CHAPTER 11
MENDEL & HEREDITY
SC STANDARD B 4:
The student will understand the molecular basis of heredity.
ESSENTIAL QUESTION
How
does segregation of alleles
contribute to genetic variation?
ORIGINS OF HEREDITARY SCIENCE

Mendel” Breeding Experiments
 Genetics: is the science of heredity &the mechanism by
which traits are passed from parents to offspring
 Mendel
 born in Austrian Empire (today Czech Republic) in 1822
 Studied physics & mathematics @ University of Vienna
 Joined monastery in 1843 where he was put in charge of
the gardens
MENDEL’S EXPERIMENTS
 Mendel spent 2 years preparing his control
plants to insure they were true breeders
 describes organisms that are homozygous for
a specific trait so always produce offspring
that have the same phenotype for that trait
MENDEL’S EXPERIMENTS
MENDEL’S EXPERIMENTS

crossed true breeding, purple blossomed pea
plants with true breeding, white blossomed
pea plants and all the offspring had purple
flowers
 Then let the offspring self-pollinate and some
of the plants in that generation had purple
flowers & some had white
MENDEL’S EXPERIMENTS
 male parts were
removed from 1st
flower
 pollen taken from
male parts of 2nd
flower
 pollen from 2nd
brushed onto female
parts of 1st flower
MENDEL’S EXPERIMENTS
Vocabulary:
- character: a recognizable inherited feature
or characteristic of an individual
- trait: one of two or more possible forms of a
character
~ phenotype: physical characteristics
~ genotype: genetic makeup , what alleles an
organism has
MENDEL’S EXPERIMENTS
Vocabulary:
- hybrid: the offspring of a cross between
parents that have contrasting traits
- generation: the entire group of offspring
produced by a given group of parents
MENDEL’S EXPERIMENTS

3 reasons why the garden pea plant was good choice:
1.
2.
3.
Several characters appear in contrasting
forms
These flowers can self-pollinate because
each flower has both male & female parts
Plant is easy to grow
1. Matures quickly
2. Needs little care
3. Produces many offspring
MENDEL’S EXPERIMENTS
 Monohybrid Cross
MENDEL’S EXPERIMENTS

Monohybrid Cross: 3 Steps
1.
2.
3.
Produced a true-breeding
parent generation (P
generation)
Produced 1st filial generation
( F 1 generation)
Produced 2nd filial generation
( F 2 generation)
MENDEL’S EXPERIMENTS
 True breeding
purple
 True breeding white
MENDEL’S EXPERIMENTS
 Step 2: cross
pollinated parents
 F 1 generation all
purple
 Self-pollinated
 F 2 generation
3 : 1 purple to
white
MENDEL’S EXPERIMENTS
 Mendel repeated these experiments with 7
different traits in pea plants:
For each of the 7 characters he found a similar
3 : 1 ratio of contrasting traits in the F 2
generation
MENDEL’S EXPERIMENTS
MENDEL’S EXPERIMENTS
 Ratios in Mendel’s Results
 F 1 generation expressed the same trait for
any of the 7 characteristics he studied
 When F 1 plants allowed to self-pollinate he
always saw a 3 : 1 ratio of contrasting traits
MENDEL’S THEORY
 Explains simple patterns of inheritance
 2 of several versions of a gene combine &
result in 1 of several possible traits
 Allele:
one of two or more alternative forms
of a gene each leading to a unique trait
MENDEL’S THEORY

Dominant: describes an
allele that is fully
expressed whenever the
allele is present
 Recessive: describes an
allele that is expressed
only when there is no
dominant allele present
MENDEL’S THEORY

Law of Segregation of Alleles:
 When an organism produces gametes, each
pair of alleles on homologous chromosomes
separate in Meiosis I and each gamete has
an equal chance of receiving either one of
the alleles
MENDEL’S THEORY:
LAW OF SEGREGATION OF ALLELES
MENDEL’S THEORY

GENOTYPE: a specific combination of alleles in an
individual….. the “genes” an individual has
 example: AA, Aa, or aa

PHENOTYPE : the detectable trait or traits that result
from the genotype of an individual….. the “physical
appearance” an individual has
 example: normal, normal, albino
MENDEL’S THEORY
GENOTYPE
DETERMINES
PHENOTYPE !
MENDEL’S THEORY
The genotype of each of the peas is ____________.
MENDEL’S THEORY
 The phenotype of each of the following is _____.
MENDEL’S THEORY

