Biology - EDEXCEL IGCSE - Reproduction and Inheritance

Reproduction and Inheritance

Asexual Reproduction is when a single parent copies its genetic information, forming a daughter which is genetically identical (clone) to itself. It involves no gametes and fertilisation.

Sexual Reproduction involves gametes which fuse to create a zygote and eventually an embryo which is not genetically identical to the parents.

Fertilisation involves the fusion of a male and female gamete to produce a zygote which then undergoes cell division and develops into an embryo.

Reproduction in Plants

Insect Pollinated Plant:

Wind Pollinated Plant:

 

Adaptations of wind and insect pollinated plants:

Feature	Insect Pollinated	Wind pollinated
Stamen	Enclosed - to force insect to make contact	Exposed - so that wind can easily blow it away
Stigma	Enclosed - insect must make contact Sticky - pollen sticks to it	Exposed - easier to catch pollen Feathery - large surface area to catch pollen
Petals	Brightly coloured, large and scented to attract insects	Small and not scented or coloured as there is no need
Nectaries	Present so as to attract insect	No nectary
Pollen grains	Large, spiky and sticky to attract insects	Small, light, and aero-dynamic so that it can be easily carried bu the wind

Fertilisation:

1.    Pollination occurs where pollen is transferred from the anthers to the stigma. The stigma must be ripe so that chemicals can be produced for the production of pollen tubes and a sugary substance for energy.

2.    The pollen tube grows down through the style and into the ovary. It enters the ovule by the micropyle.

3.    The male nucleus moves out of the pollen grain and moves down the channel created by the pollen tube and ion the ovule.

4.    Fertilisation occurs where the male and female gamete fuse to form a zygote.

5.    The zygote undergoes mitosis in a seed to create an embryo.

Seed and fruit formation:

1.    The petals and anthers die as there is no longer any need for them.

2.    The ovule wall becomes the testa which protects the embryo.

3.    The cotyledon provides the food store for the embryo.

4.    The plumule is the embryonic shoot ad the radicle the embryonic roots.

5.    The ovary becomes the fruit.

Germination:

1.    The dormant seed is buried in soil.

2.    Water is absorbed by the seed through the micropyle.

3.    Food reserve (starch) in the cotyledon is mobilised.

4.    The testa splits as the seed expands.

5.    The radicle grows downwards. The root develops and begins to absorb water directly.

6.    The plumule begins to grow upwards with the cotyledon still fueling growth.

7.    The embryonic leaves of the plumule now clear of the ground and begin to photosynthesise.

Germination requires:

1.    Heat for enzymes to act efficiently.

2.    Water for chemical reactions to take place.

3.    Oxygen for respiration to release energy.

4.    Light is not needed.

Reproduction in Humans:

Male:

Sperm is stored and created in the testes. During intercourse it travels along the sperm duct in the penis and mixes with secreted liquid from the seminal vesicle to form semen. One ovum is released into the fallopian tube each month and when it is in the tube a sperm can fertilise it.

Female:

Menstruation:

Hormones are very important for this process. First the follicle stimulating hormone (FSH) stimulates the growth of the follicle containing an ovum. At the same time FSH stimulates the release of oestrogen which begins the re-thickening of the uterus lining and also slows the release of FSH and stimulates the release of LH (lutenising hormone). When LH is at its peak, ovulation occurs where the ovum is shed by the ovary. If sexual intercourse occurs, what is left of the follicle forms a structure called the corpus lute. This releases progesterone which completes the thickening of the uterus walls and inhibits production of FSH and LH, stopping any further ovulation. If the egg is not fertilised then the corpus lute breaks down and the lining of the uterus is shed through menstruation. Progesterone is also used during pregnancy to stop menstruation. It is produced by the placenta.

