Chromosomal Basis of Inheritance
The chromosome theory of inheritance: involves the combination of mendel’s laws with cytological (studies from plant and animal cells) evidence of the process of meiosis
It is the process that involves genes. These genes occupy specific positions called loci in the terms of genetics on the chromosomes, and from this gene loci occupation, the chromosomes undergo segregation and independent assortment in the process of meiosis in gamete formation.
The chromosome theory of inheritance proposed that it is MEIOSIS that causes the patterns of inheritance that Mendel observed. For example, Mendel’s rules can be explained by independent alignment and separation of homologous chromosomes at meiosis I.
Chromosomes similar to each other that form a pair. Ex. Diploid organisms (2n) have homologous pairs of chromosomes; chromosomes that are similar to one another and they could be visualized using karyotype.
An ordered display of an individual’s chromosomes. It may be used to identify chromosomal abnormalities that are associated with inherited disorders.
In a diploid organism, the chromosomes and genes are both present in pairs.
The homologous chromosomes separate and the alleles from genes segregate
Fertilization restores the paired condition for both chromosome and gene.
Mendelian genes have specific loci on chromosomes and it is the chromosomes, not the genes that undergo independent assortment and segregation in the process of gamete formation in meiosis.
The framework
Sex chromosomes in mammals are,
XX = females
XY = male
Scientific Inquiry. Identified a specific gene with a specific chromosome
Worked with Drosophila melanogaster. Normal phenotype for a character is called the wild type. Alternative phenotypes for a character arise from mutations and are called mutant phenotypes.
Evidence for the chromosome theory of inheritance was determined from analysing eye color of flies
Discovered a mutant white-eyed male fly of which he mated with a wild-type red-eyed female,
F1 = red for all
F2 = red for females
= half red and half white for males
Concluded that the gene for eye color was located only on the X chromosome.
Sex of the fly was linked to the eye color. Sex phenotype was know to be linked to the sex chromosomes. Morgan proposed the theory of sex-linked inheritance. The X chromosome carries the genes therefore, X-linked inheritance. Principle of linkage was involved. He showed an association between the sex of the fly and eye color.
Linked genes tend to be inherited together because they are located near each other on the same chromosome:
Linked genes are those genes that are located on the same chromosome and tend to be inherited together.
If linked genes do not have a tendency to be inherited together then the phenotypic ratio would be 1:1:1:1. This phenotypic ratio was not observed; the offspring had the same phenotypes as the P generation parents therefore showing that these traits (black bodies and vestigial wings) were inherited together because their genes were located on the same chromosome.
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