Inheritance

• a gamete is an organism's reproductive cell with half the number of chromosomes as a normal cell (23)

  • females = egg
  • males = sperm
• a genome is the entire DNA of an organism
• a chromosome is a structure found in the nucleus which is made up of a long strand of DNA
• a gene is a short section of DNA that codes for a protein & therefore contributes to a characteristic

  • they control the production of proteins in a cell
• an allele is the different forms of a gene

  • humans have two alleles for each gene as they inherit one from each parent
• homozygous is when both inherited alleles are the same

  • eg two dominant or two recessive
• heterozygous is when the inherited alleles are different

  • one is dominant and one is recessive
• dominant alleles = only one is needed for it to be expressed
• recessive allele = two copies are needed for it to be expressed (a dominant allele must not be present)
• a genotype is the combination of alleles an individual has, eg Aa
• a phenotype is the physical characteristics a person has due to their genes, eg eye colour
• phenotypic features are the result of polygenic (multiple genes) inheritance rather than single genes
• offspring from sexual reproduction vary genetically as:

  • fertilisation is random
  • every gamete is genetically unique & so the resulting zygote has a unique combination of genes
• identical twins are formed from the same zygote

  • they are sometimes called monozygotic twins
  • the zygote divides by mitosis but the two genetically identical cells formed do not stay together
• the environment also affects genetic variation, eg:

  • weight & height are affected by genes and environment
  • tan is affected by environment
  • piercings, scars, language, accent
• uppercase letters are used to represent dominant alleles & lowercase letters are used to represent recessive alleles
• gregor mendel was a monk who carried out hundreds of breeding experiments with pea plants to discover the laws governing inheritance
• gregor mendel found that:

  • the gene is passed from one generation to the next
  • the gene can have alternative forms (alleles)
  • each individual must have two alternative forms per feature
  • gametes only have one alternative form per feature
  • one allele can be dominant over the other
• at the time, mendel's discoveries were not taken seriously as:

  • he was a monk, not a scientist
  • people did not know much about dna or inheritance at the time
• a punnett square diagram is used to look at the probability of offspring of two parents having certain genotypes & phenotypes, by crossing the parents genes

  
• the numbers predicted in punnet square diagrams are not always accurate as:

  • the ratios are simply predicted & the real numbers are unlikely to be exact
  • unpredictable or chance events can affect the offspring's survival
• many characteristics are actually controlled by two or more genes working together

  • this is called polygenic inheritance
• characteristics determined by polygenic inheritance

  • weight, height, skin colour
  • darker skin contains more melanin = the amount of melanin a person has is determined by several genes
• sex is determined by X and Y chromosomes = sex determining genes

  • these are contained in the 23rd pair of chromosomes
• sex is determined by the presence or absence of Y chromosomes

  • female = XX
  • male = XY
• a family pedigree is a way of showing a family history of a genetic condition

  
• Mitosis occurs during asexual reproduction as it occurs when one cell divides to form two identical daughter cells
• Mitosis:

  • stage 1 (interphase) = cell grows as organelles grow & increase in number; synthesis of protein occurs; all 46 chromosomes replicated (form X shape); energy stores increase
  • stage 2 = chromosomes line up at equator of cell; spindle fibres pull each chromosome to either side of the cell
  • stage 3 = two identical daughter cells form when the cytoplasm & cell membranes divide, each containing same 46 chromosomes as original cell
• cells in the reproductive organs divide by meiosis to form gametes

  • gametes only have one copy of each chromosome
• meiosis:

  • cell makes copies of chromosomes = so has double the amount of genetic information
  • cell divides into two cells each with half amount of chromosomes = to give full amount of 46 chromosomes (diploid)
  • each cell divides into two again to produce 4 cells, each with 23 chromosomes = as have half normal amount of chromosomes, they are haploid
• Speciation is when one population of a species that has formed due to a mutation becomes so different they can no longer interbreed to produce fertile offspring

  • this means they have become a new species
• • mitosis produces 2 daughter cells whereas meiosis produces 4 daughter cells
  • mitosis produces diploid cells whereas meiosis produces haploid cells