Question

A female fly that has miniature wings and sepia eyes is crossed with a male that has normal wings and is homozygous for red eyes. The F1 are intercrossed to produce the F2. Give the phenotypes and their proportions expected in the F1 and F2 flies from this cross.

Answer 1

Answer and Explanation:

• The gene that expresses the size of the wing is X-linked. The dominant allele expresses long wings, XM, while the recessive allele expresses miniature wings, Xm.  

• The gene that expresses the color of the eyes is autosomal. The dominant allele expresses red color, S, and the recessive allele expresses sepia color, s.

A female fly that has miniature wings and sepia eyes (XmXm ss) is crossed with a male that has normal wings and is homozygous for red eyes (XMY SS).

The F1 are intercrossed to produce the F2.

1º Cross:

Parental) XmXm ss    x    XMY SS

Gametes) Xms   Xms     Xms   Xms  

                XMS   XMS    YS    YS

Punnet Square)    Xms           Xms            Xms             Xms

                XMS   XMXmSs    XMXmSs   XMXmSs    XMXmSs

                XMS   XMXmSs    XMXmSs   XMXmSs    XMXmSs

                YS      XmY Ss      XmY Ss       XmY Ss       XmY Ss

                YS      XmY Ss      XmY Ss       XmY Ss       XmY Ss

F1) 1/2 or 50% of the progeny are females and the other 1/2 or 50% are males

  100% of the females (1/2 or 50% of the progeny) are long-winged and  red-eyed

    100% of the males (1/2 or 50% of the progeny) are miniature.winged and red-eyed.

2º Cross:

Parental) XMXm Ss      x      XmY Ss

Gametes) XMS   XMs  XmS  Xms

                XmS   Xms  YS    Ys

Punnet Square)       XMS             XMs          XmS             Xms

                  XmS    XMXm SS  XMXm Ss   XmXm SS   XmXm Ss

                  Xms    XMXm Ss   XMXm ss   XmXm Ss    XmXm ss

                   YS      XMY SS     XMY Ss      XmY SS        XmY Ss

                   Ys       XMY Ss     XMY ss       XmY Ss        XmY ss

F2) 8/16=4/8=1/2 or 50% of the progeny are females and the other 1/2 or 50% are males

Females (8/16):    

• 3/8 of the females are long-winged and red-eyed, XMX- S-

• 1/8 of the females are long-winged and sepia-eyed, XMX- ss

• 3/8 of the females are miniature-winged and red-eyed, XmXm S-

• 1/8 of the females are miniature-winged and sepia-eyed, XmXm ss

Males (8/16):    

• 3/8 of the males are long-winged and red-eyed, XMY S-

• 1/8 of the males are long-winged and sepia-eyed, XMY ss

• 3/8 of the males are miniature-winged and red-eyed, XmY S-

• 1/8 of the males are miniature-winged and sepia-eyed, XmY ss  

Note: The symbol "-" means that in its place might be a recessive allele or a dominant allele. For example, S- means that it might be SS or Ss.