Sex determination - PowerPoint Presentation

Slide1

Sex determination

CfE Advanced Higher Biology

Unit 2: Organisms and EvolutionSlide2

2biii

– Sex determination

Gender (2)Chromosomal sex determination (2)Sex linkage (5)Dosage compensation (2)Environmental sex determination (3)Changing sex (2)

Contents and number of slides for each topicSlide3

SQA mandatory

key i

nformationSex determination in mammals and Drosophila, sex ratio and resource availability. Environmental factors can affect sex determination. Change of sex through size, competition or parasitic infection. Hermaphrodites. Sex linked patterns of inheritance in carrier females and affected males in terms of gene products. Random inactivation on X chromosomes in females prevents a double dose of gene products. Half of the cells in any tissue will have a working copy of the gene in question therefore carriers remain unaffected by any deleterious mutations. Slide4

Key concepts 1

Many species are hermaphroditic. For some species environmental rather than genetic factors determine sex. Environmental sex determination in reptiles controlled by environmental temperature of egg incubation.

Sex chromosomes, such as XY in live-bearing mammals and some insects including Drosophila. In many of the mammals a gene on the Y chromosome determines development of maleness. In some species the sex ratio of offspring can be adjusted in response to resource availability. Slide5

Key concepts 2

In live-bearing mammals, the heterogametic (XY) male lacks homologous alleles on the smaller (Y) chromosome. This can result in sex-linked patterns of inheritance as seen with carrier females (

XBXb) and affected males (XbY). In the females, the portions of the X chromosome that are lacking on the Y chromosome are randomly inactivated in one of the homologous X chromosomes in each cell. This effect prevents a double-dose of gene products. Carriers remain unaffected by any deleterious mutations on these X chromosomes as the X-chromosome inactivation is random, half of the cells in any tissue will have a working copy of the gene in question. Slide6

Gender

Males and females

Many animals have separate genders.In some animal species the males and females are very similar.Other animal species show sexual dimorphism.Some plants are unisexual and have male and female plants. For example, only female hollies can produce berries.

Male holly flower

Female

holly flower

Swans:

Ralfie

/

Wikimedia, Ducks:

Keven

Law /

Wikimedia, Holly flowers:

Tigerente

/ WikimediaSlide7

Gender

Hermaphrodites

In Greek

mythology,

Hermaphroditus

was the child of Hermes and Aphrodite and had both male and female characteristics.

Some animals and plants have male and female organs in one individual.

Reproductive structures in many flowers mature at different times to prevent self-pollination.

Earthworms and most land molluscs are hermaphroditic but must meet another individual to reproduce.Slide8

Chromosomal sex determination

X and Y in humans

Karyotypes developed in early 20th Century and it took until 1956 to agree on 46 chromosomes for humans.By 1959, agreed on 44 autosomes and two sex chromosomes in humans:Females have two large sex chromosomes, both called XMales have an X and a small Y chromosome.Live-bearing mammals have this system - all mammals except Platypus and Echidna.Drosophila melanogaster also has this system with 6 autosomes and two sex chromosomes, X and Y (this is unusual for insects).Slide9

Chromosomal sex determination

SRY gene

Default setting for human embryonic development is to become female (hence male nipples!).Y-chromosome has a gene called SRY (Sex determining Region of Y-chromosome).SRY controls the development of male genitalia and male characteristics.SRY is a ‘master switch’ which produces a transcription factor to switch on other genes.Many of these genes are on the autosomes but are only transcribed in males due to SRY control.XX male due to SRY translocated to X-chromosome.XY female due to SRY deleted from Y-chromosome.Slide10

Sex linkage

X and Y are homologous chromosomes

X and Y are an exception to the homology definition.Human X-chromosome is larger than Y and has about 2000 genes compared to about 450.X and Y are homologous because they pair up during meiosis I.Pairing is due to a small area of homology around the centromere.The Y-chromosome lacks many genes found on its homologous X-chromosome.This leads to a pattern of inheritance called sex linkage.In XX females, a recessive allele on one X can be masked by a dominant allele on the other X.In XY males, a recessive allele on the X has no second copy to mask its effects.Slide11

Sex linkage

Homogametic and heterogametic

Human females produce all gametes with the same combination of chromosomes = homogametic.Human males produce gametes with two possible combinations of chromosomes = heterogametic.Because they are homologous, the sex chromosomes are separated during meiosis into different gametes.

44 + XX

22 + X

22 + X

22 + X

44 + X

Y

22 +

YSlide12

Sex linkage

White-eyed males

In the early 1900s, T. H. Morgan studied inheritance in Drosophila melanogaster to try to disprove Mendel’s theory.It took two years to find any variation in his vast fly breeding programme.Eventually a male fly was found which had white eyes. (His wife found it.)This white-eyed male was crossed with a normal red-eyed female.We will use the notation XR to show the red eye allele is on the X-chromosome and is dominant to white eye, Xr.

