Principles of Inheritance and Variation Flashcards
Who applied statistics and mathematics to problems in biology for the first time?
Gregor Mendel
Define a true breeding line.
A true breeding line is one that, having undergone continuous self pollination, shows the stable trait inheritance and expression for several generations
What was special about the plant varieties that Mendel selected?
The 14 true breeding pea plant varieties, as pairs were selected. These were similar EXCEPT FOR ONE CHARACTER WITH CONTRASTING TRAITS.
When tall pea plant was crossed with a dwarf one, what was observed?
In the first hybrid generation (F1) all the plants were tall and none of them were dwarf.
What did Mendel observe on self crossing the F1 generation?
The recessive trait (dwarf in this case) was expressed in the ratio 3:1 (tall : dwarf)
Define blending
Blending is the term used to describe offspring having in between characters of the parents.
What is meant by genotype and phenotype
Genotype: The genetic configuration of an organism
Phenotype: What is physically visible to the naked eye
Give the phenotypic and the genotypic ratios of the F1 generation
Phenotypic: 3:1 (tall : dwarf)
Genotypic: 1:2:1 (TT : Tt : tt)
Define ‘factors’ according to Mendel
Factors were something that was being stable, passed down unchanged, from parent to offspring through the gametes, over successive generations. These factors are called GENES
Define an allele.
A allele is a pair of contrasting genes controlling the same character, but producing different traits.
What is heterozygous state?
Different alleles for a gene (Aa)
What is homozygous state?
Same alleles for a gene (aa or AA)
Who developed the Punnett Square?
British geneticist Reginald C. Punnett
When F1 generation is self pollinated, comment on the gametes produced
F1–> Tt X Tt (self pollination) products T and t genotypes in equal proportion
Comment on fertilisation of F1 generation.
When fertilisation takes place, the pollen grains of genotype T have 50% chance to pollinate eggs of the genotype T as well as genotype t.
What can you infer from the Punnett square of the monohybrid cross?
Of the random fertilisations, 1/4th lead to TT
1/2 lead to Tt and
1/4th lead to tt
What did Mendel observe when he self pollinated F2 plants
Dwarf F2 pants continued to generate dwarf plants in F3 and F4 generations.
Conclusion: Genotype of the dwarfs was homozygous
What did Mendel do to solve the problem of being unable to determine the genotypic composition?
To determine the genotype of a tall plant at F2, he crossed the tall plant from F2 with a dwarf plant. This was called a test cross.
Define a test cross.
In a typical test cross, an organism showing a dominant phenotype (whose genotype is unknown) is crossed with the recessive parent instead of self crossing.
What are the advantages of a test cross?
The progenies of such a cross can easily be analysed to predict the genotype of the test organism.
Define law of dominance
The LoD is used to explain the expression of only one of the parental characters in a monohybrid cross in the F1 and the expression of both in F2
What is the basis of Law of Segregation?
Based on the fact that alleles do not show any blending and tha both the characters are recovered as such in the F2 generation though one of these is not seen in the F1 stage.
What is the key characteristic of incomplete dominance?
Sometimes the F1 had a phenotype that did not resemble either of the two parents and was in between the two. Eg: Snapdragon or Antirrhinum sp.
Let’s say that there is a gene having 2 alleles responsible for producing an enzyme. Suppose one of the allele is modified. What are the possible cases of the situation?
a) Normal/less efficient enzyme
b) Non-functional enzyme
c) No enzyme at all
In these cases, the functional/unmodified allele is the dominant allele
How many alleles control the ABO blood group gene (I)
3 alleles: I^A I^B and i (caps are dominant and small is recessive)
What does the I gene control?
Protrusion of sugar polymers from the plasma membrane of RBCs and it’s kind (kind of sugar)
Why was Mendel’s work initially not accepted?
- Communication was fucking horrible
- His concept of genes 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 as an explanation for the apparently continuous variation seen in nature
- Mathematical approach for biology was new and unacceptable
- Mendel could not provide any physical proof for the existence of genes or what they were made of
Who rediscovered Mendel’s results on the inheritance of characters?
de Vries, Correns and von Tschermak
What were Walter Sutton and Theodore Boveri’s observations?
