woesofscience
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#1
Report Thread starter 9 months ago
#1
I've been stuck trying to understand part (ii) of the answer (from the mark scheme) to the following question on genetics:


Inheritance of sex birds is also determined by a pair of sex chromosomes.

A breeding experiment was carried out to investigate the inheritance of sex in birds.

A gene found on the sex chromosomes in pigeons determines the colour of their feathers. The dominant allele, B, produces blue feathers and the recessive alleles, b, produces red feathers.

Male pigeons with red feathers were crossed with females that had alleles for blue feathers only.

(i) Use a genetic diagram to explain the expected results of this cross if birds inherit sex in the same way as mammals.

Answer:

- Genetic diagram shows) males genotype X^bY, female genotype

- genetic diagram to show correct genotypes of offspring

- all offspring have blue feathers

- punnett square (i)


(ii) The cross between male pigeons with red feathers and female pigeons with alleles for blue feathers only was carried out.

The offspring produced suggested that sex chromosomes in birds have a different effect on sex compared with the sex chromosomes in mammals.

All the male offspring had blue feathers and all the female offspring had red feathers.

Deduce how the inheritance of sex in birds differs from that in mammals.

Answer:

- females must have inherited b from their father (because the are red)

- female offspring only inherit one allele (or they would be blue)

- (in birds) females have different sex chromosomes and males have the same chromosomes

- punett square (ii)



What I don't understand is why X^B Y on the left is labelled females and X^b X^b is labelled males on the top.

And also why the answer shows cross between the offsprings; male blue feathers with female red feathers. Shouldn't it be cross between male with red feathers and female pigeons since it's asking to deduce the inheritance?


I hope I've explained my confusion and someone can give me insight on what I'm missing. It's only one mark but it's annoying me.
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BDavies1
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#2
Report 9 months ago
#2
(Original post by woesofscience)
I've been stuck trying to understand part (ii) of the answer (from the mark scheme) to the following question on genetics:


Inheritance of sex birds is also determined by a pair of sex chromosomes.

A breeding experiment was carried out to investigate the inheritance of sex in birds.

A gene found on the sex chromosomes in pigeons determines the colour of their feathers. The dominant allele, B, produces blue feathers and the recessive alleles, b, produces red feathers.

Male pigeons with red feathers were crossed with females that had alleles for blue feathers only.

(i) Use a genetic diagram to explain the expected results of this cross if birds inherit sex in the same way as mammals.

Answer:

- Genetic diagram shows) males genotype X^bY, female genotype

- genetic diagram to show correct genotypes of offspring

- all offspring have blue feathers

- punnett square (i)


(ii) The cross between male pigeons with red feathers and female pigeons with alleles for blue feathers only was carried out.

The offspring produced suggested that sex chromosomes in birds have a different effect on sex compared with the sex chromosomes in mammals.

All the male offspring had blue feathers and all the female offspring had red feathers.

Deduce how the inheritance of sex in birds differs from that in mammals.

Answer:

- females must have inherited b from their father (because the are red)

- female offspring only inherit one allele (or they would be blue)

- (in birds) females have different sex chromosomes and males have the same chromosomes

- punett square (ii)



What I don't understand is why X^B Y on the left is labelled females and X^b X^b is labelled males on the top.

And also why the answer shows cross between the offsprings; male blue feathers with female red feathers. Shouldn't it be cross between male with red feathers and female pigeons since it's asking to deduce the inheritance?


I hope I've explained my confusion and someone can give me insight on what I'm missing. It's only one mark but it's annoying me.
In humans XY is male and XX is female. This is not always the case- as in this example. The only way to explain the results is that XX is male and XY is female in this species.
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woesofscience
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#3
Report Thread starter 9 months ago
#3
(Original post by BDavies1)
In humans XY is male and XX is female. This is not always the case- as in this example. The only way to explain the results is that XX is male and XY is female in this species.
That would have never crossed my mind. I guess assumptions can be quite ignorant.

Thank you!
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