Reproduction and Variation

Reproduction

Living organisms all reproduce in order to make a new generation of organisms . There are two methods of reproduction that living organisms use:-

  1. Sexual Reproduction - A 'male' and 'female' organism each contribute 1/2 the DNA required to produce a new offspring.

  2. Asexual Reproduction - a single organism contributes all the DNA required to produce a new offspring. The new offspring is a clone of the parent.


Note - This course will focus only on Sexual Reproduction.


Recap of Cells

There are two types of cells that take part in reproduction:-

Egg Cell

The image above shows an egg cell. Egg cells are part of the female reproductive system. An egg cell is specialised in two ways:-

  1. Nucleus contains only 1/2 the DNA required to create a new organism.

  2. Has a lot of cytoplasm containing nutrients to keep the cell alive until implantation.


Sperm Cell

The image above shows a sperm cell. Sperm cells are part of the male reproductive system. A sperm cell is specialised:-

  1. Nucleus contains only 1/2 the DNA required to create a new organism.

  2. Has a tail, to help it 'swim' to the egg Cell.


Fertilisation

Fertilisation occurs when a sperm cell fuses with an egg cell. The sperm cell loses its tail and the two cell nuclei join together to form a new cell called a Zygote. A membrane then forms around the egg cell, stopping any more sperm cells entering.

Human Reproduction

Humans reproduce through sexual reproduction. A male sperm cell Fuses with a female egg cell inside the female's reproductive system. From fertilisation to birth, the process takes approximately 9 months.


Female Reproductive System

The diagram below shows the main parts of the female reproductive system:-

Function of parts:-

  1. Ovary - Contains hundreds of undeveloped egg cells (Ova). The female is born with all the ova she will ever have.

  2. Oviduct - Connects the ovaries to the uterus. They are covered in thousands of cilia cells to move the ovum towards the uterus. Fertilisation occurs in the oviduct.

  3. Uterus - The uterus is where the baby will develop over approximately 9 months until birth.

  4. Cervix - The cervix is a ring of muscle which acts as the entrance to the uterus.

  5. Vagina - The vagina is a tube of muscle that connects the cervix to the outside of the female body. The vagina is where the male sex cells enter the female's body.


Male Reproductive System

The diagram below shows the main parts of the male reproductive system:-

Function of parts:-

  1. Testes - The male sex cells (sperm) are continuously made here. Hormones are also produced here.

  2. Sperm duct - Carries the sperm cells to the urethra. The sperm cells mix with fluid from the glands to make semen.

  3. Urethra - Carries the semen from the glands to outside of the male body. This is the same tube which carries urine from the bladder, but a ring of muscle prevents these from mixing.

  4. Penis - Enters the female body during sexual intercourse to pass semen into the vagina.


Menstrual Cycle

In order to ensure that the fertilised egg has the best possible environment to develop in, the entire lining of the uterus (called the Endometrium) is replaced every 28 days in a process known as the menstrual cycle.

The diagram below shows the (approximately) 28 day menstrual cycle:-

The process of the menstrual cycle is as follows:-

  1. Day 1 - The endometrium breaks down and is lost from the body through the vagina. In the ovary, a new ovum starts to develop.

  2. Day 7 - By this point, the endometrium starts to thicken again and the ovum is almost fully mature.

  3. Day 14 - The ovum is released from the ovary in a process called ovulation. Cilia cells in the oviduct move the ovum towards the uterus.

  4. Day 21 - The endometrium has reached full thickness and is ready for implantation if the female becomes pregnant.

  5. Day 28 / 1 - If the female is not pregnant, the endometrium breaks down and is lost from the body through the vagina.


Fertilisation

The process of fertilisation takes place within the oviduct of the female reproductive system. To do this, the male sperm cells must be placed within the female through sexual intercourse.

The sperm cells 'swim' from the vagina, through the cervix and uterus to the oviduct to reach the ovum.

The video below shows the journey and difficulties a sperm cell must endure to fuse with the ovum.

Stages of Pregnancy

Once fertilisation has occurred, the process of pregnancy can begin. The single fertilised cell goes through a huge amount of change before the baby is ready to be born:-

Note - The diagrams above are not to scale, the actual sizes of the early stages are too small to show clearly on the scale of a full grown baby.

The videos linked below show short explanations of the three trimesters of pregnancy:-

Childbirth

After 37 to 40 weeks, the foetus is counted as being full term, and if born would be expected to survive without specialist treatment.

The diagram below shows the main stages of childbirth:-

The video linked below shows a short explanation of the process of birth:-

Variation

Variation is the differences that exist between individuals of a species. Genetic information received from parents determines some of our physical characteristics.

There are two types of variation:-

  1. Continuous Variation.

  2. Discrete Variation.

Presenting Variation Graphically

When presenting variation graphically, we use two versions of a bar chart:-

Inheriting Genetic Information

Previously, the chromosomes that make up the human genetic sequence was explained. In humans, as well as all other organisms, the chromosomes are organised into 23 pairs. Half of each pair came from the organism's father, and half came from the organism's mother. This is because a sex cell (gamete) contains half the genetic information needed to make a complete individual.

The chromosomes in a matching pair contain the same type of genes that code for the same characteristics. Each chromosome may have a different version of a gene. Different versions of a gene, that code for different versions of a characteristic, are called Alleles.

The physical expression of an allele pair is the Phenotype. Examples of phenotype would be brown eyes or blue eyes.

The allele pair that give each characteristic is called the Genotype. Examples of genotype would be Bb or bb or BB or bB.

Genes are passed on from parents to offspring. Sexual reproduction introduces variation because the Zygote contains a mixture of genes, and therefore characteristics, from each parent.


Genetic Expression

In the above diagram, Dominant and Recessive genes were introduced, but not explained.

We inherit 2 copies of the same gene – 1 from each parent. There can be more than one form of the same gene. These are called alleles e.g. there is an eye colour gene. One allele will code for brown eyes and another allele will code for blue eyes.

Some genes hide other genes – these are dominant genes, and the ones that are hidden are recessive.

If the brown allele is dominant and the blue allele is recessive, then the child born would be born with either brown or blue Eyes, depending on which combination of genes was passed on.

The combination of genes inherited from a parent is the GENOTYPE, e.g. BB/Bb/bb, while the physical expression of these genes, e.g. blue eyes, is called the PHENOTYPE.


There are 4 Possible ways to combine B and b together as Genotypes :-

  1. BB - Both genes code for brown eyes, the child will have brown eyes.

  2. Bb - The brown gene is dominant, hiding the effect of the recessive blue gene, the child will have brown eyes.

  3. bB - The brown gene is dominant, hiding the effect of the recessive blue gene, the child will have brown eyes.

            • bb - Both genes code for blue eyes, the child will have blue eyes.

As can be seen above, the genotype for dominant characteristics can have several different versions, due to the combinations of the genes. These different combinations can be separated into two groups:-


  1. Homozygous - Both genes are the same e.g. BB or bb.

  2. Heterozygous - Each gene is different e.g. Bb or bB.


Punnet Squares

These possible combinations of genes can be represented visually using a Punnet Square. A punnet square is a grid that shows all possible ways to combine the parent's genes together.


Example 1 -

The diagram below shows the punnet square for two homozygous parents:-

Example 2 -

The diagram below shows the punnet square for two heterozygous parents:-