c) Sex Determination

Starter Questions:

• What is the purpose of meiosis?
• How does the products of meiosis compare to the products of mitosis?
• Name 2 processes that occur during meiosis to increase variation.
• What is cytokinesis?

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Answers:

• Meiosis produces gametes and gametocytes.
• Mitosis produces two diploid cells that are identical to the parent cell. Whereas meiosis produces four non-identical daughter cells.
• Random assortment and crossing over.
• The splitting of the cytoplasm into two daughter cells.

Today we will

Learning Intention

• Learn about how sex is determined in different organisms

Success Criteria

• Describe how chromosomes determine sex and the characteristics of the different sexes
• Describe what a hermaphrodite is and the benefits its has
• Describe how the environment and other factors can determine or change the sex of an individual

Sex determination

The sex of birds , mammals and some insects is determined by the presence of sex chromosomes.

Sex chromosome: A type of chromosome that participates in sex determination. E.g. Male XY, female XX.

SRY-gene

In most mammals the SRY gene on the Y chromosome determines development of male characteristics. Heterogametic (XY) males lack most of the corresponding homologous alleles on the shorter (Y) chromosome.

SRY-gene

This can result in sex-linked patterns of inheritance as seen with carrier females ( X^BX^b ) and affected males (X^bY ).

Sex linkage

Genes are located on the sex chromosomes are described as sex linked. The study of their inheritance involves examining both the sex of the offspring and the genetic trait of interest.

Haemophilia

Duchenne muscular dystrophy

Red–green colour blindness

Rare and debatable!

It is argued that there is little room on the Y chromosome for anything other than genes controlling testes formation and function.

Haemophilia in the Royal Family

The red eye is dominant to the white eye.��In this example a carrier female is crossed with a dominant male.

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The possibility of offspring is shown in the cross.

X-Chromosome inactivation

In homogametic females (XX) one of the two X chromosomes present in each cell is randomly inactivated at an early stage of development.

X chromosome inactivation is a process by which most of one X chromosome is inactivated X chromosome inactivation prevents a double dose of gene products, which could be harmful to cells.

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Video

X-Chromosome inactivation

X carriers are less likely to be affected by any deleterious mutations on these X chromosomes.

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As the X chromosome inactivated in each cell is random , half of the cells in any tissue will have a working copy of the gene in question.

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How can x–chromosome inactivation explain tortoiseshell cats?

The orange allele is dominant to the black allele. Some patches the X chromosome has the orange allele being active. The other patches the orange allele has been inactivated, meaning the patch is black.

Hermaphroditism

Hermaphrodites are species that have functioning male and female reproductive organs in each individual. They produce both male and female gametes and usually have a partner with which to exchange gametes.

Hermaphroditism

Hermaphroditic flower Unisexual flower

Hermaphroditism

The benefit to the individual organism is that if the chance of encountering a partner is an uncommon event, there is no requirement for that partner to be of the opposite sex.

Environmental sex determination

For other species, environmental rather than genetic factors determine sex and sex ratio . Environmental sex determination in reptiles is controlled by environmental temperature of egg incubation .

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Environmental sex determination

Sex can also change within individuals of some species as a result of size , competition , or parasitic infection .

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In some species the sex ratio of offspring can be adjusted in response to resource availability.

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.

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.

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• Some species having chromosomal sex determination but the bacteria kill the males or feminize them.

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.

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Transmission electron microscope image of Wolbachia bacteria inside an insect cell

Environmental sex determination�Competition affects sex ratio

• Some 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.

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Gabriella Skollar & Rebecca Lewis / Wikimedia

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.

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.

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Stan Shebs / Wikimedia