F215 1.2 - Meiosis and Variation

Question 1

Allele

Answer 1

A particular form of a given gene i.e. one of a related set of sequences of nucleotides found at a particular locus on a particular chromosome.

Question 2

Anaphase I

Answer 2

A stage in meiosis I in which the two chromosomes (each consisting of two chromatids) in a homologous pair/bivalent are pulled towards opposite poles of the spindle apparatus.

Question 3

Anaphase II

Answer 3

A stage in meiosis in which the two sister chromatids (now technically chromosomes) are pulled towards opposite poles of the spindle apparatus after the centromere has split.

Question 4

Artificial selection

Answer 4

A type of selection process applied by one species (humans) to another species whereby individuals possessing “desirable characteristics” are specifically encouraged to produce offspring whilst those possessing less “desirable characteristics” are prevented from reproducing thus leading to an increase in the frequency of the “desirable” features in progressive generations.

Question 5

Autosome

Answer 5

Any chromosome other than a sex chromosome.

Question 6

Bivalent

Answer 6

A pair of homologous chromosomes seen during the early stages of meiosis I which align so that equivalent loci lie side by side and can undergo crossing-over.

Question 7

Centriole

Answer 7

One of a pair of microtubular structures found in animal cells in the cytosol just outside the nucleus responsible for organising the spindle apparatus during cell division.

Question 8

Chiasma (Chiasmata)

Answer 8

A point at which two aligned homologous chromatids are enzymatically cut and rejoined so that equivalent sections of the chromatids exchange places: essential for crossing-over.

Question 9

Chromatid

Answer 9

One of a pair of DNA sequences (with associated proteins) which forms half of a chromosome after semi-conservative DNA replication: each of the two chromatids so formed is held to its sister chromatid by the centromere until the appropriate stage of cell division.

Question 10

Chromosome

Answer 10

A specific length of DNA along which are found many alleles separated by other sequences. Each species has a specific set of such structures which contain the genetic “blueprint “ for that species

Question 11

Cladistics

Answer 11

A system of grouping organisms using lines of descent rather than simple structural similarities.

Question 12

Codominance

Answer 12

This occurs where the effects of both of a pair of homologous alleles are visible in the phenotype e.g. the black and orange coat colour in tortoiseshell cats.

Question 13

Continuous variation

Answer 13

A form of variation in a characteristic between individuals of a species in which there are no clear distinctions between phenotypes e.g. human height

Question 14

Crossing-over

Answer 14

The event that takes place at chiasmata leading to intra-chromosomal recombination whereby equivalent sections of DNA on sister chromatids exchange places.

Question 15

Diploid

Answer 15

An organism or cell with two full set of chromosomes. Denoted by 2n.

Question 16

Directed selection

Answer 16

A type of (usually) natural selection in which individuals having a non-average value of a particular characteristic are more likely to survive and reproduce so that over time the average for that characteristic in the population itself is shifted in the same direction e.g. if paler fur provided better concealment from predators due to a change in the background colouration then the species as a whole would tend to become paler.

Question 17

Discontinuous variation

Answer 17

A form of variation in a characteristic between individuals of a species in which there are clear distinctions between phenotypes e.g. the ABO blood group system in humans.

Question 18

Dispersive/Disruptive selection

Answer 18

A type of (usually) natural selection in which individuals (in a species) having different and distinct values of a particular characteristic are each more likely to survive and reproduce than those having more average values of the characteristic so that over time a divergence develops ultimately leading to the coexistence of distinct species. e.g. Darwin’s finches. This is linked to adaptive radiation.

Question 19

Dominant allele

Answer 19

That form of a gene that always appears in the phenotype if present in the genotype (unless epistasis prevents this) i.e. will appear in both heterozygotes and dominant homozygotes.

