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Phenotype is physical appearance of a character in a living being which depends on their genotype i.e. genetic constituents so by study of phenotype, genotype can be predicted which may or may not be exactly same but definately not wrong because every character can be homozygously dominant or hetrozygously dominant.
A dominant Allele is written YY or Homo dominant. That is completely dominant and no recessive. Yy is half or hetro dominant. Forgive me for it has been a while since i talked and brushed on this. Homo recessive is yy. You should and would never see yY. it is written Yy.
NOW. Colourblindness we will assume, is a Sex-Linked Disorder. Hence, the sex chromosomes are involved in determinig the phenotypes of the children. XY --male XX--female XbY--colourblind male XBXb --normal female Cross: (parents) Xby * XBXb F1 progeny/offspring XBXb, XbXb, XBY, XbY 50 % of the children will be colourblind. This is a very simple question. You also need to clarify within the question if it is sex linked or autosomal.
Females have XX chromosomes and males have XY chromosomes. Sex-linked traits are almost always carried on the X chromosome. Males are more likely to inherit sex-linked traits because they have only one chromosome that can carry a trait. If a male gets a recessive sex-linked trait, they will always acquire it because they have no other chromosome that will dominant over the recessive. In contrast, females have two X chromosomes which carry sex-linked traits. If they receive a recessive sex-linked trait, they always have one other chromosome that could be dominant over the other. For example: B - regular vision trait b - color blindness If a female has a colorblind father (XbY) and a mother with regular vision (XBXB), she will have XBXb. Even though the recessive colorblind trait was passed onto her, she will still have regular vision because of her other chromosome. Thus, she will be a carrier for colorblindness. Say the daughter above had children with a man with regular vision (XBY). If they have a son, there is a 50% chance he will be colorblind because of his mother who is a carrier. If they had a daughter, no matter what, she would always have regular vision with 50% chance that she will be a carrier.
Colorblindness is a sex-linked trait carried on the X chromosome. Males have an X and a Y chromosome (XY) and females have two X chromosomes (XX). Normal vision is dominant over colorblindness. This means that the man must have an allele for normal vision on his only X chromosome. This also means that the woman must have two colorblind alleles on both of her X chromosomes. If we use a capital 'B' to represent normal vision and a lowercase 'b' to represent colorblindness, the genotype of the male would be XBY, and the genotype of the female would be XbXb. Now that we know the genotypes, setting up the Punnett square should be easy. Here is a direct image link to the Punnett square: http://i.imgur.com/gDfUv.png This Punnett square tells us that there is a 50% chance of having a female that has normal vision and a 50% chance of having a colorblind male.