Question

dihybrid practice problems

1. in man, assume that spotted skin (s) is dominant over non-spotted skin (s) and that wooly hair (w) is dominant over non-wooly hair (w). cross a marriage between a heterozygous spotted, non-wooly man with a heterozygous wooly-haired, non-spotted woman. give genotypic and phenotypic ratios of offspring.

genotypes:
results (ratios)
genotypes:
phenotypes:

1. in horses, black is dependent upon a dominant gene, b, and chestnut upon its recessive allele, b. the trotting gait is due to a dominant gene, t, the pacing gait to its recessive allele, t. if a homozygous black pacer is mated to a homozygous chestnut trotter, what will be the appearance of the f₁ generation?

genotypes
results (ratios)
genotypes:
phenotypes:

1. in summer squash, white fruit color (w) is dominant over yellow fruit color (w) and disk-shaped fruit (d) is dominant over sphere-shaped fruit (d). if a squash plant true-breeding for white, disk-shaped fruit is crossed with a plant true-breeding for yellow, sphere-shaped fruit,

genotypes
results (ratios)
genotypes:
phenotypes:

Explanation:

Problem 1

Step1: Identify parent genotypes

Man: $Ssww$; Woman: $ssWw$

Step2: Find gamete combinations

Man's gametes: $Sw, sw$; Woman's gametes: $sW, sw$

Step3: Fill Punnett Square

	$Sw$	$sw$
$sw$	$Ssww$	$ssww$

Step4: Count genotypic ratios

1 $SsWw$ : 1 $ssWw$ : 1 $Ssww$ : 1 $ssww$

Step5: Count phenotypic ratios

1 spotted, wooly : 1 non-spotted, wooly : 1 spotted, non-wooly : 1 non-spotted, non-wooly

Step1: Identify parent genotypes

Black pacer: $BBtt$; Chestnut trotter: $bbTT$

Step2: Find gamete combinations

Black pacer gametes: $Bt$; Chestnut trotter gametes: $bT$

Step3: Fill Punnett Square

	$Bt$

Step4: Determine F₁ genotype/phenotype

All offspring are $BbTt$, showing dominant traits

Step1: Identify parent genotypes

True-breeding white disk: $WWDD$; True-breeding yellow sphere: $wwdd$

Step2: Find gamete combinations

White disk gametes: $WD$; Yellow sphere gametes: $wd$

Step3: Fill Punnett Square

	$WD$

Step4: Determine F₁ genotype/phenotype

All offspring are $WwDd$, showing dominant traits

Answer:

Genotypes (ratio): $SsWw:ssWw:Ssww:ssww = 1:1:1:1$
Phenotypes (ratio): Spotted wooly : Non-spotted wooly : Spotted non-wooly : Non-spotted non-wooly = $1:1:1:1$

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Problem 2