Chromosomes, Sex-Linked Traits & Gene Linkage: Key Concepts for Biology
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30 Terms
How many chromosome pairs do humans have?
23 pairs.
How many pairs are autosomes?
22 pairs.
What is the 23rd pair?
Sex chromosomes.
What combination makes a female?
XX.
What combination makes a male?
XY.
Which chromosome is bigger and carries more genes?
X chromosome.
Which chromosome is smaller with fewer genes?
Y chromosome.
What controls sex-linked traits?
Genes on sex chromosomes.
Where are most sex-linked traits located?
On the X chromosome.
Why do males express X-linked recessive traits more often?
They only have one X.
Who passes Y-linked traits?
Father → son only.
How many recessive alleles do females need to show an X-linked recessive trait?
Two.
How many recessive alleles do males need?
One.
In a typical X-linked recessive pattern, who is affected more?
Males.
Can X-linked recessive traits skip generations?
Yes.
What do affected males pass to their daughters?
The recessive allele → all daughters become carriers.
Do affected males pass it to their sons?
No.
What do affected females pass to their sons?
The recessive allele → all sons affected.
Name X-linked recessive disorders.
Red-green colorblindness, Hemophilia, Duchenne muscular dystrophy, Fragile X syndrome.
Which is the leading inherited cause of intellectual disability?
Fragile X syndrome.
Are X-linked dominant traits common?
No, rare.
What do affected males pass to daughters?
The dominant allele → all daughters affected.
Do affected males pass it to sons?
No.
Example of X-linked dominant disorder?
Incontinentia pigmenti (IP).
Who usually doesn't survive with IP?
Males.
What do affected females show?
Swirled skin pigmentation.
What is gene linkage?
Genes on the same chromosome inherited together.
What is complete linkage?
No crossing over → 2 gamete types.
What is incomplete linkage?
Crossing over → 4 gamete types (unequal amounts).
When does crossing over happen more often?
When genes are farther apart on the chromosome.
Note 
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