Chapter 9: Developmental genetics

Question 1

Hemizygote

Answer 1

one of two copies of each gene

Question 2

Sex determination

Answer 2

Sexual differentiation occurs at 6 weeks of embryo development

Question 3

If the Y chromosome is present

Answer 3

embryo will develop wolffian ducts

Question 4

Wolffian ducts

Answer 4

develop into epididymis, seminal vesicles and vas deferens

Question 5

Absence of Y chromosome

Answer 5

embryo will develop Mullerian ducts

Question 6

Mullerian ducts

Answer 6

will develop fallopian tubes, uterus and proximal vagina

Question 7

“Default” sexual differentiation

Answer 7

female differentiation

Question 8

SRY gene

Answer 8

right below telomere of the short arm of the Y chromosome.
It is a transcription regulator
Protein product promotes expressions of other genes such as SOX9

Question 9

Originally called Testes Determining Factor

Answer 9

later renamed sex-determining region of Y chromosome (SRY)

Question 10

SRY continued

Answer 10

The SRY gene codes for a transcription regulator, which will bind to its target genes and gene transcription
Missing the SRY will turn an individual with an XY complement into a female

Question 11

Examples of genetic diseases associated with sex chromosome abnormality

Answer 11

• Turner syndrome (45,X)
• Klinefelter syndrome (47,XXY)
• XXX and XXXX females
• 46,XX males

Question 12

Turner Syndrome

Answer 12

• Incidence: 1/5000 to 1/10,000
• Puffy extremities (edema, too much of fluid in the tissue of hands or feet)
• Webbed neck, low hair line, widely spaced nipples
• Monosomy X chromosome (45,X; 50%), or mosaicism (45,X/46,XX; 30%-40%)
• Occurs only in females
• Infertile due to lack of ovaries
• Missing paternal X chromosome

Question 13

Klinefelter Syndrome

Answer 13

• Incidence: 1/1000 male live birth
• First discovered in 1959; 47,XXY
• The syndrome is characterized by moderate learning difficulties, enlargement of the breasts, and infertility (100%)
• An increased incidence of carcinoma of the breasts in the adult life
• The extra X chromosome is maternal in origin in about 50% of cases
• A higher incidence of the disease is associated with advanced maternal age
• Treatment includes male hormone and mastectomy of the enlarged breast to prevent development of cancer
• 48,XXXY

Question 14

XXX and XXXX females

Answer 14

• Learning difficulties
• Tall
• Some have normal fertility and have children with normal karyotype. Others infertile
• An increase of the number of the X chromosome correlates to the severity of the learning difficulties
• The extra X chromosome in the most cases is maternal in origin
• A higher incidence of the disease is associated with advanced maternal age

Question 15

Deletion of the short arm of chromosome 9 can cause sex reversal

Answer 15

• DMRT genes are on human chromosome 9
• Deletion of the 9p24.3 region can result in sex reversal. In this case, an individual who is phenotypically a female has a 46,XY karyotype

Question 16

Monosomy

Answer 16

Loss of one member of a homologous pair of chromosomes so that there is one less than diploid number of chromosomes (2N-1)

Question 17

pseudoautosomal region

Answer 17

Area where genes that behave like autosomal genes are located as a result of being located in the on the homologous portions of the X and Y chromosomes

Question 18

TDF

Answer 18

Testes Determining Factor, later called SRY

Cloned and sequenced in the 1990s