17 Genetics I

• Genetics & Inheritance - Genetic Basis for Variation:

  • Genotypes determine phenotypes
  • Phenotypic ratios provide clues on the possible type(s) of inheritance
  • Understanding genetics & inheritance requires a combination of conceptual understanding and mathematical logic
• Organisms inherit genetic information from their parents
• Gregor Mendel was the first to describe how traits are passed on from one generation to the next through pea plant breeding
• Mendel's principles of inheritance describe the transmission of genetic traits and are the basis of understanding inheritance
• Statistical tests like the chi-squared test are used to test the significance of differences between observed and expected results of genetic crosses
• Common terms in genetics:

  • Locus, allele, dominant, recessive, codominant, incomplete dominance, homozygous, heterozygous, phenotype, genotype, linkage
• Genes are inherited from one generation to the next via germ cells or gametes
• Genotype is linked to phenotype
• Genetic diagrams are used to solve problems in dihybrid crosses, including codominance, incomplete dominance, multiple alleles, sex linkage, autosomal linkage, and epistasis
• Understanding linkage and crossing-over effects on phenotypic ratios from dihybrid crosses
• Environmental factors can affect phenotype, like diet affecting honey bee differentiation and temperature affecting fur color of Himalayan rabbits
• Genotype of F1 generation from a cross between a homozygous dominant and homozygous recessive organism is heterozygous
• F2 generation consists of offspring produced by crossing two F1 organisms
• Wild type refers to the typical genotype found in a natural population of a species
• Codominance occurs when two different dominant alleles of the same gene are both expressed and influence the phenotype, resulting in more than two possible phenotypes
• Incomplete dominance is when the phenotype of heterozygotes is intermediate between the phenotypes of individuals homozygous for either allele
• Linkage is when two or more genes on the same chromosome do not assort independently in meiosis and are inherited together
• Labelling conventions for alleles:

  • Dominant alleles are usually represented with uppercase letters and recessive alleles with lowercase letters
  • When there are multiple alleles of the same gene, the gene is allocated an uppercase letter and each allele an appropriate superscript
  • For sex-linked genes, the sex chromosomes X and Y should always be indicated, and dominant or recessive alleles of particular genes should be indicated by appropriate superscript of uppercase and lowercase letters
• From genotype to phenotype:

  • Genotype is the genetic makeup determining observed traits or phenotype
  • Phenotype is the expression of genetic information resulting in observable traits
  • Environmental factors can influence phenotypic variation
• Environmental factors affecting phenotypic variation:

  • Temperature
  • Food & Nutrition
  • UV light
  • Photoperiod
• Monohybrid crosses study the inheritance of one characteristic controlled by a single gene, involving crosses between pure-breeding plants with different traits
• Monohybrid crosses track the inheritance of a single characteristic, such as flower color in Mendel's experiments
• Mendel's famous 1964 study on attachment:

  • 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: the babies of parents/carers who had ‘sensitive responsiveness’ - (who were more sensitive to the baby’s signals) - were more likely to have formed an attachment
• Attachment is a strong reciprocal emotional bond between an infant and a primary caregiver
• Freud's superego is the moral component of the psyche, representing internalized societal values and standards
• In Mendel's flower-color experiment, the F1 generation produced only purple flowers, with no white flowers, while the F2 generation showed a ratio of 3 purple-flowered plants to 1 white-flowered plant, known as the monohybrid ratio
• Mendel studied 6 other pairs of contrasting characteristics, obtaining a ratio of approximately 3:1 for the dominant to recessive characteristic in the F2 generation for each of these pairs
• Mendel's conclusions included the idea that in each organism, there is a pair of “factors” controlling a characteristic, with the separation of these factors during meiosis known as Mendel's first law - Law of Segregation
• Genes are now known as the factors determining characteristics, with alleles representing distinct types of genes, e.g., one allele, P, for purple and another allele, p’, for white
• A diploid plant may have 2 of the same or 2 different alleles, with homozygous organisms having a pair of identical alleles and heterozygous organisms having two different alleles
• In a heterozygote, one allele may mask the presence of another, with the dominant allele being expressed in the phenotype while the recessive allele is masked
• Mendel's Law of Segregation states that alleles separate during meiosis to form gametes, distinct from the Law of Independent Assortment
• A test cross can reveal the genotype of an organism with the dominant trait by crossing it with an individual expressing the homozygous recessive trait
• Dihybrid crosses study the inheritance of two different characteristics controlled by two different genes, with Mendel obtaining a 9:3:3:1 phenotypic ratio in his experiments
• Multiple alleles: a condition where a single characteristic appears in several different forms as it is controlled by three or more alleles, of which any two may occupy the same locus on homologous chromosomes of a diploid organism
• Example of multiple alleles: ABO blood group in humans, controlled by an autosomal gene with 3 alleles: IA, IB, and IO, where IA and IB are dominant to IO and IA and IB are codominant
• In codominance, both alleles of a gene are equally expressed in the phenotype of a heterozygote, as seen in the blood group system in humans where IA and IB are codominant, resulting in blood groups A, B, AB, and O
• Incomplete dominance results in an intermediate expression of a trait in heterozygous individuals, seen in flowers like primroses, snapdragons, and four-o'clocks where red or white flowers are homozygous while pink ones are heterozygous
• Lethal alleles cause death of an organism when homozygous for that allele, disadvantaging organisms through impaired biochemical or physical functioning, as seen in chickens with a dominant lethal allele (C)
• A lethal allele causes death of an organism that is homozygous for that allele