Genetics Problems
(sex-linked traits)


The alien couple pictured above displays characteristics that have been linked to the sex chromosomes of this particular species.  Below is a chart listing some of these traits.  Use the information in the chart to solve the genetics problems that follow.

Trait Chromosome Dominant 
Body color  X (P)  purple  (p)  pink 
Ear shape X (H)  pointed (h)  round
Eye color X (E)  yellow  (e)  red 
Foot color  X (F)  matches body (f)  mismatch to body
Size of nose  X (N)  small (n)  large
Eyebrows Y

     eyebrows present

Extra arms Y

     extra arms present for 4 arms total


1.  A male with yellow eyes mates with a red-eyed female.  What is the chance that this couple will have a baby boy with red eyes?  answer

2.  A female, heterozygous for the body color trait, mates with a pink male.  What is the chance that these aliens will give birth to a purple-skinned girl?  answer

3.  A small-nosed male falls in love and marries a large-nosed female.  The couple becomes pregnant with their first child.  What is the chance that the baby will be a girl and have a large nose like the mother?   answer

4.  Two aliens with mismatched feet are expecting a baby.  What is the chance that this couple's child will have feet that match its body?  answer

5.  Mr. & Mrs. Yardaz are expecting their second child.  Both parents have pointed ears but the couple's 2-year old son has round ears.  What is the chance that their second born will have pointed ears like the parents?  answer

6.  Doug Doouge has four arms.  Doug's pregnant wife only has two arms.  Doug would really like a daughter that has four arms just like him.  What is the chance that Doug's wife will give birth to a baby girl with four arms?  answer

7.  Wendy is a female alien whose feet and body color match.  She always considered herself lucky because her father's feet did not match his body and she thought that looked strange.  Wendy would like to have children someday but is extremely concerned about mismatching feet.  She makes an appointment with a genetic counselor.  Wendy tells the counselor that she will only mate with males whose feet match their body and asks if there is any possibility that any of her children would have mismatching feet.  What does the geneticist tell Wendy? answer


















1.  male = XEY     female = XeXe

     chance of having a baby boy with red eyes (XeY) = 50%

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2.  male = XpY     female = XPXp

     chance of giving birth to a purple-skinned girl (XPXP or XPXp ) = 25%

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3.  male = XNY     female = XnXn

    chance of baby being a girl with a large nose (XnXn) = 0%

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4.  male = XfY     female = XfXf

    chance of child's* feet matching its body (XFXF or XFXf or XFY ) = 0%

      *note that gender was not a factor in this problem
          "child" refers to either a boy or a girl that would display this trait

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5.  male = XHY     female* = XHXh

      *you know mom's genotype is hybrid because of the 2-year old son with round ears

     chance of second born's* ears being pointed (XHXH or XHXh or XHY ) = 75%

      *note that gender was not a factor in this problem
          "second born" refers to either a boy or a girl that would display this trait

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6.  male = XY     female = XX

     chance of baby girl with four arms (genetic impossibility*) = 0%

      *because the extra arm trait is carried on the Y chromosome & girls don't have Y chromosomes

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7.  male = XFY     female* = XFXf

      *you know Wendy's genotype is hybrid because her dad had mismatching feet

     chance of any child having mismatching feet (XfXf or XfY ) = 25%

The geneticist tells Wendy that she has a 25% chance of having a child with mismatching feet.  Based on the genotypes, any of Wendy's future daughters would have matching feet but, because she is a carrier of the mismatching gene, the potential exists that Wendy's sons may have feet that don't match their body.  

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