X

r

Y

X

R

X

R

X

R

X

r

X

R

X

r

Y

X

R

YSlide13

Sex linkage

Carrier females

All the flies in the F

1

had red eyes.

F

lies

with genotype

X

R

X

r

are

carrier females

.

Morgan crossed the F

1

with each

other.

F

2

came out with

a 3:1

ratio of red-eyed

flies to white-eyed flies.

But all the white-eyed flies were males.

Which flies could be crossed to get white-eyed females?

X

R

Y

X

R

X

r

X

R

X

rXRXRXR Red-eyed femaleXRXr

Red-eyed carrier female

Y

XRY Red-eyed maleXrY White-eyed maleSlide14

Sex linkage

Human examples

Even though the mechanism of inheritance was not understood, examples of sex linkage had already been noticed in humans before Morgan found the white-eyed fly in 1910.Red-green colour blindness, 1798Haemophilia, 1803Duchenne muscular dystrophy, 1834Other examples have been noticed since then, such as Vitamin D resistant rickets (1937), which is unusual in being a dominant allele.Slide15

Dosage compensation

X-chromosome inactivation

Males are heterogametic (with only one X-chromosome) while females are homogametic (two X-chromosomes). Despite this, female cells do not have a double-dose of gene products from their X-chromosomes.In females, one X-chromosome is partially inactivated early in embryonic development. The regions inactivated are those that are lacking on Y-chromosome.

(

Deacetylation

of histones, methylation of bases.)

The inactivated X-chromosome shows up as a highly condensed region on the inside of the nuclear membrane – a

Barr body

.

Xi = Barr body (inactivated X)

Steffen

Dietzel

/ WikimediaSlide16

D

osage compensation

MosaicsCats show this mosaic in their fur as a coat colour gene is found on the X-chromosome.The two alleles are black and ginger and tortoiseshell cats are females, XBXG.Because of random X-inactivation, an XX female is a mosaic of cells with different X-chromosomes active and inactive.A carrier of a deleterious alleles (e.g. XDX

d

) remains unaffected because she still has a working copy of the gene in half of the cells in every tissue.

If you see a ginger cat, why is it more likely that it is male than female?

Suziecat

/ WikimediaSlide17

Environmental sex determination

Temperature affects sex ratio in reptiles

In turtles and crocodiles, the level of expression of some genes is affected the temperature during a sensitive period in development.For some reptiles, there is trend in the effect of temperature on the sex ratio. In Hermann’s Tortoises, below 31C is all males and above 32C is all females.In other reptiles, the extremes cause the production of one sex while the intermediate temperature produces the other sex.In Mississippi Alligators, males only develop at 32–34C while females are produced at temperatures below 32C and above 34C.Slide18

Environmental sex determination

Parasites can affect sex ratio

In some arthropods, particularly insects, sex ratio is affected by infection by bacteria of the genus Wolbachia.Some species having chromosomal sex determination but the bacteria kill the males or feminize them.Other insects rely on infection by Wolbachia to cause their sex determination!Research reported in 2007 showed that human mothers infected by the protist, Toxoplasma gondii, have a higher proportion of male offspring. This parasite may also affect human behaviour.

Transmission electron microscope image of

Wolbachia

bacteria inside an insect cellSlide19

Environmental sex determination

Competition

affects sex ratioSome species can actively change the sex ratio of their offspring.When a solitary female Lesser Mouse Lemur detects the urine of other females in her habitat she switches from producing one-third males to two-thirds males.Male offspring disperse away from the home range sooner than female offspring so this reduces competition.Female Seychelles

Warblers

in a high quality territory

produce 87

%

females

but in a poor territory they produce 77% males.

F

emale offspring

stay and help raise the next

brood with their mother while males disperse

away

and reduce competition.

Gabriella

Skollar

& Rebecca Lewis / Wikimedia Slide20

Changing sex

Group-living fish

Clown fish live in groups with a large dominant female and a number of smaller males.If the female is removed, the largest male becomes female.Bluehead wrasse have one male with a group of females.Removal of the male causes the largest female to grow twice its size and change its colouration.This sex change is influenced by aromatase, an enzyme that controls the androgen:oestrogen ratio.Slide21

Changing sex

Size and parasites

The sea anemone Metridium senile changes from male to female as it gets larger.Bigger females may be necessary due to the higher investment by females in producing gametes for reproduction.A nematode (Gasteromermis sp.) parasitises the aquatic larvae of a type of mayfly, Baetis bicaudatus.Females become sterile. Some parasitised males become ‘intersex’ while others become completely female.

All the

parasitised

animals (males and females) behave like females when adult and crawl into the water as if to lay eggs.

This helps the parasite return to infect the next generation.

Stan

Shebs

/ Wikimedia