AH Biology U2 KA3: Variation and Sexual Reproduction

Costs and benefits of sexual reproduction

Costs:

• males unable to produce offspring

• only half of each parent’s genome passed onto offspring,

disrupting successful parental genomes

Benefits:

• these outweigh costs due to an increase in genetic variation in the population

Importance of genetic variation

Genetic variation provides the raw material required for adaptation, giving sexually reproducing organisms a better chance of survival under changing selection pressures, e.g. genetic variability in offspring reduces the chances that all will be susceptible to infection by parasites

Use the Red Queen hypothesis to explain the persistence of sexual reproduction

The Red Queen hypothesis postulates that parasite pressure maintains sexual reproduction in the host population by selecting for the ability to produce rare genes types that are resistant to infection

Sexual reproduction

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

Asexual reproduction

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

Advantage of asexual reproduction

• Asexual reproduction can be a successful reproductive strategy as whole genomes are passed on from parent to offspring

• In asexual reproduction, just one parent can produce daughter cells and establish a colony of virtually unlimited size over time

• Offspring can be reproduced more often and in larger numbers with asexual reproduction

• Maintaining the genome of the parent is an advantage particularly in very narrow, stable niches or when re-colonising disturbed habitats

Examples of asexual reproduction in eukaryotes

Vegetative cloning in plants and parthenogenesis in lower plants and animals that lack fertilisation

Parthenogenesis

Reproduction from a female gamete without fertilisation

Parthenogenesis is more common in…

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

Evolution in asexually reproducing populations

Asexually reproducing populations are not able to adapt easily to changes in their environment, but mutations can occur that provide some degree of variation and enable some natural selection and evolution to occur.

Organisms that reproduce principally by asexual reproduction also often have

mechanisms for horizontal gene transfer between individuals to increase variation

Example of mechanisms for horizontal gene transfer

The plasmids of bacteria and yeasts

Explain why evolutionary change can be faster in prokaryotes than eukaryotes

Prokaryotes can exchange genetic material horizontally, resulting in faster evolutionary change than in organisms that only use vertical transfer.

Meiosis

Division of the nucleus that results in the formation of haploid gametes from a diploid gametocyte

In diploid cells, chromosomes typically appear as:

homologous pairs

Homologous chromosomes

Chromosomes of the same size, same

centromere position and with the same

sequence of genes at the same loci.

Describe the process of meiosis I

1. The chromosomes, which have replicated

   prior to meiosis I, each consist of two

   genetically identical chromatids attached at the centromere

2. The chromosomes condense and the

   homologous chromosomes pair up

3. Chiasmata form at points of contact between the non-sister chromatids of a homologous pair and sections of DNA are exchanged. This crossing over of DNA is random and produces genetically different recombinant chromosomes

4. 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.

5. The chromosomes of each homologous pair are separated and move towards opposite poles

6. Cytokinesis occurs and two daughter cells form

Linked genes

Genes on the same chromosome which have an association with each other

Crossing over of linked genes

Can result in new combinations of the alleles of these genes

Independent assortment

Each pair of homologous chromosomes is

positioned independently of the other pairs,

irrespective of their maternal and paternal

origin.

Meiosis II

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.

The sex of birds, mammals and some insects is determined by:

the presence of sex chromosomes

SRY gene

SRY gene on the Y chromosome in most mammals determines development of male characteristics

Explain what can result in sex-linked patterns of

inheritance

Heterogametic (XY) males lack most of the

corresponding homologous alleles on the

shorter (Y) chromosome

X chromosome inactivation during development

In homogametic females (XX) one of the two

X chromosomes present in each cell is

randomly inactivated at an early stage of

development

X chromosome inactivation

Process by which most of one X chromosome is inactivated

Purpose of X chromosome inactivation

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

Explain why carriers are less likely to be affected by deleterious mutations on X chromosomes

As the X chromosome inactivated in each cell

is random, half of the cells in any tissue will

have a working copy of the gene in question

Hermaphrodites

species that have functioning male and female reproductive organs in each individual

Hermaphrodites produce _____?____ gametes

and usually have ——?——

both male and female, a partner with which to

exchange gametes

Benefit to individual organism of having both male and female reproductive organs

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.

Environmental sex determination

For other species, environmental rather than

genetic factors determine sex and sex ratio

Environmental sex determination in reptiles

Environmental sex determination in reptiles is controlled by environmental temperature of egg incubation.

Changing of sex within individuals

Sex can change within individuals of some species as a result of size, competition, or parasitic infection

In some species the sex ratio of offspring can

be adjusted in response to…

resource availability