2. Key Area 3- Variation and Sexual Reproduction Flashcards
What is sexual reproduction?
The production of new living organisms by combining genetic information from two individuals of different types (sexes)
What is asexual reproduction?
A mode of reproduction by which offspring arise from a single organism and inherit the genes of only that parent
What are the disadvantages of sexual reproduction?
- Only half of each parents genome is passed onto the offspring, disrupting successful parental genomes
- Males are unable to produce offspring
What are the advantages of sexual reproduction?
- Benefits outweigh the costs due an increase in genetic variation in the population. Genetic variation provides the raw material required for adaptation giving sexually producing organisms a better chance of survival under changing selection pressures
Explain the benefit of sexual reproduction: raw material for adaptation in terms of the RQH
Sexual production provides genetic variation in offspring and this genetic variation provides the raw material required to keep running the Red Queen’s arms race and explains the persistence of sexual reproduction
What is a benefit of hosts reproducing sexually in terms of a parasite and hosts relationship?
If hosts reproduce sexually, the genetic variability in their offspring reduces the chances that all of the offspring will be susceptible to infection by parasites
What are the benefits of asexual reproduction?
- Asexual reproduction can be a successful reproductive strategy as whole genomes are passed from parent to offspring
- Just one parent can produce daughter cells and establish a colony of virtually unlimited size over time
- Maintaining the genome of the parent in as advantage in very narrow stable niches or when recolonising disturbed habitats.
- It avoids the disadvantages of sexual reproduction but does not lead to variation
What are examples of asexual reproduction in eukaryotes?
- Vegetative cloning in plants
- Parthenogenesis (plants and animals that lack fertilisation)
What are the benefits of vegetative cloning
- asexual reproduction allows plants to quickly and successfully recolonise disturbed habitats
- The offspring are clones, they are genetically identical to the parent plant and thus already adapted to the habitat.
- Offspring can reproduce more often and in larger numbers with asexual reproduction
What is parthenogenesis?
The reproduction from a female gamete without fertilisation
When does parthenogenesis take place?
It is more common in cooler climates which is disadvantageous for parasite or regions of low parasite density or diversity
What species use parthenogenesis to reproduce?
Komodo dragons and aphids
How do prokaryotes reproduce?
Prokaryotes have mechanisms of horizontal gene transfer which means they can exchange genetic materials between individuals of the same generation e.g plasmids in bacteria and yeast. This results in faster evolutionary change than in organisms that only use vertical gene transfer
Define meiosis
Is the division of the nucleus that results in the formation of haploid gametes from a diploid gametocyte. One diploid gametocyte divides into 4 haploid sex cells
What are the reproductive organs in plants and in animals?
Animals- ovaries and testes
Plants- Anthers and ovaries
What are the two consecutive nuclear divisions in meiosis which results in variable gametes
Meiosis I and Meiosis II
In diploid cells, what do chromosomes typically appear as?
Homologous pairs. Each homologous chromosome is inherited from a different parent, one maternal, one paternal
Homologous chromosomes are pairs of chromosomes of the same…
same size, same centromere position and with the same sequence of genes at the same loci
Describe the process of Meiosis I
- Division I starts with interphase where each chromosome undergoes DNA replication to become two identical chromatids.
- Homologous chromosomes pair up and line up along the equator of the cell.
- When homologous chromosomes pair up, they touch each other at points known as chiasmata. Crossing over occurs at the chiasmata which can result in new combinations of alleles to occur.
- Homologous chromosomes pair up and line up along the equator. The final position of one pair is completed random and these chromosomes go on to be separated irrespective of their maternal and parental original. This is known as independent assortment. This results in variation between gametes
- The chromosomes of each homologous pair are separated by spindle fibres and move to opposite poles
-Cytokinesis occurs and two haploid daughter cells form
What are linked genes?
Are those on the same chromosome
What does independent assortment result in?
Variation between gametes
What is cytokinesis?
The physical process of cell division
Describe the process of Meiosis II?
The two haploid daughter cells produced in meiosis I undergo further division during which the sister chromatids of each chromosome are separated by spindle fibres. This produces 4 haploid cells
What do sex chromosomes determine?
Whether an individual is male (XY) or female (XX)
What gene determines the development of male characteristics in most mammals?
The sex determining region Y (SRY) gene on the Y chromosome determines the development of male characteristics
How does the SRY function?
The SRY gene provides instructions for making a transcription factor called a testis-determining factor (TDS)
What is a transcription factor?
It is a protein that binds to specific regions of DNA and helps control the activity of particular genes
What are male XY chromosomes termed as and why?
XY chromosomes are termed heterogametic and lack the corresponding homologous alleles on their shorter Y chromosome
What is the result of males having XY chromosomes?
It results in a sex-linked pattern of inheritance where males have a greater chance of being affected by certain recessive conditions e.g colour blindness and haemophilia in humans
What are female XX chromosomes termed as and why?
XX chromosomes are termed homogametic as there are 2 X chromosomes, one of the 2 X chromosomes present in each cell is randomly inactivated at an early stage of development
What is X chromosome inactivation?
Is a process by which most of one X chromosome is inactivated
Why is X chromosome inactivation beneficial?
It prevents a double dose of gene products which could be harmful to cells
How does X chromosome inactivation work and give an example?
In females, parts of the X chromosome is randomly inactivated in one homologous X chromosome in each cell of the body to prevent a double dose of gene product. This means carriers will not suffer from the effects of any harmful mutations on these X chromosomes because the X chromosome inactivation is random and usually 50% of the cells in any tissue will have a normal copy of the gene in question. An example is tortoise shell for colour in cats
Why are carrier females more likely to be unaffected by a recessive allele?
Because females would need two copies of the recessive allele to be affected while males only need one copy. Females can be carriers thus having one copy of the affected gene yet remain unaffected, however they can still pass this gene onto any offspring they have. This is due to X-chromosome inactivation
What is an advantage of being a hermaphrodite?
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
What is a hermaphrodite?
An organism with functioning male and female reproductive organs. They produce male and female gametes
What can sex determination be influenced by apart from genetic control?
Environmental factors e.g temperature in reptiles
Explain sex determination in crocodiles and alligators
In extremes temperatures, offspring will be female ( below 31.7 degrees celsius and above 34.5 degrees celsius), males are born within the narrow range of both temperatures. This is because the activity of the enzyme responsible for conversion of testosterone to oestrogen is influenced by temperature during the temperature sensitive period
What are factors which can cause the sex of an organism to change?
- Size
- Social interactions including competition
- Parasitic infection
