Page 13 - Lesson Notes- Principle of inheritance & Variation

non-sister chromatics of homologous 2. It involves individual chromosomes.

chromosomes.

3. The number of linkage group can
3. The frequency of crossing over can never
never be more than haploid
exceed 50%.
Chromosome number.

4. It increases variability by forming new
4. It reduces variability.
gene combinations.

Sex Determination

Henking in 1891 observed a trace of specific nuclear structure in few insects. He also

observed that this specific nuclear structure is located on 50% of sperms only. He called

this x body. He was not able to explain its significance.
Latter it was observed that the ovum that receive the sperms with x body become

female and those not becomes males, so this x body was called as sex chromosome

and other chromosomes are called autosomes.
In humans and other organisms XY types of sex determination is seen but in some

insects like Drosophila XO type of sex determination is present.

In both types of sex determination, male produce two different types of gametes either
with or without X chromosome or some with X chromosome and some with Y

chromosomes. Such types of sex determination are called male heterogamety.
In birds ZW type of sex determination is present., two different types of gametes are

produced by females in terms of sex chromosomes; this type of sex determination is

called female heterogamety.
Sex determination in human beings XY type. Out of 23 pairs of chromosomes, 22 pairs

are exactly same in male and female called autosomes. A pair of X chromosome is
present in female and XY in male. During spermatogenesis, male produce two type of

gametes (sperms), 50% carries Y chromosome and remaining 50% contain X

chromosome. Female, produce only one kind of gamete (ovum) having X chromosomes
only.

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