17 Genetics II

• Mendel's principles of inheritance explain basic phenomena, but non-Mendelian inheritance like autosomal linkage and epistasis, and the role of the environment in determining phenotype, are also studied

  
• Statistical tests like the chi-squared test are used to test the significance of differences between observed and expected results of genetic crosses
• Genetic diagrams are used to solve problems in dihybrid crosses, including codominance, incomplete dominance, multiple alleles, sex linkage, autosomal linkage, and epistasis
• Linkage refers to genes located on the same chromosome, where alleles tend to be inherited together and do not undergo independent assortment
• Crossing over can result in the formation of recombinant gametes, disrupting the linkage group and separating alleles of linked genes
• The likelihood of crossing over between gene loci on the same chromosome increases with distance, affecting the inheritance of alleles and formation of recombinant gametes
• Thomas Hunt Morgan's fruit fly experiment discovered genetic linkage between body color and wing length, proposing that recombinant phenotypes were due to crossing over between homologous chromosomes
• T.H. Morgan's experiment on Drosophila aimed to understand genetic inheritance:

  • Heterozygous individuals in a test cross showed deviations from the expected Mendelian offspring phenotypic ratio, indicating autosomal linkage
  • Genes for body color and wing size were found to be linked on the same chromosome
  • Recombinant types (resulting from crossing over) were dependent on the proximity of the linked genes: closer genes had fewer recombinants, while distant genes had a higher percentage of recombinant offspring
• Recombination frequency is used to measure the distance between genes: 1% recombination frequency equals 1 map unit or 1 centimorgan (cM)

  • With a 17% recombination frequency, the distance between the genes for body color and wing type in Drosophila is 17 map units or 17 cM
• In epistasis, two or more genes determine a single phenotype, with one gene's expression masking another's

  • Epistatic alleles can be dominant or recessive, leading to modified 9:3:3:1 dihybrid F2 phenotypic ratios
  • Recessive epistasis masks phenotypic effects in the homozygous condition, while dominant epistasis masks effects in both homozygous and heterozygous conditions
• Example 1a: Flower colour in blue-eyed Mary (9:3:4 F2 phenotypic ratio):

  • F2 phenotypic ratio deviates from 9:3:3:1, indicating epistasis
  • Epistasis occurs when two or more different gene products are part of an enzymatic pathway
  • Allele A codes for enzyme A, converting precursor to magenta pigment; recessive allele a codes for a non-functional enzyme A
  • Allele B codes for enzyme B, converting magenta pigment to blue pigment; recessive allele b codes for a defective enzyme B
  • Genotype aa results in colorless flowers, while genotype bb results in magenta flowers
• Example 1b: Coat colour inheritance in Labrador retrievers (9:3:4 F2 phenotypic ratio):

  • Epistasis occurs between gene loci B/b and C/c'
  • Allele B produces black pigment, allele b produces brown pigment
  • Allele C allows pigment deposition, allele c' prevents deposition resulting in gold/yellow fur
  • Genotype c'c' masks gene locus B/b, genotype c'c' results in gold fur
• Example 2: Flower colour in sweet pea (9:7 F2 phenotypic ratio):

  • F2 phenotypic ratio deviates from 9:3:3:1, indicating epistasis
  • Allele C codes for enzyme C, converting precursor to intermediate; recessive allele c' codes for a non-functional enzyme C
  • Allele P codes for enzyme P, converting intermediate to purple pigment; recessive allele p' codes for a defective enzyme P
  • Genotype c'c' or p'p' results in white flowers due to lack of functional enzymes
• In a study on attachment by Schaffer and Emerson in 1964:

  • Aim: identify stages of attachment / find a pattern in the development of an attachment between infants and parents
  • Participants: 60 babies from Glasgow
  • Procedure: analysed interactions between infants and carers
  • Findings: babies of parents/carers with 'sensitive responsiveness' were more likely to have formed an attachment
• Freud's superego is the moral component of the psyche, representing internalized societal values and standards
• In genetics, the observed ratio of 9 purple flower plants : 7 white flower plants is explained by a genetic diagram showing meiosis, gametes, and the self-cross of the F1 generation
• In wheat kernel color inheritance:

  • F2 phenotypic ratio of 15:1 is explained by a genetic diagram showing the biochemical pathway and the concept of dominant epistasis
• In petal color inheritance in Primula plants:

  • F2 phenotypic ratio of 13:3 is explained by a genetic diagram showing the biochemical pathway and the concept of dominant epistasis
• In squash fruit color inheritance:

  • F2 phenotypic ratio of 12:3:1 is explained by a genetic diagram showing the biochemical pathway and the concept of dominant epistasis
• The chi-square test in genetics is used to compare observed results to expected results, determining if deviations are due to chance or other factors, leading to conclusions about the significance of the results
• In a chi-square test, one of the conclusions can be that the difference between observed and expected results is not significant, and any difference is due to chance alone (H0 is not rejected)
• Alternatively, the conclusion could be that the difference between observed and expected results is significant, and any difference is due to other factors and not chance alone (H0 is rejected)
• To carry out a chi-square test, you need to follow 5 steps:
• The chi-square formula used in the test is: χ2 = ∑((O - E)2 / E), where O = observed value, E = expected value
• Degrees of freedom (df) are calculated as df = n - 1, where n is the number of phenotype classes
• Discontinuous variation is characterized by distinct and discrete phenotypes, often caused by different alleles of a single gene or a few genes
• Continuous variation shows slight differences with phenotypic variations along a continuum, indicating polygenic inheritance and influenced by environmental factors