MENDELIAN GENETICS. GARDEN PEAS Advantages seeds easy to obtain characters easy to score crosses easily controlled short generation time large numbers.

MENDELIAN GENETICSMENDELIAN GENETICS

GARDEN PEAS Advantages

• seeds easyto obtain

• characters easyto score

• crosses easilycontrolled

• short generationtime

• large numbers of progeny

• use of statisticsin analysis

• Unit factors (genes) exist in pairs (alleles)

• Dominance/Recessiveness

• Alleles segregate into gametes in equalfrequencies

• Alleles from different gene pairs assort independently into gametes

MENDEL'S POSTULATES

pure (true) breeding line- a group of genetically identical individuals that always produce offspring of the same phenotype when mated to each other

F1 generation

- first filial generation; the progeny resulting from the first cross in a series

F2 generation

- second filial generation; the progeny resulting from a cross of the F1 generation

monohybrid cross

- a genetic cross between two individuals involving only one character (e.g. AA x aa) in which the parents possess different alleles of the character

MONOHYBRID CROSS: PARENTAL AND F1

R_ = Round

R is dominant

rr = wrinkled

r is recessive

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F1 Gametes either R or r produced in equal frequencies

LAW OF EQUAL SEGREGATION

Parents

Gametes

F1 seeds

Gametes

(pure breeding)

CONCEPT OF DOMINANT AND RECESSIVE ALLELES

Round x wrinkled

F1 all Round (selfed or crossed to each other)

F2 3/4 Round; 1/4 wrinkled

Mendel concluded that one phenotype of the pea shape character (wrinkled) is hidden in the F1 and reappears in the F2.

Seed Parent

Rr

Pollen Parent

Rr

XF1

1/2 R 1/2 r 1/2 R 1/2 r

1/4 RR 1/4 Rr 1/4 Rr 1/4 rr

1/2 Rr

3/4 Round 1/4 wrinkled

Genotypic Ratio = 1: 2: 1 Phenotypic Ratio = 3: 1

Mendel explained his data by invoking the concept of genes:

- each pea plant has a pair of alleles for one character;one allele is dominant and the other is recessive

Mendel’s First Law - Law of Equal Segregation

- different alleles of one gene segregate into different gametes in equal frequencies (i.e. each gamete carries only one allele of a gene)

- the union of gametes to make a zygote is random (it doesn’t matter which allele is in each gamete)

Proof: testcross - crossing a homozygous recessive individual to anindividual of unknown genotype to determine the unknown genotype (e.g. R? x rr)

Parents RR x rr

F1 Rr (all round seeds)

F2 3:1 R_ : rr (phenotypic ratio)

Testcross (cross F1 individuals to homozygous recessive)

Rr x rr

Gametes R or r r

1:1 Rr : rr (phenotypic ratio)

If F1 were RR, all progeny from the test cross would be Rr(phenotypically round)

dihybrid cross

- a genetic cross involving two phenotypic characters in which the parents possess different alleles of each character (e.g. round, green x wrinkled, yellow peas)

DIHYBRID CROSS

R_ Round

rr wrinkled

Y_ Yellow

yy green

Parents

Gametes

F1 Progeny

F1 Gametes

Mendel’s Second Law - Independent Assortment

- different gene pairs assort independently into gametes

(pure breeding)

PHENOTYPES OF F2 PROGENY IN DIHYBRID CROSS

Phenotypic Ratio Phenotypes Alleles Present

9 round, yellow R_Y_3 round, green R_yy3 wrinkled, yellow rrY_1 wrinkled, green rryy

Punnett Square

The 9:3:3:1 phenotypic ratio observed in the F2 is created by the random superimpositionof two independent 3:1 phenotypic ratios.

3/4 R_ 3/4 Y_

1/4 rr 1/4 yy

9/16 R_Y_

3/16 R_yy

3/16 rrY_

1/16 rryy

F1 cross: RrYy x RrYy

MENDEL IGNORED FROM 1866-1900

• Mendel was an unknown researcher

• Darwin’s Origin of Species (1859)

- continuous not discontinuous variation

• Cytology of time could not explain hypothesis

- chromosomal theory of heredity developed by Theodore Boveri and Walter Sutton in early 1900's clearly showed link between chromosome segregation during meiosis and Mendel's unit factors (genes)

• Unable to replicate results in all organisms

MENDEL REDISCOVERED IN 1900

by Carl Correns, Hugo DeVries, Eric Von Tschermak

Using branch (forked) diagrams to determine expected phenotypic ratios from a cross

Parental cross RrYy x Rryy(R and Y are dominant alleles)

1/2 Y_ 3/8 R_Y_R_ 3/4

1/2 yy 3/8 R_yy

1/2 Y_ 1/8 rrY_rr 1/4

1/2 yy 1/8 rryy

• calculate frequencies for individual genes separately* because events are independent multiply probabilities

In the cross Aa bb CC Dd Ee x Aa Bb Cc dd Ee,what proportion of the progeny will have the genotypeAA bb CC dd EE?

• calculate frequencies for individual genes

Aa x Aa 1/4 of the progeny will be AA

bb x Bb 1/2 of the progeny will be bb

CC x Cc 1/2 of the progeny will be CC

Dd x dd 1/2 of the progeny will be dd

Ee x Ee 1/4 of the progeny will be EE

* because events are independent multiply probabilities

Probability of AA bb CC dd EE individuals = 1/4 x 1/2 x 1/2 x 1/2 x 1/4 = 1/128

In the cross AaBbCc x AaBbCC the expected number of

gametes = 2 x 2 x 2 = 8 and 2 x 2 x 1 = 4

genotypes = 3 x 3 x 2 = 18

phenotypes = 2 x 2 x 1 = 4

Assumes complete dominance and recessiveness for all gene pairs and independent assortment.

Don't blindly use 2n and 3n rules described in text; they can’t be used in situations such as the one above