biology ................. meiosis and genetics

1. Meiosis and the Sexual Life cycle
Origins of genetic variation among offspring:
• Independent assortment of chromosomes,
• crossing over,
• random fertilisation

2. Origin of genetic variation among offsprings
• Genetic variation refers to the diversity in genetic material within a population or among
individuals of the same species. It is a fundamental concept in genetics and plays a
crucial role in evolution and the adaptation of species to their environments.
• Genetic variation can manifest in various forms, including differences in DNA sequences,
gene expression, and chromosomal structures. It is a measure of the genetic differences
that exist within a population.
• Genetic variation is advantageous to a population because it enables some individuals to
adapt to the environment while maintaining the survival of the population.

3. Sources of Genetic
Variation:
Mutation: Mutations are changes in the DNA sequence that can occur
spontaneously or be induced by external factors such as radiation or certain
chemicals. Mutations can lead to new alleles, providing the raw material for
evolution.
Recombination: During the process of meiosis, genetic material is shuffled
between homologous chromosomes, leading to the creation of new combinations
of alleles. This process is known as genetic recombination and occurs during the
formation of gametes (sperm and egg cells).

4. Cont
Gene Flow (Migration): Movement of individuals between populations can
introduce new alleles into a population. This gene flow can occur through
migration and interbreeding, leading to the exchange of genetic material between
populations.
Sexual Reproduction: The combination of genetic material from two parents
during sexual reproduction results in offspring with a unique combination of
alleles. This contributes to genetic diversity within a population.

5. Causes of Genetic
Variation:
Point Mutations: Changes in a single nucleotide, such as substitutions, insertions, or
deletions, can result in point mutations. These mutations may or may not have a significant
impact on the function of the encoded protein.
Chromosomal Mutations: Larger scale changes, such as deletions, duplications, inversions,
or translocations of entire segments of chromosomes, can occur, leading to chromosomal
mutations. These can have significant effects on an organism's phenotype.

6. cont
Horizontal Gene Transfer: In some cases, genetic material can be transferred between
organisms that are not parent and offspring. This is known as horizontal gene transfer and
can occur through mechanisms such as bacterial conjugation, transformation, and
transduction.
Selective Pressures: Environmental factors and selective pressures can influence the
prevalence of certain alleles in a population. Natural selection favors individuals with traits
that enhance their survival and reproductive success, leading to the propagation of those
beneficial alleles
Polyploidy: This occurs when an organism has more than two complete sets of
chromosomes. Polyploidy is common in plants and can result from errors in cell division.
.

7. Causes of genetic variation
• random fertilization
• crossing over (or recombination) between chromatids of homologous
chromosomes during meiosis
• Independent assortment

8. cont
• The genetic variation of an entire species is often called genetic diversity.
• Genetic variations are the differences in DNA segments or genes between
individuals and each variation of a gene is called an allele
• Genetic variation is essential for natural selection because natural
selection can only increase or decrease frequency of alleles that already
exist in the population.

9. Independent assortment
• Independent assortment of genes and their corresponding traits was first observed
by Gregor Mendel in 1865 during his studies of genetics in pea plants. Mendel
was performing dihybrid crosses, which are crosses between organisms that
differ with regard to two traits
• Independent assortment refers to the alleles or genes that sort into the newly
formed gametes independently of one another.
• In other words, the allele a gamete receives for one gene does not influence the
allele received for another gene.
• It (Independent assortment ) takes in meiosis in eukaryotes during metaphase I
of meiotic division.

10. Mendel’s experiment

11. cont
• This ratio was the key clue that led Mendel to the law of independent
assortment.
• That’s because a 9:3:3:1 ratio is exactly what we’d expect to see if the F1
plant made four types of gametes (sperm and eggs) with equal frequency:
YR, Yr, yR, and yr.

12. Mendel’s experiment

13. cont
• The physical basis for the law of independent assortment lies in meiosis I
of gamete formation, when homologous pairs line up in random
orientations at the middle of the cell as they prepare to separate.
• We can get gametes with different combos of “mom” and “dad”
homologues (and thus, the alleles on those homologues) because the
orientation of each pair is random.

14. Crossing over
Crossing over is the exchange of genes between two chromosomes, resulting in
non-identical chromatids that comprise the genetic material of gametes (sperm
and eggs)
Crossing over, or genetic recombination, further amplifies the genetic diversity
This phenomenon occurs during prophase I of meiosis when homologous
chromosomes exchange genetic material
.

15. cont
Enzymes facilitate the breaking and re-joining of chromatids, leading to
the creation of chromosomes with novel combinations of alleles
The result is a mosaic of genetic information, each chromosome a
chimeric entity, blending the genetic heritage of both parents.
Crossing over not only enhances genetic diversity but also promotes the
formation of genetically unique offspring, distinct from both parents

18. Significance of crossing
over
Crossing over produces a new combination of genes.
Crossing over plays an essential role in the process of evolution.
Crossing over frequency helps in the construction of genetic maps.
Crossing over provides evidence for a linear arrangement of linked genes in a
chromosome.
Crossing over provides an inexhaustible store of gene variability in sexually
reproducing organisms

19. Random fertilization
Sexual reproduction is the random fertilisazion of a gamate from the female
using a gamate from a male.
Once gametes are formed, the process of fertilization ensues, where a single
sperm cell fuses with an egg cell to form a zygote
The choice of which sperm fertilizes which egg is entirely random
With millions of sperm cells in a single ejaculation, the probability of any
specific sperm fertilizing a particular egg is minuscule.

20. cont
This randomness ensures that every possible combination of genetic
material has a chance of coming together, leading to an unparalleled
diversity among offspring even within the same mating pair.
A spem cell with over 8 million chromosome combinations, fertilizes an
egg cell which also has over 8 million chromosome combinations
That is over 64 trillion unique combinations, not counting the unique
combinations produced by crossing over