Please explain these two Biology questions. The text book explanation was too...

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Anya Rainey

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...complicated for me to understand? I really want to learn the answers to these two questions. Please explain them to me as simply as possible so that I can understand. The textbook doesn’t do such a good job.

The gene for color blindness in humans is found on the X chromosome. A boy has a color-blind father. Will the boy be color bind? Explain.

Some biology students wanted to determine whether a pair of brown mice purchased at a pet store was homozygous dominant or heterozygous for fur color. They let that mice mate and examined the offspring. All six had brown fur. Some of the students felt that this was enough evidence to prove that the mice were homozygous for brown fur color. Other students did not, so another experiment was planned. Describe the next experiment the students could conduct to determine whether the parent mice are homozygous or heterozygous. Explain your answer.
 
1. The gene for color blindness, as it is found on the X chromosome, is a sex-linked genetic disorder. A boy has the XY combination, right? So if he has a screwed up gene on his X that is ONLY on the X, he's screwed. If a girl (XX) has one screwed up X-gene, then she'll be alright because the corresponding gene on her other X can pull her through.

I'm not sure how to answer your question though, as the boy is guaranteed to get his "X" from his mom and his "Y" from the dad - was his mother color-blind?

Not sure about the second one.
 
The male child (XY) will receive the X chromosome from his mother and the Y from his father. Since color-blindness is X-linked, he will not receive the color-blindness trait from his father (since he only receives the Y from his father). The only way the child could possibly be color blind is if the mother carries the trait on one of her X chromosomes.


During the original mating, even if both brown-furred mice were heterozygous (Bb) the chance that each of the offspring would be brown is 75% (draw a punnett square of Bb x Bb=> BB, Bb, Bb, bb offsrping). To better discern whether each of the mice are homozygous (BB) or heterozygous (Bb) for fur color, they should each be separately mated with an animal that is homozygous recessive (bb...let's call that a white mouse just for argument sake). If you do the cross of the homozygous brown (BB) with a homozygous recessive (bb):

BB x bb => 4Bb animals (all brown progeny)

So you can see from the above if the original brown mouse from the store is homozygous dominant for fur color (BB), all it's progeny would be brown when it's mated with a homozygous recessive (bb).
If you crossed a heterozygous brown mouse (Bb) with a homozygous recessive (bb):

Bb x bb => 2Bb, 2bb

So you can see from the above cross if the pet store animal was heterozygous for fur color (Bb) and it was mated with the homozygous recessive (bb) there's a 50% chance that a pup would have the recessive fur color (ie: white).
 
1) The boy could be color blind because if you do a punnet square for colorblindness as in Xb(subscript), xb(subscript) thre is a 50% chance that he could be color blind but theres most likely he will because the trait is said to be dominant.
 
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