4.3 Inheritance

• Gene


  A sequence of nucleotides on DNA that codes for a particular protein
• Locus


  The site on a chromosome occupied by a gene
• Allele


  Different forms of the same gene
• Dominant


  A gene always expressed when present
• Recessive


  A gene only expressed in a homozygous pair
• Codominant


  Both alleles contribute to the phenotype
• Phenotype


  The characteristics of an organism
• Genotype


  The alleles contained in an organism
• Homozygous


  Alleles are the same e.g. HH or hh
• Heterozygous


  Alleles are different e.g. Hh
• F1


  First generation in a genetic cross that starts with a pair of homozygotes
• F2


  The second generation produced by crossing the F1 generation
• Autosomes


  A chromosome that is not a sex chromosome. In humans these are numbered 1-22
• Sex chromosomes


  Sex chromosomes, usually X and Y, in humans this is pair 23

  
• Codominance


  Both alleles are expressed in the phenotype
• Codominance - Blood typing

  • IAIA - A antigen on blood cells
  • IBIB - B antigen on blood cells
  • IAIB - Both A and B antigens on blood cells
  • ii - Neither A or B antigens on blood cells
• Incomplete dominance


  A blend of both alleles can be seen in the phenotype
• Incomplete dominance - Carnations

  • A cross between red and white carnations produces an all pink F1
  • In the F2 - a cross of 2 pink parents there is a phenotypic ratio of red, white and pink
• Mendelian inheritance


  Gregor Mendel studied inheritance in pea plants, choosing easy to distinguish characteristics controlled by single genes on different chromosomes
• Monohybrid crosses


  Single gene inheritance
• Mendel's 1st law: the law of segregation


  The characteristics of an organism are determined by factors (genes) which occur in pairs. Only one member of a pair of factors (genes) can be represented in a single gamete
• Monohybrid crosses - Colouring in cows

  • Some cows are Red (RR), some white (WW), offspring can be roan (RW)
• Monohybrid cross


  1. Identify the characteristic and designate a letter to represent the dominant and recessive allele

  2. Give the genotype and phenotype of the parents

  3. Identify the gametes produced by both parents

  4. Place the gametes in a Punnet square and show all 4 possible crosses at fertilisation. Label each new genotype with the phenotype
• The F2 shows the mendelian phenotype ratio 3:1
• All the offspring in the F1 are tall
• Dihybrid inheritance


  2 gene inheritance
• Dihybrid cross phenotypic ratio

  9 round yellow: 3 Round green:3 wrinkled yellow: 1 wrinkled green
• Independent assortment


  The independent assortment of chromosomes in meiosis explains why unlinked genes (found on different chromosomes) can combine to form all 4 kinds of gametes
• Dihybrid cross


  1. Parental phenotypes and genotypes

  2. F1 generation gametes and phenotype

  3. F2 parental genotype and gametes

  4. F2 phenotypic ratios
• If you take each characteristic individually in the dihybrid cross, the monohybrid phenotypic ratio still stands
• Mendel's second law of segregation

  The law of independent assortment
• Test crosses


  • Monohybrid - To determine if an organism showing a dominant phenotype is a homozygote or heterozygote
  • Dihybrid - cross each genotype with homozygote for both characteristics
• The mendelian 9:3:3:1 ratio is expected in any experiment where genes are not linked
• Linkage


  Genes occurring on the same chromosome and therefore being inherited together
• Chi squared


  A statistical test used to determine if the numbers and phenotypes of offspring produced in a genetic cross is close enough to the expected Mendelian ratio that any difference is due to chance and not for any other reason
• Chi squared test


  1. Null hypothesis

  2. Calculate expected numbers

  3. Calculate χ2

  4. Calculate the degrees of freedom

  5. Accept or reject the null hypothesis
• Mutation


  A spontaneous, random change in a gene
• Mutation


  • Mutation rates are increased in organisms with short life cycles or frequent cell division
  • Occurs mostly during crossing over in prophase 1 and non-disjunction in anaphase I and II
• Advantageous mutations


  Mutations affect protein synthesis and so change the phenotype of the organism, leading to variation in species that causes evolution by natural selection
• Disadvantageous mutations


  Some genes called proto-oncogenes can mutate to become oncogenes, which are involved, causing uncontrolled cell division to form a cancer