Chromosomal Organisation in Gametes

Understanding the chromosomal content of gametes requires three things. First, distinguish autosomes from sex chromosomes. Second, understand how the haploid chromosome number is composed. Third, know what each parent contributes to the zygote.

Autosomes are the 22 pairs of chromosomes, numbered 1 through 22, that are not involved in primary sex determination. Males and females have them in an identical pair arrangement. In a normal diploid somatic cell, all 44 autosomes are present as 22 homologous pairs, one member of each pair inherited from the mother and one from the father.

After meiosis, each haploid gamete, whether spermatozoon or oocyte, contains a single representative of each of the 22 autosome pairs: 22 autosomes in total. The random, independent assortment of homologous chromosome pairs during meiosis I determines the maternal or paternal origin of each autosome in that set.

The sex chromosomes, X and Y, are the 23rd chromosome pair and determine primary chromosomal sex. A normal diploid female somatic cell contains two X chromosomes, XX. A normal diploid male somatic cell contains one X and one Y chromosome, XY.

Because the sex chromosomes differ between the sexes, gametogenesis gives a fundamentally different sex chromosome outcome in males and females.

In oogenesis, every somatic and germ cell in a female contains two X chromosomes. Every oocyte produced therefore carries a single X chromosome, without exception, so oogenesis is chromosomally uniform for sex chromosome content.

In spermatogenesis, the primary spermatocyte contains one X and one Y chromosome. Meiosis I separates them, so half of the secondary spermatocytes, and therefore half of the spermatozoa, carry an X chromosome and half carry a Y chromosome.

Spermatogenesis therefore produces two chromosomally distinct populations of spermatozoa, in a theoretically equal 50:50 ratio.

Every normal human gamete, spermatozoon or oocyte, contains a total of 23 chromosomes: 22 autosomes, one representative of each autosomal pair, plus a single sex chromosome, X in every oocyte, and either X or Y in any given spermatozoon. This total of 23 is the defining haploid chromosome number in humans, exactly half the normal diploid somatic cell total of 46.

Because every oocyte carries an X chromosome and a spermatozoon carries either an X or a Y chromosome, the genetic sex of the resulting zygote is determined entirely by which type of spermatozoon happens to achieve fertilisation, rather than by any contribution from the oocyte.

Fertilisation of an X-bearing oocyte by an X-bearing spermatozoon produces a 46,XX zygote, genetically female; fertilisation of the same X-bearing oocyte by a Y-bearing spermatozoon produces a 46,XY zygote, genetically male. This is the precise chromosomal basis for the statement that genetic sex is paternally determined.

At fertilisation, each parent contributes exactly one haploid set of 23 chromosomes to the diploid zygote. This restores the normal 46 chromosome, 23 pair, complement.

Each autosomal pair in the zygote comprises one maternally derived and one paternally derived homologue. The sex chromosome pair comprises the maternally derived X together with whichever sex chromosome, X or Y, the fertilising spermatozoon carried.

The nuclear genetic contribution from each parent is numerically equal, 23 chromosomes each, but the contribution is not equal in every respect.

Essentially all of the zygote's mitochondria, and therefore its mitochondrial DNA, derive from the oocyte's cytoplasm. Sperm mitochondria, confined to the middle piece, are generally selectively degraded and eliminated from the embryo after fertilisation.

Mitochondrial inheritance in humans is therefore, for practical purposes, exclusively maternal. It is a separate pattern from the equal biparental inheritance of nuclear chromosomal DNA.