Barron Cross over Bio

[joonkimdds]

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This is a problem from Barran bio #21 on page 309

Question: A "zinger" denotes an abnormal phenotype. When zingers are interbred, a phenotype ratio of two zingers to one normal occurs; 1/4 of the offspring die. When a zinger is crossed with a mate having a normal phenotype, 1/2 of the offspring are zingers, 1/2 are normal, and none dies. The most probable explanation is that zingers

Answer: have recessive traits that are lethal when homozygous


how do u solve this problem?

 

[lintydent]

A zinger has the genotype for example Tt where T is the normal dominant gene and t is the "harmful" recessive gene.

If interbred Tt x Tt = 1TT/2Tt/1tt
Since T is dominant then 3/4 live. But having homozygous tt means you die.

If a zinger is mating with a normal nonzinger then:
Tt x TT = 2TT/2Tt
Everyone lives because the normal non-dying gene is dominant.

Hope this makes sense.

 

[joonkimdds]

A zinger has the genotype for example Tt where T is the normal dominant gene and t is the "harmful" recessive gene.

If interbred Tt x Tt = 1TT/2Tt/1tt
Since T is dominant then 3/4 live. But having homozygous tt means you die.

If a zinger is mating with a normal nonzinger then:
Tt x TT = 2TT/2Tt
Everyone lives because the normal non-dying gene is dominant.

Hope this makes sense.

how do u know that zinger is Tt instead of TT or tt?
and if it says ''interbred'', does that mean it combines with the same gene combination?

 

[lintydent]

how do u know that zinger is Tt instead of TT or tt?
and if it says ''interbred'', does that mean it combines with the same gene combination?

I think I might have explained it incorrectly...the problem says that a zinger produces an abnormal PHENotype...which mean it displays something abnormal like codominance, even though the zinger trait is recessive. In my previous example i said Tt just displays the dominant gene. But with codominance a Red (normal) and White (zinger recessive trait) display pink (abnormal zinger phenotype). So if pink and pink interbred RW x RW = 1RR (normal) / 2RW (zingers) / and 1WW (homozygous zinger)

RW = pink heterozygous zingers that live
RR = red ..homozygous dominant non zingers live
WW = white...homozygous zinger die

If they bred with someone normal RW x RR = 2RR / 2RW ...you either come up pink or red.

And you know that a zinger is heterozygous because they tell you in the problem what ratios are produced when a zinger breeds. If you had thought that maybe a zinger is RR...then when they interbreed: RR x RR = 4RR, no variation and everyone lives.

If you picked WW x WW = 4WW, no variation but they ALL die. In the problem they tell you 3/4 live, 1/4 die and two zingers (RW) and one normal (RR).

And yes "interbred" means they mate with each other.

 

[tinman831]

I think that any time you see a 1:2:1 ratio, we have to assume that there is co-dominance coming into play.