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Heredity - HEREDITY8.2 HEREDITY8.2 HEREDITY8.2 HEREDITY8.2 HEREDITY

Grade 10CBSE

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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Heredity refers to the transmission of characters or traits from parents to their offspring through genes.

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Gregor Johann Mendel, known as the Father of Genetics, performed experiments on Garden Pea (Pisum sativumPisum\ sativum) to understand the rules of inheritance.

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A Monohybrid Cross involves the inheritance of a single pair of contrasting characters. The F2F_2 generation phenotypic ratio is 3:13:1 and the genotypic ratio is 1:2:11:2:1.

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A Dihybrid Cross involves two pairs of contrasting characters (e.g., seed shape and color). The F2F_2 phenotypic ratio is 9:3:3:19:3:3:1.

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The Law of Dominance states that in a heterozygote, one trait (dominant) will conceal the presence of another trait (recessive) for the same characteristic.

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Sex determination in humans is chromosomal. Females possess two XX chromosomes (XXXX) while males possess one XX and one YY chromosome (XYXY).

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The sex of the child is determined by the type of chromosome (XX or YY) contributed by the father's sperm.

📐Formulae

Monohybrid Phenotypic Ratio=3:1\text{Monohybrid Phenotypic Ratio} = 3 : 1

Monohybrid Genotypic Ratio=1:2:1\text{Monohybrid Genotypic Ratio} = 1 : 2 : 1

Dihybrid Phenotypic Ratio=9:3:3:1\text{Dihybrid Phenotypic Ratio} = 9 : 3 : 3 : 1

Probability of Male/Female Child=12=50%\text{Probability of Male/Female Child} = \frac{1}{2} = 50\%

💡Examples

Problem 1:

In a cross between a pure tall pea plant (TTTT) and a pure short pea plant (tttt), what will be the phenotypic ratio of the F2F_2 generation?

Solution:

  1. F1F_1 Generation: TT×tt→TtTT \times tt \rightarrow Tt (All Tall).
  2. F2F_2 Generation (Selfing F1F_1): Tt×Tt→TT,Tt,Tt,ttTt \times Tt \rightarrow TT, Tt, Tt, tt.
  3. Phenotypes: 33 Tall (from TT,Tt,TtTT, Tt, Tt) and 11 Short (from tttt). Ratio: 3:13 : 1.

Explanation:

The dominant allele 'TT' masks the expression of the recessive allele 'tt'. Only the homozygous recessive 'tttt' results in a short plant.

Problem 2:

Show the sex determination in humans using a cross.

Solution:

Parents: Mother (XXXX) ×\times Father (XYXY) Gametes: Mother (XX, XX) and Father (XX, YY) Cross combinations: XYXXXXYXXXXY\begin{array}{c|cc} & X & Y \\ \hline X & XX & XY \\ X & XX & XY \end{array} Results: 50%50\% Female (XXXX) and 50%50\% Male (XYXY).

Explanation:

Since the mother always contributes an XX chromosome, the sex of the baby depends entirely on whether the father's sperm carries an XX or a YY chromosome.