Genetic Variation
Sexual reproduction gives rise to variation because, when gametes fuse, one of each pair of alleles comes from each parent.
b) In human body cells, one of the 23 pairs of chromosomes carries the genes that determine sex. In females the sex chromosomes are the same (XX); in males the sex chromosomes are different (XY).
c) Some characteristics are controlled by a single gene. Each gene may have different forms called alleles.
d) An allele that controls the development of a characteristic when it is present on only one of the chromosomes is a dominant allele.
e) An allele that controls the development of characteristics only if the dominant allele is not present is a recessive allele.
f) Chromosomes are made up of large molecules of DNA (deoxyribo nucleic acid) which has a double helix structure.
g) A gene is a small section of DNA.
h) Each gene codes for a particular combination of amino acids which make a specific protein.
i) Each person (apart from identical twins) has unique DNA. This can be used to identify individuals in a process known as DNA fingerprinting.

	Created by banski about 12 years ago

GCSE Science (Biology) Note on Reproduction, created by banski on 31/03/2014.