Science B1: You and Your genes


You and Your genes

Genes,Chromosomes and DNA

Most cells in the human body have a nucleus. Which contains your genetic material.

The human body contains 23 pairs of chromosomes which 46 chromosomes in total.

Genes control development, e.g. hair colour, and how an organism functions.

A gene can have different versions, called alleles.For example, the gene for eye colour has two alleles:brown and blue. Alleles are described as being either dominant or recessive. A dominant allele is one which controls the development of a characteristic even if it is not present on only one chromosome in a pair.A recessive allele is one which controls the deveopment of a characteristic only if a dominant allele is not present, i.e. if the recessive allele is present on both chromosomes in a pair. 


Differences between individuals of the same species are described as variations.

Variation may be due to: 

.Genes- the different characteristics an individual inherits, e.g. the colour of dogs' coats.

.Enviroment- the conditions in which an individual develops, e.g. how much someone weighs. 

Usually variation is due to a combination of genetic and enviromental causes.

Genetic Diagrams 

Genetic diagrams are used to show all the possible combinations of alleles and outcomes for a particular gene. They use captial letters for dominant alleles and lower case letters for recessive alleles.


Family trees can also be used to identify how an individual has inherited a characteristic, like blue eye colour or not being able to roll their tounge.

Sex Chromosomes

One of the 23 pairs of chromosomes in a human cell is the sex chromosomes.In females the sex chromosomes are identical;they are both X chromosomes. In males they are different;there is an X and a Y chromosome. The Y chromosome is much shorter than the X chromosome



Rare Disorders

Most characteristics are governed by a range of genes, so the presence of one fualty allele may not effect the overall


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