GCSE AQA: Biology 2.7

Additional GCSE, biology 2.7

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B2.7 Cell division and inheritance
Characteristics are passed on from one generation to the next in both plants and animals.
Simple genetic diagrams can be used to show this.
Genetic table for
crossing tall pea
with dwarf pea
Parent genotypes: TT
x tt
Genotypes of children T T
t T T
t t
t T T
t t
This gives 100% tall plants (genotype Tt), but they all carry the allele t for dwarf pea plants
The diagrams above and below give a modern genetic interpretation of Mendel's results from initially crossing a pure
line of tall pea plants with a pure line of dwarf pea plants and then cross-breeding their offspring
The first resulting offspring were all tall pea plants and these were then crossed with each other to give the second set
of offspring shown above.
This gave approximately 75% tall plants (genotype TT or Tt) and 25% dwarf pea plants (genotype tt)
Mendel found that the second cross produced tall: dwarf pea plants in the approximate ratio of 3 : 1
Homozygous- same alleles e.g. XX or TT genes
Heterozygous- different alleles e.g. XY or Tt
Phenotype- gene expression - the outcome
Genotype- gene type
Cell division
In body cells the chromosomes which contain the genetic information are normally found in pairs
Body cells divide by mitosis, when a body cell divides by mitosis:
Copies of the genetic material are made i.e. copies of the DNA, which form into identical chromosomes

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Then the cell divides once to form two genetically identical body cells, in the process both full sets of chromosomes
become enclosed in a cell membrane complete with the necessary cytoplasm, so the separation into two cells is
DNA is the acronym for deoxyribonucleic acid and these giant molecules have all the coded instructions for
reproduction and developing an organism and keeping the organism alive
In the nucleus of a cell the DNA is collected together in huge sections called chromosomes.…read more

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The cells of the offspring produced by asexual reproduction are produced by mitosis from the parental cells; in asexual
reproduction the offspring contain the same alleles as the parents
Some plants reproduce by mitosis, so all new plants have identical genes and so are identical plants
Genetic variation
Sexual reproduction gives rise to variation because, when gametes fuse, one of each pair of alleles comes from each
In human body cells, one of the 23 pairs of chromosomes carries the genes that determine sex…read more

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Cystic fibrosis- a disorder of cell membranes- 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 and can therefore be passed on by parents, neither of whom has the disorder.
Cystic fibrosis is a genetic disorder disease passed down through families.…read more


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