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Biology · 2026 · 5 marks
CBSE 2026 · Region 5 · Set 1 · Q32
(a) Work out separate monohybrid crosses up to F_2 generation between two pea plants and two Antirrhinum plants, both having contrasting traits with…
Work out separate monohybrid crosses up to $\displaystyle \mathrm{F}_{2}$ generation between two pea plants and two Antirrhinum plants, both having contrasting traits with respect to the colour of the flower. Comment on the patterns of inheritance in the crosses carried out in such two cases.
Answer the following questions :
(i)
How does a chromosomal disorder differ from a Mendelian disorder ? Write one example for each.
(ii)
Name the phenomenon that leads to situations like 'XO' abnormality in humans. Also name this genetic disorder. How are individuals with an XO chromosomal abnormality affected ? Write its symptoms as well as karyotype.
Marking-scheme solution
• In Pea plants:
Parents : WW (Violet) X ww (White)
Gametes : W and w
F1 : Ww (All Violet)
Selfing : Ww X Ww
F2 :
Phenotypic ratio :- violet : white = $\displaystyle 3$ : $\displaystyle 1$
Conclusion: This is the case of dominance
• In Antirrhinum plant
Parents : RR (Red) X rr (White)
Gametes : R and r
F1 : Rr (All pink)
Selfing : Rr X Rr
F2 :
Phenotypic ratio :- Red : Pink : white = $\displaystyle 1$ : $\displaystyle 2$ : $\displaystyle 1$
Conclusion: This is a case of incomplete dominance
(i)
| Mendelian disorder | Chromosomal disorder |
| Occurs due to mutation / alteration in single gene / follow mendelian inheritance pattern | Occurs due to addition or deletion of one or more chromosomes / Does not follow mendelian inheritance pattern |
Example of mendelian disorder: Haemophilia / Cystic Fibrosis / Sickle Cell Anaemia
/ Colour Blindness / Phenylketonuria / Thalassemia.
Example of chromosomal disorder: Turner’s syndrome / Down’s syndrome /
Klinefelter’s syndrome.
(ii)
• Aneuploidy / Monosomy
• Turner’s syndrome
• Symptoms:
lack of secondary sexual characters.
ovaries are rudimentary
sterile
short stature
underdeveloped feminine characters
• Karyotype – $\displaystyle 44$+XO ($\displaystyle 45$)
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