Inheritance of Sex

The nucleus of a cell contains chromosomes. 

In human cells, there are 23 pairs of chromosomes. Only one of these pairs carries the genes that determine sex.

In females, the sex chromosomes are the same. They are both X chromosomes.
In males, the sex chromosomes are different. One is a X chromosome and the other is a Y chromosome.

The gametes contain one chromosome of each pair.
All female gametes contain one X chromosome.
Male gametes contain either a X or Y chromosome.

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Inheritance of Characteristics

Each chromosome carries many genes. Chromosomes are made of the chemical DNA. A gene is a section of a DNA molecule.

Genes control the characteristics of the body, for example, eye colour.

DNA in a gene, codes for a particular characteristic. It is made up of very long strands which have four different compounds called bases.

A sequence of three bases, codes for a particular amino acid.

The order of bases controls the order in which amino acids are put together to form a particular protein.

Genes may have different forms, for example, there are genes for blue eyes, brown eyes, green eyes and hazel eyes. The different forms of a gene are called alleles.

The diagram shows a pair of chromosomes with genes A, B and C that would each code for a different characteristic.

A1 and A2 are different alleles of the same gene. A1 might code for red hair, whilst A2 might code for brown hair.
B1 and B2 are the same genes, coding for the same characteristic, for example, freckles.
C1 and C2 are different alleles. C1 might code for straight hair and C2 for wavy hair.

If both chromosomes in a pair have the same allele of a gene the individual is homozygous for that gene.
If the alleles on the two chromosomes are different the individual is heterozygous for that gene.

An allele which controls the development of a characteristic when it is present on only one of the chromosomes is called a dominant allele.
An allele which controls the development of characteristics only when the dominant allele is not present is a recessive allele.

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Inherited Disorders

Some diseases can be passed on from parent to offspring. These are called inherited disorders.

Sickle-cell anaemia is a disorder of red blood cells. They have a sickle shape and cannot carry as much oxygen.
It is caused by a recessive allele, so the allele has to be present on both chromosomes to cause the disease; both parents must carry the allele.

People who just have one allele for the disease are said to be carriers, because they can pass on the faulty gene to their offspring.

Carriers of the allele have some immunity to malaria. It is therefore an advantage to be a carrier if you live where malaria is common.

The diagram shows how sickle-cell anaemia is inherited.

N is the allele for normal blood cells.
n is the allele for sickle-cell anaemia.

If both parents are carriers, they both have the genotype Nn.

There is a 1 in 4 chance that the offspring will have sickle-cell anaemia, nn and a 1 in 2 chance that it will be a carrier, Nn.

Cystic fibrosis is a disorder of cell membranes caused by a recessive allele.
Thick mucus is produced in the respiratory and digestive systems, making breathing and digestion difficult.

Most inherited diseases are caused by a recessive allele, however Huntington's disease is caused by a dominant allele. Only one copy of the allele is needed to cause the disorder. 

It is a disease of the nervous system. The symptoms do not appear until middle age. They then get progressively worse resulting in loss of movement and speech, and mental confusion. Fortunately, it is very rare.

H is the allele for Huntington's disease.
h is a normal allele.

The diagram shows how the disease can be passed on if the mother is normal but the father has the faulty allele.

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