Chapter 12: Mendel’s Experiments and Heredity

Homunculus

Thought sperm had tiny “little people” inside them.

Like begets like

Offspring always looked exactly like parents.

Everything from the egg

Females gave all traits.

Paternal heredity

Males gave all traits.

Blending inheritance

Offspring were a mix or “average” of parents

Inheritance of acquired traits

Traits parents gained in life were passed to kids.

Pangenesis

Tiny particles (“gemmules”) carried traits through the body.

Gregor Mendel — Father of Genetics (1822–1884)

• From Austria (now Czech Republic).

• Grew up on a farm → liked plants and bees.

• Became a monk at the Augustinian Abbey of St. Thomas.

• Studied science and math at the University of Vienna.

• Did pea plant experiments while teaching.

• Later became Abbot (head monk) and stopped experimenting.

🥇 Mendel is called the Father of Genetics because he discovered how traits are inherited.

Why Mendel Used Pea Plants

• Had many simple traits (like flower color, seed shape, seed color).

• Could self-pollinate or be cross-pollinated easily.

• Had male & female parts in one flower (monoecious).

• Made many offspring quickly.

• Were cheap and available.

P

Parent generation (true-breeding)

F1

First offspring generation (from crossing parents)

F2

Offspring of two F1 individuals

F3, F4, ...

Later generations

Sibs

Brothers and sisters

Cross sibs

Mate siblings

Backcross

Cross offspring back with a parent

Self cross

Plant fertilizes itself

True breeding

Always produces offspring with same traits

Mendel’s Ratio

Always 3:1 for monohybrid crosses.

From monohybrid crosses, Mendel saw

• Traits do not blend.

• Traits are passed as separate particles (we now call them genes).

• Some traits mask others:

  • Dominant: hides the other trait.

  • Recessive: hidden by dominant.

• The recessive trait can skip a generation and reappear.

Mendel realized..

• Each plant has two particles (alleles) for a trait.

• These separate into gametes (egg/sperm).

• They recombine randomly at fertilization.

Trait

A characteristic (e.g. flower color, eye color)

Allele

A version of a gene (P = purple, p = white)

Dominant

Shows up in phenotype if present (P)

Recessive

Hidden if dominant allele is present (p)

Homozygous

Both alleles the same (PP or pp)

Heterozygote

Two different alleles (Pp)

Zygote

The first fertilized cell

Gene

DNA section that controls a trait

Locus

Location of a gene on a chromosome

Reciprocal cross

Switch the male and female traits in a cross

Wild type allele

Common “normal” allele

Mutant allele

Rare or changed allele

Mendel’s First Law — Law of Segregation

• Each parent has two alleles for every trait.

• These separate (segregate) during gamete (egg/sperm) formation.

• Each gamete gets one allele.

• During fertilization, alleles pair up again in offspring.

Test Cross (To Find Genotype)

🧩 Test Cross (To Find Genotype)

Why?
To see if a dominant-looking organism is homozygous (PP) or heterozygous (Pp).

How:
Cross it with a recessive homozygote (pp).

Results:

• If all offspring = dominant → parent is PP.

• If ½ dominant and ½ recessive → parent is Pp.

Expected Ratio:
1:1 (dominant : recessive)

✅ Mendel tested this with many traits — results always fit his law.

Summary — Mendel’s Key Ideas

Traits come from “particles”	Now known as genes
Two alleles per trait	One from each parent
Dominant vs. Recessive	Dominant hides recessive
Law of Segregation	Alleles separate during gamete formation
F1 all show dominant trait	Recessive “hides” but still passes on
F2 ratio = 3:1	Classic monohybrid result
Test cross	Determines if dominant is PP or Pp

Probability Basics

• Probability: how likely an event is (0 = never, 1 = always)

• Example: coin flip = ½ = 0.5 = 50%

Incomplete Dominance

Neither allele is fully dominant

Multiple Alleles

• Some traits have more than 2 alleles.

Sex-Linked Traits (X-linked)

Genes on the X chromosome.

Lethal Alleles

Some genes cause death if homozygous

➡ can be dominant or recessive.

Trihybrid Cross (3 Traits)

Use forked-line method

 Chromosomal Crossover

• Happens during meiosis.

• Homologous chromosomes exchange genes, creating new combinations (recombinants).

• Increases genetic variation.

Epistasis

One gene hides or blocks another gene’s expression.