Question 20

Epistasis

Answer 20

A form of interaction between different genetic loci whereby the effects of one locus modify the expression of the other. Very often this involves suppression of the expression of the dominant form at the second locus if the first possesses or lacks a particular allele. e.g. if a metabolic pathway has alleles whose gene products act in succession and the first is homozygous recessive, the form of the alleles at the second locus become irrelevant since their product has nothing to work on.

Question 21

Extinct

Answer 21

The term used to describe a species that once existed but now has no living representatives.

Question 22

Gamete

Answer 22

A sex cell. In humans these are eggs and sperm, haploid cells produced by meiosis.

Question 23

Gene

Answer 23

A section of DNA which encodes a particular functional product (or related family of products) usually a polypeptide but sometimes tRNA or rRNA.

Question 24

Gene pool

Answer 24

The total genetic information possessed by the reproductive members of a population

Question 25

Genetic drift

Answer 25

The gradual change over time of the frequencies of different alleles in a population ascribed simply to chance rather than the operation of specific selection mechanisms.

Question 26

Genetic variation

Answer 26

The variation in genetic information in a gene pool.

Question 27

Genome

Answer 27

All the genetic information in a cell or organism.

Question 28

Genotype

Answer 28

The description/listing of the alleles present at one or more loci for an individual. For a diploid organism each gene will be represented by two alleles except possibly for those on sex chromosomes.

Question 29

Haploid

Answer 29

An organism or cell with one full set of chromosomes. Denoted by n.

Question 30

Hardy-Weinberg equilibrium/equation

Answer 30

The situation in which, if certain criteria are met, the frequencies of the different alleles at a gene locus in a population do not change in successive generations so that the proportions of the different phenotypes can be predicted from a knowledge of the allele frequencies.

Question 31

Heterozygous

Answer 31

A eukaryotic cell or organism which has two different alleles for a particular gene.

Question 32

Homologous pair

Answer 32

The name given to the two chromosomes in a diploid cell (one from each parental gamete) that carry equivalent alleles at the same loci (except for the sex chromosomes if present)and which pair up in meiosis to form a bivalent.

Question 33

Homozygous

Answer 33

A eukaryotic cell or organism with two identical alleles for a particular gene.

Question 34

Interphase

Answer 34

That part of the cell cycle between the visible events (light microscope) of meiosis or mitosis consisting of G1,S,and G2 stages during which the cell grows and replicates its DNA, organelles etc.

Question 35

Isolating mechanism

Answer 35

Any process whereby a barrier is introduced to the free mixing of gametes between generations allowing differential selection pressures to operate on sub-populations thus making it easier for differences to develop in gene pools leading to eventual speciation.

Question 36

Linkage

Answer 36

This refers to the fact that in a particular species there are far more genes than chromosomes so that certain genes must always lie on the same chromosome. The alleles of these genes will therefore be inherited in their original combinations unless separated by crossing-over and so will appear to be linked i.e. will not show independent assortment.

Question 37

Locus/Gene locus

Answer 37

The “address” of a particular gene, alleles of which will always be found in a particular species at the same physical location. i.e. on a particular chromosome, on a particular arm of that chromosome, and at a particular position on that arm.

Question 38

Meiosis

Answer 38

A type of cell division with two stages seen in eukaryotes whereby diploid parent cells each produce four haploid daughter cells with half the chromosome number. This permits subsequent fusion of two such haploid cells later in the life cycle to restore the diploid status.

Question 39

Metaphase I

Answer 39

A stage in meiosis I in which the bivalents (homologous pairs of chromosomes) become linked to the spindle apparatus and are then positioned in the equatorial plane midway between the poles prior to chromosome separation during anaphase I.

Question 40

Metaphase II

Answer 40

A stage in meiosis II in which individual chromosomes become linked to the spindle apparatus and are then positioned in the equatorial plane midway between the poles prior to centromere splitting and chromatid separation during anaphase II.

