Inheritance

• The spread is smaller for larger samples, → standard deviation ↓ as sample size ↑. The larger the natural variation, the larger the sample & depends on the type of statistical test used.
• The ABO blood type classification system uses the presence/ absence of certain antigen on red blood cells to categorise blood into 4 types. Agglutinogens (a type of antigen) are attached to the surface of red blood cells. There are 2 diff types of agglutinogens, type "A" & "B”. Antibodies (immunoglobulins) are specific to antigens. 
• The immune system recognises 'foreign' antigens & produces antibodies in response - so if you are given the wrong blood type you will die from blood clot. Blood type O is known as the universal donor, as it has no antigens for rxn. Type AB is the universal recipient, as the blood has no antibodies which will react to AB antigens. 
• Antigens: create a defence against future invaders.

  Antibodies: created to identify, attack, & destroy the same type of antigens if they enter the body again.
• A single gene, the ABO gene, controls the ABO blood type. This gene has 3 diff alleles: A, B, O
• Dominant alleles have the same effect on the phenotype whether it is present in the homozygous/ heterozygous state. Recessive alleles only have an effect on the phenotype when present in the homozygous state. Codominant alleles are pairs of diff alleles that both affect the phenotype when present in a heterozygote. 
• Sickle Cell anaemia:

  Eg of codominance. Rmb notation used: superscripts represent codominant alleles. In codominance, heterozygous individuals have a mixed phenotype. The mixed phenotype gives protection against malaria but does not exhibit full-blown sickle cell anaemia.
• Cystic Fibrosis (CF):

  Pedigree charts can be used to trace family histories & deduce genotypes and risk of inherited gene-related disorders. Here is a pedigree chart for this family history. A mutation in the CFTR gene causes secretions (e.g. mucus, sweat and digestive juices) to become thick instead of the normal thin. Instead of acting as a lubricant, the secretions block tubes, ducts & passageways, esp in lungs & pancreas. Despite therapeutic care, lung problems from CF → early death (life expectancy: 35 - 50 yrs). 
• Huntington’s disease:

  A brain disorder that affects a person's ability to think, talk, & move. HD is caused by a mutation in a gene on chromosome 4. One copy of mutation → disease, meaning that this is a dominant disease. 
• A mutation is a Δ in an organism's genetic code. A gene mutation is a Δ in nucleotide seq of a section of DNA coding for a particular feature. Alleles of a gene are similar but have variations in the base sequence. New alleles are created by gene mutation. Mutations can be classed as beneficial, neutral (due to the degenerate nature of DNA)/ harmful. Most mutations are neutral/ harmful. Mutations that occur in the body (somatic cells) remain within the organism. Mutations that occur in gametes can be inherited by offspring: this is how genetic diseases arise. 
• Humans have 22 pairs of chromosomes which are autosomes & homologous pairs, remaining pair is the sex chromosome. XX = female gender, XY = male. X chromosome is much larger than Y. X carries many genes in the non-homologous region which are not present in Y. The presence & expression of the SRY gene on Y → male development. Chromosome pairs segregate in meiosis. Females (XX) produce eggs containing the X chromosome. Males (XY) produce sperm which contain either X or Y chromosomes.
• Sex-linked traits are carried on the X-chromosome in the non-homologous region. Genes located on the Y chromosome are always expressed in males cus there is no other paired chromosome to potentially block/ mask their expression. This means that any allele, whether it be dominant/ recessive, will be expressed in males if it is present on the Y chromosome. ∴ sex-linked genetic disorders are more common in males. 
• Haemophilia:

  Blood clotting is an eg of a metabolic pathway –  a series of enzyme-controlled biochemical reactions. It requires globular proteins aka clotting factors. A recessive X-linked mutation in haemophiliacs → one of these factors not being produced. ∴ the clotting response to injury does not work & patient can bleed to death. 
• Random Orientation vs Independent Assortment:

  Random Orientation refers to the behaviour of homologous pairs of chromosomes (metaphase I)/ pairs of sister chromatids (metaphase II) in meiosis. Independent assortment refers to the behaviour of alleles of unlinked genes as a result of gamete prod (meiosis). Due to random orientation of the chromosomes in metaphase I, the alleles of these unlinked genes have become independently assorted into the gametes. This only holds true for unlinked genes (genes on different chromosomes).
• Autosomal Gene Linkage:


  Linked genes are pairs/ groups of genes inherited together, & carried on the same chromosome.
• Recombination of alleles occurs as a result of crossing-over between non-sister chromatids. Exchange of alleles gives new genotypes of gametes. Plants which are heterozygous at both loci are test-crossed. The further apart a pair of alleles are on a chromosome, the more likely that crossing over may occur btw them → recombination. Knowing this, researchers can map pos of genes on a chromosome based on the frequency of recombination btw gene pairs: the further apart they are, the more often they cross over.  
• Polygenic Inheritance:

  Polygenic inheritance gives rise to continuous variation in the phenotype. A single characteristic controlled by multiple genes.  For eg: human skin colour & wheat kernel colour. Other eg: Susceptibility to heart disease, certain types of cancer, mental illnesses, Autism Spectrum (not as clearly polygenic as other eg but it’s suspected that gene interactions & envi factors play a large role). 
• More abt skin:

  Skin colour is thought to be controlled up to 4  separate genes, each with their own alleles.