公式化解決genetics problems (Type 1)

9.7.15

Type 1: 利用 Mendel's law of segregation / 1st law 做deduction 

有學過Mendel 做嘅monohybrid experiment 都知如果將兩棵heterozygous 高嘅植物cross，produce 出嚟嘅高既offspring同矮既offspring 以 3比1 呈現，高既trait係dominant 而矮既trait係recessive。


係呢種題型會俾呢數字你計，你計到個ratio係3:1，就可以知道3/4 offspring 有 dominant trait 而 1/4 offspring 有 recessive trait ，兩個parents 係 heterozygous。

e.g. 1 1990 HKCE I (1cii)


With respect to grain colour, deduce the genotype and phenotypes of the parent plants. 
  • The purple and yellow grains occur in a ratio of 3:1.
  • According to Mendel’s Law of Segregation, both parents must be heterozygous.
  • Purple grain colour is the dominant trait.
  • Hence the phenotype of both parents must be purple grain colour.

呢條一開始叫你數粟米粒數目，會數到數到紫色有53粒，黃色有18粒，比例大約3:1，就知紫色既allele係dominant 而黃色既allele係recessive。


e.g. 2 2002 HKAL IB (13a)


Eye colour and wing morphology of a certain species of fly are controlled by two different genes. A student made a cross using flies he collected from the field. The cross and its F1 progeny are shown below:


Deduce the recessive trait for eye colour and wing morphology. Explain your deduction.
  • In the F1 progeny, there are 108 (35+20+37+16) red-eyed and 34 (16+18) yellow-eyed flies. This shows a 3:1 ratio.
  • In the F1 progeny, there are 106 (35+16+37+18) normal-winged and 36 (20+16) vestigial-winged flies. This shows a 3:1 ratio.
  • Therefore, according to Mendel’s Law of Segregation, both parents are heterozygous for eye colour and wing morphology.
  • Under heterozygous condition, only the dominant alleles will be expressed while the recessive alleles will be masked. Hence, yellow eye and vestigial wing are recessive traits.

呢條有2個characters 但係做法一樣，可以將eye colour 同 wing morphology 分開處理。