1.1 In body cells the chromosomes are normally
found in pairs. Body cells divide by mitosis.
1.1.1 The chromosomes contain
the genetic information.
188.8.131.52 When a body cell divides by
mitosis copies of the genetic
material are made and then
the cell divides once to form
two genetically identical body
184.108.40.206.1 Mitosis occurs during growth or to produce replacement cells.
220.127.116.11.1.1 Body cells have two sets of chromosomes;
sex cells (gametes) have only one set.
18.104.22.168.1.1.1 Cells in reproductive organs – testes and ovaries in humans – divide to form gametes.
22.214.171.124.126.96.36.199 The type of cell division in which
a cell divides to form gametes is
188.8.131.52.184.108.40.206.1 When a cell divides to form gametes copies of the genetic information are made and then
the cell divides twice to form four gametes, each with a single set of chromosomes.
220.127.116.11.18.104.22.168.1.1 When gametes join at fertilisation, a single body cell with
new pairs of chromosomes is formed. A new individual
then develops by this cell repeatedly dividing by mitosis.
22.214.171.124.126.96.36.199.1.1.1 Most types of animal cells
differentiate at an early stage
whereas many plant cells are
pluripotent and so retain the
ability to differentiate throughout
life. In mature animals, cell
division is mainly restricted to
repair and replacement.
188.8.131.52.184.108.40.206.220.127.116.11 Cells from human embryos and adult bone marrow, called stem
cells, can be made to differentiate into many different types of cells.
18.104.22.168.22.214.171.124.126.96.36.199.1 Human stem cells have the
ability to develop into any
kind of human cell.
188.8.131.52.184.108.40.206.220.127.116.11.1.1 Treatment with stem
cells may be able to
help conditions such
18.104.22.168.22.214.171.124.126.96.36.199.1.1.1 The cells of the offspring produced by asexual reproduction
are produced by mitosis from the parental cells. They contain
the same alleles as the parents.
2 B2.7.2 Genetic Variation
2.1 Sexual reproduction gives rise to variation because, when gametes
fuse, one of each pair of alleles comes from each parent.
2.1.1 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).
188.8.131.52 Some characteristics are
controlled by a single gene.
Each gene may have
different forms called alleles.
184.108.40.206.1 An allele that controls the
development of a characteristic
when it is present on only one
of the chromosomes is a
220.127.116.11.1.1 An allele that controls the
characteristics only if the
dominant allele is not
present is a recessive allele.
18.104.22.168.1.1.1 Chromosomes are made up of large
molecules of DNA (deoxyribo nucleic
acid) which has a double helix structure.
22.214.171.124.126.96.36.199 A gene is a small section of DNA.
188.8.131.52.184.108.40.206.1 Each gene codes for a particular combination of
amino acids which make a specific protein.
220.127.116.11.18.104.22.168.1.1 Each person (apart from identical twins) has unique DNA.
This can be used to identify individuals in a process
known as DNA fingerprinting.
3 B2.7.3 Genetic Disorders
3.1 Some disorders are inherited.
3.1.1 Polydactyly – having extra fingers or toes – is caused
by a dominant allele of a gene and can therefore be
passed on by only one parent who has the disorder.
22.214.171.124 Cystic fibrosis must be inherited from
both parents. The parents may be
carriers of the disorder without actually
having the disorder themselves. It is
caused by a recessive allele of a gene.
126.96.36.199.1 Embryos can be screened for the alleles that cause these and other genetic disorders.