Principles of Inheritance and Variation 3 Flashcards
State law of independent assortment
‘when two pairs of traits are combined in a hybrid, segregation of one pair
of characters is independent of the other pair of characters’.
Mendel’s law of independent assortment states that the alleles of two (or more) different genes get sorted into 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.
explain law of independent assortment
RrYy plant. Consider the
segregation of one pair of genes R and r. Fifty per cent of the gametes
have the gene R and the other 50 per cent have r. Now besides each
gamete having either R or r, it should also have the allele Y or y. The
important thing to remember here is that segregation of 50 per cent R
and 50 per cent r is independent from the segregation of 50 per cent
Y and 50 per cent y. Therefore, 50 per cent of the r bearing gametes
has Y and the other 50 per cent has y. Similarly, 50 per cent of the R
bearing gametes has Y and the other 50 per cent has y. Thus there are
four genotypes of gametes (four types of pollen and four types of eggs).
why did mendel’s laws were not accepeted to easily
Firstly,communication was not easy (as it is now) in those days and his work
could not be widely publicised.
Secondly, his concept of genes (or
factors, in Mendel’s words) as stable and discrete units that controlled
the expression of traits and, of the pair of alleles which did not ‘blend’
with each other, was not accepted by his contemporaries as an
explanation for the apparently continuous variation seen in nature.
Thirdly, Mendel’s approach of using mathematics to explain biological
phenomena was totally new and unacceptable to many of the
biologists of his time.
Finally, though Mendel’s work suggested that
factors (genes) were discrete units, he could not provide any physical
proof for the existence of factors or say what they were made of.
who gave the chromosomal theory of inheritance
This theory states that individual genes are found at specific locations on particular chromosomes, and that the behavior of chromosomes during meiosis can explain why genes are inherited according to Mendel’s laws. Sutton studied chromosomes and meiosis in grasshoppers. Boveri studied the same things in sea urchins. Walter Sutton and Theodor Boveri gave the idea of a connection between chromosomes and heredity, chromosomes as the genetic material responsible for Mendelian inheritance, chromosomes carry the unit of heredity (genes).
state the main points of chromosomal theory of inheritance
- The chromosomes occur in pairs in the cell, so do the Mendelian factors.
- Homologous chromosomes pair during meiosis and only one member of each pair finds its way into given gamete. Mendelian factors segregate from one cell to another in similar fashion.
- The spatial arrangement of any given chromosome pair on the spindle is independent of the arrangement of all chromosome pairs. Mendelian factors are also supposed to assort themselves independently.
- By the process of fertilization the original number of chromosomes is restored in the zygote, similarly Mendelian factors (pairs of contrasting characters) reduced to one half their original number during germ cells formation are restored by fertilization.
why did thomas hunt morgan decide to use drosophila melanogaster
which were
found very suitable for such studies. They could be grown on
simple synthetic medium in the laboratory. They complete their life
cycle in about two weeks, and a single mating could produce a large
number of progeny flies. Also, there was a clear differentiation of
the sexes – the male and female flies are easily distinguishable. Also,
it has many types of hereditary variations that can be seen with
low power microscopes.
describe haplodiploidy
The sex determination in honey bee is
based on the number of sets of
chromosomes an individual receives. An
offspring formed from the union of a
sperm and an egg develops as a female
(queen or worker), and an unfertilised
egg develops as a male (drone) by means
of parthenogenesis. This means that the
males have half the number of
chromosomes than that of a female. The
females are diploid having 32
chromosomes and males are haploid, i.e., having 16 chromosomes.
This is called as haplodiploid sex-determination system and has special
characteristic features such as the males produce sperms by mitosis
what is pedigree analysis
The study of inheritance of genetic traits in several generations of a human family in the form of a family tree diagram is called pedigree analysis.
advantages of pedigree analysis
(i) It helps in genetic counselling to avoid disorders in future generations
(ii) it is useful to the find the origin and flow of genetic diseases in a family.
(iii) it is important to discover expressive alleles that can cause disorders like haemophilia, sickle cell anaemia, etc
(iv) controlled crosses cannot be performed in humans, so it is useful in studying the inheritance pattern of genes
(v) it is used extensively in medical research
(vi) helps us understand the nature of a genetic disorder, like whether it is autosomal or sex linked or dominant or recessive.
what are genetic maps
Alfred Sturtevant determined that genes are arranged linearly on the chromosome and prepared chromosome/ genetic maps with the position of genes on the chromosome based on percentage of recombinants
what are the contributions of Thomas hunt Morgan to genetics
- genes are arranged linearly on chromosome
- studied linkage, sex linkage, crossing over and discovered the relation between gene and chromosomes
- established chromosomes mapping
- he observed and worked on mutations
