Genetics and Chromosomal Abnormalities: Sex Chromosomes, Linkage, and Disorders

How many chromosome pairs do humans have?

23 pairs

How many human chromosome pairs are autosomes?

22 pairs

How many sex chromosomes do humans have and what are the combinations for females and males?

1 pair; XX = female, XY = male

Which sex chromosome is bigger and carries more genes?

X chromosome

Which sex chromosome is smaller and has fewer genes?

Y chromosome

What controls sex-linked traits?

Genes on sex chromosomes

Where are X-linked traits located?

On the X chromosome

Why do males express recessive X-linked traits more often?

They have only one X chromosome

Where are Y-linked traits found?

On the Y chromosome

Who can inherit Y-linked traits?

Only sons from their father

How many recessive alleles does a female need to show an X-linked trait?

Two

How many recessive alleles does a male need?

One

Who is more often affected by X-linked recessive traits, males or females?

Males

Can X-linked recessive traits skip generations?

Yes

What happens when an affected male passes on an X-linked recessive allele?

All daughters are carriers, no sons affected

What happens when an affected female passes on an X-linked recessive allele?

All sons are affected

Name an X-linked disorder affecting color vision.

Red-green colorblindness

Which X-linked disorder reduces blood clotting?

Hemophilia

Which X-linked disorder is caused by missing dystrophin?

Duchenne muscular dystrophy

Which X-linked disorder is the leading inherited form of intellectual disability?

Fragile X syndrome

Are X-linked dominant traits common or rare?

Rare

What happens when an affected male passes on an X-linked dominant trait?

All daughters affected, no sons affected

Name an example of an X-linked dominant disorder.

Incontinentia pigmenti (IP)

What usually happens to males with incontinentia pigmenti (IP)?

They often do not survive

What happens to affected females with incontinentia pigmenti (IP)?

Show skin pigmentation swirls

What are linked genes?

Genes on the same chromosome inherited together

What happens with complete linkage?

No crossing over → 2 types of gametes

What happens with incomplete linkage?

Crossing over → 4 gametes (unequal amounts)

Does crossing over happen more often between genes close together or far apart?

Far apart

What causes chromosomal number abnormalities?

Nondisjunction in meiosis

What is a trisomy?

Extra chromosome (2n + 1)

What is a monosomy?

Missing chromosome (2n - 1)

Give an example of trisomy.

Down syndrome (trisomy 21)

Give an example of monosomy.

Turner syndrome (XO)

Name two other chromosomal number disorders.

Klinefelter syndrome (XXY), Jacobs syndrome (XYY)

What is a deletion?

Segment of chromosome is lost

What is a duplication?

Segment of chromosome is repeated

What is a translocation?

Segments exchanged between nonhomologous chromosomes

What is an inversion?

Segment flipped, changing gene order