Question 41

Microtubule

Answer 41

A component of the cytoskeleton made of many tubulin subunits with multiple cellular roles, one of which is to construct the spindle apparatus responsible for organising chromosome separation during both mitosis and meiosis in eukaryotic cells

Question 42

Multiple alleles

Answer 42

These occur when there are more than two alleles available in a population any of which may be present at the relevant genetic locus e.g. there are three options at the human ABO blood group locus of which any one individual can only possess two.

Question 43

Mutation

Answer 43

A heritable change in the nucleotide sequence or the quantity of the genetic material, usually DNA (RNA in some viruses).

Question 44

Natural selection

Answer 44

The name given to the mechanism whereby evolution is believed to occur (when coupled to mutation to create fresh variation and some form of pressure whereby those organisms with the most appropriate combinations of characteristics are more likely to survive and reproduce).

Question 45

Phenotype

Answer 45

The outward expression of the inherited characteristics often modulated by the interaction between genotype and environment e.g. height, eye colour etc.

Question 46

Phylogenetic/phyletic

Answer 46

Relating to the sequence of events whereby a particular species/genus etc. is thought to have evolved from ancestral species/genera etc.

Question 47

Polygenes/polygenetic

Answer 47

A situation in which the observed phenotypic characteristic is governed by several or many genetic loci. This is often associated with a continuous distribution for the characteristic since each locus only makes a small contribution so the different variations are difficult to distinguish especially when environmental factors also come into effect to further blur distinctions.

Question 48

Prophase I

Answer 48

A stage in meiosis I in which the chromosomes first become visible (light microscope) and the centrioles (animal cells) migrate to opposite poles to begin organising the spindle apparatus. It ends with disaggregation of the nuclear membrane.

Question 49

Prophase II

Answer 49

A stage in meiosis II usually immediately following telophase I during which the (partially) decondensed chromosomes recondense and the centrioles (animal cells) again divide and migrate to define the new poles of the second stage meiotic spindles.

Question 50

Recessive

Answer 50

That form of a gene that can only appear in the phenotype when present in the homozygous (both alleles the same) condition.

Question 51

Recombination

Answer 51

In genetics the processes whereby different genes/alleles are shuffled into new combinations, usually in successive generations. These include intrachromosomal (crossing over), interchromosomal (independent assortment) and random gamete fusion mechanisms

Question 52

Sex chromosome

Answer 52

One of a pair of chromosomes, usually designated X and Y, present in species where sex is genetically-determined and carrying genes relevant to the development of individuals of different sex.

Question 53

Sex linked

Answer 53

A term used to describe a characteristic whose genetic locus happens to lie on a sex chromosome even though the characteristic itself may not be related to sex determination. The form of the characteristic seen in the phenotype will therefore tend to follow the sex chromosome in a pedigree.

Question 54

Spindle (apparatus)

Answer 54

A cytoskeletal construction, made of microtubules, which forms in metaphase and disassembles at the end of anaphase and is responsible for ensuring the proper segregation of chromosomes/chromatids into daughter cells.

Question 55

Stabilising selection

Answer 55

A type of natural selection in which the median/mean/mode value for a particular characteristic in a normal distribution is the one conferring the most suitable adaptation to the current situation so those individuals having it are most likely to survive and reproduce successfully thus maintaining that value in the population.

Question 56

Telophase I

Answer 56

The final stage of meiosis I in which the separated chromosomes gather near the poles of the dismantled spindle, (partially) decondense, nuclear membranes temporarily reform and cytokinesis (animal cells) or cell plate formation (plant cells) completes cytoplasmic division thus producing two haploid daughter cells.

Question 57

Telophase II

Answer 57

The final stage of meiosis II in which the separated chromosomes (chromatids of the original parent cell) gather near the poles of the dismantled spindles, fully decondense and become surrounded by nuclear membranes. Cytokinesis or cell plate formation completes formation of four haploid daughter cells.

Question 58

Zygote

Answer 58

A cell formed during sexual reproduction by the fusion of two gametes.