2.3 - 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 are the costs of sexual reproduction

Answer 2

males are unable to produce offspring

only half of each parent’s genome passed onto offspring, disrupting successful parental genomes

Question 3

why do the benefits of sexual reproduction outweigh the costs

Answer 3

due to an increase in genetic variation

Question 4

what does genetic variation provide

Answer 4

the raw material required for adaptation, giving sexually reproducing organisms a better chance of survival under changing selection pressures

Question 5

how does the red queen hypothesis explain the persistence of sexual reproduction

Answer 5

the red queen hypothesis postulates that parasite pressure maintains sexual reproduction in the host population by selecting for the ability to reproduce rare genotypes that are resistant to infection

Question 6

hosts have greater fitness if

Answer 6

it is able to resist and tolerate parasitism

Question 7

parasites have greater fitness if

Answer 7

it is able to feed, reproduce and find new hosts

Question 8

if hosts reproduce sexually…

Answer 8

the genetic variability in their offspring reduces the chances that all will be susceptible to infection by parasites

Question 9

what is asexual reproduction

Answer 9

a type of reproduction by which offspring arise from a single organism, and inherit the genes of that parent only; it does not involve the fusion of gametes

Question 10

why can asexual reproduction be a successful reproductive strategy

Answer 10

whole genomes are passed on from parent to offspring

Question 11

when is maintaining the genome of the parent an advantage in asexual reproduction

Answer 11

in very narrow, stable niches or when re-colonising disturbed habitats

Question 12

examples of asexual reproduction in eukaryotes

Answer 12

vegetative cloning in plants

parthenogenesis in lower plants and animals that lack fertilisation

Question 13

what is parthenogenesis

Answer 13

asexual reproduction whereby embryos develop from an unfertilised egg

Question 14

where is parthenogenesis more common

Answer 14

in cooler climates which are disadvantageous to parasites, or regions of low parasite density or diversity

Question 15

asexually reproducing populations are not able to adapt easily to changes in their environment, what provides some degree of variation

Answer 15

mutations that can occur which enable some natural selection and evolution to occur

Question 16

what is vegetative progation/cloning

Answer 16

a form of asexual reproduction that takes place on some plants, resulting in offspring that are genetically identical to the parent plant e.g. bulbs and runners

Question 17

organisms that reproduce principally by asexual reproduction also often have mechanisms for…

Answer 17

horizontal gene transfer between individuals to increase variation, for example the plasmids of bacteria and yeasts

Question 18

what does prokaryotes exchanging genetic material horizontally result in

Answer 18

faster evolutionary change than in organisms that only use vertical transfer

Question 19

what is meiosis

Answer 19

a type of cell division where four haploid gametes are produced from one diploid gamete mother cell

Question 20

how do chromosomes appear in diploid cells

Answer 20

homologous

Question 21

what are homologous chromosomes

Answer 21

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

Question 22

which stages is meiosis split into

Answer 22

meiosis I

meiosis II

Question 23

where are reproductive organs produced in animals

Answer 23

ovaries and testes

Question 24

where are reproductive organs produced in plants

Answer 24

anthers and ovaries

Question 25

what is the first step of meiosis I

Answer 25

interphase - each undergoes DNA replication to become two identical chromatids

Question 26

what is the second step of meiosis I

Answer 26

homologous chromosomes pair up and line up along the equator of the cell

Question 27

what is the third step of meiosis I

Answer 27

crossing over occurs at points called chiasmata

Question 28

what does chiasmata mean

Answer 28

points of contact between non-sister chromatids of a homologous pair

Question 29

what does crossing over lead to

Answer 29

it shuffles sections of the DNA between the homologous pairs, allowing the recombination of alleles to occur thus increasing variation

Question 30

what are linked genes

Answer 30

genes that are on the same chromosome

Question 31

what is the fourth step of meiosis I

Answer 31

independent assortment - spindle fibres attach to the homologous pairs and line them up at the equator of the spindle
the orientation of the pairs of homologous chromosomes at the equator is random
each pair of homologous chromosomes is positioned independently of the other pairs, irrespective of their maternal and paternal origin

Question 32

what is the fifth step of meiosis I

Answer 32

the chromosomes of each homologous pair are separated and move towards opposite poles
cytokinesis occurs, the division of the parent cytoplasm, dividing the cell into two haploid daughter cells

Question 33

what happens in meiosis II

Answer 33

each of the two cells produced in meiosis I undergoes a further division during which the sister chromatids of each chromosome are separated
a total of four haploid cells are produced

Question 34

how is the sex of birds, mammals and some insects determined

Answer 34

the presence of sex chromosomes

Question 35

what is a sex chromosome

Answer 35

a type of chromosome that participated in sex determination

e.g. male - (XY) female - (XX)

Question 36

what determines the development of male characteristics

Answer 36

the SRY gene on the Y chromosome

Question 37

what does SRY stand for

Answer 37

sex determining region of the Y chromosome

Question 38

what do heterogametic (XY) males lack

Answer 38

most of the corresponding homologous alleles on the shorter Y chromosome

Question 39

what does heterogametic mean

Answer 39

dissimilar sex chromosomes e.g. mammalian males where the Y chromosome is much smaller than the X chromosome

Question 40

what does lacking most of the corresponding homologous alleles on the shorter Y chromosome lead to

Answer 40

sex-linked patterns of inheritance as seen with carrier females (XBXb) and affected males (XbY)

Question 41

what is sex linkage

Answer 41

when males have a greater chance of being affected by certain recessive conditions because females would need two copies of the recessive allele to be affected whereas males only need one

Question 42

what does homogametic mean

Answer 42

sex chromosomes which do not differ e.g. mammalian females have 2 X chromosomes (XX)

Question 43

what happens to one of the two X chromosomes present in each cell in females

Answer 43

it is randomly inactivated at an early stage of development

Question 44

what is X chromosome inactivation

Answer 44

the process by which most of one X chromosome is inactivated

Question 45

why is X chromosome inactivation important

Answer 45

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

Question 46

why are X carriers less likely to be affected by any deleterious mutations on these X chromosome

Answer 46

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

Question 47

what are hermaphrodites

Answer 47

an organism with both functioning male and female reproductive organs

Question 48

what is the advantage of hermaphrodites

Answer 48

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 49

other than genetic factors, what determines sex and sex ratio

Answer 49

environmental factors

Question 50

environmental sec determination in reptiles is controlled by…

Answer 50

environmental temperature of egg incubation

Question 51

how can the sex of an organism change

Answer 51

as a result of size, competition, or parasitic infection

Question 52

how can the sex ration of offspring be adjusted

Answer 52

in response to resource availability