Week 4 Flashcards
Fill in the blank:
The ___ of the arms of X & Y chromosomes share a small region of ______
THe “tips” of the arms of X & Y chromosomes share a small region of “homology”
What do the tips of the X and Y chromosomes allow them to do during meiosis
It allows them to pair during meiosis
Are X-linked recessive alleles expressed in females/males.
& why?
X-linked recessive alleles are expressed in males because males have only 1 X chromosome
Where can you see a recessive mutation in a male?
a recessive mutation is visible in the male’s phenotype
- Y does not carry an allele of this gene to compensate
Explain how a male with a X-linked recessive mutation can be transmitted to another male.
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
True or False:
During Meiosis in a female, there are only X-bearing eggs
True
FIll in the blank:
During Meiosis in a male, there is a : ratio of X-bearing and Y bearing sperm
1:1 ratio
During the segregation of sex chromosomes, there is random fertilization.
What is the unexpected ratio of XX(female) to XY(male) progeny
1/2 XX
1/2 XY
What was the significance of the Fruit Fly experiment?
- It led to the discovery of X-linked inheritance
2. the 1st experimental evidence that genes are on chromosomes
Explain what happened to each generation in the Fruit fly experiment with a conclusion
-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
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
c) Both would have a ratio 1R:1W
What happens to normal chromosome separation in each egg?
each egg containing a single X chromosome
What did rare non disjunction help prove in the fruit fly experiment?
It helped prove that the inherited factor for eye colour was on the X chromosome
Define non-disjunction
eggs w/ either 2X or no X chromosome
What would nondisjunction of the X chromosome result in if a White F is paired with a R male?
= in XXY female progeny w/ white eyes
= in X_ male progeny w/ red eyes
What would happen during a heterozygous cross of a female carrier and a normal male
F = heterozygous carrier M = normal
=1/2 daugthers are expected to be heterozygous
=1/2 sons expected to be affected
What would happen if you cross between an affected male and a normal female?
M must pass nutant X to female
- daughters = heterozygous
- sons = normal
Why must there be dosage compensation
b/c females have twice as many X’s than males
XX vs XY
What are the 3 different mechanisms that adjust for the unequal dosage of X-linked genes in male and female animals?
- inactivating 1 of 2 X chromosomes in females
- in mammels - hyperactivating the single X chromosome in males
- in Drosophila - each of the 2 X chromosomes has reduced activity, that are collectively equal to the single X in males
- in C elegans (a nematode)
Define barr body
the inactive X is a highly condensed chromosome
Explain what happens with an X-Linked recessive trait
- usualy more males than females affected
- affected sons are usually born to unaffected moms
- trait skips generations
- =1/2 of a carrier (heterozygous) mom’s sons are affected
- all daughters of affected fathers are carriers
Explain what happens with an X-Linked Dominant Trait
- both males and females are usually affected
- usually more females than males - doesn’t skip generations
3a. affected son from affected mom
3b. affected daughter by affected mom/dad - affected dads pass trait to all daughters
- affected moms (if heterozygous) pass trait to 1/2 sons and 1/2 to daughters
Explain Y-Linked Inheritance
- only males affected
- passed from father to son
- doesn’t skip generations
Why does neither “ recessive” nor “dominant” term apply to Y-Linked Inheritance?
b/c theres only ever 1 Y chromosome
What is the function of an Ultrasound?
+ what can it help detect?
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
Explain the steps for Amniocentesis
- under guidance of ultrasound, a sterile needle is inserted through the abdominal wall into the amniotic sac
- a small amount of amniotic fluid = withdrawn through the needle
- the amniotic fluid contains fetal cells, which are separated from the amniotic fluid…
- … & cultured
- test = then performed on the cultured cells
- chemical analysis, dna analysis, chromosomal analysis
What some key characteristics of amniocentesis?
- 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
Explain the steps for Chorionic villus sampling
- CVS can be performed early in pregnancy
- under the guildance of ultrasound, a catheter is inserted throughthe vagina and the cervix and into the uterus
- …where it is placed into contact w/ the chorion
Chorion = the outer layer of the placenta - suction removes a small piece of the chorion
- cells of the chorion are used directly for many genetic tests & the culturing is not required
What are some key characteristics of Chorionic villus sampling?
- 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 )
How does fetal cell sorting work and what is the benefit to fetal cell sorting?
-fetal cells are sorted from maternal cells
zero risk to fetus
Explain a pre-implantation genetic diagnosis.
When can it be done?
Done prior to egg being fertilized
