Question

In placental mammals, either the maternal or paternal X chromosome is inactivated, by chance. Each is equally likely to be inactivated. However, there is an evolutionary conflict. The chromosomes “compete” against each other. It would be in the evolutionary interests of each parent to inactivate the chromosome from the other parent. In placentals, the two sexes have evolved to a draw, and both chromosomes become inactivated (the idea of selfish genes and chromosomes is covered in more detail in Evolution).

In marsupial mammals, it is different. They also inactivate one of the X chromosomes, but it always the paternal X that gets inactivated. Your assignment is to figure out how this will alter the result of a sex-linked cross in marsupials. Show a cross with a dominant and recessive gene in reciprocal crosses. How would the result or ratios differ between placental and marsupial mammals?

Answer 1

in placental mammals

female (XX)    X males (XY)

normal cross

	X	Y
X	XX (FEMALE)	XY (MALE )
X	XX (FEMALE)	XY(MALE)

Due to inactivation in x chromosome in either the maternal or parental

if inactivation takes place in maternal i.e, onlu sinlge x is there in maternal chromosome which is activated

	X	Y
X	XX	XY

when x chromosome is inactivated in both the parents then the female will have only one X and male will have just Y chromosome, that is only a boy will be the resultant progeny

	y
x	XY (MALE)

IN MARSUPIAL MAMMALS

the X chromosome of male is inactivated always i.e, males only have Y chromosome but females bears both x chromosome

	Y
X	XY
X	XY

CONSIDER A RECIPROCAL CROSS

reciprocal cross in which the parental generation is reversed and than crossed

i.e, female (XY) and male (XX) and since it is given that the paternal X chromosome is inactivated so male (X) crossed with female(XY)

	X (MALE)
X (FEMALE GAMETE)	XX (MALE)
Y(FEMALE GAMETE)	XY(FEMALE)

IN RECIPROCAL CROSS THE RATIO OF MALE TO FEMALE IS 1:1

while before it was only male in f1 generation

the ratios of marsupial and placental mammals are different totally since, placental mammals both chromosomes are inactivated so there is no chance of getting a female progeny in f1 generation but in reciprocal cross of marsupial mammals so we get both male and female in 1 : 1 but it was not true for true breeding in marsupial mammals because there we have one x chromosome inactivate in paternal so we only have only males in that cross so if we see in both marsupial and placental then the progeny ratio is same but different with respect to reciprocal cross.