punnett square

Punnett square

The Punnett square is a diagram designed to determine the probability (or ratio) of an offspring having a particular genotype. It is made by comparing all the possible combinations of alleles from the mother with those from the father.
Typical monohybrid cross *希望個example可以幫到你 =)*

圖片參考：http://upload.wikimedia.org/wikipedia/commons/thumb/1/17/Punnett_square_mendel_flowers.svg/550px-Punnett_square_mendel_flowers.svg.png

The Punnett square shows every possible combination when combining one maternal allele with one paternal allele. In this example, both organisms have the genotype Bb. They can produce gametes that contain either the B or b alleles. (It is conventional in genetics to use capital letters to indicate dominant alleles and lower-case letters to indicate recessive alleles.) The probability of an individual offspring having the genotype BB is 25%, Bb is 50%, and bb is 25%.





Maternal

B
b

Paternal
B
BB
Bb

b
Bb
bb
It is important to note that Punnett squares only give probabilities for genotypes, not phenotypes. The way in which the B and b alleles interact with each other to affect the appearance of the offspring depends on how the gene products (proteins) interact (see Mendelian inheritance). For classical dominant/recessive genes, like that which determines whether a rat has black hair (B) or white hair (b), the dominant allele will mask the recessive one. Thus in the example above 75% of the offspring will be black (BB or Bb) while only 25% will be white (bb). The ratio of the phenotypes is 3:1, typical for a monohybrid cross.

做你份功課, 首先要定出後代既phenotypic ratio. 由於係實驗結果, 係好難100%準確, 都大約可以分成幾類: 1 個character既可以係1:1 或者3:1. 而2個character既可以係1:1:1:1或者9:3:3:1. 咁呢4個比例應該夠你份功課用住先, 你可以填好左phenotypic ratio個直行先.
好嘞, 咁你一定知phenotype 同genotype 係唔一定相同既. phenotype 係recessive既, genotype 一定係rr. phenotype係dominant 既, genotype 可能係RR 或者Rr喎.
係一個dominant/recessive既系統裡面, 睇單一個character,
phenotypic ratio 係有2種可能性, dominant trait : recessive trait = 1:1 或者 3:1.
假如parental type 係 Rr x Rr 後代phenotype出來就係 3:1 而genotypic ratio就係(RR : Rr : rr) = 1:2:1








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r



R


RR


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rr

假如parental type 係 Rr x rr 後代phenotype出來就係 1:1 而genotypic ratio就係(Rr : rr) = 1:1








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Rr



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rr

睇兩個characters, 就要假設呢個係純independent segregation, 冇linkage. phenotypic ratio 係有幾種可能性, 2 dominant traits : 1 dominant 1 recessive : 1 recessive 1 dominant : 2 recessive traits = 9:3:3:1 或者 1:1:1:1 又或者有其他比例.
先睇9:3:3:1, parental genotype要係RrYy:RrYy 至會得出呢個比例. 所以後代既genotype 有RRYY : RRYy : RRyy : RrYY : RrYy : Rryy : rrYY : rrYy : rryy (一共9款) 比例係1:2:1:2:4:2:1:2:1









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RRYY


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RRyy


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ry


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rryy

再睇1:1:1:1, parental genotype要係RrYy:rryy 至會得出呢個比例. 所以後代既genotype 只有RrYy : Rryy : rrYy : rryy 比例係1:1:1:1








ry



RY


RrYy



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rryy

其實上面提過, 仲可以有其他parental genotype產生出不同的phenotype比例, 不過你份功課用唔到, 我就唔講住啦.


為免出唔到圖, 太長或補充唔到, 你來我blog睇下啦.
http://hk.myblog.yahoo.com/jw!RKor5m6REQOD7kJlL8VM7A--