Question

Merfolk have three pairs of autosomes and one sex chromosome; males are XO and females are XX. In merfolk the genes controlling tail fin shape and scale colour are on different autosomes. Solid tail fin (Ff) is dominant to forked tail fin (ff), and the Fallele is recessive lethal. There are two alleles of the scale colour gene - CB(blue) and CR (red)- CB and CR show incomplete dominance with heterozygotes being purple. The silent trait is X-linked and dominant (XS) to normal voice (X+). Atolla is a male merfolk, he has a forked tail fin, blue scales, and is voiceless (silent). Brachy is a female merfolk with a solid tail fin, purple scales and a normal voice.

Answer 1

1. As Brachy is female having normal voice, so it carries genes for voice on its X-chromosomes, X+X+ and Atolla is male carrying genes for silence in single sex chromosome as XsO.

Mating between Brachy and Atolla:

Parents   X+X+ x XsO

Gametes	X+	X+
Xs	XsX+ Silent female	XsX+ Silent female
O	X+O Normal male	X+O Normal male

So, the cross indicates all the daughters of this mating be silent or in other words, it is 100% that daughter born be silent. So, the probability is 1.

2. As the phenotype of Brachy is Solid tailfin (Ff) and purple scale (CBCR), its genotype will be FfCBCR.

The phenotype of Atolla is forked tail fin (ff) and blue scales (CBCB), its genotype will be ffCBCB.

As both the genes are present of different chromosomes, so they will assort independetly during game formation.

Parents: FfCBCR     X ffCBCB

​​​​Gametes: FCB    fCR fCB   fCB

Progeny:  

Gametes	fCB	fCB
FCB	FfCBCB Solid tail, Blue scales	FfCBCB Solid tail, Blue scales
fCR	ffCBCR Forked tail, Purple scales	ffCBCR Forked tail, Purple scales

The cross showing 50% of the offsprings have Solid tail and blue scales and 50% have Forked tail fin and Purple scales. As the chromosomes are located on autosomes, it do not correlate with sex of individual.

So, the propability of child with Forked tail and purple scales is 1/2.

3. As given in the question, it is asked to ignore the phenotypes and one thing to keep in mind that genes for silent or normal voice present on sex chromosomes, thus having direct correlation with sex of an individual. But the genes present on autosomes inherit independent of sex of an individual.

Looking forward, female heterzygote for silent will carry genes for this trait only on one of its X-chromosomes. Its can be represented as XSX+. Mating of this female is to be done with silent male having chromosomes, XsO.

Mating between heterozygote silent female and silent male:

Parents:   XsX+ x XSO  

Gametes:   Xs     X+    Xs    O

Progeny:

Gametes	Xs	X+
Xs	XSXS Silent female	XSX+ Silent female
O	XSO Silent male	X+O Normale male

*** Now, to find overall probability, cross female with blue scale (CBCB) and carrier of forked tail (Ff) with male having blue scale (CBCB) and carrier of forked tail (Ff).

NOTE: Carriers are those who inherits a recessive trait but do not express it due to presence of dominant gene.

Parents: FfCBCB     x   FfCBCB

​​​​​​Gametes:   FCB   fCB FCB fCB

Progeny:

Gametes	FCB	fCB
FCB	FFCBCB Solid tail, Blue scales	FfCBCB Solid tail, Blue scales
fCB	FfCBCB Solid tail, Blue scales	ffCBCB Forked tail, Blue scales