7. Reproduction & Meiosis

Success Criteria

Your learning has been successful if you can do the following:

I can explain how genes are inherited through reproduction.
I can compare advantages and disadvantages of sexual and asexual reproduction.
I can describe the processes and products of meiosis.
I can explain how independent assortment and crossing over increases genetic variation.

Vocabulary

Learn these so you can communicate this concept well.

Asexual reproduction: Type of reproduction that involves only one parent and creates genetically and phenotypically identical offspring.
Diploid (2n): Having two sets of each chromosome.
Gametes: Sex cells (sperm, egg, pollen)
Genetic variation: Differences in DNA / genes / alleles.
Haploid (n): Having only one set of each chromosome.
Homologous chromosomes: Pairs of chromosomes that carry the same genes in the same place. One comes from mum, the other from dad.
Meiosis: The type of cell division that creates haploid gametes for reproduction.
Mitosis: Type of cell division that creates diploid daughter cells for growth and repair.
Random fertilisation: The egg and sperm that end up fusing together are randomly selected from a larger pool of gametes.
Sexual reproduction: Type of reproduction that involves two parents combining their DNA and creates genetically unique offspring.
Zygote: Fertilised egg.

Hei Mahi (Do Now)

Do Now in your OneNote/Notebook:

In ONE sentence, link mutations to nucleotides, genes and alleles.

Hei Mahi (Do Now)

Do Now in your OneNote/Notebook:

What is the genotype of the unknown individual? How do you know?

Exit Task

In your Learning Journal:

Re-write this interpreting question so it is asking about Mitosis & Meiosis:

How is it different?

Then, write an answer for it.

Exit Task

In your Learning Journal:

Re-write this interpreting question so it is asking about Genetic Variation:

How does this relate to...?

Then, write an answer for it.

Two Types of Reproduction - Asexual & Sexual

Reproduction involves individuals passing on their genetic material (genes) to the next generation). There are two types: 1) ASEXUAL REPRODUCTION, and 2) SEXUAL REPRODUCTION.

Reproduction is essential for the survival of a species. Some species reproduce only sexually. Others reproduce both sexually and asexually.

Asexual reproduction

Asexual reproduction involves one parent passing on its DNA.

Offspring are genetically and phenotypically identical to each other and the parent (no variation). No new allele combinations are introduced into the population.

For example, identical aphids (one genotype and phenotype) are the result of asexual reproduction.

Advantages:

Reproduction is simple and rapid.
Population numbers can increase rapidly in ideal environmental conditions.

Disadvantages:

The lack of phenotypic variation, means the population is at risk of being wiped out in adverse environmental conditions.

Sexual reproduction

Sexual reproduction involves two parents combining their DNA.

Offspring are genetically unique, because new allele combinations are introduced into the population. (This is different to mutations). So it produces a variety of phenotypes.

Sexual reproduction gambles on new combinations of alleles (different phenotypes). Not every phenotype will be successful.

For example, varied rabbits (many phenotypes) are the result of sexual reproduction.

Advantages:

Some of the different phenotypes (variation) may survive adverse environmental conditions.

Disadvantage

Reproduction is slower, so the population cannot increase rapidly.
Reproduction takes more energy (need to find a mate).

Meiosis (is different to Mitosis!)

MEIOSIS is another type of cell division to make GAMETES (egg and sperm in animals and egg and pollen in plants). In humans meiosis happens in either a woman’s ovaries to make eggs or a man’s testes to make sperm.

A cell goes through two divisions and ends up with half the number of chromosomes as a body cell - we call this 'n' or HAPLOID (a normal body cell is '2n' or DIPLOID). This means the gametes have 23 chromosomes. In humans a body cell has 46 chromosomes or 23 pairs of chromosomes.

Parts of the HOMOLOGOUS CHROMOSOMES are shuffled during meiosis and this creates GENETIC VARIATION. Four, genetically different gametes are produced.

S1.9 (7) Meiosis

Meiosis for Sexual Reproduction

MEIOSIS is the cell division that produces unique GAMETES (sperm or eggs) used in sexual reproduction. The daughter cells are HAPLOID, meaning they have half the number of chromosomes.

Mitosis for Asexual Reproduction

MITOSIS is the cell division that produces identical daughter cells used in asexual reproduction. The daughter cells are DIPLOID, meaning they have a full number of chromosomes, identical to the parent cells.

Tying it all together

SEXUAL REPRODUCTION involves combining GAMETES from 2 parents to create a new individual. Each parent contributes a single copy of each chromosome to a gamete. Therefore, when these are combined (forming a ZYGOTE), the offspring has combined information from 2 individuals.

A gamete is a sex cell formed during meiosis, with one set of chromosomes (HAPLOID) instead of the normal 2. As the chromosomes are shuffled randomly during meiosis (crossing over and independent assortment), every gamete is different. And so each individual (even from the same 2 parents) is a unique combination of its parents’ alleles. In this way, meiosis increases VARIATION.

The gametes produced in sexual reproduction have only one set of chromosomes, and so these can be combined with another parent to make a unique individual. This process of a random male and random female gamete joins to produce zygote (fertilised egg) is called RANDOM FERTILISATION. Random fertilisation produces new combinations of alleles and thus GENETIC VARIATION between zygotes / individuals.

ASEXUAL REPRODUCTION does not use gametes and so offspring are not varied. That means one disease could potentially harm the entire population. The advantage of genetic variation to a species is that it may enable some individuals to survive a change in the environment, passing on favourable alleles to the next generation. Over many generations, this genetic advantage will rise /accumulate in the population, allowing the survival of the species.