Ch 5. Principles of Inheritance and Variations (Stack 2) Flashcards
What is dihybrid cross
It is a cross between two pure organisms of a species in order to study the inheritance of two pairs of alleles.
Mention the phenotypic ratio of dihybrid cross
9:3:3:1
State the law of independent assortment
Mendel’s Law of Independent Assortment states that ‘when two pairs of traits are combined in a hybrid, segregation of one pair of characters is independent of the other pair of characters.’
Mention dihybrid test cross ratio.
1: 1: 1: 1
Distinguish between monohybrid cross and dihybrid cross
Monohybrid cross
(i) It is a cross between two pure organisms in order to study the inheritance of a single pair of alleles.
(ii) It produces a phenotypic monohybrid ratio of 3:I in F2 generation.
It produces a genotypic ratio of 1:2:1 in F2.
Dihybrid cross
(i) It is a cross between two pure organisms of a species in order to study the inheritance of two pairs of alleles.
(ii) It produces a phenotypic monohybrid ratio of 9:3:3:I in F2 generation.
(iii) It produces a genotypic ratio of 1:2:1:2:4:2:1:2:1
State the law of segregatio n and law of independent assortment
Law of segregation states that ‘though the parents contain two alleles during gamete formation, the factors or alleles of a pair segregate from each other such that a gamete receives only one of the two factors.”
Mendel’s Law of Independent Assortment states that ‘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 work was not recognized during his time.
Give any three reasons.
- Communication was not easy (as it is now) in those days and his work could not be widely publicized.
- 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.
- Mendel’s approach of using mathematics to explain biological phenomena was totally new and unacceptable to many of the biologists of his time.
- 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.
Mention the similarities between the behaviors of chromosomes and genes according to the chromosomal theory of inheritance.
- Both chromosome and genes occur in pairs.
- Segregate at the time of gamete formation such that only one of each pair is transmitted to a gamete.
- Independent pairs segregate independently of each other
Mention three features of Drosophila melanogaster which makes it suitable for genetic experiments
Drosophila melanogaster, which was found very suitable for genetic experimental studies because
- They could be grown on a 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.
- There was a clear differentiation of the sexes - the male and female_ flies are easily distinguishable.
- It has many types of hereditary variations that can be seen with low power microscopes.
Define linkage
Linkage is defined as physical association of genes on a chromosome.
Define recombination
Recombination is defined as the generation of non-parental gene combinations.
Even when genes are grouped on the same chromosome, some genes show very low recombination. Give reason
The nearer two genes are on a chromosome, the lower the chance of recombination as recombination frequency depends directly on the distance between the genes.
What are the conclusions drawn by T H Morgan from the crossing experiment in Drosophila with respect to linkage?
Morgan and his group also found that even when genes were grouped on the same chromosome, some genes were very tightly linked (showed very low recombination), while others were loosely linked (showed higher recombination).
Morgan and his group knew that the genes were located on the X- chromosome and saw quickly that when the two genes in a dihybrid cross were situated on the same chromosome, the proportion of parental gene combinations were much higher than the non-parental type.
Morgan attributed this due to the physical association of the two genes called linkage to describe this physical association of genes on a chromosome and the term recombination to describe the generation of non-parental gene combinations.
Morgan and his group also found that even when genes were grouped on the same chromosome, some genes were very tightly linked (showed very low recombination) (Cross A) while others were loosely linked (showed higher recombination) (Cross B). For example, he found that the genes white and yellow were very tightly linked and showed only 1.3 percent recombination while white and miniature wings showed 37.2 percent recombination.
What is polygenic inheritance?
Inheritance of traits which are controlled by three or more genes is called polygenic inheritance.
