genetics and evolution Notes

Genetics and evolution

Approaching the topic

  • Understanding the concepts

  • Application of these concepts


Fates of meiosis

  • Replication of chromosome and joining of two copies of chromosomes with each other by centromere.
    Figure 10.1 Chromosomes before and after replication
  • Formation of bivalent chromosome during prophase I. Random orientation and alignment on equatorial plate at metaphase stage followed by separations in next
  • Two homologous chromosomes are not identical because each of the two chromosomes is received from different parent. But sister chromatids are exactly
  • Crossing over occurs during prophase I resulting in the exchange in DNA material between non-sister chromatids. It is an enzyme-mediated process and the enzyme involved is
  • Chiasma forms while crossing over. It is a point of adherence of two non-sister chromatids for efficient exchange of genetic material. The chromosomes fall apart eventually but chiasma sites restrict it from parting, therefore the smaller portion of the chromosomes gets
    Figure 10.2 Chiasma formation and crossing over between the two chromosomes containing allele Ab and aB respectively

Law of independent assortment: It states that the separation of one pair of alleles between the daughter cells is independent of another pair of alleles. This law was put forward by Gregor Mendel in his studies of genetic
How it is co-related with meiosis?

  • The process of meiosis ensures this law. The random orientation of chromosomes during metaphase results in the situation which results in independent assortment of alleles during anaphase
  • There are 4 possibilities of alignment of chromosomes making sure that each allele has 50% chance of moving to the same And the chances of two alleles coming together at the same pole are 25%.


  • Dihybrid cross
    • Law of Segregation: It states that during the formation of the gametes the pair of alleles for a specific trait separates such that the offspring receive one allele from each This law is one of the conclusions of dihybrid cross.
    • The crossing of the two contrasting characters is called dihybrid
    • First ever dihybrid cross was also done by Mendel. He considered two characters; seed shape and seed colour of the pea plant to show his
    • In Pisum sativum, seed shows wrinkled and round shape among which, round shape is dominant and shows yellow colour as dominant over green colour (recessive).
  • Let us consider ‘R’ allele for round shape and ‘r’ for wrinkled, ‘Y’ for yellow and ‘y’ for green.
  • The combinations; RRYY and RrYy represents round yellow seed, rryy represents wrinkled green seeds.


  • When two genes are found on the same chromosome i.e. really close to each other then they are called linked. The tendency of these genes to pass on to the next generation together is called
  • When the linked genes are found on the autosomes then it is called autosomal linkage and it is called sex linkage when they are present on the X-chromosome.
  • Linkage is mostly studied in Drosophila melanogaster (fruit fly) and it was found that it has grey or black body and long or short
  • The alleles for these character are; ‘G’ and ‘g’ for grey and black body respectively, ‘L’ and ‘l’ for long and short wings
  • The combinations; GGLL and GgLl represent grey bodied and long winged fly, while ggll represent black bodied and short winged fly. Same law of dominance applied.
  • Crossing over can still occur and may result in some unpredictable

Types of variation

  • Two types: Continuous and discrete or discontinuous variation.
  • Continuous variation deals with the spectrum of phenotypes and shows and unbroken range of phenotypes of a particular character. While discontinuous variation deals with few phenotypes and can show two or more form of a particular character. While discontinuous variation deals with few phenotypes and can show two or more form of a particular character.
  • When an inheritance involves two or more genes influencing one character then it is called polygenic inheritance. It is an example of continuous variation.

Chi squared tests

  • These are the tests done to actually check the accuracy of possible ratios come by
  • It has been always seen that observed values (experimental results) are differing from the expected values (calculated). So it is the test to find out whether the difference between the observed and expected values are quite significant or not,

How it works?

  • There are two concepts (hypothetically);
    • Ho – null hypothesis, it suggests that traits assort independently and the values therefore can be deviated.
    • H1 – suggests that traits do not assort independently.
  • Degree of freedom is the value of (total classes – 1). For example – in a monohybrid cross there are two classes and I dihybrid cross there are 4, so there d.f. will be 1 and 3
  • There are some probability values but the convention is to look for 05 (5%).
  • The formula used is; 𝜒2 = ∑ (𝑂−𝐸)/𝐸
    where, O is observed value and E is expected value.
    c is Greek letter called chi.
  • Here are the critical values of c2 up to 3 degree of freedom:
  • Application of these values are such like that, if calculated c2 > critical c2 then reject null hypothesis and vice versa if calculated c2 < critical c2.

Gene pools and speciation

  • Allele frequencies and evolution
    • Gene pool can be defined as all the genetic information present in the reproducing members of a population at a given
    • Allele frequency of a population may change due to certain causes like mutation by the time, indeed it does not remain same for long. Change in the allele frequency leads to
    • Hardy-Weinberg equation; It is the equation to calculate the allele frequency, genotypes or phenotypes within a It states that sum of frequency of dominant allele and frequency of recessive allele must add up to 1.
      Let us consider, ‘p’ to be frequency of dominant allele and ‘q’ to be the frequency of the recessive allele. The equation will be,
      𝑝 + 𝑞 = 1
      Considering diploid organism with two copies of the genes the equation would be,
      (𝑝 + 𝑞)2 = 1
      𝐼𝑡 𝑒𝑥𝑝𝑎𝑛𝑑𝑠 𝑖𝑛 𝑎 𝑚𝑎𝑡ℎ𝑒𝑚𝑎𝑡𝑖𝑐𝑎𝑙 𝑒𝑞𝑢𝑎𝑡𝑖𝑜𝑛 𝑎𝑠,
  • Variation in allelic frequency by natural factors leads to different types of selection of equilibrium:
    • Directional selection- When a phenotype is chosen as the most appropriate by natural selection than the other, it is called directional selection.
    • Stabilizing selection- When one phenotype is chosen as the most appropriate by natural selection than two most extremes, it is called directional selection.
    • Disruptive selection- When two extreme phenotypes are chosen as the most appropriate by natural selection rather than one, it is called directional selection.

Reproduction isolations

Some factors cause the hindrance in a major proportion which fall apart the populations and causes barrier in reproducing together.

These isolations include:

  • Geographical isolations- It includes physical barriers like lakes, mountains or any such land or water formations which prevent opposite partners from mating. Once majorly separated, two kinds of populations are
  • Temporal isolation- Time reference acts as the barrier in this kind of isolation. The maturity of an individual may attain before the other, while the other is still in its juvenile
  • Behavioural isolation- The behavioural patterns causes separations. One individual in a population may be so different in its behaviour that the opposite gender may not find it attractive or worth it for reproducing

Polyploidy can also cause reproductive isolation, because if in one population some individuals have three sets of chromosome (3n) i.e. triploid and some individual have two sets (2n) or four sets (4n) of chromosomes then they will be evolving differently.

All the reproductive isolation factors lead to speciation.

Rate of speciation

  • The rate of speciation can be judge by two ways:
    • Gradualism: The changes happen are small, continuous and occur at a very slow This idea was adopted by Charles Darwin while purposing his ideas about natural selection.
    • Punctuated equilibrium: This idea suggests that the changes happen quickly and no or subtle change happens