Heredity and Mendel's Laws
NCERT Class 10 • Science • Chapter "Heredity and Evolution"
Heredity and Mendel's Laws
Heredity is the passing of characters from parents to offspring. The rules governing it were worked out by Mendel through experiments on the pea plant, long before anyone knew what a gene was, and his conclusions still form the base of genetics.
Mendel's Experiments and Laws
- Mendel chose the pea plant because it has clearly contrasting characters, a short life cycle and is easy to cross.
- A gene is the unit of heredity, located on a chromosome, and it carries the instruction for a character.
- Different forms of the same gene are called alleles; one may be dominant and the other recessive.
- A dominant allele shows its effect even when only one copy is present; a recessive allele shows only when both copies are recessive.
- Genotype is the genetic makeup of an organism while phenotype is its visible appearance.
- In a monohybrid cross of two contrasting pure traits, the first generation shows only the dominant trait, and the second generation shows the ratio 3 to 1 by appearance and 1 to 2 to 1 by genotype.
- The law of dominance says one allele masks the other; the law of segregation says the two alleles separate during gamete formation.
- The law of independent assortment, seen in a dihybrid cross, says different characters are inherited independently of one another.
Sex Determination in Humans
- Humans have 23 pairs of chromosomes, of which one pair is the sex chromosomes.
- A female has XX and a male has XY.
- The mother always contributes an X chromosome, so the sex of the child is determined by whether the father's gamete carries X or Y.
- A child receiving X from the father becomes a girl and one receiving Y becomes a boy.
- The belief that the mother decides the sex of the child is therefore scientifically wrong.
- Acquired traits developed in a lifetime are not inherited, while inherited traits carried in genes pass on.
| Term | Meaning |
|---|---|
| Gene | Unit of heredity on a chromosome |
| Allele | An alternative form of a gene |
| Genotype | Genetic makeup |
| Phenotype | Visible appearance |
Recessive trait — a trait that appears only when both alleles for it are recessive, as its effect is masked by a dominant allele.
Hindi me samjhein
बच्चे का लिंग पिता के युग्मक से तय होता है, क्योंकि माता सदैव X गुणसूत्र देती है। पिता X दे तो बालिका और Y दे तो बालक होता है।
Exam me kaise aata hai
- Mendel worked on which plant — the pea plant
- Phenotypic ratio of a monohybrid cross in the second generation — 3 to 1
- Chromosomes of a human male are — XY
- The sex of a child is determined by the — father
UPSC/State PSC ke liye
- Mendel succeeded because he studied one character at a time and counted large numbers, which turned biology into a quantitative science.
- Acquired characters are not inherited because they do not change the genes in the reproductive cells, which is what the next generation actually receives.
Yahan confuse hote hain
✗ The mother determines the sex of the child | The father's gamete determines it | ✓
✗ Genotype and phenotype are the same | Genotype is genetic makeup; phenotype is visible appearance | ✓
✗ Acquired traits are inherited | Only traits carried in genes are inherited | ✓
Ek nazar me
- Genes on chromosomes carry heredity; alleles may be dominant or recessive.
- Monohybrid cross gives 3 to 1 by appearance in the second generation.
- Mendel's three laws: dominance, segregation and independent assortment.
- The father's gamete determines the sex of the child.