D3.2 Inheritance

• Diploid


  The condition when a cell has two versions of every chromosome, the two versions of each chromosome are homologous. In humans, 2n=46.
• Haploid


  The condition when a cell has only one version of each chromosome. These are normally gametes (sex cells) such as sperm and eggs. In humans, n=23.
• Zygote


  The first cell formed after fertilisation. In animals, this formed when the sperm and ovum fuse.

  This first cell is totipotent.
• Gene


  A length of DNA that codes for a chain of amino acids (a protein). This protein determines some characteristic or trait of the organism – for example, hair colour, eye colour, behaviours.
• Allele


  An alternative version of the same gene. These may differ by one or a few bases and occupy the same locus on the same chromosome.
• Locus


  The specific location of a gene on a chromosome
• Recessive

  Alleles that are only expressed in the phenotype if two copies are present. The allele is always represented using a lower case letter. For example, t, b or h.
• Dominant


  Alleles that are always expressed in the phenotype even if only one copy is present. The allele is always represented using a capital letter. For example, T, B or H.
• Genotype


  Describes the alleles in a cell for a certain feature. For example, TT, Bb or hh.
• Phenotype


  The visual characteristic that is a result of the genotype. For example, tall plant or blue eyes.
• Homozygous


  An organism with two copies of the same allele. For example, TT, BB or hh.
• Hetrozygous


  An organism with two different copies of the same allele. For example, Tt, Bb or Hh.
• P generation


  Abbreviation for ‘parent generation’.
• F1


  Abbreviation for ‘first filial generation’. The offspring of the P generation.
• F2


  Abbreviation for ‘second filial generation’. The offspring of the F1 generation.
• Phenotypic plasticity


  The ability of individual genotypes to produce different phenotypes when exposed to different environmental conditions.

  For example, domesticated dogs are descended from the gray wolf (Canis lupus) and show broad _________ _________.
• Phenylketonuria (PKU)


  _________ is an inherited autosomal recessive disorder. It is caused by mutations in the PAH gene, which can result in non-functional phenylalanine hydroxylase.

  This means phenylalanine is not converted to tyrosine.
• Single-nucleotide polymorphisms


  This is the substitution of a single nucleotide at a specific position in the genome.
• Codominance


  In __________, heterozygotes have a dual phenotype.

  For example, the AB blood type (IAIB).
• Incomplete dominance


  In _________ __________ heterozygotes have an intermediate phenotype.

  For example, the pink colouration marvel of Peru (Mirabilis jalapa) from red and white parent flowers.
• X-linked


  A gene that is carried on the X-chromosome.

  Recessive traits are therefore more common in males than females.
• Autosomes


  Chromosomes that do not determine sex (i.e. normal chromosomes).

  These are the other 22 chromosomes in humans.
• Sex chromosomes


  The chromosomes that carry the information that determines the sex of the individual.

  Human females have two X chromosomes (XX) and males have an X and a Y chromosome (XY).
• Haemophilia


  Sex-linked disease – just like colour blindness.

  The blood of the sufferers is unable to clot properly, leading to exsanguination (death via blood loss after comparatively minor injuries).
• Continuous variation


  When the value of a trait can take any of an infinite number of values within a certain range.

  For example, height or mass.
• Polygenic characteristics


  Characteristics affected by multiple genes. For example, these are factors such as height, intelligence and behaviour.
• Box and whisker plot

  A) lower quartile

  B) upper quartile

  C) max

  D) min

  E) median

  
• Law of segregation


  Mendel's first law.

  This states that one unit or allele for each trait is inherited from each parent to give a total of two alleles for each trait.

  The separation of alleles in each pair takes place when the gametes are formed and some alleles code for phenotypes that are dominant over others.
• Law of independent assortment


  Mendel's second law.

  This states that different traits are inherited independently of each other. This means that the inheritance of alleles for one phenotype has nothing to do with the inheritance of alleles for another characteristic.
• Unlinked genes


  Genes that segregate independently as a result of meiosis.

  For example, skin colour and attached ear lobes. There is no linkage between these two genotypes.
• Dihybrid crossing


  A cross using two genes and each has two alleles.

  If the two cross-bred organisms are heterozygous at both alleles then the ratio would be 9:3:3:1.
• Linked genes


  Genes located on the same chromosome, which are therefore commonly inherited together.

  For example, skin colour and eye colour.
• Chiasma (plural chiasmata)


  The point where two sister chromatids cross over, forming a cross-shape.
• Crossing over


  Occurs during prophase I.

  Non-sister chromatids of an homologous pair, exchange sections. This mixes up genes from the mother with those of the father on one chromosome
• Recombinant


  Gametes are produced that are different from either of the original parents. A new combination of alleles.
• Non-mendelian ratio


  Phenotypic ratio that does not fit the expected pattern of independent assortment.
• Drosophila spp.


  Fruit fly. A common organism used in the study of genetics.
• Null hypothesis


  A hypothesis that suggests there is no significant difference between the 2 sets of data, or the 2 means.
• Chi squared


  This statistical test analyses the association between two pieces of categorical data.

  For example, this can analyse if there is an association between the distribution of two species or if two genes are located on the same chromosome (linked) and therefore commonly inherited together.
• Critical value


  A _________ ________ is a value compared to the test statistic to determine whether to reject the null hypothesis. Commonly in science, the value at 5% is used.

  This means there is a 5% chance that your result occurred by fluke and a 95% that there is a biological mechanism involved.