Chapter 20

• Etiolation

  • When plants are grown in dark there is a lot of phytochrome; almost all in form of Pr 
  • When Pr is exposed to sunlight it is converted to Pfr 
  • Has been shown that in presence of Pfr, seedlings grow normally, however in absence of Pfr, seedlings grow tall (etiolated) 
  • Somehow the Pr triggers the plant to share available materials in favour of growing tall - stopping production of chlorophyll (chlorosis)
• What is chlorosis?

  • when cells stop producing normal amounts of chlorophyll
  • this lack of chlorophyll can result in leaves looking pale/ yellow
• Causes of chlorosis

  • lack of light -  in absence of light plants will turn off chlorophyll production to conserve resources - chlorosis only occurs in dark
  • mineral deficiencies - e.g. lack of iron or magnesium that help produce chlorophyll
  • virus infections - interfere with plant metabolism & chlorophyll synthesis
• How is animal body mass determined?

  • combo of genetic & environmental factors
  • environmental - determined by amount of food consumed, quantity of exercise, presence of disease
  • genetic - mutations can cause pattern of fat deposition in body to be altered
• Genotype

  genetic makeup of an organism 
• Phenotype

  observable characteristics of an organism
• Dominant allele

  version of the gene that will always be expressed if present in an organism 
• Recessive allele

  version of gene that will only be expressed if 2 copies are present in an organism 
• Homozygous gene  

  • they have 2 identical alleles for a characteristic
  • can be homozygous dominant (contain 2 alleles for dominant phenotype) or homozygous recessive (contain 2 alleles for recessive phenotype)
• Heterozygous gene

  • have 2 different alleles for a characteristic
  • the dominant allele will be expressed
• Discontinuous variation

  • a characteristic that can only appear in specific values
  • qualitative
  • limited number of traits, no immediate forms
  • controlled by one or 2 genes usually with multiple alleles
  • mostly genetic causes - environment has little effect
  • presented on bar chart
• Continuous variation

  • a characteristic that can take any value within any range
  • quantitative
  • range or gradation of characteristics
  • controlled by 2 or more genes (polygenic)
  • environment has significant effect
  • presented on line graph
• Examples of discontinuous variation

  • Human blood groups 
  • Albinism 
  • Round & wrinkled pea shapes
  • Antibiotic resistance 
• Examples of continuous variation

  • stem height in plants
  • milk yield in cows
  • animal mass
  • skin colour
• How does sexual reproduction lead to genetic variation within a species?

  • Crossing over 
  • Reassortment of chromosomes
  • Reassortment of chromatids 
  • Fertilisation 
  • Mutation 
• Crossing over

  • at prophase I non-sister chromatids wrap around each other at points called chiasmata & these sections swap over
  • these sections contain same genes but diff alleles
• Reassortment of chromosomes

  • metaphase I - maternal & paternal chromosomes align at random
  • each gamete acquires a diff mixture of maternal & paternal chromosomes
• Reassortment of chromatids

  • metaphase II - sister chromatids align randomly
  • this determines how they segregate at anaphase II
  • results in sister chromatids no longer being genetically identical
• Fertilisation

  • genetic material from 2 unrelated individuals is combined to make a zygote
• 2 main causes of genetic variation

  • mutation
  • sexual recombination
• How does mutation cause genetic variation?

  • Deletion, addition or substitution of a nucleotide 
  • Deletion or translocation of part of a chromosome
  • Aneuploidy - loss or gain of a single chromosome 
  • Polyploidy - addition of whole chromosome sets 
• Why is variation essential in selection?

