General GeneticsAyesha M. Khan

Spring 2013

The mechanism of sex determination controls the inheritance of sex-linked characteristics.

Most genes on the X chromosome are different from genes on the Y chromosome.

Males and females do not possess the same number of alleles at sex-linked loci. This difference in the number of sex-linked alleles produces the distinct patterns of inheritance in males and females.

Sex determination and Sex linked characteristics

In sexual reproduction, parents contribute genes to produce an offspring that is genetically distinct from both parents. In eukaryotes, sexual reproduction consists of meiosis, which produces haploid gametes, and fertilization, which produces a diploid zygote.

Sex determination

We define the sex of an individual in terms of the individual’s phenotype—ultimately, the type of gametes that it produces.

The cells of female humans normally have two X chromosomes, and the cells of males have one X chromosome and one Y chromosome. ◦ A few rare examples: Genetically female, having

two X chromosomes but male phenotype.

Sex determination (contd)

Dioecious-organism has either male or female reproductive structures.

Monoecious-organisms that bear both male and female reproductive structures.

Hermaphroditism-both sexes are present in the same individual.

Sex determination mechanisms

Hermann Henking-1891: Peculiar structure termed the X body found in nuclei of male insect cells.

Clarence E. McClung-1901: Accessory chromosome, which eventually became known as the X chromosome.

Nettie Stevens and Edmund Wilson-1905: In grasshoppers and other insects, the cells of females have two X chromosomes, whereas the cells of males have a single X.◦ Two X chromosomes were found in female cells, whereas a

single X chromosome plus a smaller chromosome, which they called Y, was found in male cells

Sex determination (history)

Stevens and Wilson also showed that the X and Y chromosomes separate into different cells in sperm formation; half of the sperm receive an X chromosome and half receive a Y. All egg cells produced by the female in meiosis receive one X chromosome.

A sperm containing a Y chromosome unites with an X-bearing egg to produce an XY male, whereas a sperm containing an X chromosome unites with an X-bearing egg to produce an XX female.

Chromosomal Sex-Determination Systems:Sex chromosomes and non-sex chromosomes (autosomes)

XX-XO system: • XX – female• XO – male• grasshoppers

XX-XY system:• XX – female• XY – male• Mammals

ZZ-ZW system:• ZW- female • ZZ-male • birds, moths, some

amphibians, and some fishes.

Heterogametic sex: two different types of gametes with respect to the sex chromosomes.Homogametic sex: gametes that are all the same with respect to the sex chromosomes.

The X and Y chromosomes are homologous only at pseudoautosomal regions.

Haplodiploidy system:No sex chromosomes; sex is based on the number of chromosome sets found in the nucleus of each cell. Males develop from unfertilized eggs,and females develop from fertilized eggs.• Haploid set – male• Diploid set – female• Bees, wasps, and ants

Chromosomal Sex-Determination Systems

Genic Sex-Determining System

No sex chromosomes, only the sex-determining genes.

Genotypes at one or more loci determine the sex of an individual

No obvious differences in the chromosomes of males and females.

In both genic sex determination and chromosomal sex determination, sex is controlled by individual genes; the difference is that, with chromosomal sex determination, the chromosomes that carry those genes appear different in males and females.

Environmental Sex Determination

Sex is determined fully or in part by environmental factors.Marine mollusk Crepidula fornicata (slipper limpet)-sequential hermaphroditism each individual animal can be both male and female, although not at the same time. -In limpets sex is determined environmentally by the limpet’s position in the stack.

Although most snakes and lizards have sex chromosomes, in many turtles, crocodiles, and alligators, temperature during embryonic development determines sexual phenotype.

Environmental Sex Determination (contd)