Sex Chromosomes

Question 1

X and Y Chromosomes

Answer 1

• Children receive an X chromosome from their mother, but
  either an X or Y chromosome from their father
• Results in approximately1: 1 ratio of females-to-males

Question 2

What determines maleness in a single gene?

Answer 2

SRY (sex determining region of Y)
SRY is always present in XX males
SRY is always nonfunctional in XY females

Question 3

What is located at the end of a Y chromosome?

Answer 3

Pseudoautosomal regions (PARs)

• Enables sex Chromosomes to pair during meiosis 1

Question 4

What determines gender in humans?

Answer 4

Presence or absence of Y chromosome

Abnormal numbers of X or Y chromosomes have different effects in
humans and flies

Question 5

Heterogametic sex

Answer 5

sex
with two different kinds of gametes (for example, XY males in humans, ZW females in birds)

Question 6

Homogametic sex

Answer 6

sex with one type of gamete (for example, XX females in humans, ZZ males in birds)

Question 7

Wild type allele

Answer 7

Denoted with +

Question 8

Recessive mutant alle

Answer 8

lowercase (w)
if it is wild-type (w+)

Question 9

Dominant mutant

Answer 9

Upper case
Wild-type (B+)
mutant (Bar)

Question 10

When does crisscross inheritance occur?

Answer 10

occurs with X-linked recessive traits

daughters inherit the phenotype of their
fathers, sons inherit the phenotype of their mothers

Question 11

Inheritance and Sex-Linked Traits

Answer 11

-In males (XY), genes on the X chromosome are
hemizygous because there is no corresponding allele on
the Y chromosome.
- If a gene on the X chromosome is defective, males will
express the trait because they have no second copy to
mask the defect (e.g., color blindness, hemophilia).
- Hemizygosity refers to the presence of only one copy of
a gene or chromosome region in an organism, instead of the typical two copies found in diploid cells.

Question 12

Meiosis mistake..

Answer 12

help confirm the chromosome theory

Question 13

Nondisjunction

Answer 13

• produced XXY females that could have their chromosomes traced
• three sex chromosome pair and segregate in 2 ways, producing progeny w/ unusual sex chromoms complements

Question 14

Pedigrees

Answer 14

X-linked recessive
* Mutation never passes from father to son
* Daughters of affected males are carriers. ½ of sons of
carriers will inherit the trait.

X-linked dominant
* Trait seen in every generation
* Affected male will produce affected daughters and
unaffected sons.

Y-linked
Only in males (hypertrichosis- excessive ear hair)

Question 15

Dosage compentsation

Answer 15

Females (XX), males (XY)
* Dosage compensation offsets what would be double the gene
products in the homogametic versus heterogametic sex
In female cells, one X is randomly inactivated
* The inactive X chromosome is
condensed into a Barr body

Question 16

Turners Syndrome

Answer 16

• 45, x
• Short, inferitility

Question 17

Klinefelter Syndrome

Answer 17

• 47, XXY
• tall, learning disablity, reduced test