Chromosomal Basis of Inheritance

Chromosomal Basis of Inheritance

• Mendelian inheritance is physically based on the behavior of chromosomes during sexual life cycles.
• Diploid cells have paired chromosomes and genes.
• During gamete formation, chromosomes separate, and allele pairs segregate.
• Fertilization restores the diploid chromosome and gene condition.
• The chromosomal theory of inheritance arose from these observations.

Genetics and Cytology Timeline

• Genetics:
  • In the 1860s, Mendel proposed discrete inherited factors that segregate and independently assort during gamete formation.
  • In 1900, Mendel's work was rediscovered.
  • In 1902, cytology and genetics converged, showing similarities between inherited factors and chromosomes.
• Cytology:
  • In 1875, cytologists worked out the basics of mitosis.
  • In 1890, cytologists worked out the basics of meiosis.

Linked Genes

• Linked genes tend to be inherited together because they are located on the same chromosome.
• Genes on the same chromosome do not assort independently.
• The 9:3:3:1 ratio is not observed in linked genes.

Linking Genes to Chromosomes: The Work of Morgan

• Thomas Hunt Morgan traced a gene to a specific chromosome.
• Morgan worked with fruit flies (Drosophila melanogaster).

Genetic Symbols

• Alleles are named after the non-wild-type (mutant) phenotype.
• Wild-type alleles are dominant, and mutant alleles are recessive.
• Wild-type traits are designated with a superscript "+" sign.
• For example, in eye color, the wild-type phenotype is red (+), and white eyes are recessive (w).

Mutant Phenotypes

• The discovery of sex-linked genes was made through experiments.
• Morgan crossed a white-eyed male with a red-eyed female.
• In the F1 generation, the white-eyed trait disappeared, indicating the red-eyed trait was dominant.
• In the F2 generation, the white-eyed trait reappeared in males only.
• This indicated that the eye color gene is located on the sex chromosomes.

Sex-linked Genes

• Genes located on the sex chromosomes are sex-linked genes.

Genetic Recombination

• Independent assortment of chromosomes and crossing over cause genetic recombination.
• Recombination of unlinked genes is a result of independent assortment.

Testcross Example

• A testcross (YyRr x yyrr) will demonstrate parental and recombinant offspring

Recombination Frequency

• The recombination frequency is the percentage of recombinant offspring, determined by recombination events.
• This frequency is used to estimate the relative distance between genes on a chromosome.

Sex-Linked Traits in Humans

• Sex is a phenotypic character determined by inherited chromosomes.
• Different systems determine chromosomal sex (e.g., X-Y, X-0).
• Males are often heterogametic (having two different sex chromosomes), while females are homogametic.

The Z-W System

• In birds/fish/insects, females are heterogametic (ZW), and males are homogametic (ZZ).

The Haplo-diploid System

• In insects (ants/bees), sex is determined by fertilization. Males develop from unfertilized eggs.

Transmission of Sex-linked Genes

• Fathers pass their X chromosome to their daughters, and mothers pass their X chromosome to both sons and daughters.
• Recessive sex-linked traits appear more frequently in males.

Problems in Heredity

• Errors during meiosis or the effect of mutagens can lead to chromosomal changes.

Changes in Chromosome Number

• Aneuploidy and polyploidy result from nondisjunction during meiosis (meiosis I or II) or mitosis.
• Aneuploidy includes trisomy (extra chromosome) and monosomy (missing chromosome).
• Down syndrome is an example of trisomy.
• Polyploidy includes triploidy (3N) and tetraploidy (4N).

Explore the fundamental concepts of the chromosomal basis of inheritance, including Mendelian inheritance and linked genes. This quiz covers the timeline of genetics and cytology, as well as the processes involved in gamete formation and fertilization. Test your understanding of how chromosomes behave in sexual life cycles.