Maternal Effect

 Write about maternal effect?




Answer

The development of some characters in many organism is government or markly influence by the genotype of the female parents, this is known as maternal effect. Although such characters are governed by nuclear genes they still show the following features 

1) Reciprical difference in F1.

2)Reciprical difference in most cases disappear in F2 

3)It comsiderably samller variation in F2 as compare to that in F3 .

In some extream cases, there may be no phenotype segregation in F2 , but it does Become evident only in the F3. Many important characteristics of both plants and animals show maternal effects of which some example are describe below An extreame example of maternal effect is known in the snail limnaea .. In this snail, the direction of coiling of it's shape cell is control by a single nuclear gene D/d, the dominant allele - D produces right handed coiling, while its recessive allele d produces left handed coiling. The direction of cell coiling in an individuals is governed by the genotype of its female parent. and not by its own genotype. As a result, pre cross between female with left handed coil (dd) and males having right handed (DD) produced. F₁ progeny (Dd) which left handed coil, since the genotype of the female parent was dd. In F₂ segregation of Dd produces three genotypes (Dd. dd, DD) in ratio 1:2:1 but all the F₂ snail exhibit right handed coiling since there female parent had the genotype Dd, which directs right handed colling in the progeny. The F3 progeny from the F₂ Individuals with the genotypes DD and Dd will show right handed colling, while Individuals will exihibit left handed individuals, those produces typical 3:1 ratio(right handed : left handed in F3.


The reciprocal cross (♂ right handed DD X ♀ left handed dd), on the other hand, right handed coiling in F1 as well as F₂ But on F3, a 8:1 ratio is optained as was cross at the previous. Thus inheritance coiling of limnaea cell show......

   1) Differences between Fi from receprocal cross

(2) No segregation in F₂ 

(3)Appearence of typical F₂, F3 generation.


  The 3:1 ratio is clearly indicates the coiling of cells is governed by single nuclear gene. But the segregation of this nuclear gene appeared to be delayed by one generation. In fact, the effect of the segregation of nuclear gene Dd is normal and occurs in the F₂ generation itself. But the phenotypic effects of this segregation are visible only one generation after the actual segregation of the alleles. These is because the direction of colling. in this snail is determined by the plan or the direction of the first meiotic division of the zygote, which is determined by some substances already present in the egg.

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