Homologous chromosomes separate in meiosis I

Prophase I

• DNA has already replicated and?condenses?and?becomes visible?as chromosomes
• Each chromosome consists of two sister?chromatids?joined together by a centromere
• The chromosomes are arranged side-by-side in homologous pairs
  • A pair of homologous chromosomes is called a?bivalent
• As the homologous chromosomes are very close together the crossing over of non-sister chromatids may occur. The point at which the crossing over occurs is called the?chiasma?(chiasmata; plural)
• In this stage?centrioles migrate?to opposite poles and the spindle is formed
• The?nuclear envelope breaks down?and the?nucleolus disintegrates

Metaphase I

• The?bivalents line up along the equator of the spindle, with the spindle fibres attached to the centromeres
• The bivalents line up by?independent assortment?(random orientation)

Anaphase I

• The homologous pairs of chromosomes are separated?as microtubules pull whole chromosomes to opposite ends?of the spindle
• The centromeres do not split

Telophase I

• The chromosomes arrive at opposite poles
• Spindle fibres start to break down
• Nuclear envelopes form?around the two groups of chromosomes and nucleoli reform
• Some plant cells go straight into meiosis II without reformation of the nucleus in telophase I

Meiosis I is reduction division

• Meiosis I is referred to as?reduction division?because homologous chromosomes separate and move to opposite poles of the cell.
• Therefore, the number of chromosomes per cell is?reduced by a factor of 2

The different stages of meiosis I in an animal cell

• During metaphase I, an event occurs that is an important source of genetic variation in the gametes formed by meiosis
• Independent assortment is the?production of different combinations of alleles?in gamete cells due to the random alignment of homologous pairs along the equator of the spindle during metaphase I
  • This random alignment is sometimes referred to as?random orientation
• In prophase I homologous chromosomes pair up and in metaphase I they are pulled towards the equator of the spindle
  • Each pair can be arranged with either chromosome on top, this is?completely?random
  • The orientation of one homologous pair is independent / unaffected by the orientation of any other pair
• The homologous chromosomes are then separated and pulled apart to different poles during anaphase
• The combination of alleles that end up in each daughter cell?depends on?how the pairs of homologous chromosomes were lined up
• To work out the number of different possible chromosome combinations the formula 2ⁿ?can be used, where?n?corresponds to the number of chromosomes in a haploid cell
• For humans this is 2²³?which calculates as?8 388 608 different combinations
  • This may seem like a lot of combinations, but by contrast,?crossing over?introduces an almost infinite amount of variation

Independent assortment of homologous chromosomes leading to different genetic combinations in daughter cells

NOS: Careful observation and record keeping turned up anomalous data that Mendel’s law of independent assortment could not account for

• Gregor Mendel (of pea plant fame) devised the?law of independent assortment?in 1866
• In this, he states that characteristics are inherited?completely independently?of others
• This was refined by later scientists to state that the allele that gets sorted into a particular gamete cell?is not influenced?by the allele received for another gene
• In many cases, this holds true, and can be observed in?Mendelian ratios?of offspring in certain crossing experiments
  • Mendel observed a 9:3:3:1 ratio in many dihybrid crosses of sweet pea plants
• As a result,?Mendel's findings were not challenged?until the early 20th century, when Bateson and Punnett found?seemingly anomalous?ratios of offspring
• For which they could offer no explanation
• These became named as?non-Mendelian ratios?because they did not follow the pattern as predicted by Mendel

Bateson and Punnetts' experiment (1905)

• Also working with sweet peas,?two pairs of alleles?were identified (Dominant alleles in capital letters)
  • Flower colour:
    • P?= purple,?p?= red
  • The shape of pollen grains:
    • R?= long,?r?= round
• Their first cross involved pure-bred purple-flowered plants with long pollen grains (PPRR) with pure-bred red-flowered plants with round pollen grains (pprr)
• As expected, this cross resulted in 100% purple-flowered plants with long pollen grains, with the double-heterozygous genotype?PpRr?in the F₁?generation
• However, when individuals from the F₁?generation were crossed with each other, Bateson and Punnett?would have expected a 9:3:3:1 mix of phenotypes, in line with Mendel's law of independent assortment

Modelling the expected ratios using a Punnet square

• Expected phenotype ratios for the second generation:
  • 9 purple flowers, long pollen grains
  • 3 purple flowers, round pollen grains
  • 3 red flowers, long pollen grains
  • 1 red flowers, round pollen grains

Actual Results from Bateson and Punnetts' Experiment

• In the F₂?generation of offspring, there were...
  • Very many?of the grandparent phenotype (purple-flowered, long pollen grains) produced
  • Many?of the grandparent phenotype (red-flowered, round pollen grains) produced
  • Very few (but not zero)?other phenotypes (purple/round or red/long) produced
• This?appeared as an anomaly?to Bateson and Punnett, though they did not find an explanation

Thomas Hunt Morgan developed the notion of linked genes to account for the anomalies

• Later in the 20th century (approx. 1910-1940), an American biologist,?Thomas Hunt Morgan, put forward an explanation for the results previously observed by Bateson and Punnett
• From his findings, he devised the theory of?linkage
• Morgan worked on?fruit flies?(Drosophila melanogaster) thanks to their ability to reproduce?quickly, in?large numbers, in a small physical space and with?observable heritable characteristics?like eye colour and wing shape
• In fact, Morgan bred them selectively for many years to develop a range of phenotypes by natural mutation
• Morgan's work identified?sex-linked characteristics?from genes carried on the X or Y chromosomes that determine gender
• This work led Morgan to develop the idea of?autosomal linkage
• This is where?unexpected patterns of inheritance?are caused by separate alleles being inherited?together, from the same chromosome (an autosome)
• Morgan then elaborated his work by developing the theory of?crossing over
  • As a way of accounting for unexpected (recombinant) genotypes