SPI Biology · Genetics and Evolution

Heredity and Mendel's Crosses

Each trait is set by a pair of genes, one from each parent; when the two differ, the dominant one shows, and Mendel's crosses predict the ratios in the next generation.

All Genetics and Evolution notes

Why this matters

Three questions in the bank: one MODERATE (counting one kind of plant in a dihybrid F2) and two HARD (the father of human genetics, and what therapeutic cloning rests on, from the 2022 paper). The cross is pure method and always gives 9 : 3 : 3 : 1; the other two are names to learn.

Concept 1 of 2: Monohybrid and dihybrid crosses

Mendel crossed pea plants that differed in one trait, then in two. Each parent gives one gene of each pair to its offspring. In the second generation the hidden (recessive) trait comes back in a fixed share: one in four for one trait, and a 9 : 3 : 3 : 1 pattern for two traits.

Definition

A gene controls a trait; its two forms are alleles. A capital letter (T) is dominant, a small letter (t) is recessive.
Genotype is the gene pair (TT, Tt, tt); phenotype is what shows (tall or dwarf). Tt looks like TT.
Monohybrid (one trait): TT $\times$ tt gives F1 all Tt (all tall). Selfing F1 gives F2 phenotypes 3 : 1 and genotypes 1 : 2 : 1 (TT : Tt : tt).
Dihybrid (two traits): RRYY $\times$ rryy gives F1 all RrYy. Selfing F1 gives F2 9 : 3 : 3 : 1 out of 16.
The 9 shows both dominant traits, each 3 shows one dominant and one recessive, the 1 shows both recessive.
To count plants: multiply the class's share by the total, such as $\frac{3}{16}$ of the F2.

F2 ratios

\text{monohybrid } 3:1,\quad \text{dihybrid } 9:3:3:1,\quad \text{number in a class} = \frac{\text{class share}}{16} \times \text{total}

Worked example

Tall (T) is dominant over dwarf (t). A pure tall pea plant is crossed with a dwarf one, and the F1 plants are selfed to give 400 F2 plants. How many F2 plants are dwarf?

Practice this conceptself-check · 4 quick reps

The same idea in a practice question:

Practice questionModerate

Example 1 · Genetics and Evolution · Heredity and Mendel's Crosses

In a plant, smooth seeds(S) are dominant over wrinkled seeds(s) and green seeds (G) are dominant over yellow seeds (g). A plant homozygous for smooth and green seed is crossed with a plant having wrinkled and yellow seeds. The

\text{F}_1

offspring are self crossed to produce

\text{F}_2

generation. If a total of 160 offspring are produced, how many plants are expected to be having wrinkled and green seeds in

\text{F}_2

generation, according to a typical Mendelian cross?

Using 9/16 for a mixed class

A class with one dominant and one recessive trait is one of the two '3' classes, so its share is 3/16. Only the class with both dominant traits is 9/16.

Reading a capital letter as a fixed colour

Which trait is dominant is given in the question. If it says green is dominant over yellow, then G is green, even though Mendel's own peas had yellow dominant.

Concept 2 of 2: Names in genetics and cloning

A few scientists are remembered by a title, such as 'father of genetics'. Learn each title with its owner, and learn what the two kinds of cloning make.

Definition

Gregor Mendel: father of genetics (laws of inheritance, pea plants).
Sir Archibald Garrod: father of human genetics (inherited disorders, 'inborn errors of metabolism').
T H Morgan: worked on fruit flies and showed genes lie on chromosomes.
Har Gobind Khorana: helped crack the genetic code.
Watson and Crick: the double helix model of DNA.
Cloning makes a genetically identical copy. Reproductive cloning makes a whole animal (Dolly the sheep, 1996). Therapeutic cloning grows stem cells to repair a patient's tissues.

Scientist	Known for
Gregor Mendel	Father of genetics
Sir Archibald Garrod	Father of human genetics Mendel is the father of genetics; Garrod of human genetics.
T H Morgan	Genes on chromosomes, fruit flies
Har Gobind Khorana	The genetic code
Watson and Crick	Double helix of DNA
Ian Wilmut	Dolly, the first cloned mammal

Practice this conceptself-check · 4 quick reps

The same idea in a practice question:

Practice questionHard

Example 2 · Genetics and Evolution · Heredity and Mendel's Crosses

Father of Human genetics is

Mendel for human genetics

Mendel worked on peas and is the father of genetics as a whole. The father of human genetics is Sir Archibald Garrod.

Therapeutic cloning makes a whole animal

Reproductive cloning makes a whole animal, like Dolly. Therapeutic cloning stops at stem cells, which are grown into the tissue a patient needs.

Drill 1 more on names in genetics and cloning

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Formulas (1)

Monohybrid and dihybrid crosses
F2 ratios
$\text{monohybrid } 3:1,\quad \text{dihybrid } 9:3:3:1,\quad \text{number in a class} = \frac{\text{class share}}{16} \times \text{total}$

Reference tables (1)

Names in genetics and cloning6 rows

Scientist	Known for
Gregor Mendel	Father of genetics
Sir Archibald Garrod	Father of human genetics Mendel is the father of genetics; Garrod of human genetics.
T H Morgan	Genes on chromosomes, fruit flies
Har Gobind Khorana	The genetic code
Watson and Crick	Double helix of DNA
Ian Wilmut	Dolly, the first cloned mammal

Watch out for (4)

Using 9/16 for a mixed class
→ Monohybrid and dihybrid crosses
Reading a capital letter as a fixed colour
→ Monohybrid and dihybrid crosses
Mendel for human genetics
→ Names in genetics and cloning
Therapeutic cloning makes a whole animal
→ Names in genetics and cloning

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