SEX-LINKED inheritance involves genes which are found only on the X chromosome.  The pattern of inheritance differs from somatic genes, because males have only one X chromosome.

• Review sex chromosomes (p. 206 and 178 in Freeman). 
  • In humans and fruit flies, 
    • females have two X chromosomes
    • males have one X and one Y.
    • females get an X from the mother and another X from the father
    • males get  an X from the mother and a Y from the father
  • In humans, the Y chromosome is relatively small and contains only a few genes, among them the important "SRY" gene, which contains the instructions for turning on genes on other chromosomes to activate the signals for making male hormones and male anatomical peculiarities.  The SRY gene and some other "Y" chromosome genes are not present on the X chromosome
• In humans and also fruit flies, the X chromosome contains many genes which are not present on the Y chromosome.  For these genes males have only one allele.  
• Here is an example for Duchenne Muscular Dystrophy, which is sex-linked and recessive:
  

SEX	GENOTYPE	PHENOTYPE
female	XDXD	normal
female	XDXd	normal
female	XdXd	has muscular dystrophy
male	XDY	normal
male	XdY	has muscular dystrophy

• Which of the individuals above can pass the defective allele to their children?

 

• Here are some Punnett Squares for sex-linked inheritance; for each describe the phenotypes of the parents and offspring:

#2	XD	Xd
Xd	XD Xd	Xd Xd
Y	XD Y	Xd Y

• The Freeman textbook has an example for sex-linkage in flies on pp. 206-207

    flies

• SAMPLE PROBLEMS:

  1. In Punnett Square # 1 above, what is the probability that their first child will have Duchenne muscular dystrophy?

  2. In Punnett Square # 1 above, what is the probability that their first son will have Duchenne muscular dystrophy?

  3. In Punnett Square # 1 above, what is the probability that their first daughter will be a carrier of  Duchenne muscular dystrophy?

  4. In Punnett Square # 2 above, what is the probability that their first daughter will be a carrier of  Duchenne muscular dystrophy?

  5. In Punnett Square # 2 above, what is the probability that their first son will be a carrier of  Duchenne muscular dystrophy?

  6. In Punnett Square #2 above, what is the probability that their first child will be a carrier of  Duchenne muscular dystrophy?

  • answers

    1. 1/2

    2. 1 (100%)

    3. 100%

    4. 1/2

    5. 0

    6. 1/4

NEED HELP?

LINKS TO EXTRA HELP X&Y http://vector.cshl.org/dnaftb/9/animation/fs.html sex-linked fruit fly problem demo and other fly mutations: http://vector.cshl.org/dnaftb/10/animation/fs.html human hemophilia (pedigree included) and eye color "near-truth" instead of the lies your teachers and professors told you about brown and blue eye color: http://vector.cshl.org/dnaftb/13/animation/fs.html Map of the X-chromosome http://www.ncbi.nlm.nih.gov/cgi-bin/Entrez/maps.cgi?org=hum&chr=X