Unit 2 - Variation and Sexual Reproduction

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.

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.

Asexual reproduction in plants, found in addition to sexual reproduction. eg. bulbs (daffodils), corms (crocus), tubers (potato), stolons or runners (strawberries) and rhizomes (ginger)

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.

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

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.

Huge increase in genetic variation within the population. Sexually reproducing organisms have a better chance of surviving changing selection pressures.

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

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

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

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.

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.

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.

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.

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.

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.

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.

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.

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.

Crossing over Independent assortment Fertilisation, combining genetically variable gametes from 2 parents.

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.

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).

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)

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.