This article explains the various mechanisms of sex determination found in different organisms such as mammals, insects, eukaryotic microorganisms, and plants. It covers topics such as heterogametic and homogametic sexes, XX-XY and XX-XO sex determination systems, ZZ-ZW sex determination system, haploid-diploid mechanism of sex determination, and sex determination in plants.
One
pair of chromosomes is associated with sex determination in majority of
animals. In such cases one sex male or female is heterogametic i.e. produces
two types of gamete in equal number and the other sex male or female is
homogametic i.e. produces only one type of gametes. It may be:
(a)
Male Heterogamy (b) Female Heterogamy
Male Heterogamy
Males
are heterogametic and produce two type of gametes in equal number. The females
are homogametic and produce only one type of eggs.
1. XX XY Type
In
mammals including man and in Drosophila, sex is determined by X and Y
chromosome. Male is heterogametic sex. The sperm may have either X or Y
chromosome. Female is homogametic sex. All the ova produced have only X
chromosome. The sex is determined at the time of fertilization, by the type of
sperm fusing with ovum. The ovum with X- chromosome when fertilized by a sperm
with X- chromosome develops into female (XX chromosome). The ovum with X-
chromosome when fertilized by sperm having Y chromosome develops into a male
(XY chromosome), chances of male and female offspring is equal.
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| Sex determination in drosophila |
2. XX XO Type
It
is found in many insects e.g., grasshopper. The female is homogametic (XX) and
the number of chromosomes is 24. It produces eggs, all of one type having
X-chromosome. The male is hemigametic. It has 23 numbers of chromosomes. It has
only one type of sex chromosome i.e. X. It produces two types of sperms. 50
percent having X chromosomes and the other 50 percent having only autosomes and
no sex chromosome. I.e. XO. During meiosis half the sperms have eleven pairs
and half have 12 pairs of chromosomes.
Female Heterogamy
Females
produce two type of eggs and are heterogametic. Males produce only one type of
sperm and are homogametic.
ZZ ZW Type
It
is found in butterflies, moths, fishes, reptiles and some birds. The male has
homomorphic sex chromosomes ZZ. It produces sperms having Z-chromosomes. The
female is heterogametic. Its sex chromosomes are heteromorphic represented by
ZW. It produces eggs of two types having Z or W sex chromosome.
Haploid Diploid
Mechanism of Sex Determination
In
insects, belonging to order Hymenoptera e.g. honey bees, wasps, and ants and
saw flies, the fertilized diploid egg develops into a female, but an
unfertilized egg develops into male. Thus, the females are diploid and males
are haploid. The development of egg without fertilization is called
parthenogenesis.
Sex Determination in
Haploid Organisms
In
Liverworts and Mosses females have only X chromosome and males have only Y
chromosome.
Sex Determination in
Eukaryotic Microorganisms
Many
species of eukaryotic microorganism like Yeasts do not have sex chromosome and
depend on genetic system for sex determination. Sexes are specified by simple
allelic differences at a small number of gene loci e.g. a and X are the two
mating types i.e. sexes in yeast is controlled by MATa and MATα alleles
respectively.
Sex Determination in
Plants
Plants
have sex chromosomes. Some plants species like date palms, spinach, marijuana
and Ginkgo are dioecious having plants of separate sexes. Male plants produce
flowers with only stamens and female plants produce flowers with only carpels
having X-Y system of sex determination. In Cannabis and Melandrium the male
plants are heterogametic because it has been seen that sparse pollination in
Cannabis produces excess of males while in Melandrium it produces excess of
female. A heterogametic female is expected to produce male and female plants in
equal proportion. The male plants of Malandrium have XY chromosomes and female
plants have XX female. Wild strawberry have the Z-W system i.e. eggs of two
types having Z and W sex chromosomes.
