MR. SURRETTE
VAN NUYS HIGH SCHOOL
CHAPTER 19: GENETICS
CLASS NOTES
GREGOR MENDEL
Gregor Mendel published his work on the genetics of peas in 1866. Mendel chose traits that were clear
and distinct: seed shape – round or wrinkled; seed color – yellow or green; seed coat color – grey or
white; pod shape – inflated or constricted; pod color – green or yellow; flower position – axial or
terminal; stem length – tall or dwarf.
MONOHYBRID CROSS
Mendel tested the cross between a pure yellow-seeded plant, one whose family showed only the yellow
seed trait, and a pure green-seeded plant. In his notes, the letter P represented the parent plants. The
resulting offspring, the F1 generation (F stands for filial), were all yellow-seeded.
MONOHYBRID CROSS
From this, Mendel concluded that yellow seeds were controlled by a dominant factor, and that the
factor that caused the seeds to be green was recessive.
MONOHYBRID CROSS
Next, he took two of the F1 plants and crossed them. The F2 offspring showed a ratio of three yellowseeded plants for each green-seeded plant. His notes show 6,022 yellow seeds and 2,001 green seeds.
FAMILY TREE FOR PEA PLANTS
LAW OF SEGREGATION
Mendel came to the conclusion that each genetic trait was controlled by a distinct factor within each
plant. Parents had two of these factors that were separated during reproduction so that a set of
chromosomes from each parent was given to each offspring.
LAW OF SEGREGATION
Mendel’s Law of Segregation explained this separation. He called these factors genes. When the
genes come in two or more forms for a trait, they are called alleles for that trait.
1|Page
INTEGRATED SCIENCE
MR. SURRETTE
VAN NUYS HIGH SCHOOL
HOMOZYGOUS VERSUS HETEROZYGOUS
Mendel discovered that pure yellow-seeded parent plants (P) had two dominant alleles for yellow (Y/Y).
They were homozygous for the dominant trait. The green-seeded parent plant was homozygous for the
recessive green gene (y/y). F1 was always heterozygous (Y/y).
MONOHYBRID CROSS
The terms pure (homozygous) and hybrid (heterozygous) are also used by breeders. The F1 cross
between two parents each hybrid for a given trait is called a monohybrid cross.
PUNNETT SQUARE
A Punnett square is divided vertically and horizontally in half. If parental plants hybrid for seed color
are the subject, then the letters Y and y represent their possible genetic contribution.
PUNNETT SQUARE
The letters Y and y from one parent are placed above each column, and the letters Y and y from the other
parent are placed beside each row. The possible offspring in the four squares take a gene from each
parent.
PHENOTYPE VERSUS GENOTYPE
Of the four possible kinds of offspring, one is homozygous yellow (Y/Y), two are heterozygous yellow
(Y/y), and one is homozygous green (y/y). In a monohybrid cross, the dominant to recessive
phenotypic ratio is three to one. Phenotype refers to the appearance and genotype to whether it is
homozygous or heterozygous.
MONOHYBRID PEA CROSS
TEST CROSSING
To determine if a yellow-seeded plant was pure or hybrid, Mendel used a test cross. He crossed the
unknown plant, either pure (Y/Y) or hybrid (Y/y), with a plant possessing green seeds. Green seeds
indicated the pure recessive genotype. If the offspring were all yellow, the unknown parent must be
homozygous.
DYHYBRID CROSS
Consider the inheritance of two traits: seed shape and plant height. Round is dominant, so use R for
round and r for wrinkled. Tall is dominant, so use T and t. Cross a homozygous tall-round parent (T/T
R/R) with a homozygous short-wrinkled parent (t/t r/r).
2|Page
INTEGRATED SCIENCE
MR. SURRETTE
VAN NUYS HIGH SCHOOL
DYHYBRID CROSS
The gametes (sex cells) of the first parent would carry T/R. The gametes of the second parent would
carry t/r. The F1 offspring can have only one genotype (T/t R/r). This is called a dihybrid. It would
have the phenotype tall and round.
LAW OF INDEPENDENT ASSORTMENT
Crossing two of these dihybrid F1s gets interesting. Each F1 can produce the gametes: T/R, T/r, t/R, and
t/r. Mendel’s Law of Independent Assortment describes the results. For this dihybrid cross, the
Punnett Square is divided into quarters each way.
LAW OF INDEPENDENT ASSORTMENT
Across one side put T/R, T/r, t/R, and t/r. The same goes on the other side; there are sixteen possible
offspring types.
LAW OF INDEPENDENT ASSORTMENT
Filling in the sixteen squares shows that the phenotypic ratio is nine F2 with the dominant phenotype
(T/- R/-), three with a dominant-recessive phenotype (T/- r/r), three with a recessive-dominant
phenotype (t/t R/-), and one with the recessive phenotype (t/t r/r).
DIHYBRID PEA CROSS
3|Page
INTEGRATED SCIENCE
MR. SURRETTE
VAN NUYS HIGH SCHOOL
PARTIAL DOMINANCE
Partial dominance, also called incomplete dominance, is when a heterozygous individual shows a
third phenotype, often a blended phenotype. For example, a red snapdragon flower (R/R) crossed with a
white (W/W) will produce pink offspring R/W. Partial dominance is easier to work with because there
are no hidden traits.
HAPLOID VERSUS DIPLOID
Most organisms are diploid, which means they have two sets of chromosomes in their nuclei. One set
comes from one parent, and the other comes from the other parent. Having one set of chromosomes is
called haploid. Bacteria, some primitive plants and gametes are haploid.
HUMAN SEX CHROMOSOMES
In humans, the sex chromosomes are called “X” and “Y.” Females have a pair of X chromosomes, and
males have an X and a Y chromosome. The X chromosome is larger than the Y with more genes on it.
Human males are therefore haploid for some traits.
MEIOSIS
Meiosis is the cell division process that produces gametes.
STAGES OF MEIOSIS
4|Page
INTEGRATED SCIENCE