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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 yellowseeded 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 yellowseeded.
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
yellow-seeded 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.
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).
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 dominantrecessive 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
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