Sex Determination and Sex Linkage

Sex Determination and Sex Linkage

Objectives

• To analyze the causes of sex determination in various organisms

Additional Reading

• H. Henking, C. E. McClung, and E. B. Wilson all contributed to investigations relating chromosomes to sex determination.

Sex Determination

• Most mechanisms for sex determination are under genetic and hormonal control.
• Four types of chromosomal sex-determining mechanisms exist: XY, ZW, XO, and compound chromosomal mechanisms.
• Humans and most other mammals have two different sex chromosomes (X and Y), which are heteromorphic.
• The presence of the Y chromosome determines maleness.
• Normal males have 22 pairs of autosomes, plus X and Y chromosomes.
• Males produce two kinds of gametes (X and Y) and are considered the heterogametic sex.
• Females have 22 pairs of autosomes and two X chromosomes.
• Females produce only one kind of gamete and are considered the homogametic sex.

XY Method

• In Drosophila and humans, normal females and males have an XX and an XY sex chromosome composition, respectively.
• In Drosophila, the Y chromosome contains genes active in forming sperm in adults, but plays no significant role in sex determination.
• The SRY gene, located on the short arm of the Y chromosome, encodes a gene product called testis-determining factor (TDF).
• SRY is highly conserved in mammals.
• The SRY gene, in combination with several other genes, encodes a DNA-binding protein that activates the expression of testicular development and fertility.

Heterogametic Females (ZZ-ZW)

• This method of sex determination is found in many species, including most birds, butterflies, moths, and some fish.
• The mode is identical to the XX-XY mechanism.
• Females are heterogametic (ZW), while males are homogametic (ZZ).

The Genic Balance in Drosophila

• C. B. Bridges showed that sex determination in Drosophila was more complicated.
• Female determinants are located on the X chromosomes, and male determiners are on the autosomes.
• Male-determining genes are located on the three autosomal chromosomes of Drosophila.
• The genic balance theory of sex determination explains the mechanics of sex determination in Drosophila melanogaster.
• One X and two haploid sets of autosomes (2A) produce a normal male with a ratio of 0.5 (1X : 2A).
• Two X chromosomes and two sets of autosomes (2X:2A) produce a normal diploid female with a ratio of 1.0.
• An individual with 2X : 3A has a ratio of 2:3 or 0.67 and are intersexes.
• A ratio of XY;3A are metamales.
• Intersex flies are sterile and have sexual characteristics intermediate between the male and female.
• Ratios above 1.0 produce sterile metafemales or superfemales.
• Ratios below 0.5 produce sterile metamales.
• The ratio determines sex by activating sex-specific gene expression of several genes, including sex-lethal (Sxl), transformer (tra), and doublesex (dsx).

Haplodiploidy in Hymenoptera

• Hymenoptera includes insects, such as ants, wasps, and bees, utilize more involved sex determination mechanisms.
• In several species, males develop parthenogenetically from unfertilized eggs and have a haploid chromosome number (16 in the drone honeybee).
• The queen and the workers arise from fertilized eggs and carry the diploid chromosome number of 32.
• In the wasp Bracon hebetor, females are diploid with 20 chromosomes, and males are haploid with 10 chromosomes.
• Females originate from fertilized eggs, and males from unfertilized eggs.
• Because normal males are haploid and normal females are diploid, this mechanism of sex determination is often referred to as haplodiploidy.

Mosaics and Gynandromorphs

• Abnormal chromosomal behavior in insects can result in the formation of gynandromorphs or sexual mosaics, in which some parts of the animals are female and other parts are male.

Environmental Factors and Sex Determination

• Sex determination in some lower animals is non-genetic and depends on factors in the external environment.
• Males and females have similar genotypes, but stimuli from environmental sources can initiate development toward one sex or the other.
• In some reptiles, the temperature at which the fertilized eggs are incubated prior to hatching determines the sex of the offspring.
• In the turtle Chrysema picta, high incubational temperatures result in mostly female progeny.
• In the lizard Agama agama, high incubation temperatures result in mostly male offspring.

Diversity of Sex Determination

• The above examples illustrate the diversity of sex determination mechanisms that exist in nature.

Sex Linkage or X-linked

• Any trait determined by a gene on the X chromosome is sex-linked or X-linked.
• Any gene located on the X chromosome (in mammals, Drosophila, and others) or on the analogous Z chromosome (in birds and other species) is said to be sex-linked.

Morgan's Discovery of Sex Linkage in Drosophila

• The first extensive experimental evidence for sex linkage came in 1900 by T. H. Morgan with a white-eyed mutant in Drosophila.
• Morgan's work showed the presence of a white-eyed (mutant) in a culture of red-eyed (wild type) flies.
• The white-eyed male was mated with a red-eyed female.
• All the F₁ generation exhibited red-eye, expected for flies heterozygous for a recessive white-eye allele.
• The F₂ generation included both red and white-eyed flies in a proportion of about 3:1
• All the white-eyed flies in the F₂ generation were males.
• About half of the F₂ males had white eyes, and half had red eyes.
• All females had red eyes.
• The recessive allele was expressed only in males.
• Morgan associated this "gene" with the X chromosomes.
• These observations were correlated with a difference found in chromosome composition between male and female Drosophila.
• Females possess two rod-shaped homologues called the X chromosomes (XX), while males possess a single X chromosome and a J-shaped chromosome (XY).
• The white-eye trait is recessive to the red-eye trait, and this discovery was a natural consequence of the Mendelian assortment of chromosomes.

Important Points About Sex-linked Traits

• In normal diploid organisms with sex-determining mechanisms like that of humans or Drosophila, a trait governed by a sex-linked recessive gene usually manifests itself more frequently in the male than in the female of the species.
• Such a recessive trait fails to appear in females unless it also appeared in the paternal parent.
• A trait governed by a sex-linked dominant gene usually manifests itself more frequently in the female than in the male of the species.
• Sex-linked dominant genes are found in all female offspring of a male which shows the trait.
• Sex-linked dominant genes failing to be transmitted to any son from a mother which did not exhibit the trait itself.

Variations of Sex Linkage

• The sex chromosomes (X and Y) are of unequal size, shape, and/or staining qualities.
• Pairing occurs during meiosis, indicating that they contain homologous segments.
• Genes on the homologous segments are said to be incompletely sex-linked or partially sex-linked.
• Incompletely sex-linked or partially sex-linked genes may recombine by crossing over in both sexes just as it happens on homologous autosomes.
• Genes on the nonhomologous segments of the X chromosome are said to be completely sex-linked and exhibit the peculiar mode of inheritance described above.
• A few genes reside in the nonhomologous portion of the Y chromosome in humans.
• In such cases, the trait would be expressed only in males and would always be transmitted from father to son.
• Completely Y-linked genes are called holandric genes.