2

1/2

1/41/4

1/41/4

1/4

1/4

1/4

1/4

9/163/16

3/161/16

YR

YR

YR

YRyr

yr

yr

yr

1/43/4

Yr

Yr

yR

yR

YYRR YyRr

YyRr yyrr

YYRR YYRr YyRR YyRr

YYRr YYrr YyRr Yyrr

YyRR YyRr yyRR yyRr

YyRr Yyrr yyRr yyrr

Segregation ofalleles into eggs

Segregation ofalleles into sperm

Sperm

Eggs

1/2

1/2

1/21/2

1/41/4

1/41/4

Rr Rr

R

R

RR

R

R

r

r

r

r r

r

Probability of YYRR

Probability of YyRR

1/4 (probability of YY)

1/2 (Yy)

1/4 (RR)

1/4 (RR)

1/16

1/8

Probability of YYRR

Probability of YyRR


1/2 (Yy)

1/4 (RR)

1/4 (RR)

1/16

1/8

Probability of yyrr = ?A. 1/8 B. 1/16 C. 1/32

Probability of YYRR

Probability of YyRR


1/2 (Yy)

1/4 (RR)

1/4 (RR)

1/16

1/8

Probability of YYrr = ?A. ¼ B. 1/8 C. 1/16

Probability of YYRR

Probability of YyRR


1/2 (Yy)

1/4 (RR)

1/4 (RR)

1/16

1/8

Probability of YxRr = ?(x can be Y or y)A. ½ B. 3/4 C. 3/8 D. 1/16

Chance of at least two recessive traits

ppyyRr

ppYyrr

Ppyyrr

PPyyrrppyyrr

1/4 (probability of pp) 1/2 (yy) 1/2 (Rr) 1/4 1/2 1/2 1/2 1/2 1/2 1/4 1/2 1/2 1/4 1/2 1/2

1/16

1/16 2/16

1/16

1/16

6/16 or 3/8

Documents

Gregor Mendel – 1822-1884