Chromosomes are made from DNA. Genes are short sections of DNA. Genetically identical cells are produced by a type of cell division called mitosis. In sexual reproduction, a male gamete fuses with a female gamete to produce a new cell. This is called fertilisation. Gametes are produced by a type of cell division called meiosis. They contain a single set of chromosomes, whereas body cells contain two sets of chromosomes.

DNA

DNA (deoxyribose nucleic acid) molecules are large and complex. They carry the genetic code that determines the characteristics of a living thing.

Except for identical twins, each person’s DNA is unique. This is why people can be identified using DNA fingerprinting. DNA can be cut up and separated, forming a sort of 'bar code' that is different from one person to the next.

Genes

A gene is a short section of DNA. Each gene codes for a specific protein by specifying the order in which amino acids must be joined together.

Chromosomes

The cell’s nucleus contains chromosomes made from long DNA molecules.

The diagram shows the relationship between the cell, its nucleus, chromosomes in the nucleus, and genes.

Nucleus, chromosome and gene

Mitosis is the type of cell division that leads to growth or repair. When a cell divides by mitosis:

• two new cells form
• each cell is identical to t

  Stages of mitosis

  
  1. parent cell
  2. chromosomes make identical copies of themselves
  3. they line up along the centre
  4. they move apart
  5. two daughter cells form with identical chromosomes to the parent cell

Mitosis is the type of cell division that leads to growth or repair. When a cell divides by mitosis:

• two new cells form
• each cell is identical to t

  Stages of mitosis

  
  1. parent cell
  2. chromosomes make identical copies of themselves
  3. they line up along the centre
  4. they move apart
  5. two daughter cells form with identical chromosomes to the parent cell

Gender

Human body cells have 23 pairs of chromosomes in the nucleus. One of these pairs controls the inheritance of gender - whether offspring are male or female:

• In males, the two sex chromosomes are different. They are XY.
• In females, the two sex chromosomes are the same. They are **.

Chromosomes from a female

Chromosomes from a male

Gametes

Gametes are sex cells. The male gametes are the sperm, and the female gametes are the eggs.

Gametes contain one set of genetic information, while body cells contain two sets of genetic information.

Fertilisation

Fertilisation is the joining or fusion of a male gamete and a female gamete. The new cell that is formed divides over and over again by mitosis. This creates the many cells that eventually form a new individual.

Meiosis - higher

Gametes are formed from cells in the reproductive organs by a type of cell division called meiosis.

MaleFemale reproductive organ testes ovaries gametes sperm eggs

The cells that are formed by meiosis have half as many chromosomes as the cell that formed them. Human body cells contain 23 pairs of chromosomes, while human gametes contain 23 single chromosomes.

The main features of meiosis are:

• the chromosomes are copied
• the cell divides twice, forming four gametes

The diagram outlines how this works.

When an egg and sperm cell come together, the now fertilised egg contains 23 pairs of chromosomes. Sex chromosomes are responsible for certain genetic traits.

Sex cells and chromosomes

Human body cells each contain 23 pairs of chromosomes. Parents pass on theirgenes to their offspring in their sex cells.

• female sex cells are called egg cells, or ova
• male sex cells are called sperm.

Process of fertilisation

A pair of chromosomes carry the same genes in the same place, on each chromosome within the pair. However, there are different versions of a gene calledalleles. These alleles may be the same (homozygous) on each pair of chromosomes, or different (heterozygous), for example, to give blue eyes or brown eyes.

Sex cells only contain one chromosome from each pair. When an egg cell and sperm cell join together, the fertilised egg cell contains 23 pairs of chromosomes. One chromosome in each pair comes from the mother, the other from the father.

Which chromosome we get from each pair is completely random. This means different children in the same family will each get a different combination. This is why children in the same family look a little like each other and a little like each parent, but are not identical to them.

Sex chromosomes

A set of chromosomes can be separated from its cell, spread out on a microscope slide and magnified many thousands of times. When stained and photographed, they look like this:

Chromosomes from a female

Chromosomes from a male

The highlighted pair of chromosomes are called the sex chromosomes; they are a pair. The longer sex chromosome is called the X chromosome, the shorter one the Y chromosome.

• females are **
• males are XY.

When sex cells form, the pairs of sex chromosomes (** and XY) are separated. Remember that females carry **, males XY. This means:

• all normal egg cells produced by a human ovary have an X chromosome
• half the sperm carry an X chromosome, and half a Y.

So a human baby’s gender is determined by the sperm that fertilises the egg cell. The baby will be a girl if it carries an X chromosome. It will be a boy if the fertilising sperm carries a Y chromosome. Study the animation below to test your understanding of this.

Notice that half of the babies should be male, and half female. Individual families can have more or less of each sex, but in a large population there will be roughly equal numbers of boys and girls. If you toss a coin many times you will get roughly equal numbers of ‘heads’ and ‘tails’ for the same reason.

Ideas about science - choosing gender

Some societies prefer to have male children. It is now possible in some countries to choose the sex of a child using IVF. Some people think parents should be able to choose the sex of their future children, especially if they had a child that died, or already have three or four children of the same sex. Other people think we should not be able to choose, because this could affect the balance of males and females in society, or because they believe it is against God or nature.

Different values - Higher tier

Decisions of this kind are called values. Science can provide information and data, but it cannot answer questions about values. Values often result in different people coming to different decisions. This is why some people think we should be able to choose the sex of our children while others do not.

The Y chromosome - Higher tier

The Y chromosome carries a gene called the ‘sex-determining region Y’, or SRY for short. The SRY gene causes testes to develop in an XY embryo. These produceandrogens: male sex hormones. Androgens cause the embryo to become a male: without them, the embryo develops into a female.

Genetic diagrams

Genetic diagrams are used to show the possible outcomes of a particular cross. A dominant allele is shown by a capital letter, and a recessive allele by a lower case letter.

Cystic fibrosis

Cystic fibrosis is an inherited disorder caused by a recessive allele. This genetic diagram shows the possible outcomes when both parents are heterozygous for the faulty allele. There is a one in four chance of the offspring being homozygous for the faulty allele, and so having cystic fibrosis.

Mother and father both have one F and one f allele

This genetic diagram shows the possible outcomes when only one parent carries the faulty allele. There is no chance of the offspring being homozygous for the faulty allele and therefore having cystic fibrosis.

Mother has two F alleles, father has one F and one f allele.

Gender determination

Female gametes (eggs) contain X chromosomes, while male gametes (sperm) contain either X chromosomes or Y chromosomes. This genetic diagram shows that equal numbers of male and female offspring should be produced.

Females are ** and males are XY

Some characteristics of a living thing are caused by the environment, some by inherited factors, while others are caused by a combination of environment and inherited factors.

Male mammals carry XY sex chromosomes - female mammals carry ** sex chromosomes. Inherited disorders are caused by faulty genes on these chromosomes.

Different versions of a gene are called alleles, and these alleles can be dominant or recessive. Genetic diagrams can show the possible outcomes of a particular cross.

Environmental and inherited characteristics

Some characteristics of an individual are caused by the environment. For example, the language we use or whether we have scars are environmental characteristics. Other characteristics are inherited.

Examples of inherited characteristics include:

• the shape of the earlobes
• eye colour
• nose shape.

Whether you have lobed or lobeless ears is due to genetic causes

Some characteristics - including intelligence, body mass and height - are the result of both environmental and inherited factors. But there is debate about the relative importance of these two types of factor in some human characteristics, such as intelligence, health and sporting ability.