Human Biology - EDEXCEL IGCSE - Reproduction and Heredity

Reproduction and Heredity

The process of fertilisation involves the fusion of a male and female gamete (sex cell) to produce a single cell called a zygote.

Reproductive System:

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 (luteinising 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 luteum 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.

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.
• Haploid cells - cells with chromosomes not in a homologous pair is said to be haploid.

Meiosis:

1. Each chromosome in the nucleus duplicates itself.
2. The cell divides into two as in mitosis.
3. The cell divides again to form four cells containing half the number of chromosomes. This results in the formation of genetically different haploid (half the number of chromosomes) cells that are not in homologous pairs.

A zygote is formed when two gametes (sex cells) formed through meiosis fuse to form a cell with a full chromosome count. Following this, mitosis duplicates the cell millions of times to form an embryo.

The embryo in the uterus develops a placenta which not only anchors the embryo to the uterus but also allows the embryo to obtain nutrients such as oxygen and glucose from the mother’s blood. It also allows the embryo to get rid of waste products such as urea and carbon dioxide. An embryo also is enclosed by a membrane called the amnion. This secretes amniotic fluid which protects the embryo from jolts and bumps.

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 develop.
• Menstruation starts.
• Growth of armpit and pubic hair.

Birth Process:

1. Cervix dilates to allow the baby to pass through. The muscles of the uterus contract strongly and rupture the amnion, allowing the amniotic liquid to escape. This is called the water break.
2. Strong contractions of the uterus push the baby head first through the cervix and vagina.
3. After the birth, the uterus continues to contract to push out the placenta and the amnion. This is called the afterbirth.

Breastfeeding:

Advantages:

• Perfect food for healthy growth of the baby.
• Contains antibodies which protect the baby against infection diseases.
• Forms an emotional bond between the mother and the baby.

Growth and development:

gametes—> zygote—> embryo—> foetus—> baby—> child —> adolescent —> puberty—> adult

Contraception:

Hormonal - oral contraceptive pill such as the combined pill (oestrogen and progesterone) or the mini pill (progesterone). The mini pill creates a thickening of the mucus in the cervix which acts as the barrier and the combined pill prevents the production of FSH and LH, preventing menstruation. Its advantage is that it has a low failure rate however it must be taken every day and at a certain time.

Barrier - uses a barrier to prevent sperm from reaching the ovum. Examples include the condom and femidom. The advantages are that they are easy to obtain and use and that they also protect against STI. However, they may slip off during intercourse.

Natural - withdrawal from intercourse or having intercourse during a “safe period” is easy however there is a high failure rate and women will also have to have a regular cycle and will need to keep track of it.

Inter-uterine - an IUD (inter-uterine device) or coil is inserted through the cervix into the uterus. It is a piece of plastic or copper that prevents a fertilised egg from implanting in the uterus. Its main advantage is that it has no effect on intercourse however it does have to be fitted by a doctor and cause heavy periods.

Sterilisation - a surgical process that prevents sperm from passing to the penis or eggs from passing to the uterus. In men, it is called a vasectomy and is where the sperm ducts are cut and tied under general anaesthetic. In women, a similar process occurs in the fallopian tubes and is called tubal ligations. Its advantage is that it has a very low failure rate while its disadvantages are that it is non-reversible and that it has to be performed by a doctor.

The nucleus of a cell contains chromosomes which contain DNA. A small section of DNA that determines a particular feature by instructing cells to produce a particular protein are called genes. In humans, the diploid number of chromosomes is 46 and the haploid number is 23.

ABO blood groups are determined by multiple alleles (more than two alleles) with each allele determining which antigens are on red blood cells. The alleles are: I^A, I^B, I^O

 I^A and I^B are codominant and I^O is recessive to both.

To show patterns of inheritance we often s a genetic diagram called a pedigree.

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:

There are certain diseases which are sex-linked. This means that they exist on the X chromosome and means that often boys are more susceptible to them. An example of this is a blood disorder called haemophilia. The allele for this is recessive and is “h”. It is only found on the X chromosome. A woman with the gene X^HX^h would not have haemophilia but would be a carrier.

A carrier female and a healthy male have a 25% chance of a having a haemophiliac boy but no chance of a haemophiliac girl.

Another example of a sex-linked disease is red-green colour blindness.

The offspring formed through intercourse vary genetically because of the huge variation in sex cells. It is also because of the random nature of fertilisation where over a billion different sperms can fertilise one of the thousands of ova

Variation can be produced both by genes and by the environment. e.g. body mass, height, skin colour, intelligence.

Mutations are rare, random genetic changes to the genetic material that can be inherited.

Most mutations are harmful, some are natural 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. These bacteria live for longer and can, therefore, multiple more.

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