2. Key Area 3- Variation and Sexual Reproduction

Question 1

What is sexual reproduction?

Answer 1

The production of new living organisms by combining genetic information from two individuals of different types (sexes)

Question 2

What is asexual reproduction?

Answer 2

A mode of reproduction by which offspring arise from a single organism and inherit the genes of only that parent

Question 3

What are the disadvantages of sexual reproduction?

Answer 3

• Only half of each parents genome is passed onto the offspring, disrupting successful parental genomes
• Males are unable to produce offspring

Question 4

What are the advantages of sexual reproduction?

Answer 4

• 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

Question 5

Explain the benefit of sexual reproduction: raw material for adaptation in terms of the RQH

Answer 5

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

Question 6

What is a benefit of hosts reproducing sexually in terms of a parasite and hosts relationship?

Answer 6

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

Question 7

What are the benefits of asexual reproduction?

Answer 7

• 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

Question 8

What are examples of asexual reproduction in eukaryotes?

Answer 8

• Vegetative cloning in plants
• Parthenogenesis (plants and animals that lack fertilisation)

Question 9

What are the benefits of vegetative cloning

Answer 9

• 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

Question 10

What is parthenogenesis?

Answer 10

The reproduction from a female gamete without fertilisation

Question 11

When does parthenogenesis take place?

Answer 11

It is more common in cooler climates which is disadvantageous for parasite or regions of low parasite density or diversity

Question 12

What species use parthenogenesis to reproduce?

Answer 12

Komodo dragons and aphids

Question 13

How do prokaryotes reproduce?

Answer 13

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

Question 14

Define meiosis

Answer 14

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

Question 15

What are the reproductive organs in plants and in animals?

Answer 15

Animals- ovaries and testes
Plants- Anthers and ovaries

Question 16

What are the two consecutive nuclear divisions in meiosis which results in variable gametes

Answer 16

Meiosis I and Meiosis II

Question 17

In diploid cells, what do chromosomes typically appear as?

Answer 17

Homologous pairs. Each homologous chromosome is inherited from a different parent, one maternal, one paternal

Question 18

Homologous chromosomes are pairs of chromosomes of the same…

Answer 18

same size, same centromere position and with the same sequence of genes at the same loci

Question 19

Describe the process of Meiosis I

Answer 19

• 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

Question 20

What are linked genes?

Answer 20

Are those on the same chromosome

Question 21

What does independent assortment result in?

Answer 21

Variation between gametes

Question 22

What is cytokinesis?

Answer 22

The physical process of cell division

Question 23

Describe the process of Meiosis II?

Answer 23

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

Question 24

What do sex chromosomes determine?

Answer 24

Whether an individual is male (XY) or female (XX)

Question 25

What gene determines the development of male characteristics in most mammals?

Answer 25

The sex determining region Y (SRY) gene on the Y chromosome determines the development of male characteristics

Question 26

How does the SRY function?

Answer 26

The SRY gene provides instructions for making a transcription factor called a testis-determining factor (TDS)

Question 27

What is a transcription factor?

Answer 27

It is a protein that binds to specific regions of DNA and helps control the activity of particular genes

Question 28

What are male XY chromosomes termed as and why?

Answer 28

XY chromosomes are termed heterogametic and lack the corresponding homologous alleles on their shorter Y chromosome

Question 29

What is the result of males having XY chromosomes?

Answer 29

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

Question 30

What are female XX chromosomes termed as and why?

Answer 30

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

Question 31

What is X chromosome inactivation?

Answer 31

Is a process by which most of one X chromosome is inactivated

Question 32

Why is X chromosome inactivation beneficial?

Answer 32

It prevents a double dose of gene products which could be harmful to cells

Question 33

How does X chromosome inactivation work and give an example?

Answer 33

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

Question 34

Why are carrier females more likely to be unaffected by a recessive allele?

Answer 34

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

Question 35

What is an advantage of being a hermaphrodite?

Answer 35

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

Question 36

What is a hermaphrodite?

Answer 36

An organism with functioning male and female reproductive organs. They produce male and female gametes

Question 37

What can sex determination be influenced by apart from genetic control?

Answer 37

Environmental factors e.g temperature in reptiles

Question 38

Explain sex determination in crocodiles and alligators

Answer 38

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

Question 39

What are factors which can cause the sex of an organism to change?

Answer 39

• Size
• Social interactions including competition
• Parasitic infection