Download Note Review Sex-Linked Traits

Name: _____________________________
Note Review: Sex-Linked Traits
Use your in-class notes AND p. 307-8 to fill in the blanks and answer the questions.
Chromosome Numbers
Humans have _____ chromosomes:
2 are the sex chromosomes:
XX or XY
_____ _____
The other 44 are ________________
●Remember, however, that gametes (sperm/eggs/pollen) carry half of the DNA. So, in a
human, sperm and eggs carry 23 chromosomes (one from each pair of chromosomes).
In addition to 22 other chromosomes (autosomes)...
•Egg cells carry one X chromosome.
•Sperm cells can carry either an X or a Y chromosome.
With that in mind, which parent determines the sex of the child? _______________
Some genes are carried only on the X chromosome...
“Sex-Linked” Traits
Examples:
____________________________ & _________________________
Since males have only one copy of the X chromosome, any traits found on the X (even
recessive ones) are automatically expressed.**
Wait. What??
Think about it…
How many alleles* would a female have for the gene that determines red/green color vision?
2- one on each X chromosome
The “normal” gene is dominant and the colorblind gene is recessive. The only way a female
could be colorblind is if she receives 2 copies of the defective gene (a pretty rare event).
* An allele is a form of the gene, like “T” or “t”
However…
How many alleles for the same gene does a male receive?? (Remember that this gene is carried
ONLY on the X chromosome…)
1!
If his X chromosome carries the gene for colorblindness, he is automatically colorblind
because he doesn’t have a “back-up” X chromosome like females do…
Statistically, inheriting 1 colorblindness gene is MUCH more likely than inheriting 2.
So, men are much more likely to develop these recessive sex-linked characteristics than women
are.
Do you REALLY get that???
Let’s say it another way:
Who is more likely to inherit a sex-linked trait, men who get only 1 copy of the X chromosome,
or women who get 2 copies of the X chromosome?
Men, because they do not have a second X chromosome to protect them if their first X
chromosome is faulty.
Other info about chromosomes and Inheritance:
Linked Genes
●Pairs of genes that tend to be _______________________________
•Why?
•Found on the same chromosome
Crossing Over
•The exchange of pieces of DNA between pairs of
chromosomes during ____________________ formation.
•The further apart genes are on a chromosome, the more likely
they are to recombine during crossing over.
•Increases ____________________
Chromosome Map
Shows the order of genes on a chromosome
1 map unit = frequency of recombining during crossing over 1 percent of the time