Week 4

Question 1

Fill in the blank:

The ___ of the arms of X & Y chromosomes share a small region of ______

Answer 1

THe “tips” of the arms of X & Y chromosomes share a small region of “homology”

Question 2

What do the tips of the X and Y chromosomes allow them to do during meiosis

Answer 2

It allows them to pair during meiosis

Question 3

Are X-linked recessive alleles expressed in females/males.

& why?

Answer 3

X-linked recessive alleles are expressed in males because males have only 1 X chromosome

Question 4

Where can you see a recessive mutation in a male?

Answer 4

a recessive mutation is visible in the male’s phenotype

- Y does not carry an allele of this gene to compensate

Question 5

Explain how a male with a X-linked recessive mutation can be transmitted to another male.

Answer 5

Trick question!!
1. Males can’t transmit to males in the next generation

an x chromosomes present in a male in 1 generation must be transmitted to a female in the next generation, then in the next generation after, it can be transmitted back to a male
Male –> Female –> Male

Question 6

True or False:

During Meiosis in a female, there are only X-bearing eggs

Answer 6

True

Question 7

FIll in the blank:

During Meiosis in a male, there is a : ratio of X-bearing and Y bearing sperm

Answer 7

1:1 ratio

Question 8

During the segregation of sex chromosomes, there is random fertilization.

What is the unexpected ratio of XX(female) to XY(male) progeny

Answer 8

1/2 XX

1/2 XY

Question 9

What was the significance of the Fruit Fly experiment?

Answer 9

1. It led to the discovery of X-linked inheritance

2. the 1st experimental evidence that genes are on chromosomes

Question 10

Explain what happened to each generation in the Fruit fly experiment with a conclusion

Answer 10

-fruit fly eye colour
P generation:
-Mom = Red, Dad=White
F1 generation
-F and M were red eyes
F2 generation
-F = all red, M= 1R:1W ratio 

only males were affected
- suggests inheritance of eye colour is linked to sex chromosomes

Question 11

What would happen if F=w+,w- paired with a w-,Y in the F1 generation

a) F + M would be all red
b) F + M would be all white
c) Both would have a ratio 1R:1W
d) only M would be all white

Answer 11

c) Both would have a ratio 1R:1W

Question 12

What happens to normal chromosome separation in each egg?

Answer 12

each egg containing a single X chromosome

Question 13

What did rare non disjunction help prove in the fruit fly experiment?

Answer 13

It helped prove that the inherited factor for eye colour was on the X chromosome

Question 14

Define non-disjunction

Answer 14

eggs w/ either 2X or no X chromosome

Question 15

What would nondisjunction of the X chromosome result in if a White F is paired with a R male?

Answer 15

= in XXY female progeny w/ white eyes

= in X_ male progeny w/ red eyes

Question 16

What would happen during a heterozygous cross of a female carrier and a normal male

Answer 16

F = heterozygous carrier
M = normal

=1/2 daugthers are expected to be heterozygous
=1/2 sons expected to be affected

Question 17

What would happen if you cross between an affected male and a normal female?

Answer 17

M must pass nutant X to female

• daughters = heterozygous
• sons = normal

Question 18

Why must there be dosage compensation

Answer 18

b/c females have twice as many X’s than males

XX vs XY

Question 19

What are the 3 different mechanisms that adjust for the unequal dosage of X-linked genes in male and female animals?

Answer 19

1. inactivating 1 of 2 X chromosomes in females
   - in mammels
2. hyperactivating the single X chromosome in males
   - in Drosophila
3. each of the 2 X chromosomes has reduced activity, that are collectively equal to the single X in males
   - in C elegans (a nematode)

Question 20

Define barr body

Answer 20

the inactive X is a highly condensed chromosome

Question 21

Explain what happens with an X-Linked recessive trait

Answer 21

1. usualy more males than females affected
2. affected sons are usually born to unaffected moms
3. trait skips generations
4. =1/2 of a carrier (heterozygous) mom’s sons are affected
5. all daughters of affected fathers are carriers

Question 22

Explain what happens with an X-Linked Dominant Trait

Answer 22

1. both males and females are usually affected
   - usually more females than males
2. doesn’t skip generations
   3a. affected son from affected mom
   3b. affected daughter by affected mom/dad
3. affected dads pass trait to all daughters
4. affected moms (if heterozygous) pass trait to 1/2 sons and 1/2 to daughters

Question 23

Explain Y-Linked Inheritance

Answer 23

1. only males affected
2. passed from father to son
3. doesn’t skip generations

Question 24

Why does neither “ recessive” nor “dominant” term apply to Y-Linked Inheritance?

Answer 24

b/c theres only ever 1 Y chromosome

Question 25

What is the function of an Ultrasound?

+ what can it help detect?

Answer 25

it measured the thickness of the fluid buildup under the skin at the back of the baby’s neck
-can help detect 90% of trisomy 21 Down Syndrome

Question 26

Explain the steps for Amniocentesis

Answer 26

1. under guidance of ultrasound, a sterile needle is inserted through the abdominal wall into the amniotic sac
2. a small amount of amniotic fluid = withdrawn through the needle
3. the amniotic fluid contains fetal cells, which are separated from the amniotic fluid…
4. … & cultured
5. test = then performed on the cultured cells
   - chemical analysis, dna analysis, chromosomal analysis

Question 27

What some key characteristics of amniocentesis?

Answer 27

• 14-18 weeks of gestation
• embryotic cells cutured
• about 20 cells karyotyped
• only suitable for large-scale chromosomal anomalies
• detects about 800 / 5000 known conditions

Question 28

Explain the steps for Chorionic villus sampling

Answer 28

1. CVS can be performed early in pregnancy
2. under the guildance of ultrasound, a catheter is inserted throughthe vagina and the cervix and into the uterus
3. …where it is placed into contact w/ the chorion
   Chorion = the outer layer of the placenta
4. suction removes a small piece of the chorion
5. cells of the chorion are used directly for many genetic tests & the culturing is not required

Question 29

What are some key characteristics of Chorionic villus sampling?

Answer 29

• 10-12 weeks of gestation
• karyotyped directly ( results = sooner b/c culturing is not required )
• slightly less accurate, higher risk of miscarriage ( 0.8% vs 0.3% w/ amniocentesis )

Question 30

How does fetal cell sorting work and what is the benefit to fetal cell sorting?

Answer 30

-fetal cells are sorted from maternal cells

zero risk to fetus

Question 31

Explain a pre-implantation genetic diagnosis.

When can it be done?

Answer 31

Done prior to egg being fertilized