I love to tell one of my friends that he and his family just throw Punnet squares right out the window. His mom has blue eyes and blonde hair, his dad has brown eyes and brown hair. My buddy is the only one of 4 kids to have brown/brown-all the other boys are blond/blue. He married a girl with brown eyes and brown hair, and everybody in her immediate family is brown/brown.
My buddy and his wife had a little girl. She's blonde/blue, looks just like Dad's brothers and mom! Can somebody break this down for me?
Thanks,
towanda_ajt
Genetically, how do two brown-eyed parents have blue-eyed kids? Eye colour and hair colour are quantitative traits, which means that they are regulated by the expression of several genes and the Punnet square cannot be applied in a simple way...
We have 3 genes (2 alleles for each, which means actually we have like 6 genes) controlling the eye colour. Imagine it as dark-dark-dark.
The "sum" of these genes determines our eye colour: 6 dark genes means dark-brown eyes, 5 means brown, 4 means brown-green, 3 means green, 2 means dark-blue and 1 means light-blue. When they assort on the spindle and homologous chromosomes have to segregate, it can happen that 2 dark genes from the mother went to one cell (remember we have 6 in total) and 1 dark from the father went to one cell. If you imagine these two cells to fuse (sperm and egg), the little girl got 5 dark genes, resulting in a brown-eyed boy. This is the way iy works. It could be that with 3 dark genes you have blue eyes.The same is true for hair colour.
I hope I helped you.
Genetically, how do two brown-eyed parents have blue-eyed kids? If both parents contain the genes where Brown eyes is the Dominant trait [B] and Blue eyes is the Recessive trait [b], if you use a Punnett square, each offspring has a 25% chance of having blue eyes.
%26gt;%26gt;B b
B BB Bb
b Bb bb
25% chance kid will carry two dominant Brown eyes genes (and have brown eyes). [BB]
50% chance kid will have Brown eyes and be the carrier of a blue eyes gene like mom and dad. [Bb]
25% chance kid will have two recessive blue eyes genes (and have blue eyes). [bb]
Genetically, how do two brown-eyed parents have blue-eyed kids? I don't know the exact specifics, but if that blue eyed gene is recessive, and they both have it, then there is a chance that their children can have blue eyes. It would only be a 25% chance, but obviously it can happen.
Genetically, how do two brown-eyed parents have blue-eyed kids? Simple genetic trait patters can be mapped in a Punnet square, each parent has two genes for the particular trait, resulting in 4 possible combinations. For two brown eyed parents to have a blue eyed baby they would both have to have one brown eyed gene and one blue eyed gene. Since brown is the dominate gene, they would both be brown eyed. Bb (B=Brown, b=Blue) Mom = Bb (she has brown eyes), Dad=Bb ( he too has brown eyes). the 4 combinations of their offspring would be; BB (Brown Brown resulting in Brown eyes) Bb (Brown from dad, blue from mom resulting in Brown eyes), Bb (Brown from mom, blue from dad resulting in Brown eyes), or bb (Blue from dad, blue from mom resulting in blue eyes). The fact that the blue eyed gene, (same for blond hair, red hair etc) is recessive and the brown gene is dominate is the reason blue eyes (or green, or grey) are much less common than brown.
Genetically, how do two brown-eyed parents have blue-eyed kids? Well, in order to answer this question fully, there are a few thing that need to be explained before-hand.
Alleles - an allele is a "flavor" of a gene. If, for example there was a gene that controlled the flavor of ice-cream, one population of ice-cream might have the vanilla allele whereas another might have the chocolate allele.
Genotype - Although this idea can become very complicated, for a single locus with two alleles it is quite simple. An individual who is "homozygous" for a trait has two of the same allele type, one from the father and the other from the mother, whereas an individual who is heterozygous got a different allele from either the mother or the father.
Phenotype - This idea is simple no matter how many genes are involved, this is the outward manifestation of the genotype, in this case the trait called brown eyes.
Dominance/Recessivity - For this simple example traits that are dominant overshadow or cover up traits that are recessive. In some cases, where there are several genes in the genotype there may be multiple dominance or co-dominance
For this situation there are a few of possibilities, one could be that one of the people in the cross (genetics term for having sex) could be homozygous for the trait while the other is heterozygous. In such a case the Punnett Square would actually be something of a Punnett rectangle. If we assign letters to the alleles in the genotype we could call the homozygote BB and the heterozygote Bb. Since the BB really has only one allele, B, that is the only thing this person can contribute to the cross. The Bb person however has two alleles B and b. Either of these alleles are just as likely to be passed on throught the cross such that these outcomes are possible:
BB(Brown)xBb (Brown)= B(from the the BB)B(from the Bb) and B(from the BB)b(from the Bb) for a final result of BB and Bb. Both of these offspring would have the brown phenotype since we could arbitrarily assign brown as the dominant trait.
If on the other hand the cross was BbxBb, the result would be BB, Bb, Bb and bb (1:2:1 genotypic ratio) giving 3 Brown and one new blue phenotype (3:1 phenotypic ratio)
The other alternative, although not important in this case, is the bbxbb cross which would only result in bb.
For hair color the example would work exactly the since dominance gene loci rules apply equally. I hope this answer helps.
Genetically, how do two brown-eyed parents have blue-eyed kids? My mother has blue eyes my dad brown . I got screwed with brown eyes. So i decided to have the newcoloriris surgery and now have green eyes ! :)
