Download Chapter 8 “Mendel and Heredity”

Chapter 11
“Mendel and Heredity”
• Heredity is the passing of
traits from parents to their
offspring.
• The study of heredity is
called Genetics.
• Gregor Mendel was an
Austrian monk who first
studied heredity by breeding
different varieties of garden
peas.
• Mendel is considered the
“Father of Genetics” because
he was the first to develop
rules that accurately predict
patterns of heredity.
Useful Features in Peas
1. Seven traits that
have two clearly
different forms that
are easy to tell
apart.
• For example, the
flower color is either
purple or whitethere are no
intermediate forms.
2. Mating of
the gardenpea flowers
are easily
controlled• Both male
and female
gametes
are on the
same
flower.
• Mendel carefully
cross-pollinated his
pea plants by
removing the
stamen (male
reproductive organ
that produces
pollen) from the
flower of one plant.
• Then he dusted the
pistil (female
reproductive organ
that produces eggs)
of that plant with
pollen from a
different pea plant
3. The garden pea is small, grows easily,
matures quickly and produces many
offspring.
Mendel’s Work Became a Theory
of Heredity
• Before Mendel’s experiments, many
people thought offspring were just a blend
of the characteristics of their parents.
• Mendel’s results did not support the
blending hypothesis. He correctly
concluded that each pea has two separate
genes for each trait- one from each parent.
Mendel’s Hypotheses
1. For each inherited trait, an individual has two copies of
the gene- one from each parent.
2. There are alternative versions of genes. For example,
the gene for flower color can be purple or white.
Different versions of a gene are called alleles.
3. When two different alleles occur together, one of them
may be completely expressed, while the other may
have no effect on the organism’s appearance.
• The expressed forms of a trait is dominant.
• The trait that is not expressed when a dominant trait is
present is described as recessive.
4. When gametes are formed, the alleles for
each gene in an individual separate
independently of one another.
• So, gametes only carry one allele for each
trait.
Genetics
• Dominant alleles are written by capitalizing
the first letter of the trait.
• Recessive alleles are also written using
the first letter of the trait but it is not
capitalized.
• If the two alleles of a particular gene
present in an individual are the same, the
individual is homozygous for that trait.
• If the two alleles of a particular gene are
different, the individual is heterozygous for
that trait.
• In heterozygous individuals, only the
dominant allele is expressed.
• The physical appearance of a trait is called
a phenotype
• The set of alleles that a person has is
called the genotype
Mendel’s Ideas Gave Rise to the
Laws of Heredity
• Law of Segregation- during meiosis, the
two alleles for a trait separate when
gametes are formed.
• Law of Independent Assortment- the
inheritance of one trait does not influence
the inheritance of any other trait.
• For example, the alleles for plant height
separate independently of the alleles for
flower color.
Studying Heredity
• Punnett square is a diagram that predicts
the expected outcome of a genetic cross
by considering all the possible
combinations of gametes in the cross.
• Monohybrid Cross- a cross that
considers one pair of contrasting traits
between two individuals.
• Use a punnett square to predict the
outcome of a cross between a pea plant
that is homozygous for yellow seed color
(YY) and a pea plant that is homozygous
for green seed color (yy).
Pedigrees
• A pedigree is a diagram that shows how a family
trait is inherited over several generations.
• They are helpful if the trait is a genetic disorder
and the family members want to know if they are
carriers.
• Carriers are individuals who are heterozygous
for an inherited disorder but do not show
symptoms of the disorder.
• Carriers can pass the allele for the disorder to
their offspring.
•
•
•
•
“Squares” = males
“Circles” = females
Horizontal lines = matings
Vertical lines indicate offspring (arranged from left to
right in order of their birth).
• Colored symbols represent affected individuals
Autosomal or Sex-linked?
• Autosomal traits will appear in both sexes
equally. Autosomes are chromosomes other
than an X or Y sex chromosome.
• Sex-linked traits are located on the X
chromosome. Most sex-linked traits are
recessive.
• Because males have only one x chromosome, a
male who carries a recessive allele on the X
chromosome will have the condition.
• Autosomal dominant traits are expressed if
the individual has a parent with the trait.
• If the autosomal trait is recessive, one,
both or neither of the parents may have
the trait.
• Do page 343 “Evaluating a pedigree”.
Autosomal recessive or autosomal
dominant?
Complexity of Heredity
• Polygenic Traits- several genes influence
a trait.
• The genes may be scattered along the
same chromosome or located on a
different chromosome.
• Many different combinations appear in
offspring.
Intermediate Traits
• Examples include:
eye color, height,
weight and
hair/skin color.
• All have
intermediate
conditions between
one extreme and
the other.
Blue eyes
Blue/black eyes
Incomplete dominance
• Intermediate traits- a
blending of traits from
female and male.
• Snapdragons
example: red flower X
white flower = pink
flower
Codominance
• Two dominant traits
are expressed at the
same time.
• A cross between a
homozygous red
horse and a
homozygous white
horse results in a
heterozygous
offspring with both red
and white hairs,
producing the mixed
color roan.
Multiple Alleles
• Genes with 3 or more alleles.
• ABO blood groups (which produces 4
different blood types) are determined by 3
alleles: IA, IB and i.
• IA and IB are dominant over recessive i.
When both IA and IB are present, they are
codominant.
Traits influenced by the
environment
• Examples: soil acidity
determines flower color
in hydrangeas.
• Arctic fox’s fur changes
color according to
changes in the
temperature.
• Enzymes make pigments
in the summer to darken
the coat.
• In the winter, no pigment
is made so that it can
blend with the
environment.