Principles Of Inheritance And Variation - Class 12 Medical Biology - Extra Questions

• Mutation in a single nucleotide base of a DNA segment is called a point mutation.
• It can introduce a premature stop codon, or a nonsense codon, by addition or deletion of the base, in the transcribed mRNA (nonsense mutation) or changes a codon specifying a different amino acid by substitution of one base (missense mutation).
• For example, replacement of A by T at the 17$$^{th}$$ nucleotide of the Hbb gene changes the codon GAG (glutamic acid) to GTG (which encodes valine) and causes sickle cell anaemia.

• Pedigree analysis is the study of a particular trait that is inherited from one generation to another.
• It helps to know the trait of inheritance for a particular trait, and also know whether the trait is actually getting inherited or not.
• Thus, it can be used to predict the characteristics of the future generations of a particular species, like colour, stature, height, etc.

• Finding the sex of an individual as it begins life is known as sex determination. X chromosome carry genes for sex. X chromosome carries female determining genes and Y chromosome has male determining genes. The number of X and Y chromosome determines the female or male sex of an individual. Autosomes carry genes for the somatic characters that do not have any relation with the sex. Human has XX-XY type of sex determination where there is male heterogamy. The male hs\as XY chromosome and the females have XX chromosomes.

Theodor Boveri and Walter Sutton are the scientists credited with an extension of Mendel's laws, known as the chromosome theory of inheritance. In essence, this theory, which was developed in the early 1900s, explains what Mendel observed in his pea plants, otherwise known as the Boveri and Sutton chromosome theory. The Chromosomal Theory of Inheritance was consistent with Mendel’s laws and was supported by the following observations:

·         During meiosis, homologous chromosome pairs migrate as discrete structures that are independent of other chromosome pairs.

·         The sorting of chromosomes from each homologous pair into pre-gametes appears to be random.

·         Each parent synthesizes gametes that contain only half of their chromosomal complement.

·         Even though male and female gametes (sperm and egg) differ in size and morphology, they have the same number of chromosomes, suggesting equal genetic contributions from each parent.

·         The gametic chromosomes combine during fertilization to produce offspring with the same chromosome number as their parents.

 So, chromosome theory of inheritance was proposed by "Theodor Boveri and Walter Sutton".

• Since long it has been considered that females are responsible for determining the sex of the developing foetus. However, it is evident that human females are homogametic. 
• They produce the same types of ova, each having the X chromosome. The human males are heterogametic. They have XY as sex chromosomes and so they produce 50% sperms with X chromosome and another 50% with a Y chromosome. 
• If the sperm having X chromosome fuses with the ovum, a female foetus will be formed. If the sperm having Y chromosome fuses with an ovum, a male foetus is formed. So, the deciding factor is contributed by the males and the sex of the foetus depends on the probability. 
• Hence, we all must take efforts to stop female foeticide and mother is not responsible for a girl child.

• We first check whether the pedigree is sex-linked. If not then it is taken as autosomal.
  
• The trait is not Y-linked as in Y linked, only males are affected.
  
• The trait could be X-linked. But, it is not X-linked recessive because there are no generation skips.

• The female in the parent is unaffected and the male has only one X chromosome, so the genotypes could be XX and X$${c}$$X. But since in the next generation both the females are not similarly affected according to the above cross, the trait must be autosomal. 
• In autosomal traits, both males and females are affected equally regardless of their sex. 
  

$$\textbf{Solution:}$$

$$\textbf{ (A)}$$

$$\bullet$$ It is an individual which contains the two contracting factors of a character or two different alleles of a gene on its homologous chromosomes. It is not pure and is called hybrid for that character . Heterozygote does not breed true on self fertilization eg Tt.

$$\textbf{(B)}$$

$$\bullet$$ It is an individual when contains identical alleles of gene or factors of a character on its homologous chromosomes . The homozygote is pure for the character and breeds true,that is ,hence it give rise to offspring having the same trait on self breeding eg: TT or tt . It is of two types homozygous dominant and homozygous recessive.

$$\textbf{(C)}$$

• The trait is autosomal as both males and females are affected equally.
  
• The trait is autosomal dominant because it has no generations skipped.
  
• If the trait is 'A' for dominant and 'a' for recessive, The genotype of the female parent is 'aa'. 

• The pedigree shows a autosomal dominant trait.
  
• The genotype of the affected individual is AA/Aa and unaffected individual is aa.
• The genotype of the father is 'Aa' and that of mother is 'aa'.
• The genotype of the third child in the first generation is 'Aa'.
• The genotype of the first child in the third generation is 'Aa'. 

