SEX DETERMINATION AND SEX CHROMOSOMES

I. The Chromosome Theory of Inheritance and Sex Linkage

A. Sutton and Boveri’s chromosome theory of inheritance proposed in 1902—Genes are located on chromosomes

B. Just previous to this (end of 19^th century) biologists had discovered that half of all sperm cells carry a structure called an X body.

C. In 1905 the X body were determined to be chromosomes—X chromosomes.

D. Then the Y chromosome was also discovered in 1905.

E. Together the X and Y chromosomes are known as the sex chromosomes.

F. All other chromosomes are called autosomes.

G. Systems of sex chromosomes

1) XX-XO system (Protenor mode)

(a) Female—XX

(b) Male—X

(c) Occurs in some insects like grasshoppers

2) XX-XY System (Lygaeus mode)

(a) Female—XX (homogametic sex)

(b) Male XY (heterogametic sex)

(c) Occurs in Drosophila, mammals and some plants

3) ZZ-ZW System

(a) Female—XY (heterogametic sex)

(b) Male—XX (homogametic sex)

(c) Occurs in birds, butterflies and some fishes

4) X-Y-XY System

(a) Occurs in organisms with alteration of generations (e.g., liverworts and vascular plants)

(b) Male gametophytes—Y

(c) Female gametophytes—X

(d) Sporophytes—XY

H. Morphology and pairing of X and Y

1) Each type of sex chromosome has two regions

(a) Pairing region

(i) During synapsis of meiotic prophase I, the pairing regions combine

(ii) Some genes occur in these pairing regions

(iii) These genes exhibit X-and-Y linkage

(b) Differential region

(i) Differential regions do not pair during synapsis

(ii) Differential region genes are either

1. X-linked

2. Y-linked

(iii) Any gene X or Y chromosome is said to be sex linked

I. Sex determination in Drosophila

1) Sex is determined by the ratio of the number of X chromosomes to number of autosomal sets

2) Scheme

(a) X/A = 1.0—female

(b) X/A = 0.5—male

(c) X/A > 1.0—metafemale

(d) X/A < 0.5—metamale

J. Sex determination in humans

1) The presence or absence of the Y chromosome determines sex

2) A gene on the Y chromosome called the SRY gene codes for the testis-determining factor

(a) It controls the production of maleness

(b) Works by inducing development of the medulla of the gonadal primordial, pairs of ridges on the embryonic kidneys

(c) XY individuals who lack the part of the Y chromosomes with this gene are females

(d) XX individuals who carry a tiny piece of the Y chromosome with this gene on an X chromosome are male (though sterile)

(e) The X chromosome contains what appears to be a homologous region, though perhaps the genes there are pseudogenes

(f) Dosage compensation in mammals, including humans

(i) Bar bodies

1. XYAA—0 Barr bodies

2. XXAA—1 Barr body

3. X0AA—0 Barr bodies

4. XXXAA—2 Barr bodies

5. XXYAA—1 Barr body

(ii) Lyon hypothesis

1. One X chromosome in each cell becomes inactivated early in development

2. Which one is deactivated is random

3. All cells derived from cell with deactivated chromosome will have that chromosome deactivated

4. This results in genetic mosaics

5. Examples

a. Calico cats

b. Sweat gland distribution in females heterozygous for extodermal dysplasia

(g) Anomalies

(i) Hermaphrodites

1. Have both ovaries and testes

2. External genitalia are ambiguous

3. Generally, true hermaphrodites are sterile, but not always

4. In 1978 (at The American Society of Human Genetics meeting in Vancouver, B.C.) as case was reported in which

a. 25-year-old hermaphrodite delivered a stillborn child after 30 weeks gestation

b. Earlier in life, this person had engaged in male sexual activity

c. True hermaphrodites are genetic mosaics—some cells are XX, while others are XY

(ii) Pseudohermaphrodites

1. Pseudohermaphrodites have either testicular or ovarian tissue, but not both

2. Generally the tissue is rudimentary

3. External genitalia are often ambiguous

4. Some are genetically female, but may look like males

5. Some are genetically male, but may look like females and lead normal female sex lives

(iii) Turner’s Syndrome in humans (45, XO)

1. Due to nondisjunction in male or female parents to produce gametes without X chromosome

2. Incidence: 1/5000 female births

3. High proportion of spontaneous abortions are Turner’s and most Turner’s individuals are aborted

4. Phenotypic features:

a. Short stature

b. Webbed neck—web of skin between neck and shoulders

c. Breast development absent or nearly so

d. Some cognitive functions affected, but intelligence often about normal

e. Pubic and axillary hair reduced or absent

f. Infantile genitalia

g. Usually sterile

(iv) Kleinfelter’s Syndrome (47, XXY)

1. Due to nondisjunction of X chromosome in male or female parent

2. Incidence: 1/1000 male births

3. Tends to be maternal age effect

4. Sometimes have more than two X chromosomes

5. Phenotypic features

a. Long arms

b. Breast development

c. Little or no sperm production

d. Small testes

e. Usually mentally retarded

(v) XYY Condition

1. Due to nondisjunction of Y chromosome in male

2. Incidence: 1/1000 male births

3. Phenotypic features:

a. Above average height

b. Fertile

c. Sometimes (but not always) retarded

d. May be correlation with delinquency

(vi) Poly-X Females (XXX, XXXX, XXXXX,…)

1. Incidence: 1/1000 female births

2. Maternal age effect