l A geneticist is studying two traits in the fruit fly eye c

l. A geneticist is studying two traits in the fruit fly eye color and wing shape. They perform a dihybrid cross. If the genes for these traits are not linked, what proportion of the F2 offspring will be expected to display the dominant phenotype for eye color and wing shape? a. 1/2 b. 3/4 e. This can not be determined; more information about genotype is needed 2. In one strain of mouse, coat color can be black or white. Black is the dominant phenotype and white is the recessive phenotype. Given a mouse with a black coat (the dominant phenotype) which of the following crosses would a geneticist perform to determine the genotype of the mouse? a. cross this mouse with one that has a black coat b. cross this mouse with one that has a white coat c. cross this mouse with a sibling that also has a black coat d. cross this mouse with one that is true-breeding for a black coat e. none of the crosses listed will work, but some other mating(s (not given) will work. Questions 3 5 are based on the following information A student is studying 4 unlinked genes (referred to simply as gene 1, gene 2, gene 3, and gene 4. The alleles of gene 1 are represented as A and a; the alleles for gene 2 are represented with the letters B and b; the alleles of gene 3 are C and c; the alleles for gene 4 are D and d. An individual (P1) with the genotype Aa Bb Cc Dd is crossed to an individual (P2) with the genotype Aa bb Cc dd 3. What proportion of the gametes from the second individual 2) are expected to have all recessive alleles? a. 1/2 c. 1/6 d. 1/8

Solution

A geneticist is studying two traits in the fruit fly eye color and wing shape. They perform a dihybrid cross . if the genes for these traits are not linked what proportion of the f2 offspring will be expected to display the dominant phenotype for eye color and wing shape

Answer = 9/16

expected ratio =9:3:3:1 phenotypic ratio in the F2 generation

9/16 ---------dominant for eye color , dominant for wing shape

3/16 --------dominant for wing shape, recessive for eye color

3/16 ----------recessive for wing shape , dominant for eye color

1/16 ---------–recessive for wing shape , recessive for eye color

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2. answer .b cross this mouse one that has white coat

It is a TEST CROSS

Black Bb      x bb white

RATIO 1:1

====================

Black BB      x bb white

ALL OFFSPRING WILL BE BLACK

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3

P1-------GENEOTYPE     Aa Bb     Cc Dd

P2-------GENEOTYPE     Aa bb     Cc dd

Number of types of gametes =2^n

N= number of heterozygous loci

What proportion of the gametes form the second individual P2 are expected to have all recessive allele

P2-------GENEOTYPE     Aa bb     Cc dd

Number of types of gametes =2^n

N= number of heterozygous loci =2

2^n =4------types of gametes

Aa

bb

Cc

dd

Aa

bb

Cc

dd

½                     ½

1

½               ½

1

Hence =1/2 *1/2 =1/4 -------answer

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4 Answer =16 x8 =128 cells

P1

P2

GENEOTYPE     Aa Bb     Cc Dd

GENEOTYPE     Aa bb     Cc dd

2^4 =16------types of gametes

N= number of heterozygous loci =2

2^n =4------types of gametes

4X2 =8

16

8

Answer =16 x4 =64 cells

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5

P1

P2

GENEOTYPE     Aa Bb     Cc Dd

GENEOTYPE     Aa bb     Cc dd

Aa

½

Aa

1/2

1/4

Ba

½

bb    

1

1/2

Cc

½

Cc

1/2

1/4

Dc

½

dd

1

1/2

0.015625

Answer =1/16 x1/4 =64

There are 36 distinct genotypes

aabbccdd: 4
aabbccdd: 4
aabbccdd: 8
aabbccdd: 4
aabbccdd: 8
aabbccdd: 8
aabbccdd: 4
aabbccdd: 8
aabbccdd: 8
aabbccdd: 8
aabbccdd: 16
aabbccdd: 8
aabbccdd: 16
aabbccdd: 16
aabbccdd: 8
aabbccdd: 16
aabbccdd: 4
aabbccdd: 4
aabbccdd: 4
aabbccdd: 4
aabbccdd: 8
aabbccdd: 8
aabbccdd: 8
aabbccdd: 8
aabbccdd: 4
aabbccdd: 4
aabbccdd: 8
aabbccdd: 4
aabbccdd: 8
aabbccdd: 8
aabbccdd: 4
aabbccdd: 8
aabbccdd: 4
aabbccdd: 4
aabbccdd: 4
aabbccdd: 4

	Aa	bb	Cc	dd
	Aa	bb	Cc	dd
	½                     ½	1	½               ½	1