A2 OCR Biology: What Determines Sex

A2 OCR Biology: What Determines Sex

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  • Created by: Hamed
  • Created on: 25-05-11 00:06
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What Determines Sex?
Newly hatched chicks need to be sexed by farmers; in the Plymouth rock breed of chickens
there is a dominant sex-linked allele, B, which causes the normally black feathers to have
a white bar. Newly hatched chicks with the allele B have barred feathers, seen as a white spot
on an otherwise black head. Those with only the allele b have non-barred feathers.
In birds males are homogametic (XX) and females are heterogametic (XY)
A female with a white spot must have the genotype XBY
A male with a black head must have the genotype XbXb
B = allele for barred (white) feathers
b = allele for non-barred (black) feathers
Parental Phenotypes Male with Black Head Female with White Spot
Parental Genotypes XbXb XBY
Gametes All Xb XB Y
Crossing Plymouth Rock Chickens
Male Gametes
Female Gametes Xb Xb
XB XBXb ­ Male//White Spot XBXb ­ Male//White Spot
Y XbY ­ Female//Blackhead XbY ­ Female//Blackhead
We can see that the offspring phenotype ratios are 50% females with black heads and
50% males with white spots; the ratio is therefore 1:1
A. If we were to cross males (XBXB) with females (XbY) then all the offspring will have a
white spot meaning that it'll be more difficult to identify the females from the males.

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Male Gametes
Female Gametes XB XB
Xb XBXb ­ Male//White Spot XBXb ­ Male//White Spot
Y XBY ­ Female//White Spot XBY ­ Female//White Spot
Pale Brindled Moths
There is a rare black form of the pale brindled moth; they are all female and when such
a female mates with a normal male, all her female offspring is black and all the male
offspring are normal.
A.…read more

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Cross female (XbY) and male (XBXb)
o G1 = |XBXb ­ Male//Black|,|XbXb ­ Male//Brown|
|XBY ­ Female//Black|,|XbY ­ Female//Cinnamon|
Cross female (XbY) and male (XbXb) ­ (G1)
o G2 = |XbXb ­ Male//Brown|,|XbXb ­ Male//Brown|
|XbY ­ Female//Cinnamon|,|XbY ­ Female//Cinnamon|…read more


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