Unit 2 - Variation and Sexual Reproduction

Question 1

Asexual reproduction

Answer 1

Results in low genetic variation within a population, as whole genomes are passed from parents to offspring.

Cost effective way to maximise offspring production, which are produced frequently and in large numbers.

It is an advantage in narrow or stable niches, and is useful for rapidly re-colonising disturbed habitats.

Populations cannot adapt easily to change.

Question 2

Asexual reproduction - unicellular organisms

Answer 2

Bacteria, archaea and single celled eukaryotes such as yeast divide to produce new cells by asexual reproduction.

Horizontal gene transfer, eg. of plasmids, allows exchange of genetic material, increasing variation.

Question 3

Vegetative cloning

Answer 3

Asexual reproduction in plants, found in addition to sexual reproduction.

eg. bulbs (daffodils), corms (crocus), tubers (potato), stolons or runners (strawberries) and rhizomes (ginger)

Question 4

Parthenogenesis

Answer 4

Asexual reproduction in animals.

Offspring are produced from a female gamete, without fertilisation occurring.

Aphids and stick insects produce multiple female clones, and can produce males by deleting a sex chromosome.

It is rare in vertebrates - whiptailed lizards and some geckos only. Modified meiosis results in diploid eggs.

Parthenogenesis is found in areas with cool climates and a low parasite density.

Question 5

Sexual reproduction

Answer 5

The genetic material of 2 parents is combined to create offspring.

Question 6

Sexual reproduction - disadvantages

Answer 6

Half the population (males) are unable to produce offspring, reducing the reproductive fitness of the population as a whole.

Only half of each parent’s genome is passed on, disrupting successful parental genomes.

Question 7

Sexual reproduction - advantages

Answer 7

Huge increase in genetic variation within the population.

Sexually reproducing organisms have a better chance of surviving changing selection pressures.

Question 8

Meiosis

Answer 8

Division of the nucleus that results in the formation of haploid gametes from a diploid gametocyte (gamete producing cell).

Question 9

Diploid

Answer 9

All body cells are diploid - they have 2 sets of chromosomes, one set from each parent.

Question 10

Haploid

Answer 10

Gametes are haploid as they carry only one set of chromosomes.

Question 11

Fertilisation

Answer 11

The haploid nuclei of 2 gametes fuse together to form a new diploid nucleus.

This results in a new combination of alleles in the offspring, increasing variation.

Question 12

Homologous chromosomes

Answer 12

Matching chromosomes - one from each parent.

Homologous chromosomes are the same size, have a centromere in the same position and carry the same genes at the same loci (positions), although the alleles may be different.

Question 13

Meiosis

Answer 13

Cell division resulting in the production of haploid gametes.

Occurs in the sex organs in specialised diploid cells known as gametocytes (gamete mother cells)

During the S-phase of interphase, the homologous chromosomes within the gametocytes replicate.

This is followed by 2 rounds of cell divsion - meiosis I and II, which creates 4 haploid cells, which differentiate to form gametes.

Question 14

Meiosis I

Answer 14

Replicated homologous chromosomes pair up and are aligned gene by gene to form a bivalent.

Crossing over occurs, the nuclear membrane breaks down, spindle fibres form and homologous pairs line up on the equator.

They line up at random, irrespective or maternal/paternal origin - known as independent assortment.

The replicated chromosomes of homologous pairs are separated by the shortening of spindle fibres. (Centromeres remain intact).

Chromosomes move to opposite poles, creating 2 new nuclei, the nuclear membranes reform and cytokinesis occurs, resulting in 2 daughter cells.

Question 15

Crossing over

Answer 15

Chiasmata form between non-sister chromatids within a homologous pair when they align during meiosis I.

This allows equivalent sections of DNA to be exchanged with their homologous partner, resulting in new combinations of alleles.

This increases variation in gametes.

Question 16

Linked genes

Answer 16

Linked genes are located on the same chromosome, and are generally inherited together.

They can be separated by chiasmata during crossing over.

The further apart the linked genes are, the more likely they are to be separated by chiasmata.

Recombination frequencies can be used to calculate the distance between linked genes.

