SEX DETERMINATION (CHAPTER 6)

COMPETENCY TEST MATERIAL A. SEX DETERMINATION B. SEX LINKAGE C. MULTIPLE ALLELE D. INBREEDING

SEX DETERMINATION A sex-determination system is a biological system that determines the development of sexual characteristics in an organism.biologicalsexual characteristicsorganism Most sexual organisms have two sexes.sexes In many cases, sex determination is genetic: males and females have different alleles or even different genes that specify their sexual morphology.malesfemalesalleles genesmorphology In animals, this is often accompanied by chromosomal differences.animals chromosomal In other cases, sex is determined by environmental variables (such as temperature) or social variables (the size of an organism relative to other members of its population).environmentaltemperatureorganismpopulation

Sex determinATION the combination of sex chromosomes that determine the sex of an offspring; in humans the sex chromosomes of a normal female are XX and the sex chromosomes of a normal male are XY.

SEX DETERMINATION 1. XY TYPE DROSOPHILA HUMAN 2. XO type 3. ZW type 4. ZO type 5. Haplodiploid type

1. XY TYPE  DROSOPHYLA

SEX COMB

Drosophila melanogaster have 8 chromosomes (4 pairs) : 3 pairs autosome  A 1 pairs sex chromosome X Y

Dm Chromosome formula 3AAXX 3AAXY

nondisjunction

3AAXXX  female 3AAXXY  female 3AAXO  male, infertil 3AAYO  letal

Role of X,Y Dm chromosome X … Y …

ABNORMAL Dm Ginandromorf : - Intersex : 3AAAXX Super male : 3AAAXY …X chromosome: 3AAXXY

Dm SEX INDEX (X/A) Chromosome formula Sex indexsex AAXXX3/2 = 1.5Super female AAXX…female AAAXX…intersex AAXY…Male AAAXY…Super male

HUMAN 22AAXX 22AAXY

22AX  GINOSPERMIUM 22AY  ANDROSPERMIUM SEX CHROMATIN..?

Chromosome abnormality Turner Syndrome 22AAXO Klinefelter Syndrome 22AAXXY Super woman 22AAXXX XYY man

2. XO TYPE In this variant of the XY system, females have two copies of the sex chromosome (XX) but males have only one (X0). The 0 denotes the absence of a second sex chromosome. This system is observed in a number of insects, including the grasshoppers and crickets of order Orthoptera and in cockroaches (order Blattodea). insectsOrthopteraBlattodea The nematode C. elegans is male with one sex chromosome (X0); with a pair of chromosomes (XX) it is a hermaphroditenematodeC. eleganshermaphrodite

3. ZW Type The ZW sex-determination system is found in birds and some insects and other organisms.birdsinsects organisms The ZW sex-determination system is reversed compared to the XY system: females have two different kinds of chromosomes (ZW), and males have two of the same kind of chromosomes (ZZ).chromosomes In the chicken, this was found to be dependent on the expression of DMRT1DMRT1

4. ZO Type CHICKEN ZZ = Z0 =

5. Haplodiploid Type Haplodiploidy is found in insects belonging to Hymenoptera, such as ants and bees. HaplodiploidyinsectsHymenopteraantsbees Unfertilized eggs develop into haploid individuals, which are the males.haploid Diploid individuals are generally female but may be sterile males. Diploid Thus, if a queen bee mates with one drone, her daughters share ¾ of their genes with each other, not ½ as in the XY and ZW systems.bee This is believed to be significant for the development of eusociality, as it increases the significance of kin selection. eusocialitykin selection This[clarification needed] is common also in wasps that are parasitic and in the male greenflies.clarification needed