Chapter 8: Heredity
Heredity and variation
The passing of characters from parents to offspring is heredity, and the differences among offspring are variation. Mendel experimented on pea plants and discovered the rules of heredity.
Mendel's experiments
Monohybrid: tall (TT) × dwarf (tt) → all tall (Tt) in F1. In F2: 3 tall : 1 dwarf, and the genotypes are 1 TT : 2 Tt : 1 tt. Dihybrid: round yellow × wrinkled green → all round yellow in F1. In F2 the ratio is 9 : 3 : 3 : 1, which shows that traits are inherited independently.
Genes and expression of traits
Genes are sections of DNA that carry information to make proteins, and proteins decide body traits. A dominant trait appears in F1, while a recessive trait stays hidden. Acquired traits are not inherited.
Sex determination
Humans have 23 pairs of chromosomes: 22 pairs are autosomes and 1 pair is the sex chromosomes. Females have XX and males have XY. The mother’s egg always carries X, so the sex of the child is decided by the father’s sperm (X or Y).
Important questions
Q1. What is heredity?
The transmission of characters from parents to offspring.
Q2. What is variation?
The differences seen among individuals of the same species.
Q3. Who is called the father of genetics?
Gregor Johann Mendel.
Q4. Why did Mendel choose the pea plant for his experiments?
Pea has many contrasting traits, a short life cycle, it self-pollinates and cross-pollination is easy to do.
Q5. What is a dominant trait?
A trait that appears in the F1 generation even when the contrasting trait is present, such as tallness in pea plants.
Q6. What is a recessive trait?
A trait that stays hidden when the dominant trait is present and shows up in some F2 offspring, such as dwarfness.
Q7. What kind of plants were obtained in F1 when tall and dwarf pea plants were crossed?
All the plants were tall.
Q8. What is the ratio in the F2 generation of a monohybrid cross?
The phenotypic ratio is 3 tall : 1 dwarf. The genotypic ratio is 1 TT : 2 Tt : 1 tt.
Q9. What are phenotype and genotype?
Phenotype is the visible appearance of the organism (like tall). Genotype is its genetic makeup (like Tt).
Q10. What is the ratio in the F2 generation of a dihybrid cross?
9 : 3 : 3 : 1, that is round yellow : round green : wrinkled yellow : wrinkled green.
Q11. What does independent assortment show?
That different traits are inherited independently of each other, so new combinations of traits are formed.
Q12. What are acquired traits? Are they inherited?
Traits gained during life through the environment or practice, such as losing weight. They are not inherited because they do not change the DNA of germ cells.
Q13. How many pairs of chromosomes are there in a human cell?
23 pairs, that is 46 chromosomes in all.
Q14. What are sex chromosomes?
The pair of chromosomes that decide sex. Females have XX and males have XY.
Q15. How is the sex of a baby determined?
The mother’s egg always carries X. The father’s sperm can carry X or Y. An X sperm gives a girl and a Y sperm gives a boy.
Q16. Is the mother responsible for the sex of the baby?
No, because the sex is decided by the chromosome (X or Y) of the father’s sperm.
Q17. What is a gene?
A section of DNA that carries the information for making a protein or a trait.
Q18. How do genes control the traits of the body?
Genes carry the information for making proteins, and proteins control body features, for example the amount of a hormone in a plant decides its height.
Q19. Why does variation arise in sexually reproducing organisms?
Because of small errors in DNA copying and because the genes of the two parents form new combinations.
Q20. In a cross of a pure tall (TT) plant with a pure dwarf (tt) plant, what is the genotype of F1?
Tt, and all the plants will look tall because T is dominant.
Quiz
1. Who is the father of genetics?
Gregor Mendel is called the father of genetics.
2. What is the phenotypic ratio in the F2 of a monohybrid cross?
The F2 has 3 dominant : 1 recessive.
3. The sex of a child is determined by...
The father’s sperm carries X or Y.
4. How many pairs of chromosomes are there in a human cell?
Humans have 23 pairs of chromosomes.
5. Which trait appears in the F1 generation?
The dominant trait appears in F1.