Question 17

Independent assortment

Answer 17

Homologous pairs are aligned independently on the equator during meiosis I, irrespective of their maternal/paternal origin.

There is no control over which chromosome of a pair is pulled to each pole, and which cell it ends up in.

This results in many possible combinations of maternal/paternal chromosomes in daughter cells, increasing variation.

Question 18

Meiosis II

Answer 18

The nuclear membranes of the 2 new daughter cells break down and spindle fibres form.

Chromosomes line up on the equator, the centromeres break and sister chromatids are separated (similar to mitosis).

Chromatids gather at opposite poles, nuclear membranes form and cytokinesis occurs, resulting in 4 haploid daughter cells.

Once chromatids reach the poles, thay are known as chromosomes.

Question 19

Gamete formation

Answer 19

Human males - meiosis starts at puberty, when haploid cells develop into sperm cells.

Human females - at birth, each female already has a million gamete mother cells at the chiasmata stage of meiosis I.

After puberty, several of these cells divide further during the menstrual cycle, although only one reaches metaphase II.

Division of the cytoplasm is uneven, as a future ovum needs more cytoplasm to sustain the early embryo.

Meisois II is only completed on fertilisation.

Question 20

Mechanisms to increase variation

Answer 20

Crossing over

Independent assortment

Fertilisation, combining genetically variable gametes from 2 parents.

Question 21

Hermaphrodite

Answer 21

Individuals with functioning male and female sex organs, producing both male and female gametes.

They usually have a partner to exchange gametes with, and don’t fertilise themselves.

eg. flowering plants have stamens and carpels in the same flower, but they mature at different times.

Benefit - if the chance of encountering a partner is remote, there is no need for them to be the opposite sex.

Question 22

Sex chromosomes

Answer 22

Determine the biological sex of an individual.

In mammals, females have one homologous pair of X chromosomes (XX), males have one X and a smaller Y chromosome (XY), but they are able to line up during meiosis due to an area of homologous genes near the centromere.

Gametes carry one sex chromosome each - all female gametes are XX, male gametes can be X or Y (in mammals).

Question 23

Homogametic/heterogametic

Answer 23

2 sex chromosomes the same = homogametic (XX in female mammals, ZZ in male birds)

2 different sex chromosomes = heterogametic (XY in male mammals, ZW in female birds)

Question 24

SRY gene

Answer 24

Found on the Y chromosome of mammals.

Causes the embryo to develop male characteristics.

Acts as a master switch, triggering a cascade which activates all male developmental genes in the genome.

Lack of an SRY gene results in a default female embryo.

Question 25

Sex linkage

Answer 25

The X chromosome in mammals has many genes which do not have homologous alleles on the smaller Y chromosome.

This results in sex-linked patterns of inheritance - males always express the allele on the X chromosome, as it is not masked by a matching allele on the Y chromosome.

eg. alleles for eye colour in Drosophila and haemophilia in humans are found on the X chromosome.

In haemophilia, the allele for the disease is recessive. Males either have the condition or not (2 possible genotypes), females can be normal, carriers or sufferers (3 possible genotypes).

Question 26

X chromosome inactivation

Answer 26

During early embryonic development, most of the genes on one X chromosome are inactivated.

The few homologous genes shared by X and Y chromosomes are left switched on.

X chromosome in activation is random - tortoishell cats are always female, as coat colour is carried on the X chromosome.

Due to X chromosome inactivation, half the cells will express black fur and the other half ginger fur (at random).

Question 27

Reasons for X chromosome inactivation

Answer 27

Cells have a single working copy of the X chromosome genes.

Dosage compensation - females have the same level of gene products as males, since a double dose of gene products could be harmful to cells.

Question 28

Examples of environmental sex determination

Answer 28

Environmental factors can change the sex of embryos or adults in some species.

Reptiles - egg incubation temperature determines whether egges devlop as male or female.

Clown fish - social group size causes fish to cjange sex

Mouse lemur - male : female sex ratio adjusted due to competetition

Parasitic infection of insect egges by Wolbachia bacteria kills/feminises males.