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Genes and inheritance

Age range 14-16 Age range 16-19

DNA & Inheritance

How the sex cells divide - meiosis

diagram of meiosis

Meiosis is the process by which new cells with half the usual number of chromosomes are produced. In humans the full complement of 46 chromosomes is called the diploid number. Meiosis produces cells with 23 chromosomes, known as the haploid number, and these are the gametes, i.e. sperms for males and ova

In meiosis there are two lots of cell division so that a single diploid cell gives rise to four haploid cells. During fertilisation two haploid nuclei join to give a zygote with the diploid number of chromosomes. The zygote then grows and develops by mitosis.

How characteristics are passed on

diagram eye colour 1
Genetic diagram for eye colour

Genetics is the study of inheritance, that is, how characteristics are passed from one generation to the next. Gametes are haploid with only one of each chromosome and therefore one of each gene. When two gametes fuse at fertilisation the zygote now has two of each gene (that is a pair of alleles for each gene) one from the mother and one from the father. Where the alleles are different, one may be recessive and the other dominant and the dominant allele will determine the characteristic.

We can use genetic diagrams to show how a characteristic is passed on. Letters are used to represent the genes and a capital letter means an allele is dominant, and a small letter means a recessive allele. The diagram right shows the possible gametes and offspring for parents who both have homozygous alleles for eye colour, one for blue eyes and one for brown eyes. Note that, any child with Bb alleles will have brown eyes but carries the allele for blue eyes which could be passed on to a later generation.

For parents with heterozygous alleles for eye colour, the outcome is different as shown in the diagram below.

diagram eye colour 2

The ability to roll the tongue is something an individual can or cannot do and is passed on genetically. It cannot be learned. In the diagram below, the parents are heterozygous for tongue rolling.

The inheritance of blood group can be represented by the simplified diagram below. It shows the possible blood groups of offspring when the parents are heterozygous A and heterozygous B.

Male or female?

Many characteristics are controlled by a gene (or sometimes a few genes). The gender of the next generation, however, is determined by a whole chromosome. Human chromosome pair number 23 differs between male and female. It can be XX , which is female, or XY, which is male. In the following genetic diagram the letters represent chromosomes.

It shows that the ratio of boys to girls is 1:1, that is, there is always an equal chance of getting a boy or a girl.

Question 3

Quiz Print
Study the sentences below, then drag and drop
the missing word or phrase to complete the sentence.
 - full complement of 46 chromosomes for a human.
chromosome 23
 - number of chromosomes in sex cells.
Haploid number
 - study of characteristics passed between generations.
 - determines the gender of the next generation.
 - represents the chromosome pairing for females.
Diploid number
 - represents the chromosome pairing for males.

The type of cell division, which occurs in the ovaries and testes, to produce cells with a haploid number of chromosomes.
Diploid number
The number of chromosomes in most ordinary body cells. In humans this number is 46.
Haploid number
The number of chromosomes found in the sex cells (sperms and ova), which is half the number found in the majority of body cells. In humans the haploid number is 23.
The collective name for the sex cells, ova and sperms.
The male sex cell or gamete. The full name is spermatazoan, abbreviated to sperm cell or sperm.
The plural of ovum.
A fertilised cell produced as the result of the combination of an ovum and a sperm.
A chromosome is like a packet of coiled up DNA. Humans have 23 pairs of chromosomes. They are in the nucleus of every human cell.
The science of genes, heredity and variation.
The transfer of characteristics from parents to children through their genes.
A short piece of DNA which is responsible for the inheritance of a particular characteristic. It codes for the production of a specific protein.
Alternative forms of the same gene.
The description of a allele which is overridden by an equivalent dominant allele.
The description of a allele which overrides an equivalent recessive allele.
Genetic diagram
A diagram to represent the alleles and gametes present for a particular characteristic in parents and the possible allele combinations in their children.
The description of an individual who has two identical alleles for one particular gene.
The description of an individual who has two different alleles for one particular gene.