patterns of inheritance

• what is genetic variation

  is caused by either mutations or sexual reproduction
• what is environmental variation

  is caused by changes by the environment, some environmental factors prevent gene expression
• examples of genetic variation:

  • skin colour
  • eye colour
  • hair colour
• examples of environmental variation:

  • etiolation: this is when plants grow abnormally long and spindly because they are not getting enough light
  • chlorosis: don't produce enough chlorophyll and turn yellow caused by several environmental factors eg lack of Mg in the soil
• meiosis makes gametes with a unique assortment of alleles through crossing over and the independent assortment of chromosomes and random fusion of gametes during fertilisation also increasing genetic variation
• examples of genetic and environmental factors:

  • body mass in animals - partly genetic and is strongly influenced by environmental factors like diet
  • does not contain enough nutrients, body mass is likely to be lower than what is determined by your gene
• certain physical, biological and chemical agents can increase the rate of mutations = mutagens
• examples of physical mutagens

  • x-ray
  • gamma rays
  • UV light
• examples of chemical mutagens

  • nitrous gas
  • aromatic amines
  • colchicine
  • benzopyrene
• examples of biological agents

  • some viruses
  • transposons
  • food contaminants
• what are the 7 types of chromosome mutations that may occur during meiosis:

  • translocation
  • inversion
  • deletion
  • polyploidy
  • aneuploidy
  • duplication
  • non-disjunction
• translocation

  • a section of the chromosome is broken off and attached to another chromosome
• inversion

  • a section of the chromosome is broken off, turned 180 degrees and joined again
• deletion

  • part of the chromosomes is lost
• polyploidy

  • diploid gamete is fertilised by a haploid gamete
• aneuploidy

  • the chromosome number is not an exact multiple of the haploid number
• duplication

  • a chromosome can be duplicated
• non-disjunction

  • one pair of chromosome fails to separate leaving a gamete with an extra chromosome eg down syndrome
• what is an allele

  • a different version of a gene
• what is a locus?

  • the fixed position of a gene on a chromosome
• co-dominance


  where both alleles presents in the genotype o heterozygous individual contribute to the individuals phenotype
• what are sex chromosomes

  • 1 of the 23 pairs of chromosome is a sex chromosomes, the other 22 is called autosomes
• what are examples of X linked disorder

  • haemophilia
  • colour blindness
• what are males more likely to show recessive phenotypes for genes that are sex linked

  • males only have 1 copy of the X chromosomes, the recessive allele will more likely to be expressed as the Y chromosome is too small to carry allele
• what does it mean if autosomal genes are linked?

  • autosomal genes are genes located on same autosome 
  • linked = they are on the same autosome and stay together during independent assortment and alleles passed onto offspring together 
  • they won't stay together if crossing over occurs and splits them up however if two genes on the same autosome are close together they are more likely to be linked, crossing over is less likely to split them up
• what is the ratio for codominance heterozygous


  1:2:1
• what is the ratio for dihybrid inheritance

  9:3:3:1
• what is the expected ratio if genes were not linked

  9:3:3:1
• if they are linked the ratio would

  phenotypic ratio: 3:1

  genotypic ratio 1:2:1
• what do large numbers in results for autosomal linkage mean

  large numbers of the original parental types show that there must be linkage
• what do smaller numbers in results for autosomal linkage mean

  • there are some new combinations of phenotypes that were not in the original parents = recombinants due to a process called crossing over
• what does low percentage of recombinants mean

  the loci of the 2 genes are extremely close on the chromosome, crossing over is less likely to occur
• what does high percentage of recombinants mean

  the loci of the 2 genes are not that close on the chromosome, crossing over more likely to occur
• epistasis


  interaction of genes on different chromosomes where 1 masks the expression of the other
• hypostatic


  description of a gene that expression id prevented due to another gene
• a recessive allele is being expressed
• monogenic


  determined by a single gene
• polygenic


  determined by many different genes
• discontinuous 


  discontinuous variations describe characteristics that can be one of a limited number of options

  • eg inherited
• continuous


  describes characteristics that can change that can change over a range of values