Behaviour of chromosomes was parallel to the behaviour of genes.They used chromosome movement to explain Mendel’s laws.
Note: Chromosomes as well as genes occur in pairs
What is the chromosomal theory of inheritance?
Genes are found at specific locations on chromosomes, and that the behavior of chromosomes during meiosis can explain Mendel’s laws of inheritance.
What organism did Thomas Hunt Morgan use to experimentally verify the chromosomal theory of inheritance?
Fruit flies i.e., Drosophila melanogaster
Why did Morgan use fruit flies for his experiment?
- Could be grown on simple synthetic medium in lab
- complete life cycle in 2 weeks
- Single mating could produce a large number of progeny flies
- Male and female flies are easily distinguishable
- Many types of hereditary variations which could be seen with low power microscope
Give example of the fruit flies that Morgan chose
Males: Brown bodied, red eyed
Females: Yellow bodied, white eyed
What did Morgan observe in the F1 generation of the mentioned example?
The 2 genes did not segregate independently of each other
F2 ratio deviated very significantly from the 9:3:3:1, which is expected when the two genes are independent.
What happened when 2 genes in a dihybrid cross were situated on the same chromosome?
The proportion of the parental gene combinations were much higher than the non parental type.
Give the appropriate terms:
1. physical association or linkage of the 2 genes
2. describe the generation of non parental gene combinations
- linkage
- recombination
How did Alfred Sturtevant map the positions of genes on a chromosome?
He used the frequency of recombination between gene pairs on the same chromosomes as a measure of the distance between genes.
What are polygenic traits?
Traits that are controlled by three or more genes are called as polygenic traits. Polygenic inheritance also takes into account the influence of the environment.
What is meant by additive effect of each allele in polygenic inheritance?
Additive effect of an allele means that the more alleles an individual has that contribute to a particular trait, the greater the expression of that trait.
What is meant by a pleiotropic gene?
Instances where a single gene can exhibit multiple phenotypic expressions.
What is the underlying mechanism of pleiotropy?
In most cases, it is the effect of a gene on metabolic pathways which contribute towards different phenotypes.
What is phenylketonuria caused by?
Phenylketonuria is caused by mutation in the gene that codes for the enzyme phenyl alanine hydroxylase.
What are characteristics of phenylketonuria?
Mental retardation and a reduction in hair and skin pigmentation
What were the observations made by Henking in regards to X body in a few insects?
The X body was a specific nuclear structure, which after spermatogenesis 50% of the sperms received and 50% did not.
What are the two different types of gametes produced by males?
a) either with or without X chromosome
b) some with X and some with Y chromosomes.
What is male heterogamety?
The type of sex determination where the sex of the offspring depends on the chromosome (x or y) of the male.
What is female heterogamety?
The type of sex determination where the sex of the offspring depends on the chromosome (z or w) of the female.
In humans, what is the determinant of the male characteristic?
The presence of an X and Y chromosome are determinant of the male characteristic.
What are the types of gametes produced after spermatogenesis?
50% of the total sperm produced carry X chromosome and the remaining 50% carry Y chromosome.
What organism is the result of parthenogenesis?
Male honey bee (drone) is developed from an unfertilised egg by parthenogenesis.
What is the sex determination system in honeybees?
haplodiploid sex-determination system.
What are the special features of haplodiploid system?
Males produce sperms by mitosis, they do not have a father and thus cannot have sons, but have a grandfather and and can have grandsons.
What is mutation?
Mutation is a phenomenon which results in alteration of DNA sequences and consequently results in changes in the genotype and the phenotype of an organism
Where are chromosomal aberrations commonly observed?
in cancer cells
What is point mutation?
In addition to alteration in chromosomes, mutation also arises due to change in a single base pair of DNA. This is known as point mutation.
What are the causes of frame-shift mutations?
Deletions and insertions of base pairs of DNA.
What is the main function of pedigree analysis?
Study of the family history about the inheritance of a particular trait over several generations of a family.
What are Mendelian disorders?