Homozygous: describes an individual that carries
two identical alleles of a gene
 Example: PP or pp

Heterozygous: describes an individual that carries two
different alleles of a gene
 Example: Pp
MENDEL’S THEORY
 Mendel’s 2nd Experiments

Dihybrid crosses: involves test crossing two
characters

Law of Independent Assortment: during gamete
formation, the alleles on non-homologous
chromosomes segregate independently
MENDEL’S THEORY
PROBLEM SOLVING:
PRODUCING TRUE-BREEDING SEEDS
 Textbook





page 271
Work in table groups
Define the problem
Organize information
Create solution
Present to class
MENDEL’S THEORY
 When genes are close together on same
chromosome they will rarely separate
independently so are said to be
“linked”.
MODELING MENDEL’S LAWS
 Punnett Square:
a graphic used to predict the
results of a genetic cross
MODELING MENDEL’S LAWS
 A Punnett Square shows all the genotypes that could
possibly result from any given cross match.
MODELING MENDEL’S LAWS
 Monohybrid Homozygous Cross
 Draw a Punnett Square crossing
homozygous Y (for yellow seed color) with
homozygous y (for green seed color)
 What is the ratio of yellow to green seeds ?
 Monohybrid Heterozygous Cross
 Draw a Punnett Square crossing 2 plants
that are heteroygous for Y
 What is the ratio of yellow to green seeds?
MODELING MENDEL’S LAWS
 Test Cross:
used to test an individual whose
phenotype for a given characteristic is dominant but
its genotype is unknown
 Individual is crossed with a known homozygous
recessive
If unknown is homozygous dominant all
offspring will show dominant phenotype
If unknown heterozygous for the trait then ½ the
offspring will show dominant phenotype & ½
will show recessive trait
MODELING MENDEL’S LAWS
 Using Probability
 Probability:
the likelihood that a specific
event will occur; expressed in mathematics
Probabilities are used to predict the
likelihood that specific alleles will be
passed down to offspring
QUICK LAB: PROBABILITIES: PAGE 268
 Notebook: page
15
 Everyone completes this:
 Follow procedure
 Answer analysis questions 1 - 2
MODELING MENDEL’S LAWS
Pedigree:
a diagram that shows the
occurrence of a genetic trait in several
generations of a family
Genetic Disorder:
an inherited disease
that is caused by a mutation in a gene or
by a chromosome defect
PEDIGREES
PEDIGREES
PEDIGREES
MODELING MENDEL’S LAWS

Pedigrees can help answer 3 aspects of
inheritance:
1. Sex linkage
2. Dominance
3. Heterozygocity
MODELING MENDEL’S LAWS
1.
Sex-Linked Gene
1. Gene located on either the X or Y chromosomes
2. Females have 2 X chromosomes so rarely show
the recessive phenotype; males have just 1 X
chromosome so will show the trait for a single
recessive allele for genes on the X chromosome
3. If find a trait that is more common in males than
females it is likely sex-linked
GENES ON SEX CHROMOSOMES

Sex-Linked
MODELING MENDEL’S LAWS
2. Dominant or Recessive?

If a child shows a trait and neither parent
shows the trait it is likely a recessive trait
MODELING MENDEL’S LAWS
3.

Heterozygous or Homozygous?
Recessive trait in a child shows parents
had to be heterozygous for the trait
BEYOND MENDELIAN HEREDITY
Polygenic Character:
a character
influenced by more than 1 gene
 includes many characters in humans
Eye color
Skin color
Height
BEYOND MENDELIAN HEREDITY
 Incomplete Dominance:
the phenotype for a
heterozygous individual is intermediate
between the homozygous dominant phenotype
and the homozygous recessive phenotype
BEYOND MENDELIAN HEREDITY
 Genes that are said to have 3 or more possible alleles
are said to have multiple alleles
 Example: human’s ABO blood types
BEYOND MENDELIAN HEREDITY
 Codominance:
a condition in which both
alleles for a gene are fully expressed in the
phenotype
BEYOND MENDELIAN HEREDITY
 Genes Affected by the Environment
Nutrients available or temperature can affect the
expression of the genotype
 Examples
Some animals have fur that changes color with
the seasons

BEYOND MENDELIAN HEREDITY
 During meiosis,
genes that are close together
on the same chromosome are less likely to be
separated than genes that are far apart
 Genes that are close together and the traits
they determine are said to be linked (not just
sex-linked)
BEYOND MENDELIAN HEREDITY
 Linked Genes