Question

practice 3
answer the following questions about monohybrid crosses. darken the circle that corresponds to your
answer choice.

1. if a professor crosses two heterozygous tall pea plants, what are the most probable genotypic outcomes that he

will observe?
a 25% homozygous tall, 50% heterozygous, 25% homozygous short
b 50% homozygous tall, 25% heterozygous, 25% homozygous short
c 75% heterozygous, 25% homozygous short
d 100% heterozygous

1. two heterozygous tall pea plants are crossed. what is the expected phenotypic outcome for the offspring?

a 100% tall b 50% tall, 50% short c 75% tall, 25% short d 100% short

1. in fruit flies, straight wings (s) are dominant, and curled wings (s) are recessive. a fruit fly with a genotype of

ss is crossed with a fruit fly with a genotype of ss. if the parent flies have 20 offspring, about how many will
have curled wings?
a 5 b 10 c 15 d 20

1. in fruit flies, straight wings are dominant over curled wings. a homozygous fruit fly is crossed with a

heterozygous fruit fly. a punnett square predicts what percentage of the offspring will be heterozygous?
a 25% b 50% c 75% d 100%

1. in guinea pigs, short fur (s) is dominant, and long fur (s) is recessive. a female guinea pig with short fur is

mated with a male with long fur. half of the guinea pigs in the litter have short fur, and the other half have long
fur. what are the genotypes of the mother and father guinea pigs?
a ss and ss b ss and ss c ss and ss d ss and ss

1. in guinea pigs, brown fur is dominant to white fur. if conducting a testcross with a female brown guinea pig,

which of the following male guinea pigs would you select for the experiment?
a homozygous white b homozygous brown c heterozygous white d heterozygous brow

Explanation:

Step1: Analyze the cross of two heterozygous tall pea plants (Tt x Tt)

Using a Punnett square:

	T	t
t	Tt	tt

The genotypes are \( TT:Tt:tt = 1:2:1 \). Converting to percentages: \( 25\%\) homozygous tall (\( TT\)), \( 50\%\) heterozygous (\( Tt\)), \( 25\%\) homozygous short (\( tt\)).

Step2: Determine the phenotypic outcome of Tt x Tt

Since \( T\) (tall) is dominant over \( t\) (short), \( TT\) and \( Tt\) are tall. The ratio of tall to short is \( 3:1\), so \( 75\%\) tall and \( 25\%\) short.

Step3: Cross \( Ss\times ss\)

Using a Punnett square:

	S	s

The probability of \( ss\) (curled wings) is \( 50\%\). For \( 20\) offspring, \( 20\times0.5 = 10\).

Step4: Cross \( SS\times Ss\)

Using a Punnett square:

	S	S
s	Ss	Ss

The ratio of heterozygous (\( Ss\)) is \( 50\%\).

Step5: Analyze the guinea - pig cross

The male has long fur (\( ss\)). Since half the offspring have short fur (\( Ss\)) and half have long fur (\( ss\)), the female must be \( Ss\).

Step6: Understand test - cross

A test - cross is used to determine the genotype of an individual with a dominant phenotype. It is done with a homozygous recessive individual. For brown fur (dominant), the test - cross is with homozygous white (recessive).

Answer:

1. A. \( 25\%\) homozygous tall, \( 50\%\) heterozygous, \( 25\%\) homozygous short
2. C. \( 75\%\) tall, \( 25\%\) short
3. B. \( 10\)
4. B. \( 50\%\)
5. D. \( Ss\) and \( ss\)
6. A. homozygous white