  • when environmental conditions change, genetic variety enables those individuals which are best adapted, to survive & reproduce, passing on their advantageous alleles to their offspring
  • Plus more of the offspring may have features that are MORE beneficial to the farmers who are growing/ rearing them
• Monogenic inheritance

  characteristic inherited on a single gene
• Test Cross (not on spec but comes up in application Q so useful to know)

  • Determines whether an organism showing the dominant phenotype is homozygous or heterozygous
  • Involves crossing the organism w another that is homozygous recessive for the trait 
  • If any of the offspring show the homozygous recessive trait in the phenotype, the parent must have been heterozygous
• Steps in drawing a genetic cross

  • State phenotype of both parents 
  • State genotype of both parents
  • State gametes of both parents 
  • Draw punnett square
  • State proportion of each genotype
  • State corresponding phenotype 
• Codominance

  when 2 diff alleles for a gene are equally dominant so are both expressed in the phenotype of a heterozygote
• Example of codominance 


  • an allele codes for red flowers
  • an allele codes for white flowers
  • 3 colours can be produced
  • red flowers - plant is homozygous for allele coding for red
  • white flowers - plant is homozygous for allele coding for white
  • pink flowers - plant is heterozygous - single allele present which codes for red pigmentation produces enough pigment to produce pink flowers

  
• How is blood group determined?

  • By a gene with multiple alleles 
  • as an organism carries only 2 versions of the gene (one on each homologous chromosome) only 2 alleles can be present in an individual
  • Gene I codes for production of diff antigens on surface of RBC 
  • I^A - results in production of antigen A 
  • I^B - production of antigen B 
  • I^O - production of neither antigen 
  • I^A & I^B are codominant whereas I^O is recessive to both of the other alleles - diff combos of these alleles result in 4 blood groups 
  • group A - IAIA or IAIO
  • group B - IBIB or IBIO
  • group AB - IAIB
  • group O - IOIO

  
• Determining sex in mammals vs birds

  • in mammals females are homogametic (XX) & males are heterogametic (XY)
  • in birds, butterflies & moths, females are heterogametic (XY) & males are homogametic (XX)
• Linkage

  2 or more genes present on the same chromosome that are normally inherited together
• Sex linkage

  • characteristics determined by genes carried on sex chromosomes - these genes are sex linked
  • as the Y chromosome is smaller than X chromosomes , there are a number of genes in the X chromosome that males only have one copy of
  • means any characteristic caused by a recessive allele on the section of the X chromosome (which is missing on Y) occurs more frequently in males - because many females will also have a dominant allele present in their cells
• Example of a sex-linked genetic disorder

  haemophilia
• Haemophilia

  • patients have blood clots caused by a recessive allele that encodes non-functioning Factor VIII (protein blood-clotting factor)
  • gene for Factor VIII is located on the X chromosome - haemophilia is sex linked
  • if a male inherits recessive allele that codes for haemophilia they cannot have a corresponding dominant allele on Y chromosome & so develop condition
  • females who are heterozygous for gene are known as carriers
• Dihybrid inheritance

  inheritance of 2 genes
• Dihybrid cross

  • shows inheritance of 2 diff characteristics caused by 2 genes, which may be located on diff pairs of homologous chromosomes
  • each of these genes can have 2 or more alleles
• What can we work out from a dihybrid cross?

  • the diff phenotypes expected
  • the ratio of diff phenotypes and genotypes expected from the cross
• Stages of dihybrid cross

  • F1 cross
  • gamete formation
  • F2 cross
• F1 cross

  • 2 homozygous parents w opposite alleles are bred together - one has dominant alleles, other has recessive alleles
  • all offspring in F1 generation will have a heterozygous genotype (e.g. YyRr)
  • 2 homozygous parents w opposite alleles are bred together - one has dominant alleles, other has recessive alleles
  • all offspring in F1 generation will have a heterozygous genotype (e.g. YyRr)

  
• Gamete formation (dihybrid cross)

  • each F1 parent (e.g. RrYy) can produce 4 possible gametes due to independent assortment
  • RY, Ry, rY, ry
• F2 cross

  • 2 heterozygous parents are bred together (using offspring of F1)
  • identifies possible combinations of alleles
  • expected ratio is 9:3:3:1