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3. Inheritance and Variation

-CREATED BY-

PROF. DESHMUKH A. B.

ASST. TEACHER

AGASTI ARTS, COMMERCE AND DADASAHEB RUPWATE SCIENCE JUNIOR COLLEGE, AKOLE, TAL. – AKOLE, DIST. - AHMEDNAGAR

3.11 Sex Determination

A. B’s. Biology

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3.11 Sex Determination :

  • The mechanism by which sex is established is termed as sex determination.
  • The term sex refers to sexual phenotype.
  • In some species, both male and female reproductive organs are present in same organism.
  • It is described as bisexual or hermaphrodite or monoecious.
  • On the other hand, some species in which the organism has either male or female reproductive organs, is said to be dioecious or unisexual.
  • Humans are dioecious.

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  • German biologist, Henking (1891), studied spermatogenesis of the squash bug (Anasa tristis).
  • During the study, he noted that 50% of sperms receive the unpaired chromosomes while other 50% sperm do not receive it.
  • Henking gave a name to this structure as the x-body but he could not explain its role in sex determination.
  • Further investigations by other scientists led to conclusion that the “x-body” of Henking was infact a chromosome and gave the name ‘X-Chromosome’.

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a. Sex Determination in human beings :

  • The chromosomal mechanism of sex determination in human beings is XX-XY type.
  • In human beings, the nucleus of each somatic cell contains 46 chromosomes or 23 pairs of chromosomes.
  • Out of these, 22 pairs are autosomes and one pair of sex chromosomes.
  • Human female has a pair of XX, homomorphic sex chromosomes while male has XY, heteromorphic sex chromosomes.

X

X

X

Y

Females:

Males:

44 Autosomes

+

44 Autosomes

+

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  • Thus genotype of :

Female = 44 Autosomes + XX

Male = 44 Autosomes + XY

  • During gamete formation in male, the diploid germ cells in testis undergo spermatogenesis to produce two types of haploid sperms,
    • 50% sperms contain 22 autosomes and X chromosome while,
    • 50% sperms contain 22 autosomes and Y chromosome.

Germ Cell (2n )

(44 +XY)

Spermatogenesis

22 + X

22 + Y

22 + X (50%)

22 + Y (50%)

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  • In Female, the diploid germ cells in ovaries undergo oogenesis to produce only one type of egg.
  • All eggs contain 22 autosomes and X chromosome.
  • Thus human male is heterogametic and female is homogametic.

​

Germ Cell (2n )

(44 +XX)

Oogenesis

22 + X

22 + X

22 + X (All)

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  • If sperm containing X chromosome fertilizes egg (ovum), then diploid zygote is formed, that grows into a female child.
  • If sperm containing Y chromosome fertilizes the egg, then diploid zygote is formed that grows into a male child.
  • This indicates that the sex of a child depends on the type of sperm fertilizing the egg.
  • Hence, the father is responsible for determination of sex of child and not the mother.
  • Due to lack of knowledge, women are often blamed for giving birth to female child.

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b. Sex Determination in birds :

  • In birds, the chromosomal mechanism of sex determination is ZW-ZZ type.
  • In this type females are heterogametic and produce two types of eggs;
    • 50% eggs carry Z- chromosome,
    • 50% eggs carry W- chromosome.
  • Males are homogametic and produce one type of sperms.
  • Each sperm carries a Z chromosome.
  • Thus sex of individual depends on the kind of egg (ova) fertilized by the sperm.

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c. Sex Determination in honey bees :

  • In honey bees, chromosomal mechanism of sex determination is haplo-diploid type.
  • In this type, sex of individual is determined by the number of set of chromosomes received.
  • Females are diploid (2n=32) and males are haploid (n=16).
  • The female produces haploid eggs (n=16) by meiosis and male produces haploid sperms (n=16) by mitosis.
  • If the egg is fertilized by sperm, the zygote develops into a diploid female (2n=32) (queen and worker) and unfertilized egg develops into haploid male (n=16) (Drone) by way of parthenogenesis.

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  • The diploid female gets differentiated into either worker or queen depending on the food they consume during their development.
  • Diploid larvae which get royal jelly as food develops into queen (fertile female) and other develops into workers (sterile females).

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