OB: Genetics, Conceptions and Fetal Development

Question 1

What are the two most common aneuploid conditions?

Answer 1

1. Monsomies
2. Trisomies

Question 2

What is the difference between a monosomy and a trisomy?

Answer 2

• A monosomy is the product of a union between a normal gamete and a gamete that is missing a chromosome. Limited data available given that an embryo never survives.
• A trisomy is the product of a union between a normal gamete and a gamete with a extra chromosome. An example is down syndrome. Risk of child having down syndrome increases with age.

Question 3

What is autosomal recessive inheritance disorders?

Answer 3

Disorders are those which both genes of a pair must be abnormal for the disorder to be expressed. Heterozygous individuals have only one variant allele and aer unaffected clinically are known as carriers of the recessive trait.

Question 4

What is x-linked dominant inheritance disorders?

Answer 4

Disorders that occur in males and heterozygous females. Because of X inactivation, affected females are usually less severly affected than affected males, and they are more likely to transmit the variant allele to their offspring.

Two examples: vitaminD-resistant rickets and fragile X syndrome

Question 5

What are x-linked recessive inheritance disorder?

Answer 5

Disorders that are carried on the X chromosome. Females may be heterozygous or homozygous for traits carried on the X chromosome given that they have two chromosomes. Males are heyzygous because that have only one X chromosome. Most often occur in males.

Examples: Hemophilia, color blindness, and Duchenne muscular dystrophy

Question 6

What are primary germ layers?

Answer 6

Germ layers are layers that form in the third week after conception from an embryonic disk. They are called the ectoderm, mesoderm, and endoderm. All tissues and organs of the embryo develop from these three layers.

Question 7

Which tissue does the ectoderm form?

Answer 7

It gives rise to:

• the epidermis, glands (anterior, pituitary, cutaneous, and mammary),
• nails and hair,
• central and peripheral nervous system,
• eye lens
• tooth enamel
• floor of the amniotic cavity

Question 8

Which tissue does the mesoderm form?

Answer 8

It forms:

• bones and teeth
• muscles (skeletal, smooth, cardiac)
• dermis
• connective tissue
• cardiovascular system
• spleen
• urogenital system

Question 9

Which tissue does the endoderm form?

Answer 9

It forms:

• respiratory tract
• digestive tract
• liver and pancreas
• urethra and bladder
• vagina
• roof of the yolk sac

Question 10

When does the fetal stage begin?

Answer 10

Begins from 9 weeks (when embryo becomes recognizable as a human being) until pregnancy ends. In this stage, the fetus is less vulnerable to teratogens, except for those that affect central nervous system.

Question 11

When is the fetus viable (capable of surviving outside the uterus)?

Answer 11

With modern technology and advances in maternal and neonatal care, approximately 20 weeks after conception (22 weeks LMP), fetal weight of 500 g or more. Limitations on survival are based on central nervous system function and oxygenation capability of the lungs.