Placenta:

The placenta allows the embryo to obtain oxygen and nutrients and get rid of CO2 and excretionary waste (e.g. urea). The umbilical cord used to carry the blood containing the substances towards and away from the foetus. The placenta is also responsible for secreting progesterone in order to maintain a thick layer of endometrium in the uterus.

Amnion:

The amnion encloses the foetus and secretes amniotic fluid which protects the foetus from bumps while the woman is moving.

Secondary Sexual Characteristics:

Boys: controlled by testosterone

•   Growth of penis and testes.

•   Growth of facial and body hair.

•   Muscle development.

•   Breaking of the voice.

Girls: controlled by oestrogen

•   The breast develops.

•   Menstruation starts.

•   Growth of armpit and pubic hair.

Inheritance:

DNA or deoxyribonucleic acid contains two strands of alternating sugar and phosphate groups coiled to form a double helix. The strands are linked by two nitrogen bases at each "rung". There are four different bases: Adenine(A), Thymine(T), Cytosine(C), and Guanine(G). In DNA the bases are always paired. So if on one strand the base is A, the other must be T and likewise if on one strand it is C then on the other it must be G.

 

The nucleus of cells contains chromosomes on which genes are located. A gene is a section of a molecule of DNA which dictates a certain characteristic of the organism.

A certain genes can take different forms which although creates the same characteristic, doesn't create the same exact feature. For example, a gene may code for green eyes while the same gene might code for blue eyes. These different forms of the same gene are called alleles. Alleles give rise to differences in inherited characteristics.

Definitions:

•   Genes - a small section of DNA that determines a particular feature by instructing cells to produce a particular protein are called genes.

•   Alleles - an alternative form of a gene which gives rise to differences in inherited characteristics.

•   Dominant - a feature will always have two alleles. If one allele’s characteristic is present while the other is not then it is said to be dominant.

•   Recessive - if one allele is dominant then the other is said to be recessive.

•   Homozygous - contains two copies of one allele (e.g. TT, aa).

•   Heterozygous - contains two different alleles (e.g. Tt, Aa).

•   Genotype - describes the alleles each cell has for a certain feature.

•   Phenotype - a feature that results from the genotype.

•   Codominance - if two alleles are expressed in the same phenotype.

•   Diploid cells - cells with chromosomes in homologous pairs are said to be diploid. In humans the diploid number is 23 meaning each cell has 23 chromosomes.

•   Haploid cells - cells with chromosomes not in a homologous pair is said to be haploid. In humans the haploid number is 46.

The sex of a person is determined by a pair of chromosomes, XY in a male and XX in a female. The overall ratio of male and female births is 1:1

This can be shown by this diagram:

	X	X
X	XX (female)	XX(female)
Y	XY(male)	XY(male)

Mitosis is the nuclear division of somatic (body) cells to create genetically identical cells which are used for growth, repair, asexual reproduction and replacing worn out cells.

Meiosis occurs to produce haploid cells that are used in sexual reproduction. It involves a similar process to mitosis except that there are two divisions compared to one in mitosis. Meiosis creates four cells, each with half the number of chromosomes and creates genetically different gametes.

A human male can produce millions of genetically different sperm cells and a female holds thousands of genetically different egg cells. This large pool of genetically different gametes and the fact that fertilisation is random allows genetic variation of the offspring. Not only is variation produced by genetics, it can also be produced through the environment.

A mutation is a rare, random change in genetic material which can be inherited. Many mutations are harmful but some are neutral and a few are beneficial.

Mutations that are beneficial can cause the mutant organism to increase in population through natural selection. An example of this is in bacteria that have mutated to be resistant to antibiotics. The variation in the species is that there are bacteria that are resistant and those that are not. The bacteria that are resistant live for longer and can therefore multiple more while the non-mutated species die out as they lack the advantage. This means they eventually populate the entire species.

The chances of mutations can be increased through mutagens. Examples of these are ionising radiation such as ultraviolet light, X-rays and gamma rays and many different chemicals, both natural and manmade (e.g. benzene).