Question

practice: dihybrid cross
complete a dihybrid punnett square for the following scenarios. for each scenario be sure to clearly list the genotypes of the parents, the phenotypic ratio, and answer any additional questions.
tall sunflowers are dominant to short while yellow - colored flowers are dominant to orange. cross a plant that is heterozygous for both traits with a plant that is heterozygous for height but homozygous dominant for flower color.
problem a
parent genotypes: x
phenotypic ratio:

1. a farmer knows that most people prefer tall and yellow flowers. if he plans to grow 100 sunflowers from these two parents, how many can be expected to be tall and yellow like he desires?
2. what should be the genotypes of the parent flowers (p generation) if he wants to ensure that all of the flowers in the f1 generation are tall and yellow?

problem b
in humans, free earlobes are dominant to attached earlobes, and a straight thumb is dominant to a hitchhiker’s thumb. cross two people that are heterozygous for both traits.
parent genotypes: x
phenotypic ratio:

Explanation:

Problem A

Step1: Determine parent genotypes

Let \(T\) represent tall (dominant) and \(t\) represent short, \(Y\) represent yellow (dominant) and \(y\) represent orange. One parent is heterozygous for both traits (\(TtYy\)) and the other is heterozygous for height and homozygous - dominant for flower color (\(TtYY\)).

Step2: Set up Punnett - square

For the cross \(TtYy\times TtYY\), we first find the gametes. The first parent (\(TtYy\)) can produce \(TY\), \(Ty\), \(tY\), \(ty\) gametes. The second parent (\(TtYY\)) can produce \(TY\) and \(tY\) gametes. The Punnett - square will have 8 boxes.

Step3: Fill in Punnett - square and find phenotypic ratio

	TY	tY
Ty	TTYy (tall, yellow)	TtYy (tall, yellow)
tY	TtYY (tall, yellow)	ttYY (short, yellow)
ty	TtYy (tall, yellow)	ttYy (short, yellow)

The phenotypic ratio of tall and yellow : short and yellow is \(6:2 = 3:1\).

Step4: Calculate number of desired plants

If the farmer grows 100 sun - flowers, and the proportion of tall and yellow is \(\frac{3}{4}\), the number of tall and yellow sun - flowers is \(100\times\frac{3}{4}=75\).

Step5: Determine genotypes for all tall and yellow \(F_1\)

To ensure all \(F_1\) are tall and yellow, the parent genotypes should be \(TTYY\times\) any genotype (since \(TTYY\) will pass on the dominant alleles for both traits). A common choice could be \(TTYY\times ttyy\) (the other parent being homozygous recessive for both traits) so that all \(F_1\) are \(TtYy\) (tall and yellow).

Problem B

Step1: Determine parent genotypes

Let \(E\) represent free earlobes (dominant) and \(e\) represent attached earlobes, \(S\) represent straight thumb (dominant) and \(s\) represent hitchhiker's thumb. The parents are both heterozygous for both traits, so their genotypes are \(EeSs\times EeSs\).

Step2: Set up Punnett - square

The first parent (\(EeSs\)) can produce \(ES\), \(Es\), \(eS\), \(es\) gametes. The second parent (\(EeSs\)) can also produce \(ES\), \(Es\), \(eS\), \(es\) gametes. The Punnett - square will be a \(4\times4\) grid with 16 boxes.

Step3: Fill in Punnett - square and find phenotypic ratio

	ES	Es	eS	es
Es	EESs (free, straight)	EEss (free, hitchhiker's)	EeSs (free, straight)	Eess (free, hitchhiker's)
eS	EeSS (free, straight)	EeSs (free, straight)	eeSS (attached, straight)	eeSs (attached, straight)
es	EeSs (free, straight)	Eess (free, hitchhiker's)	eeSs (attached, straight)	eess (attached, hitchhiker's)

The phenotypic ratio of free earlobes and straight thumb : free earlobes and hitchhiker's thumb : attached earlobes and straight thumb : attached earlobes and hitchhiker's thumb is \(9:3:3:1\).

Answer:

Problem A

Parent Genotypes: \(TtYy\times TtYY\)
Phenotypic Ratio: \(3\) tall and yellow : \(1\) short and yellow

1. 75
2. \(TTYY\times\) any (e.g., \(TTYY\times ttyy\))

Problem B

Parent Genotypes: \(EeSs\times EeSs\)
Phenotypic Ratio: \(9\) free earlobes and straight thumb : \(3\) free earlobes and hitchhiker's thumb : \(3\) attached earlobes and straight thumb : \(1\) attached earlobes and hitchhiker's thumb