Sex Determination

  • Created by: rosieevie
  • Created on: 24-05-17 11:54

Parthenogenetic Reproduction

Parthenogenesis - development of an egg without fertilisation

Example - aphids (females reproduce by mitosis)

Example - plant vegetative propagation (spider plants)

Example - some lizards

  • Over 80 species of unisex reptiples
  • Female produce ovum with full set of genes provided solely by mother
  • Meiosis can occur for genetic variation
1 of 16

Ways of Reproducing


  • Gametes produced by single individual
  • May be self-fertile
  • Plants, annelids, molluscs, invertebrates
  • Simultaneously or sequential
  • Used to regulate genes but also effect by environment


  • Individuals only have male or female reproductive organs
  • Mammals, birds etc
2 of 16

Environmental Determination

Example - larval or juvenile marine spoon worms

  • Larvae settle into different sexes depending on landing from planktonic larvae
  • Larvae settle on bed = FEMALE
  • Larvae settle on female worm proboscis = MALE
  • Male detaches from proboscis and enters uterus of female - lives in androecium
  • During life males do not develop mouth or gut = parasitic

Example - slipper limpet

  • Larvae settle onto sea bed =  FEMALE
  • Females attract larvae by chemicals
  • Larvae settle on top of female = MALE
  • Male fertilises female
  • When bottom female slipper limpet dies, next limpet up becomes female
  • Slipper limpets capable of changing sexes = efficient ratio of males to females
3 of 16

Environmental Temperature-Dependent Sex Determinat

Example - egg incubation in crocodiles, turtles and lizards

  • Temperature-sensitive aromatase converrs androgen to oestrogens
  • Different species have different determination profiles
  • Implications for global warming
4 of 16

Haplo-diploidy Chromosomal Determination

Example - bees and ants

  • Queen and workers diploid females
  • Males are haploid 
  • Have no sex chromsomes
  • Males produce spermatozoa via mitosis
  • Females produce unfertillised and fertilised eggs via meiosis, depending on demand
5 of 16

Sex Chromosome Systems

**-XO Protenor System

  • Grasshoppers, protenor bugs
  • Male = XO - males have only 1 sex chromosome
  • Females = **
  • No Y chromosome


  • Mammals
    • Y promotes male development in embryo
    • Gene SRY on Y chromosome switches from females to males
  • Drosophila
    • Ratio of X chromosomes to sets of autosomes determines sex
    • <0.5 = MALE
    • >1 = FEMALE
    • Y no role for development - sperm motility
    • X causes female development
6 of 16

Sex Chromosome Systems 2


  • Birds, lepidoptera, snakes
  • ZW - heterogametic FEMALES
  • ZZ - homogametic MALES
7 of 16

Sex Determination in **-XY Systems

Use of sexual aneuploids (XO) can determine the role of sex chromsomes in sex determination

Drosophila - XO

  • Morphologically male
  • Sterile
  • = Y no role for male development but sperm motility, X for female development

Human - XO

  • Morphologically female (TURNER SYNDROME) 
  • Sterile
  • = Y promotes male development - gene SRY on Y switch from female to male

Mouse - XO

  • Functionally female

Grasshopper - XO - normal male

8 of 16

Human/Mice Maleness - SRY Gene

Presence of portion of Y chromosome essential for testis development

Y chromosome contains testis determinign factor (TDF)

Translocations and deletions of human chromosomes indicates TDF resides in short arm of Y

Deletion mapping w/ male-specific probes shows TDF is in region 1 of small arm of Y

Studies of XY females, ** males and transgenic mico localized TDF to singke region of region 1 containing SRY gene

Mice with SRY mutations develop as females despite having XY chormosmes

1 in 10,000 males have ** - ilegitimate cross over and carrying SRY gene

** males entirely normal except for sterile and small heights

9 of 16

Sex Determination in Plants

Monoecious plants - hermaphrodite flowers (male and female flowers) on same plant e.g. lily, maize

  • Certain cells are male or female - control which genes are turned on/off depending on environment

Dioecious - male and female flowers on seperate plants e.g. date palm, spinach

  • Y chromosome represses female development and induces male development
  • X chromosome induces female development
10 of 16

Sex Linkage

Sex-linked genes:

  • Locus on sex chromosome
  • Usually X
  • Nothing to do with gender
  • Gender influences expression levels
  • No heterozygous form in males
11 of 16

X-Liked Recessive Traits

Example - red-green colout blindness, haemophilia A, muscular distrophy

Males Xb Y exhibit trait (hemizygous)

Female Xb ** behave like autosomal paired alleles

Sons of affected females show phenotype

Daughters of affected males have allele

Sons cannot inherit trait from fathers (sons inherit Y chromosome from father)

No heterozygous form in males

12 of 16

X-Linked Dominant Traits

Affected males (XB Y) produce all affected daughters but no affected sons

50% chance that heterozygous affected female (XBxb) will pass on trait to either son or daughter

Homozygous females (XBXB or XbXb) pass on trait to all offspring

Expressed in heterozygous females

Males often more severely affected 

Typically associated with miscarriage or lethality in males

13 of 16

Sex Linkage on Y Chromosomes



14 of 16

Sex-Limited Inheritance

Sex-limited inheritance - affecting individuals of one sex only

  • Autosomal gene
  • Allele may not be able to be expressed if individual is wrong sex
  • Due to levels of hormones 
  • Beard growth, breast size, horn growth, milk yield

Genes have no visible effect on one sex as machinery necessary is not present

15 of 16

Sex-Influenced Inheritance

Sex-influenced inheritance - denoting to autosomal trait that is expressed differently, in either in frequency or degree, in males and females

  • Dominance relationships 
  • Autosomal alleles
  • Pattern baldness - dominant in males but recessive in women
16 of 16


No comments have yet been made

Similar Biology resources:

See all Biology resources »See all Genetics resources »