Term 2 Lecture 16: Sex Determination And Sex Linked Characteristics Flashcards
Often referred to as male/female
Whilst this is usually the case it is not always that simple. We are not talking about gender here but biological sexual phenotypes
See Nature: sex redefined, the idea of two sexes is oversimplified
There are variations in the X and Y chromosome
Across populations and these are known as DSDs differences in sexual development
Approximately 0.2-0.5% of the human population have DSDs
Sexual reproduction - why bother?
Twofold cost of males
(Williams C.G. 1988)
1) populations reproducing sexually grow at half the rate of asexual ones
2) to procreate asexually (essentially females breeding females without mating) then supporting ‘non-reproductive’ members of the population (males) becomes a matter of survival for the community
There are advantages to asexual reproduction but 99.99% eukaryotes don’t do it - why?
The evolution of sex - why do we have sexual phenotypes?
1) Red Queen hypothesis - a species needs to evolve as fast as it can to keep ahead of co-evolving species - to keep evolutionary advantage
2) Sex evolves when organisms are less adapted to their environment - poorly adapted phenotypes can be improved by recombination
3) Sex evolves when population numbers are finite - improves response to selection
4) sex evolves when selection varies over space e.g. migration produces a genetic disadvantage
Difference between sex linked/limited characteristics
Sex linked and sex limited characteristics are encoded by autosomal genes whose expression is affected by the sex of the individual organism possessing the genes.
Sex-linked characteristics are encoded by genes on the sex chromosomes
Sex determination
Sexual reproduction alternates between haploid and diploid states.
Most organisms have two sexual phenotypes known as male and female
Male and female gametes differ in size, in humans the sperm only delivers a packet of DNA the rest is provided by the large egg cell.
sex chromosomes can be heteromorphic and the two in the pair can look very different from each other e.g. X&Y
Sex determination mechanisms
Hermaphrodites have both sexes in the same organism, they are ‘monoecious’ having both male and female reproductive structures in the same organism
e.g. monoecious flowers can self pollinate they have both female attributes: stigma, style, ovary, ovules
and male attributes: filaments, anthers, pollen
Dioecious organisms are either male or female and have only the attributes of one gender
Sex determination systems
Sex chromosomes, genic, environmental
Sex determination: sex chromosomes
In humans the X & Y chromosomes specifically define male and female phenotypes
The psuedo autosomal regions at the ends of the telomeres of the X & Y chromosomes are homologous allowing XY pairing in meiosis in males.
Not all animals have the same system e.g. in grasshoppers females are XX and males are referred to as XO as they only have one chromosome (O signifies a lack of a chromosome)
Bees wasps and ants also have a haploidy system - diploid female and haploid male.
BIrds, snakes, butterflies and some amphibians and fish have a ZZ homogenetic male form and a ZW heterogametic female
i.e. the opposite to mammals.
Sex determination: Genic system
In some situations there are no sex chromosomes only sex determining genes. Genotype at one or more loci determine the sex of the individual.
This is found in some plants, fungi, protozoans and fish.
Sex determination: Environmental: slipper limpets
e.g. in Crepidula fornicata (common slipper limpet) sex is determined by an environmental factor - the limpets position in a stack of limpets. Sequential hermaphroditism - the last limpet on top is always male.
Limpet sex determination:
1) A larva that settles on an unoccupied substrate develops into a female which produces chemicals that attract other larvae
2) The larvae attracted to the female settle on top of her and develop into males, which become mates for the original female
3) Eventually the mates on top switch sex becoming female
4) this attracts additional larvae which settle on top of the stack and become females
Sex determination: environmental: temperature
group/ male producing temp/ female producing temp
( degrees Celsius) :
crocodilians/ >34 / <30
Turtles / 23-27 / 30-33
Lizards/ 29-33/ 24-29
Teleosts/ 17-25/ 11-19
Teleosts are ray finned bony fish
climate change can have a drastic impact on these species upsetting gender ratio
Nevara, K.J. (2018) The truth about Nemo’s dad: Sex change behaviours in fishes ( In: choosing sexes, Fascinating life sciences, Springer)
In the case of Nemo the death of his mother would have triggered his father to turn into a female ( due to the lack of a dominant female presence)
Nemo, like all clownfish would have hatched out as a hermaphrodite. As there would be only two fish on the little coral island (the barracuda ate the other eggs) Nemo would have developed into a male as there was only one other (female) present and then mated with his father.
When his father died of old age Nemo would change into a female.
^So that is how sex is influenced in some fish. It depends on the presence of a dominant sexual phenotype, usually the female, dictating who takes over
Common sex determining systems in eukaryotes
system/mechanism/ Heterogenetic sex/ organisms
XX-XO/ ♀XX ♂X / ♂ / some grasshoppers & other insects
XX-XY/ ♀XX ♂XY / ♂/ many insects, fish, amphibians, reptiles and mammals including humans
ZZ-ZW/ ♀ZW ♂ZZ / ♀/ butterflies, birds, some reptiles and amphibians
genic sex differentiation/
no distinct sex chromosomes, sex is determined by genes on undifferentiated chromosomes/
varies/ some plants, fungi, protozoans & fish
environmental sex determination/
Sex determined by environmental factors/
None/ some invertebrates, turtles and alligators
Differences between chromosomal/ genic and environmental sex determination
chromosomal sex determination
males and females have sex chromosomes that are distinguishable
Genic sex determination
sex is determined by genes, but the chromosomes of males and females are indistinguishable
Environmental sex determination
sex is determined fully or partly by environmental effects