Question

general biology
biol& 160

genetics problems

1. in humans, brown eyes (b) are dominant over blue (b)*. a brown-eyed man marries a blue-eyed woman and they have three children, two of whom are brown-eyed and one of whom is blue-eyed.

(* actually, the situation is complicated by the fact that there is more than one gene involved in eye color, but for this example, well consider only this one gene.)

a) if blue is recessive, what must the womans genotype be?
a. bb
b) if thats her genotype, what kind(s) of gametes (eggs) can she produce?
a. only b
c) if the man has brown eyes, but has a blue-eyed child what must his genotype be?
a. bb
d) if thats his genotype, what kind(s) of gametes (sperm) can he produce?
a. b and b
e) draw the punnett square to show the possible genotypes of the offspring.

50% - bb
50% - bb

1. in dogs, there is an hereditary deafness caused by a recessive gene, \d.\ a kennel owner has a male dog that she wants to use for breeding purposes if possible. the dog can hear, so the owner knows his genotype is either dd or dd. if the dogs genotype is dd, the owner does not wish to use him for breeding so that the deafness gene will not be passed on. this can be tested by breeding the dog to a deaf female (dd).

draw the punnett squares to illustrate these two possible crosses. in each case, what percentage/how many of the offspring would be expected to be hearing? deaf? how could you tell the genotype of this male dog? also, using punnett square(s), show how two hearing dogs could produce deaf offspring.

ddxdd: 100% hearing, 0% deaf

Explanation:

🆕 New Concept Discovered: Mendelian Genetics & Punnett Squares
Predicting offspring traits using dominant and recessive alleles.

Step 1: Analyze the first cross (DD x dd)

The male dog can hear, so his genotype is either homozygous dominant (\(DD\)) or heterozygous (\(Dd\)). The female dog is deaf, which is a recessive trait (\(dd\)).

If the male dog is \(DD\):

	d	d
D	Dd	Dd

• Genotypes of offspring: 100% \(Dd\) (heterozygous)
• Phenotypes of offspring: 100% hearing, 0% deaf

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Step 2: Analyze the second cross (Dd x dd)

If the male dog is \(Dd\):

	d	d
d	dd	dd

• Genotypes of offspring: 50% \(Dd\), 50% \(dd\)
• Phenotypes of offspring: 50% hearing, 50% deaf

---

Step 3: Determine how to identify the male dog's genotype

To determine the male dog's genotype, the owner performs a testcross by breeding him with a deaf female (\(dd\)):

• If any of the resulting puppies are deaf (\(dd\)), the male dog must carry the recessive allele and have the genotype \(Dd\).
• If a large number of puppies are born and all of them can hear, the male dog is highly likely to have the genotype \(DD\).

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Step 4: Show how two hearing dogs can produce deaf offspring

For two hearing dogs to produce a deaf puppy (\(dd\)), both parents must carry the recessive deafness allele without showing the trait themselves. This means both parents must be heterozygous (\(Dd\)):

	D	d
d	Dd	dd

• Offspring Phenotypes: 75% hearing (\(DD\) or \(Dd\)), 25% deaf (\(dd\))

---

Answer:

1. Punnett Squares for the Two Possible Crosses:

• Case 1: \(DD \times dd\)

$$
LATEXBLOCK0
$$

• Expected Offspring: 100% hearing, 0% deaf.

• Case 2: \(Dd \times dd\)

$$
LATEXBLOCK1
$$

• Expected Offspring: 50% hearing, 50% deaf.

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2. How to Tell the Genotype of the Male Dog:

Breed the male dog with a deaf female (\(dd\)). If even a single puppy in the litter is born deaf, the male dog's genotype is \(Dd\). If all offspring can hear (across a statistically significant sample size), his genotype is \(DD\).

---

3. How Two Hearing Dogs Can Produce Deaf Offspring:

Both hearing parents must be carriers of the recessive trait (heterozygous, \(Dd\)).

$$
LATEXBLOCK2
$$

This cross yields a 25% chance of producing a deaf offspring (\(dd\)).