Term 2 Lecture 16: Sex Determination And Sex Linked Characteristics

Question 1

Often referred to as male/female

Answer 1

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

Question 2

There are variations in the X and Y chromosome

Answer 2

Across populations and these are known as DSDs differences in sexual development
Approximately 0.2-0.5% of the human population have DSDs

Question 3

Sexual reproduction - why bother?

Answer 3

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?

Question 4

The evolution of sex - why do we have sexual phenotypes?

Answer 4

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

Question 5

Difference between sex linked/limited characteristics

Answer 5

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

Question 6

Sex determination

Answer 6

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

Question 7

Sex determination mechanisms

Answer 7

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

Question 8

Sex determination systems

Answer 8

Sex chromosomes, genic, environmental

Question 9

Sex determination: sex chromosomes

Answer 9

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.

Question 10

Sex determination: Genic system

Answer 10

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.

Question 11

Sex determination: Environmental: slipper limpets

Answer 11

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

Question 12

Sex determination: environmental: temperature

Answer 12

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

Question 13

Nevara, K.J. (2018) The truth about Nemo’s dad: Sex change behaviours in fishes ( In: choosing sexes, Fascinating life sciences, Springer)

Answer 13

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

Question 14

Common sex determining systems in eukaryotes

Answer 14

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

Question 15

Differences between chromosomal/ genic and environmental sex determination

Answer 15

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

Question 16

Sex determination in Drosophila melanogaster

Answer 16

-Fruit fly have 8 chromosomes - 3 pairs of autosomes and one pair of sex chromosomes
- Genic balance system
- Initially proposed by Calvin Bridges X: a ratio (X, a number of X chromosomes. A number of haploid sets of autosomes)
- Present research suggests that its about timing of developmental events controlled by autosomal chromosomes
- Length of time sex-determining genes on X chromosome have to be active is critical
- Presence of 3 autosomal chromosome sets shortens a critical development stage- intersex phenotypes can result if not enough time is given for X to express sex - intersex means neither male nor female
In drosophila it can occur that half the body of the fly is female and the other half is male.

Question 17

Sex chromosomes in humans

Answer 17

-The X chromosome contains genetic information essential for both sexes, at least one copy of an X chromosome is required in all individuals
- The male determined gene is located on the Y chromosome. A single Y even in the presence of several X chromosomes will still produce a male phenotype.
- Absence of a Y chromosome results in female phenotype
- Human Y chromosome has the SRY gene, responsible for many of the characteristically male phenotypes - without this gene we would not have diversity of phenotypes

Question 18

Syndromes occur due to abnormal numbers of sex chromosomes

Answer 18

Turner syndrome occurs in XO females - women with only one X chromosome ( occurs 1/3000 births) women with this syndrome tend to be smaller in stature and sterile

Klinefelter syndrome occurs in males with multiple X chromosomes XXY, XXXY, XXXXY or additionally with another Y XXYY. Occurs 1/1000 male births, men with multiple X chromosomes tend to be sterile and unusually tall.

Poly X females also occur 1/1000 female births.

Question 19

Sex linked characteristics determined by genes on the sex chromosomes

Answer 19

X linked characteristics
- white eyes an X linked recessive trait in drosophila
- red/green colour blindness in humans X linked recessive trait in humans

In male peacocks (ZZ) cameoplumage results from Z^ca allele recessive to the Z^ca+ allele the wildtype that determines metallic blue plumage. Z^caZ^ca males still have a fan tail but have brown plumage like the females.

Y linked characteristics

The X and Y chromosome are thought to have developed from an autosomal pair of chromosomes. Mutations of genes on one chromosome caused maleness and over time suppression of crossing over kept male traits linked to the Y chromosome and a lack of crossing lead to the deterioration of the Y chromosome to become smaller to carry just the male gene traits.

X linked characteristics

In females even though two X chromosomes are present one is normally inactivated (silenced) randomly
So females are hemizygous at a cellular level.

Lyon Hypothesis: inactivated X chromosome is visible as a Barr body ( a dark spot on the edge of the nucleus) in the nuclei of female cells

Number of Barr bodies increases with the number of inactive X chromosomes e.g. in a male with Klinefelter XXXY two Barr bodies will be visible in the nucleus - despite being male - because two X chromosomes in the nuclear DNA are inactive.