🧬Biology Unit 10 Genetics

• Mendelian Inheritance is the study of inheritance of characteristics at the whole organism level

  
• Gregor Mendel


  Austrian monk who investigated inheritance in pea plants and published his results in 1866
• Mendel's experiments


  • Investigated simple qualitative characteristics, varied one trait at a time, used an organism whose sexual reproduction he could easily control, repeated crosses hundreds of times and applied statistical tests, studied two generations of peas at a time
• Mendel's conclusions: no mixed colours, a characteristic can disappear for a generation but then reappear, the outward appearance (phenotype) is not necessarily the same as the inherited factors (genotype), one form of a characteristic can mask the other (dominant and recessive), the F2 ratio is always close to 3:1
• Genes


  Mendel's factors, found on chromosomes
• Alleles


  Alternative forms of a gene, found on homologous pairs of chromosomes

  
• Homozygous


  Having the same alleles at a locus
• Heterozygous


  Having different alleles at a locus
• Dominant allele


  Allele that is expressed in the heterozygous state
• Recessive allele


  Allele that is only expressed in the homozygous state
• Monohybrid cross


  Breeding experiment involving just a single characteristic
• Mendel's First Law (principle of segregation): individuals carry two discrete hereditary factors (alleles) controlling each characteristic, and the two alleles segregate during meiosis so each gamete carries only one of the two alleles

  
• Monohybrid test cross


  Cross with a pure-breeding homozygous recessive to determine if a parent is homozygous dominant or heterozygous

  
• Pedigree diagram


  Shows the inheritance of a particular characteristic through a family
• Genes code for proteins which actually control phenotype
• Dominant allele


  Codes for a functional protein
• Recessive allele


  Codes for a non-functional protein
• The dominant allele is usually expressed over the recessive allele in a heterozygous cell because some functional protein will be made
• Sex determination


  Females have XX sex chromosomes, males have XY sex chromosomes, resulting in a 1:1 ratio of males to females

  
• Heterozygous


  Some functional protein will be made, and this is usually enough to have the desired effect
• Enzyme reactions are not usually limited by the amount of enzyme, so a smaller amount in heterozygotes will have little effect on phenotype
• Genotypes and phenotypes


  • homozygous dominant (RR)
  • homozygous recessive (rr)
  • heterozygous (Rr)

  
• Heterosomes


  Non-homologous sex chromosomes (X and Y)

  
• Autosomes


  The other 22 pairs of chromosomes
• Sex determination in humans

  1. All embryos start developing as females

  2. If the sex-determining "SRY" gene on the Y chromosome is expressed, male hormones are produced causing male development

  3. In the absence of male hormones, the embryo continues to develop as a female

  
• X inactivation


  One of the two X chromosomes in female cells is completely inactivated and condensed into a Barr body

  
• X chromosome


  • Contains over a thousand genes that have nothing to do with sex, coding for important products
  • Females have two copies of each gene on the X chromosome (diploid)
  • Males have one copy of each gene on the X chromosome (haploid)

  
• Males always inherit their X chromosome from their mothers, and always pass on their X chromosome to their daughters
• Sex-linked characteristics


  Characteristics where the inheritance is different for males and females due to the X chromosome

  
• Codominance


  Neither allele is completely dominant over the other, so the heterozygous genotype has its own phenotype
• Genotypes and phenotypes for flower colour in snapdragons

  • homozygous (CRCR)
  • Homozygous (CWCW)
  • heterozygous (CRCW)

  
• Sickle cell anaemia


  Caused by a mutation in the haemoglobin gene, where the abnormal haemoglobin molecules link together distorting the red blood cells
• Sickle cell phenotypes


  • HbAHbA (Normal)
  • HbSHbS (Sickle cell anaemia)
  • HbAHbS (Sickle cell trait)
• Lethal alleles


  Alleles that are fatal early in development, so the embryo doesn't develop and is absorbed by the mother
• Genotypes and phenotypes for Manx cats

  • SNSN (Normal)
  • SNSA (Manx Cat)
  • SASA (Lethal)

  
• Multiple alleles


  More than two alleles of a gene can exist in a population
• Blood group alleles and phenotypes

  • IA (A)
  • IB (B)
  • Io (O)
• Mendel's dihybrid cross


  1. Crossing two traits (seed shape and seed colour)

  2. Produces 4 phenotypes in a 9:3:3:1 ratio
• Mendel's Second Law (independent assortment)

  Alleles of different genes are inherited independently

  
• Dihybrid test cross


  Crossing the original genotype with a double recessive to distinguish the 4 possible genotypes