Mendelian disorders are type of genetic disorders primarily resulting due to alterations in one SINGLE gene.
How can mendelian disorders be traced in a family?
by pedigree analysis
List examples of Mendelian disorders
Haemophilia, Cystic fibrosis, Sickle-cell anaemia, colour blindness, Phenylketonuria, thalassemia etc.
What does X linked recessive trait show?
The X linked recessive trait shows transmission from carrier female to male progeny.
Why is colour blindness caused?
It’s a sex linked recessive disorder due to defect in either red or green cone of the eye resulting in failure to discriminate between red and green colour. This defect is due to mutation in certain genes present in the X chromosome
Why does colour blindness occur in 8% of males and 0.4% of females?
Because the genes that lead to red-green colour blindness re on the X chromosome.
List characteristics of Haemophilia.
- Sex linked recessive disease which shows its transmission from unaffected carrier female to some of the male progeny
- Single protein, which is a part of the cascade of proteins involved in clotting of blood is affected.
- A simple cut can result in non stop bleeding.
- Heterozygous female (carrier) for haemophilia may transmit the disease to sons.
List characteristics of sickle-cell anaemia.
- Autosome linked recessive trait that can be transmitted from parents to the offspring when both the partners are carriers of the gene.
- The disease is controlled b a single pair of allele, Hb^A and Hb^S
- Only homozygous individuals for Hb^S show the diseased phenotype
- Hb^A and Hb^S individuals appear apparently unaffected but they are carriers of the disease
- There is a 50% probability of transmission of the mutant gene to the progeny.
What is sickle cell anaemia caused by?
Substitution of glutamic acid (Glu) by Valine (Val) at the sixth position of the beta globin chain of the haemoglobin molecule. The substitution of amino acid in the globin protein results due to the single base substitution at the sixth codon of the beta globin gene from GAG TO GUG.
How to the RBCs change shape in SCA?
The mutant Hb molecule undergoes polymerisation under low oxygen tension causing the change in the shape of the RBC from biconcave to elongated sickle like structure.
What is the result of phenylketonuria?
Phenylalanine is accumulated and converted into phenylpyruvic acid and other derivatives. Accumulation of these in the brain results in mental retardation. These are excreted through urine because of its poor absorption by the kidney.
What the most basic feature of Thalassemia?
Mutation or deletion which ultimately results in reduced rate of synthesis of one of the globin chains that make up haemoglobin
Alpha thalassemia is controlled by genes _____ present on ______
a) HBA1 and HBA2
b) Chromosome 16 of each parent.
It is observed due to mutation or deletion of one or more of the four genes
Beta thalassemia is controlled by gene/s ____ located on ______
a) HBB
b) Chromosome 11 of each parent
It occurs due to mutation of one or both the genes
How does thalassemia differ from SCA?
Thalassemia: Quantitative problem of synthesising too few globin molecules
SCA: Qualitative problem of synthesising an incorrectly functioning globin
What are chromosomal disorders?
Disorders caused due to absence or excess or abnormal arrangement of one or more chromosomes
What is aneuploidy?
Failure of segregation of chromatids during cell division cycle results in the gain or loss of a chromosome(s).
What does Down’s syndrome result in?
Gain of extra copy of chromosome 21.
What does Turner’s syndrome result due to?
Loss of an X chromosome in human females
What is polyploidy?
Failure of cytokinesis after telophase stage of cell division results in an increase in a whole set of chromosomes in an organism. This phenomenon is called as polyploidy. This is often seen in plants.
What are the characteristics of Down’s syndrome?
Affected individual is:
1. Short statured
2. Small round head
3. Furrowed tongue
4. Partially opened mouth
5. Palm is broad with characteristic palm crease
6. Physical, psychomotor and mental development is retarded
Describe Klinefelter’s syndrome
Caused due to presence of an additional copy of X chromosome resulting into a karyotype of 47, XXY.
Such an individual has overall masculine development, however, feminine development is also expressed. Such individuals are sterile
Describe Turner’s syndrome
Caused due to absence of one X chromosome i.e., 45 with XO. Such females are sterile as ovaries are rudimentary
