Key Area 3
In this section, we will explore the determination of sex in offspring - you thought it was all about genetics? Time to get back to the old David Attenborough DVDs and listen closely!
Here is an excellent overall summary of what is to come in this key area! Sex determination through chromosomes (sometimes the male and sometimes the female is responsible for sex determination) and sometimes environmental factors. Sometimes males don't even have a father. Enjoy the "more complicated than you thought" world of sex determination.
A really important message to be appreciated and reflected upon in this TED talk. At times, SQA notes and Science lessons are not enough. The world is complicated - not black and white - and the concept of 2 biological sexes is simply inaccurate. Grab a cup of tea and a Twix and listen to this.
Task 36
Before moving on, try this quick hangman game to test your knowledge of some of the terms associated with sex determination (choosing my words carefully here!!)
Now that you have completed the KeyWord activity above, have a quick flick through the Google Slides below to ensure you feel confident with these terms.
Task 37
Complete the task box in your notes to record these keywords and their definitions. Answers are available here.
Sex Chromosomes
The sex of birds, mammals and some insects is determined by the presence of sex chromosomes.
In most mammals, the SRY gene on the Y chromosome determines the development of male characteristics.
Because they contain an area of homology around the centromere, the X and Y chromosomes are said to be homologous and will pair up during meiosis. The sex chromosomes are then separated into different gametes.
Homogametic
Human females produce all gametes with the same combination of chromosomes.
Heterogametic
Human males produce gametes with two possible combinations of chromosomes.
Task 38
Human females produce all gametes with the same combination of sex chromosomes (X) so are homogametic. However, males are said to be heterogametic because their sex chromosomes are dissimilar (XY). The males lack homologous alleles on the smaller (Y) chromosome. This can result in sex-linked patterns of inheritance. Why does this mean that males have a greater chance of being affected by certain sex-linked conditions, e.g. colour blindness and haemophilia in humans?
Answers are available here.
Task 39
An example of a sex-linked gene in humans is haemophilia. This is a condition where the affected individual does not produce the blood clotting agent Factor VIII. Haemophilia is the result of a recessive gene carried on the X chromosome. Using the letter "N" for normal and "n" for affected, predict the possible genotypes of offspring produced by a normal male parent and a carrier female parent.
Answers are available here.
As a result of random inactivation, every female individual consists of a mosaic of two different cell populations, in which either the maternally or paternally-derived X chromosome is inactivated.
As the X-chromosome harbours more than a 1000 genes, of which many are implicated in human disease when mutated, this mosaicism has important disease implications.
A fantastic video talking through X-chromosome inactivation.
As we learned back in Topic 1, colour vision is the wonderful gift of our retinal cones cells.
Colour-blindness is an X-linked condition and therefore affects more males than females.
Try the quick test (right) but scrolling through the carousel - how did you get on? What did you see hidden in the coloured circles.
Task 40
In females, the parts of the X chromosome that are absent from the Y chromosome are randomly inactivated in one of the homologous chromosomes in each cell of the body. Why is this necessary?
Answers are available here.
Changing Sex
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.
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.
It's getting hot in here!
With many organisms, including nile crocodile (above) and turtles, environmental temperature during egg incubation affects sex determination of offspring. For crocodiles, if egg incubation temperatures rise above 34.5°C or below 31.7°C, all offspring will be female; offspring will be male is the egg incubation temperature sits within these limits.
With global temperatures rising, can you predict a sinister future ahead for these organisms?
Social cues and other environmental factors
Sex can change within individuals of some species as a result of size, competition, parasitic infection and temperature. In some species, the sex ratio of offspring can be adjusted in response to resource availability.
Male shrimps have been found to change into females in response to parasitic infection.
Clowning around
In anemone fish, all juveniles are males.
On forming monogamous pairs, one of the fish will become female. Should the female die, the male in the pair becomes female and a new juvenile male moves into the pair.
Blue-banded Goby
In the blue-banded goby, when a dominant male dies, a large female will take his place, changing gender to do so. The female starts to display aggressive behaviour and undergoes changes in hormone levels.
Task 41
Give an example of how the changes in population density, parasitic infection or size can affect sex ratio in a species.
Answers are available here.
Now go to SCHOLAR to practice some questions and to consolidation your learning on "3.3 Sex Determination".
Now is also the time to consolidation your learning for the whole of Key Area 3 so try:
3.4 Learning points
3.5 Extended response question
3.6 End of topic test.
Time to create your own content for our online class blog and become a published author! Write a piece that showcases your learning of Key Area 3 on Variation and Sexual Reproduction.
Your teacher might now issue you with Learner Check 15 to check your learning of Topic 2, Key Area 3.
Well done on what you have achieved so far! You are nearly there! Time to move onto Key Area 4, a short (and fun!) section on Sex & Behaviour.
