Genetics Problems

[silveryhair]

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Can anyone explain how to solve these problems? I am really bad at genetics. Thanks in advance!

1. In Mendel's peas, yellow seed color (Y) is dominant to green 👍. To identify most quickly the genotype of yellow-seeded pea plants as either homozygous dominant (YY) or heterozygous (Yy), you could do a test cross with plants of genotype ______. Explain why this genotype is best vs. other genotypes.


2. The common grackle is a species of robin-sized blackbirds that are fairly common (hence the name) over most
of the United States. Suppose that long tails (L) were dominant to short tails in these birds. A female short-tailed
grackle mates with a male long-tailed grackle who had one parent with a long tail and one parent with a short tail.
What is the male's genotype? How many of the resulting offspring will have short tails?


3. In northeast Kansas there is a creature know as a wildcat. It comes in three colors, blue, red, and purple. Blue
wildcats crossed with red wildcats always produce purple wildcats. Two purple wildcats when crossed produce
blue, red, and purple offspring in the ratio of 1 blue: 2 purple: 1 red. What pattern of inheritance does this trait
fall under? Would it be possible to produce a pure breeding strain of purple wildcats? Explain


4. A woman with type A blood marries a man of unknown blood type. They produce 3 children with the following blood types: Type A, Type A, and O. What is the mother's genotype? What are the possible blood types for the
father? Identify (and explain why) what the most likely blood type is for this male. What can be said regarding part of his genotype?


5. In fruit flies, red eyes are dominant over purple eyes. Tan body color is dominant over ebony body color. If you mated a completely heterozygous tan fly with red eyes, with a heterozygous tan fly with purple eyes, what
would the offspring look like?

 

[chromosome21]

some of these questions are really easy. I'll try to answer them.

Can anyone explain how to solve these problems? I am really bad at genetics. Thanks in advance!

1. In Mendel's peas, yellow seed color (Y) is dominant to green 👍. To identify most quickly the genotype of yellow-seeded pea plants as either homozygous dominant (YY) or heterozygous (Yy), you could do a test cross with plants of genotype ______. Explain why this genotype is best vs. other genotypes.
When you see the words "test cross" or "back cross" , you should immediately realize that homozygous recessive genotype is being used to find the genotypes of the given thing. Now, what is the meaning of a "test"? In science, we always perform "tests" to "taste" something! jk!! We generally perform a test on an individual genotype for which we are not sure if the phenotypic trait is b/c of the homozygous dominant (here, YY) or hetrozygous (Yy). In short, we are given Y_ scenario. When we cross this given genotype (Y_) with a test cross - homozygous recessive - yy, let's see what we end up with:

IF we have homozygous dominant (YY):

P: YY x yy
F: Yy (all offsprings are heterozygous, and the their phenotype are the same b/c of the dominant Y gene)

If we have heterozygous parent (Yy):

P: Yy x yy
F: 50% Yy : 50% yy (it means 50% offsprings are hetrozygous, and 50% will be homozygous recessive)

So b/c of the test cross we could easily find out what genotype the parent had contained. hope this makes sense?


2. The common grackle is a species of robin-sized blackbirds that are fairly common (hence the name) over most
of the United States. Suppose that long tails (L) were dominant to short tails in these birds. A female short-tailed
grackle mates with a male long-tailed grackle who had one parent with a long tail and one parent with a short tail.
What is the male's genotype? How many of the resulting offspring will have short tails?
Can someone plz tell me when we use autosomal recessive vs. the sex-linked?

Here's how I tried to solve this question:

step 1: first thing I noticed: Long tails dominant over short tails. therefore, L_ vs. ll

step 2: secondly, a possible given genotypes: a female short-taliled ll vs. male L_

step 3: Male long-tailed grackle had one parents with a long tail (L_) and another parent with a short tail (ll)
therefore, L_ x ll
and our male grackle is long trailed which is dominant, so he must have received that one L from one of his parents, and b/c another parents had recessive phenotype/genotype (ll), he must receive one "l" from one of the parents. Therefore, our male grackle must have a genotype of Ll

step 4: let's go back to step 2 and re-write the genotypes of our female tailed grackle. and from step 3 we got genotype of our male grackle.

P: ll x Ll
F: 50% Ll : 50% ll

(you can make that little table to get to the F generation. It'd be easier to visualize it that way)

(this cross we did, does it sound familiar? it's kinda like test cross we did in problem #1. Here, we are not doing any test cross, it's just that the genotypes of one of the parents are happen to match to the test cross genotypes)

to answer your question now, there are 50% offsprings which will have ll short-something trait.

3. In northeast Kansas there is a creature know as a wildcat. It comes in three colors, blue, red, and purple. Blue
wildcats crossed with red wildcats always produce purple wildcats. Two purple wildcats when crossed produce
blue, red, and purple offspring in the ratio of 1 blue: 2 purple: 1 red. What pattern of inheritance does this trait
fall under? Would it be possible to produce a pure breeding strain of purple wildcats? Explain
These wildcats experience what is called an "incomplete dominance". It's not quite possible, in my view, to produce a pure breeding purple wildcats b/c they are kind of "heterozygous". It's best to explain this with an example.

Blue wildcats are always BB, red wildcats are always RR, and their combination purple wildcats are BR. (If you do the punnett square, it'd be so much easier for you to visualize this.)

P: BB x RR
F: All BR (purple)

Now you tell me. If we cross BR with a BR, what will we genotypes and so the phenotypes of the F2 generation would be?

Can we have pure breed of purple wildcats?

Let's try out:

BR x BB BR x RR BR x BR

In any of the crosses, we won't have ALL of the offspring of BR genotype. Some blue and red genotype offspring will be seen.

4. A woman with type A blood marries a man of unknown blood type. They produce 3 children with the following blood types: Type A, Type A, and O. What is the mother's genotype? What are the possible blood types for the
father? Identify (and explain why) what the most likely blood type is for this male. What can be said regarding part of his genotype?
I superscript A is a proper notation. I'll just go with A.

A woman has blood type A. It means she's got either AA or AO genotype.
3 children:
child # 1: Type A (AA or AO),
child # 2: Type A (AA or AO),
child # 3: Type O (OO) (OMG!! If mother had AA genotype, this kid must receive one A from her! and therefore, this kid's type should have become an A. but that's not the case here. Mom must have AO genotype for this kid to get an O type)

Mother's genotype is AO

based on first two children's genotypes, we can say that the father might have an AA or AO or OO, but b/c of the type of child 3 genotype, we can say that the father can either have AO genotype or OO genotype for child 3 to get a type O.

type O ppl are universal donor!


5. In fruit flies, red eyes are dominant over purple eyes. Tan body color is dominant over ebony body color. If you mated a completely heterozygous tan fly with red eyes, with a heterozygous tan fly with purple eyes, what
would the offspring look like?

This one's easy too. There are two traits here. eye and body color. We just have to know which trait has what is dominant.
Red eye is dominant over purple eyes..
Tan body color is dominant over ebony body color.

Completely heterozygous TAN fly with RED eyes: Tan is dominant and this fly is heterozygous, so Tt ; red eyes + heterozygous, so Rr ==> TtRr

vs.

heterozygous tan fly: Tt ; purple eyes rr ==> Ttrr

all you need to do is a dihybrid cross.
TtRr x Ttrr
I'm sure it wouldn't be too hard for you from this point on. Good luck! 🙂

 

[silveryhair]

Thanks a bunch!!!!!!!! your so smart! 🙂