Chapter 17 — Heredity: Multiple Choice Questions
1. The science of heredity and variation is called ______.
2. The Father of Genetics is ______.
3. An organism with two identical alleles for a trait is called ______.
- Homozygous: Both alleles are the same (TT or tt). Also called pure breeding.
- Heterozygous: Two different alleles (Tt). Also called hybrid.
- Dominant allele: Expressed when present (T).
- Recessive allele: Only expressed when homozygous (tt).
4. In Mendel's pea experiments, the F₁ generation of crossing tall (TT) × short (tt) plants gave ______.
5. The ratio of phenotypes in F₂ generation of a monohybrid cross is ______.
- Genotype ratio: TT : Tt : tt = 1:2:1
- Phenotype ratio: 3 dominant : 1 recessive = 3 tall : 1 short
The 9:3:3:1 ratio is for a dihybrid cross (two traits).
6. The genotype that shows the recessive phenotype is ______.
7. Mendel's Law of Independent Assortment applies to genes that are located on ______.
8. The sex of a human child is determined by ______.
- Mother: XX → gives only X chromosome in egg
- Father: XY → gives either X or Y chromosome in sperm
If sperm with X fertilises egg: XX = Girl If sperm with Y fertilises egg: XY = Boy
Therefore, the father's sperm determines the sex of the child. It is scientifically incorrect to blame the mother for the child's sex.
9. Chromosomes that determine sex in humans are called ______.
- 22 pairs = autosomes (non-sex chromosomes)
- 1 pair = sex chromosomes (allosomes): XX in females, XY in males
The Y chromosome carries SRY gene that triggers male development.
10. A dihybrid cross produces the F₂ phenotypic ratio of ______.
- 9 : dominant for both traits
- 3 : dominant first trait, recessive second
- 3 : recessive first trait, dominant second
- 1 : recessive for both
This ratio confirms independent assortment of two pairs of genes.
Quick Revision — Genetics Terms
| Term | Definition |
|---|---|
| Gene | Unit of heredity on a chromosome |
| Allele | Alternative form of a gene (T or t) |
| Dominant | Allele expressed in heterozygote (T) |
| Recessive | Allele masked in heterozygote (t) |
| Homozygous | Both alleles same (TT or tt) |
| Heterozygous | Different alleles (Tt) |
| Genotype | Genetic makeup (TT, Tt, tt) |
| Phenotype | Observable trait (tall or short) |
| Monohybrid cross | One trait; F₂ ratio = 3:1 |
| Dihybrid cross | Two traits; F₂ ratio = 9:3:3:1 |
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Frequently Asked Questions
Law of Segregation: Each organism has two alleles for every trait. During gamete formation, these two alleles separate (segregate) and each gamete receives only one allele. Example: Tt (heterozygous tall) produces T gametes and t gametes in equal ratio.