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Principles of Inheritance and Variation

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Class 10 Class 12

What are 'true breeding lines' that are used to study inheritance pattern of traits in plants?


True breeding line is one that, having undergone continuous self-pollination, shows the stable trait inheritance and expression for several generations.

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Explain the genetic basis of blood grouping in human population.


In humans, the blood groups are controlled by a gene called gene I. It has three alleles, namely IA, IB and i.  The plasma membrane of the red blood cells has sugar polymers that protrude from the surface and the kind of sugar is controlled by the gene. The alleles IA and IB produce a slightly different sugar while i do not produce any sugar. Since humans are diploid, each person can have different combinations of the three alleles. The alleles IA and IB are dominant over i therefore a combination of IA and iproduce blood group A, IB and I produce blood group B. The combination of IA, IB produces blood group AB which has both A and B types of sugar, because of co-dominance. The combination of IA and IA produce blood group A, IB and IB produce B blood group.

The type of blood group produced by the different combinations is shown in the table below:

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What is the inheritance pattern observed in the size of starch grains and seed shape of Pisum sativum? Workout the monohybrid cross showing the above traits. How does this pattern of inheritance deviate from that of Mendelian law of dominance?


The starch synthesis in pea plants is controlled by a single gene. It has two alleles B and b. BB homozygotes produce starch effectively and therefore produce large starch grains. Whereas the bb homozygotes are less efficient in starch production and hence produce smaller grains. After maturation the BB seeds were found to be round and bb was wrinkled. Heterozygotes produce round seeds, and so B was considered to be the dominant allele. But when they were crossed the resultant progeny were intermediate-sized Bb seeds.

P generation                      BB × bb

                                            ↓

F1 Generation                     Bb × Bb

 

B

b

B

BB

(long grains)

Bb

(intermediate grains)

b

Bb

(intermediate grains)

bb

(small grains)

 















Deviation from Mendel's law of dominance: If starch grain size is considered as the phenotype, then one can see that the alleles show incomplete dominance. Thus, dominance is not an autonomous feature of a gene, it depends on gene product and production of particular phenotype from the product.
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(a) Why are thalassemia and haemophilia categorized as Mendelian disorders ? Write the symptoms of these diseases. Explain their pattern of inheritance in humans.

(b) Write the genotypes of the normal parents producing a haemophilic son.


(a) Thalassaemia and haemophilia are categorised as Mendelian disorders because they occur by mutation in a single gene. Their mode of inheritance follows the principles of Mendelian genetics. Mendelian disorders can be

  • autosomal dominant (muscular dystrophy)
  • autosomal recessive (thalassaemia)
  • sex linked (haemophilia)

Symptoms of Thalassaemia

  • Thalassaemia minor results only in mild anaemia, characterised by low haemoglobin level.
  • Thalassaemia major is also known as Cooley's anaemia. In this disease, affected infants are normal but as they reach 6 to 9 months of age, they develop severe anaemia, skeletal deformities, jaundice, fatigue, etc.


Symptoms of Haemophilia

  • Person suffering from this disease does not develop a proper blood clotting mechanism.
  • A haemophilic patient suffers from non-stop bleeding even on a simple cut, which may lead to death.

Pattern of Inheritance of Thalassaemia
Pair of alleles HbA and HbT controls the expression of this disease.
Conditions for thalassemia:

  • HbA and HbA: Normal
  • HbA and HbT: Carrier
HbT and HbT: Diseased

Let us assume that both father and mother are the carriers (HbA HbT) of beta thalassaemia.

Parents

 

HbAHbT  
(Father)

x

HbAHbT
(Mother)

Offsprings

HbAHbA
Normal child

HbAHb
Carrier child
with thalassaemia trait

HbAHbT 
Carrier child
with thalassaemia trait

HbTHbT
Child with 
severe thalassaemia


Pattern of Inheritance of Haemophilia:
Haemophilia is an X-linked genetic disorder. Compared to females, males have higher chances of getting affected because females have XX chromosomes while males have only one X with Y chromosome. Thus, for a female to get affected by haemophilia, she has to have the mutant gene on both the X chromosomes while males can be affected if they carry it on the single X chromosome.
Conditions for haemophilia:
XY; XX: Normal
XhY: Haemophilic
XhX: Carrier
XhXh: Haemophilic
Let us assume that a carrier female (XhX) is married to a normal male.


Parents

 

XY  
(Male)

x

XhX
(Female)

Offspring

XhX
Carrier
female

XX
Normal
female


XhY
Haemophilic
male
 

XY
Normal
male


Parents

 

XY  
(Male)

x

XhX
(Female)

Offspring

XhX
Carrier
female

XX
Normal
female


XhY
Haemophilic
male
 

XY
Normal
male



1803 Views

A child by the family from Thalassemia is born to a normal couple. But the mother is being blamed by the family for delivering a sick baby. (a) What is Thalassemia?

(b) How would you counsel the family not to blame the mother for delivering a child suffering from this disease? Explain.

(c) List the values your counseling can propagate in the families.

(a) Thalassemia - is an autosomal recessive blood disease which can occur due to deletion of the genes controlling the formation of globin chains (commonly Alpha and Beta) of haemoglobin.

(b) Mother cannot be blamed for the disease as, it is an autosomal recessive blood disorder. The genes for the synthesis of globin chains are present on autosomes.

(i) Formation of Alpha chain is controlled by 2 genes present on chromosome 16 and

(ii) Formation of Beta-chain is controlled by one gene present on chromosome 11manifestation of the disease occurs when the progeny receives defective genes from both the parents (as thalassemia is expressed is homozygous recessive condition only).

(c) Values that can be propagated in families are

(i) One should be aware of such autosomal diseases and should get themselves checked for the same before marriage.

(ii) Manifestation of the disease occurs due to defective genetic set up of both the parent (so it is not only that the mother should be blamed.
526 Views

(a) Explain a monohybrid cross taking seed coat colour as a trait in Pisum sativum. Work out the cross upto F2 generation.

(b) State the laws of inheritance that can be derived from such a cross.

(c) How is the phenotypic ratio of F2 generation different in a dihybrid cross?


(a) Monohybrid cross is the cross in which only one trait is taken into account.

 For example, if pea plant with yellow seed coat is crossed with pea plant having green seed coat then in the F1 generation all the plants produce yellow seeds.



(b) Two laws can be derived from such a cross

Law of Dominance- According to this law, characters are controlled by discrete units called factors, which occur in pairs with one member of the pair dominating over the other in a dissimilar pair. It explains the expression of only one of the parental character in F1generation and expression or appearance of both in F2 generation.

In the given cross, the allele for yellow seeds is dominant over the allele that produces green seeds. In F1 generation all offsprings showed yellow colour of seed (dominant character) and expression of both yellow and green in F2 generation.

Law of Segregation-This law states that the two alleles of a pair segregate or separate during gamete formation such that a gamete receives only one of the two factors. In homozygous parents, all gametes produced are similar; while in heterozygous parents, two kinds of gametes are produced in equal proportions. The alleles for yellow and green seeds in the F1 generation segregate and the gamete either contains yellow allele or green allele giving rise to different combination in the F2 generation. Thus, showing that the gametes receive only one of the two factors.

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