Chapter 3 partI

8/7/2019 Chapter 3 partI

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Chapter 3 Independent

Assortment of Genes

Dihybrid Crosses


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Independent Assortment of Genes

What about looking at the inheritance of more then onegene in combination with other genes? How are allelesrecombined in gametes?

Recombination = bringing together of two or more pairsof alleles into new combinations

Recombinant products with new combinations ofparental alleles are generated by:

independent assortment of alleles on nonhomologouschromosomes depending how they line up metaphase I

Crossing over between genes occurs in meiosis I


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Independent assortmentConsequence of independent alignment

of chromosomes in meiotic bivalents

A/A ; B/B x a/a ; b/b

A/a ; B/b

A ; B P

A ; b R

a ; B R

a;b

P

OR

Alternate bivalants

A

Bb B

a aA

b


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Recombination For genes on different (nonhomologous) pairs ofchromosomes, recombinant frequency is always 50%

A/a

B/b

a/a

b/b

A/A

B/B

A

B

a

b

A

B

a

b

A

b

a

B

parental (P) genotypes recombinant (R) genotypes

parental genotypes

meiosis meiosis

meiosis


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What Dihybrid crosses tell us

Alleles at different loci(chromosomes)

segregate independently of one another

Hetrozygous both

In dihybrid meiosis, 50% recombinants

indicates either that genes are on different

chromosomes or that they are far apart on

the same chromosome. Predictable ratios for progeny of dihybrids


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Dihybrid testcross

Best way to study recombination is in adihybrid testcross

only dihybrid produces recombinant genotypes

all homozygous recessive tester gametes alike

Typical 1:1:1:1 ratio a result of independentassortment in dihybrid

Observed by Mendel in testcrosses with twopairs of traits


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Dihybrid testcross (1) Determines genotype of dihybrid by

crossing to homozygous recessive

testerA/A ; b/b x a/a ; B/B

A/a ; B/b x a/a ; b/b testcross

Parental

F1

F1 gametes

tester

gametes

a ; b

progeny

proportions

progeny

phenotypes

A ; B A/a ; B/b AB

A ; b A/a ; b/b A b

a ; B a/a ; B/b a B

a ; b a/a ; b/b a b

1:1:1:1

ratio


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Dihybrid selfing Cross between two A/a ; B/b dihybrids

recombination occurs in both members of cross

recombination frequency is 50%

A ; B A ; b a ; B a ; b

A ; B A/A ; B/B A/A ; B/b A/a ; B/B A/a ; B/b

A ; b A/A ; B/b A/A ; b/b A/a ; B/b A/a ; b/b

a ; B A/a ; B/B A/a ; B/b a/a ; B/B a/a ; B/b

a ; b A/a ; B/b A/a ; b/b a/a ; B/b a/a ; b/b

Ratio: 9 A/ ; B/ 3 A/ ; b/b 3 a/a ; B/ 1 a/a ; b/b


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Deducing genotypes from ratios

Genetic analysis works in two directions predict genotypes in offspring

determine genotypes of parents in cross

Specific expectations, e.g., 1:1:1:1 and 9:3:3:1

can be used to deduce genotypes

Testcross example:

Phenotype # observed

A/ ; B/ 310

A/ ; b/b 295

a/a ; B/ 305

a/a ; b/b 290

The observed results are

close to 1:1:1:1, allowingthe deduction that the

tested genotype was a

dihybrid.


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Read text pg 89-109

Try these problems

Problem #1 page 121

Problem #9 page 121

Problem # 17 page122

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Chapter 3 partI