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Biology - Genetics

Grade 10ICSE

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

🔑Concepts

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Genetics is the study of heredity and variation. Heredity involves the transmission of characters from parents to offspring via DNADNA.

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A Gene is the basic unit of heredity located at a specific locus on a ChromosomeChromosome.

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Alleles are alternative forms of a gene (e.g., TT for tallness and tt for dwarfness). A dominant allele masks the expression of a recessive allele in a heterozygous state.

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Homozygous refers to having identical alleles (TTTT or tttt), while Heterozygous refers to having different alleles (TtTt).

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Phenotype is the observable physical characteristic (e.g., tall), while Genotype is the genetic makeup (e.g., TtTt).

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Mendel's Law of Dominance: In a cross between a pair of contrasting characters, only one (dominant) is expressed in the F1F_1 generation.

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Mendel's Law of Segregation: During gamete formation, the two alleles of a pair separate so that each gamete receives only one allele.

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Mendel's Law of Independent Assortment: Distribution of alleles of one character into gametes is independent of the distribution of alleles of another character.

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Sex Determination in humans: Males have XYXY sex chromosomes (heterogametic) and females have XXXX sex chromosomes (homogametic). The sex of a child is determined by the sperm: 22+X22 + X or 22+Y22 + Y.

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Mutation is a sudden change in the sequence of DNADNA or the structure/number of chromosomes.

📐Formulae

Monohybrid Phenotypic Ratio (F2)=3:1\text{Monohybrid Phenotypic Ratio } (F_2) = 3 : 1

Monohybrid Genotypic Ratio (F2)=1:2:1\text{Monohybrid Genotypic Ratio } (F_2) = 1 : 2 : 1

Dihybrid Phenotypic Ratio (F2)=9:3:3:1\text{Dihybrid Phenotypic Ratio } (F_2) = 9 : 3 : 3 : 1

Diploid number (2n)=46 in humans\text{Diploid number } (2n) = 46 \text{ in humans}

Haploid number (n)=23 in humans\text{Haploid number } (n) = 23 \text{ in humans}

P(Male offspring)=12P(\text{Male offspring}) = \frac{1}{2}

💡Examples

Problem 1:

A pure tall pea plant (TTTT) is crossed with a pure dwarf pea plant (tttt). Find the phenotypic and genotypic ratios of the F2F_2 generation.

Solution:

  1. F1F_1 Generation: TT×tt→TT \times tt \rightarrow All TtTt (Tall).
  2. F2F_2 Generation (Selfing Tt×TtTt \times Tt):
  • Genotypes: 1TT1 TT (Pure Tall), 2Tt2 Tt (Hybrid Tall), 1tt1 tt (Pure Dwarf).
  • Phenotypes: 33 Tall, 11 Dwarf.
  • Phenotypic Ratio: 3:13 : 1.
  • Genotypic Ratio: 1:2:11 : 2 : 1.

Explanation:

According to the Law of Segregation, the alleles TT and tt separate during gamete formation and combine randomly during fertilization to produce the 1:2:11 : 2 : 1 genotypic distribution.

Problem 2:

Determine the probability of a couple having a girl child using a genetic cross.

Solution:

  1. Mother's Gametes: XX and XX.
  2. Father's Gametes: XX and YY.
  3. Punnett Square:
  • X×X=XXX \times X = XX (Female)
  • X×Y=XYX \times Y = XY (Male)
  • X×X=XXX \times X = XX (Female)
  • X×Y=XYX \times Y = XY (Male)
  1. Probability = 24=50%\frac{2}{4} = 50\%.

Explanation:

Sex in humans is determined by the sex chromosome carried by the sperm. Since 50%50\% of sperms carry the XX chromosome and 50%50\% carry the YY chromosome, there is an equal chance for a male or female child.