Chapter 15 Genetics and traits

• in any population 


  easy to see that individuals differ from one another
• when you look at individuals in same family


  it is obvious that there is a reason for similarities between them

  understanding variations between related individuals
• characteristics of domestic plants and animals 


  were passed down from parent to offspring

  exactly how these characteristics, or traits, were transmitted was not clear
• first clear explanation of patterns of inheritance

  was provided by Austrian monk, Gregor Mendel, in 1865

  spending 2 years at University of Vienna, he returned to his monastery as a schoolteacher

  able to link nature and math, through careful study of reproductive behaviour of pea plants
• after 10 years of research 


  Mendel put forward two principles relating to inheritance
• Mendels principles (p.t. 1)


  various hereditary characteristics were controlled by factors (genes) that occurred in pairs
• Mendels principles (p.t. 2)


  during formation of gametes, pairs of factors seperate

  each gamete receives only one set of factors, or genes, with the other set going to another gamete

  gametes unite at fertilisation, allowing different combinations of genes to come together
• Mendels findings went

  unnoticed for 35 years before their significance was fully appreciated

  at same time as his work was being rediscovered, scientists were making considerable advances in cytology, the study of cells
• Walter Sutton, American graduate student 


  was able to link work of Mendel to that of the cytologists

  his observations of the behaviour of chromosomes during meiosis, and Mendel's speculation on the separation of the hereditary factors during the formation of gametes, lead Sutton to suggest that the hereditary factors (genes) were located in the chromosomes

  important hypothesis led to chromosome theory of hereditary
• Mendel conducted

  breeding experiments with edible garden pea, Pisa sativum, and was impressed by the fact that it possessed a number of characteristics, or traits, that were expressed in contrasting forms

  he studied seven pairs contrasting characteristics in which the alternative were easily identifiable
• before beginning experiment

  mendel made sure his plants were pure-breeding for characteristic he wished to study

  pure breeding plants are those that produce the same characteristic in each succeeding generation when bred among themselves

  e.g. plants pure-breeding for yellow seeds were crossed with plants pure-breeding for green seeds
• he found that the

  offspring (progeny) resembled only one of the parents

  these offspring were referred to as hybrids because they have genetic information for green seed as well as genetic info for yellow seed, even though they are all yellow

  therefore, only one of the pair of contrasting characteristics appeared in offspring
• Mendel referred 


  dominant trait as characteristic shown by hybrid as it masked the appearance of other characteristic, which is recessive trait
• hybrid plants self-pollinate 


  second generation of plants was produced

  characteristics appeared in ratio of 3:1 with dominant being three and recessive being one

  Mendel concluded that genes were unchanged as they passed from one generation to the next

  each pea plant has two hereditary factors for each characteristic
• principle of segregation


  during formation of gametes, these factors are separated with each gamete receiving only one factor (gene) for each trait
• as offspring are formed by 


  union of male and female gamete, each offspring receives one gene for each characteristic from each parents
• cross 


  mating of two organism
• monohybrid cross


  only one pair of contrasting characteristics is studied e.g. yellow and green pod colour in peas
• genes for particular characteristic

  represented by two letters, one for the gene that originated from the female parent and one for the gene that originated from the male parent

  if gene is dominant it is shown as capital letter; if it is recessive one, a lower case is used
• genotypes of peas

  pure breeding of green pod is GG, pure breeding of yellow pod is gg

  hybrids, with one of each gene type, have Gg

  alternative forms of gene for pod colour- G and g- are alleles
• genotype 


  combination of alleles for a particular trait
• pair of contrasting characteristics 


  two alleles may occur in one of three possible combinations: GG, gg, Gg
• homozygous 


  two of the three genotypes the alleles are the same (GG, gg)
• heterozygous 


  hybrid with one of each allele (Gg)
• three genotypes produce 


  only two types of pod colour

  GG and Gg for green, yellow for gg
• phenotype 


  physical appearance
• during formation of gametes 


  pairs of chromosomes seperate, with one of each pair going to each gamete

  pairs of alleles segregate with only one allele for a characteristic carried by each gamete
• offspring are referred to 


  first filial generation, denoted by symbol F1

  F1 is self-pollinated, a second set of offspring is produced, this is the second filial generation
• in genetics

  Punnett squares model an individual cross and pedigree chart shows the appearance of a certain trait in a family
• Punnett squares 


  model a cross to calculate the probability of genotypes and phenotypes of offspring

  named after R.C. Punnett, British geneticist of early 20th century, who devised a square to use in his work on heredity
• each pea plant produces many seeds

  so many potential offspring, appropriate to talk about proportion of each genotype or phenotype that should occur in offspring a cross

  more accurate to refer to probability of an offspring having a certain genotype or phenotype
• more accurate to refer to

  probability of an offspring having a certain genotype or phenotype

  means that the chance that any one seed will grow into long-stemmed plant is 3/4 while probability that seed will produce plant that is homozygous and long-stemmed is 1/4
• principles of Mendelian inheritance and Punnett squares


  can also be applied to human traits

  while some characteristics are influenced by more than one gene, there are many examples that are appropriate e.g. long eyelashes dominant over short eyelashes
• Punnett squares effective way to


  model single possible cross

  not useful to look at phenotypes that have been produced in a family
• pedigree (family tree)


  used to look at phenotypes that have been produced in family
• most frequent reasons for constructing pedigree


  to investigate the pattern of inheritance of a genetic disorder
• single-gene disorders


  disorders caused by inheritance of single defective gene

  pattern of inheritance follows basic laws of heredity

  severity of disorder is often variable and difficult to predict

  more than 4000 different disorders of this type has been identified in humans
• humans cells have 


  46 chromosomes except gametes

  22 pairs are autosomal chromosome and 1 pair are sex chromosomes
• autosomal inheritance relates to 


  traits, including disorders, that are inherited on autosomal chromosomes
• traits controlled by

  dominant alleles are easily passed on, as only one allele is needed for trait to be shown

  dominant alleles that cause severe defects in people are rarely passed on, because people with such alleles frequently die before opportunity to reproduce