Chapter 8

Heredity

Introduction

Reproduction produces individuals that are similar but subtly different due to variations.
Variation Sources:
- Asexual reproduction: Minor variations from DNA copying inaccuracies.
- Sexual reproduction: Greater variations due to combining DNA from two parents.
Examples: Sugarcane fields show little variation (asexual), while humans show distinct individual differences (sexual).
This chapter examines how variations are created and inherited, focusing on heredity mechanisms.

Each generation inherits a common body design with subtle changes from the previous generation (Fig. 8.1).
Asexual Reproduction:
- Produces very similar offspring (e.g., bacterial division yields four similar bacteria with minor DNA copying variations).
Sexual Reproduction:
- Generates greater diversity as offspring inherit variations from both parents, plus new variations.
Survival Advantage:
- Not all variations ensure survival; environmental factors select advantageous variants (e.g., heat-resistant bacteria survive heat waves).
- Variation drives evolutionary processes by enabling adaptation.

Heredity governs the reliable inheritance of traits, ensuring similar body designs across generations.
The rules of heredity explain how traits and variations are passed from parents to offspring.

Children share basic human features but differ from parents due to variations (e.g., not identical to either parent).
Activity 8.1 (Earlobes):
- Record free vs. attached earlobes (Fig. 8.2) in class, calculate percentages, and compare with parents’ earlobes.
- Suggests inheritance rules: Earlobe type may depend on parental contributions, hinting at dominant/recessive patterns.
Human populations show significant trait variation, reflecting heredity’s complexity.

Gregor Mendel (1822-1884):
- Studied pea plants, counting trait occurrences across generations, blending science and mathematics.
- Used contrasting traits: round/wrinkled seeds, tall/short plants, white/violet flowers.
Mendel’s Experiments:
- Crossed tall and short pea plants; F1 generation was all tall, showing no intermediate heights (Fig. 8.3).
- F1 self-pollination produced F2 with 3:1 tall:short ratio, indicating both traits were inherited but only tallness expressed in F1.
- Conclusion: Traits are controlled by two gene copies (alleles); one may dominate (e.g., T for tall) over the other (t for short).
Terminology:
- Dominant Trait: Expressed with one copy (e.g., Tt or TT = tall).
- Recessive Trait: Expressed only with two copies (e.g., tt = short).
Activity 8.2:
- To confirm F2’s 1:2:1 ratio (TT:Tt:tt), cross F2 plants with short plants (tt). TT yields all tall (Tt); Tt yields 1:1 tall:short; tt yields all short.
Mendel’s work established that traits are inherited via discrete factors (genes), with dominant/recessive patterns.

Genes and Proteins:
- DNA segments (genes) code for proteins; traits result from protein functions.
- Example: Tallness in plants depends on a hormone controlled by an enzyme. Efficient enzyme (T gene) produces more hormone (tall); less efficient (t gene) produces less (short).
Inheritance Mechanism:
- Each parent contributes one gene copy, so offspring have two copies per trait.
- Germ cells have one copy, formed by separating chromosome pairs, ensuring independent trait inheritance (Fig. 8.5).
Independent Inheritance:
- Crossing tall/round-seeded (RRYY) with short/wrinkled-seeded (rryy) plants yields F1 (RrYy, tall/round).
- F2 shows new combinations (e.g., tall/wrinkled, short/round), proving traits like seed shape and height are inherited independently.
Genes on separate chromosomes allow independent trait assortment, creating diverse offspring.

Sex determination varies by species:
- Environmental: Temperature affects sex in some reptiles.
- Flexible: Snails can change sex.
- Genetic: Human sex is determined by genes.
Human Sex Chromosomes (Fig. 8.6):
- Humans have 22 paired chromosomes plus sex chromosomes.
- Females: XX (two X chromosomes).
- Males: XY (one X, one Y).
Inheritance Pattern:
- Mother contributes X to all children.
- Father contributes X (girl, XX) or Y (boy, XY), determining sex.
- Result: ~50% boys, ~50% girls.
Sex determination in humans is a genetic process driven by paternal chromosome contribution.

Dominant/Recessive Traits:
- Mendel’s crosses (e.g., tall × short) produced F1 all tall, showing tallness (T) is dominant; F2’s 3:1 ratio revealed recessive shortness (tt).
Independent Inheritance:
- Crossing tall/round × short/wrinkled yielded F2 with new combinations (tall/wrinkled, short/round), showing traits assort independently.
Blood Group Dominance:
- A father (A) and O mother producing an O daughter (OO) suggests O is recessive, as the daughter inherited O from both parents. However, without knowing the father’s genotype (AA or AO), dominance cannot be confirmed.
Sex Determination:
- In humans, the father’s X (girl, XX) or Y (boy, XY) chromosome determines the child’s sex; mothers always contribute X.

Tall/Violet × Short/White Cross:
- All F1 have violet flowers, half are short, suggesting tall parent is heterozygous for height (Tt) and homozygous for flower color (WW). Answer: (d) TtWw.
Light Eye Color Dominance:
- Light-eyed children with light-eyed parents suggest inheritance but not dominance. Recessive traits require both parents to carry the gene, but dominant traits could also be inherited if parents are heterozygous. More data (e.g., dark-eyed parents) needed.
Project: Dominant Dog Coat Color:
- Collect data on dog coat colors and parentage from breeders or shelters.
- Cross dogs with different coat colors (e.g., black vs. brown) and record F1 offspring colors.
- Analyze F1 and F2 (from F1 self-crosses) ratios. If all F1 are one color (e.g., black) and F2 shows 3:1 (black:brown), black is dominant.
Equal Genetic Contribution:
- Each parent contributes one chromosome per pair via meiosis, forming germ cells with half the chromosome number. Fertilization restores the full set, ensuring equal DNA contribution from both parents.