NAME:_____________________________ PERIOD:__________


DATE:_____________________  DATE DUE:_______________




INTRODUCTION: A sex-linked characteristic is determined by an allele that is carried only on the X chromosome. The shorter Y chromosome does not carry an allele for the sex-linked trait.  Most sex-linked traits are recessive. Since there is only one X chromosome in the male, a male who carries a particular recessive allele on the X chromosome will have the sex-linked condition. A female who carries a recessive allele in one X chromosome will not have the condition if there is a dominant allele on her other X chromosome.  She will express the recessive condition only if she inherits two recessive alleles-one from each parent.  Her chances of inheriting a sex-linked condition are thus significantly less than those of a male.


PROCEDURE: PART I: Study the pedigree for Duchenne’s muscular dystrophy shown in Figure 1 below.  Duchenne’s muscular dystrophy is a disorder where the person’s muscles gradually weaken.  In a pedigree, a square represents a male and a circle represents a female.  If it is darkened, the person has Duchenne’s muscular dystrophy, if clear, the person has normal muscle tone.







1. How many males represented by this pedigree have muscular dystrophy?



2. How many males represented by this pedigree are normal?



3. How many females in this pedigree have muscular dystrophy?



4. How many females in this pedigree are normal?


The letters out to the side (sometimes show up are Roman numerals) indicate the different generations.


5. How many generations are represented in this pedigree?


A marriage is indicated by a horizontal line connecting a circle to a square.


6. How many marriages are indicated in the pedigree?


A line perpendicular to a marriage line indicates the offspring.  If the line ends with either a circle or square, the couple had only one child.  However, if the line is connected to another horizontal line, then several children were produced, each indicated by a short vertical line connected to the horizontal line.  The oldest child appears to the left and the youngest child to the right.


7. How many children did the couple in generation A have?


8. Was the oldest child a male or a female?


9. How many of their children were female?  Male?


10. How many children did the first married couple in generation C have?


11. Was the youngest child a male or a female?


The genotypes of the males in a pedigree for Duchenne’s muscular dystrophy are easy to determine, since normal muscle tone (D) is dominant and muscular dystrophy is recessive (d). Since these alleles are on the X chromosome only, a male represented by clear square will have the genotype denoted by XDY. A male represented by a darkened square will have the genotype denoted by XdY. Label the genotypes of all the squares in the pedigree in Figure 1.


12. How many males have the genotype XDY?


13. How many males have the genotype XdY?


Females who have Duchenne’s muscular dystrophy have an easy genotype to identify. They are XdXd.  Both recessive alleles must be present for a female to have muscular dystrophy.  If one dominant allele is present, the individual must be normal for muscle tone.  Label all the females with muscular dystrophy as genotype XdXd on the pedigree.


14. How many females have the genotype XdXd in this pedigree?


Females who do not show the trait for muscular dystrophy may be homozygous dominant (XDXD) or heterozygous (XDXd).  A heterozygous female is called a carrier.  Examination of traits among the offspring can often determine which genotype the parents have.  If any child (son or daughter) has muscular dystrophy, then the female must be heterozygous XDXd). If her son has muscular dystrophy, he has the genotype XdY.  He inherited the Y from the father, so the other allele in his genotype (Xd) had to come form the mother.  If a daughter has muscular dystrophy (XdXd), she inherited an Xd from each parent, thus making the genotype of the normal mother XDXd.  Label all females XDXd who have children with muscular dystrophy.


15. What is the genotype of the female in the first generation?


16. Are there any other females with this same genotype?


A female who has 3 or more sons, with none exhibiting muscular dystrophy, is likely to have a genotype XDXD.  If she has had fewer than 3 sons, her genotype is less certain.  In such cases, her genotype is labeled as XDX?.  Label the rest of the females in the pedigree as XDXD or XDX?.


17. Do any of the females definitely have the genotype XDXD?


18. How many of the females have the genotype XDX??


19. Which sex usually inherits a sex-linked condition?  Explain.




20. How can you tell whether a female has a genotype XDXD , XDXd or XDX?.







21. How many genotypes are possible in a pedigree of sex-linked traits?  What are they?




PART II: Study the pedigree for hemophilia shown in Figure 2 below.  Hemophilia is a sex-linked recessive trait located on the X chromosome (Xh).  Normal blood clotting is controlled by the dominant allele (XH).If a person has hemophilia then their blood fails to clot properly.  If a square or circle is shaded in, then they have hemophilia.  If the square or circle is clear then the person has normal blood clotting. Using the information you learned about Duchenne’s muscular dystrophy, label all of the genotypes in the pedigree in Figure 2.  FILL OUT THE PEDIGREE BELOW WITH THE CORRECT GENOTYPES!!!!!



PART III: Study the pedigree for cystic fibrous in Figure 3 below. Cystic fibrous is an autosomal recessive trait. FILL OUT THE PEDIGREE BELOW WITH THE CORRECT GENOTYPES!!!!!