The disorder that results from the monosomy of X chromosome is known as Turner syndrome. This disorder develops only in female individuals These females lack an X chromosome and have the karyotype 44+XO. Some symptoms that are characteristic of this disorder are swollen limbs, short stature, obesity, flat feet, heart defects, rudimentary ovaries, infertility, scoliosis, etc. 

Genes are units of genetic material present on the chromosomes. They code for molecules of RNA called mRNA that code for a particular protein. This protein expresses itself either in the form of a phenotypic character or as a metabolite that plays a role in some metabolic pathway. Mutations in the genetic material of an organism often result in alteration of the expression of a gene. This alteration could silence the gene, reduce the expression of the gene, or enhance expression of a gene. Such alterations are often observed in the form of diseases.

• Aneuploidy refers to the condition in which an abnormal number of chromosomes is present within a  cell (either 46 0r 47 chromosomes instead of the normal 46 chromosomes).
• An allosome is a chromosome that is different from the rest of the chromosomes (autosomes) in its size and structure (X and Y chromosomes).
• In hyperdiploidy, the number of chromosomes is higher than the normal (46) which is caused due to the disjunction at the time of meiosis and results in trisomy, etc. of a chromosome.
• In an allosomal hyperneuploid disorder, there is an increase in the number of either the X chromosome or the Y chromosome. 
• For example, in Klinefelter’s syndrome, the karyotype of an individual is 44+XXY as an extra X chromosome is present in the male. 
• Some common characters of this syndrome include infertility, high-pitched voice, gynecomastia, small-sized testicles, language and learning difficulties, etc.
  

1. Change in the number of chromosomes.
2. Change in the structure of chromosomes.

• From the table formed we can see that there are 3 'D' alleles and so 75% of offspring will have the dominant phenotype out of a total 100%.
  DD, Dd, Dd shows Dominant phenotype.
  Hence, the percentage of offspring which will have the dominant phenotype when Dd and dd individuals mate is 75%.
  

(i) Female XX with male XO is male heterogamety. For example, grasshopper.

(ii) Female ZW with male ZZ is female heterogamety. For example, birds.

• Nucleotides are the biomolecules comprising nitrogenous base sugar and a phosphate group in an arrangement
• Nucleotides are the basic unit of genetic material for all living organisms.
• It can be a base in another molecule, such as adenosine triphosphate (ATP). It is also found in coenzymes like NAD and NADP, that plays an important role in metabolism. 
• Another molecule that contains a nucleotide is cyclic AMP (cAMP), a messenger molecule. 

In male Drosophila one X chromosome, one small Y chromosome, and two autosomes are present. The gene for the eye colour is located on the X chromosome.

Explanation:

• In the given pedigree, in generation (i), the male parent is affected, but the female parent is not. 
• As both male and female progenies are affected in generation (ii); this means that the disorder is autosomal. 
• In generation (iii), alleles have surely passed from the affected parents of generation (ii), still we see that none of the progeny is affected. 
• This is because presence of recessive alleles at both the locus is required but only one parent is affected and passing only one allele to the progeny. This means that the disorder is recessive.
•  So, the type of given pedigree is autosomal recessive.

Having missing or extra chromosomes is a condition called aneuploidy. The risk of having a child with an aneuploidy increases as a woman ages.

Trisomy is the most common aneuploidy. In trisomy, there is an extra chromosome. A common trisomy is

 trisomy 21 (Down syndrome), there are 3 copies of chromosome 21: abnormal features of the face and body, medical problems such as heart defects, and intellectual disability occur. Many children with Down syndrome live to adulthood.

trisomy 13 (Patau syndrome) there are 3 copies of chromosome 13: causes serious problems with the brain and heart as well as extra fingers and toes, cleft palate and lip, and other defects. Most infants with trisomy 13 die within the first year of life.

trisomy 18 (Edwards syndrome) there are 3 copies of chromosome 18: causes severe intellectual disability and serious physical problems such as a small head, heart defects, and deafness. Most of those affected with trisomy 18 die before birth or within the first month of life.

 Monosomy is another type of aneuploidy in which there is a missing chromosome. A common monosomy is Turner syndrome, in which a female has a missing or damaged X chromosome.It causes a webbed neck, short height, and heart problems but does not usually cause developmental delays.

• Short stature
• Small round head
• Furrowed tongue and partially open mouth
• Palm is broad with characteristic palm crease.
• Physical, psychomotor, and mental development is retarded in the person.