Question

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Which of the following statements is not true?

If a phenotype is due to a dominant allele, affected offspring always have affected parents.

If a phenotype is due to a dominant allele, affected parents always have affected offspring.

If you make a cross between homozygous parents, and all the offspring have the phenotype of one parent, then that trait is dominant.

If a single copy of an allele is sufficient to produce the phenotype, then the trait is dominant.

The “wild type” allele may produce a dominant or a recessive phenotype.

Answer 1

Statement 2 is incorrect. According to it, if a phenotype is due to dominant allele , affected parents always have affected offspring. Now, let us consider the the ,allele of the parents for a particular trait be as follows -

From the above diagram we can see that in A all the offspring will be affected as D (which is the dominant allele for a trait ) is present in both the parents.

Dd DD Both parents are affected all offspring affected Dd DD Dd dd only one parent affected (Dd) dd only one offspring affected. Dd Dd Dd Both affected. only one affected Dd dd

In Diagram B only one parent is affected and therefore only one offspring will be affected while those carrying recessive gene will remain normal.

In diagram C , both the parents are heterozygous and carry one dominant (D) and one recessive (d ) allele in the pair. The offspring here too will be one normal and other affected. Thus, although both parents are affected they do not always produce affected offsprings . Also inheritance depends on nature of allele whether it is present on autosomal chromosome or on sex chromosome.

To understand the cross between two homozygous parents let us consider the Mendel's monohybrid cross-

We can observe that the progeny in F₁ generation are heterozygous (two different alleles ) but shows dominant phenotype.

Here, RR refers to round seeds while rr to wrinkle seeds Rr Rr Rr Rr Fig Punnet Square. R. R.

Wild type alleles are considered as normal alleles and it may lead to recessive allele by loss of function mutation and dominant allele by gain of function.