What is the difference between genotype and phenotype?

Genotype is the genetic constitution (alleles) of an organism (e.g., TT, Tt, tt). Phenotype is the observable physical characteristic (e.g., tall, dwarf). Same phenotype can have different genotypes (TT and Tt are both tall). Environment can also affect phenotype. Test cross (× tt) is used to determine if a dominant phenotype is TT or Tt.

Why are males more commonly affected by X-linked recessive disorders?

Males are XY — they have only ONE X chromosome (hemizygous). If that X chromosome carries a recessive allele for haemophilia or colour blindness, there is no second X chromosome with the normal allele to mask it — so the recessive disorder is expressed. Females (XX) need two copies of the recessive allele (one on each X) to show the disorder — much less likely.

What is a test cross?

A test cross is a cross between an individual showing a dominant phenotype (unknown genotype: TT or Tt) and a homozygous recessive individual (tt). If all offspring are tall → parent was TT (homozygous dominant). If offspring show 1 tall : 1 dwarf ratio → parent was Tt (heterozygous). Test crosses are used to determine the genotype of individuals with dominant phenotype.

Mendel's Laws of Inheritance

Principles of Inheritance and Variation • Class 12 Biology • NCERT • CBSE

Mendel's Laws: (1) Law of Dominance — dominant allele masks recessive. (2) Law of Segregation — alleles separate during gamete formation. (3) Law of Independent Assortment — genes on different chromosomes assort independently. Monohybrid ratio 3:1; Dihybrid ratio 9:3:3:1.

Frequently Asked Questions

What is the difference between genotype and phenotype?: Genotype is the genetic constitution (alleles) of an organism (e.g., TT, Tt, tt). Phenotype is the observable physical characteristic (e.g., tall, dwarf). Same phenotype can have different genotypes (TT and Tt are both tall). Environment can also affect phenotype. Test cross (× tt) is used to determine if a dominant phenotype is TT or Tt.
Why are males more commonly affected by X-linked recessive disorders?: Males are XY — they have only ONE X chromosome (hemizygous). If that X chromosome carries a recessive allele for haemophilia or colour blindness, there is no second X chromosome with the normal allele to mask it — so the recessive disorder is expressed. Females (XX) need two copies of the recessive allele (one on each X) to show the disorder — much less likely.
What is a test cross?: A test cross is a cross between an individual showing a dominant phenotype (unknown genotype: TT or Tt) and a homozygous recessive individual (tt). If all offspring are tall → parent was TT (homozygous dominant). If offspring show 1 tall : 1 dwarf ratio → parent was Tt (heterozygous). Test crosses are used to determine the genotype of individuals with dominant phenotype.

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