Transcript Chapter 5: Patterns of Inheritance - ahs

UNIT 2: Genetic Processes
Chapter 4: Cell Division and Reproduction
Chapter 5: Patterns of Inheritance
How are traits inherited, and how can
inheritance be predicted?
Chapter 6: Complex Patterns of Inheritance
UNIT 2 Chapter 5: Patterns of Inheritance
5: Patterns of Inheritance
Canola (Brassica napus) is a
Canadian success story. It was
developed in a traditional selective
breeding program in the 1970s. It is
now a valuable Canadian crop that
benefits from continued modern,
molecular genetics research.
UNIT 2 Chapter 5: Patterns of Inheritance
Section 5.1
5.1 Understanding Inheritance
While people bred animals and plants for thousands of years without
understanding the mechanisms of inheritance, eventually theories and
explanations of how breeding worked were proposed.
The first widely accepted theory was pangenesis, proposed by Aristotle. It
suggested that sperm and egg contained tiny particles from all body parts.
Others thought that only the sperm had such an essence. In fact, it was
proposed that an entire miniature human being was inside the sperm!
By the 1800s, people settled on the idea that traits from the
parents were irreversibly blended in the offspring.
None of these theories was based on scientific evidence.
UNIT 2 Chapter 5: Patterns of Inheritance
Section 5.1
Gregor Mendel’s Experiments
Gregor Mendel (1822–84), an Augustinian monk, used scientific methods
to solve the mystery of how traits were inherited. Before his time at the
monastery, he studied botany and mathematics, which proved invaluable
to his observations.
Between 1856 and 1863,
Mendel bred, tended, and
analyzed more than 28 000
pea plants in his monastery
garden.
One of the keys to his discovery was the plant type he chose to work
with: pea plants. Pea plants come in many varieties and show different
traits (characteristics exhibited by an organism). In addition, they usually
self-fertilize, which allowed Mendel to start with plants that were true
breeding (same outcome traits every generation). He carefully crosspollinated true-breeding pea plants.
UNIT 2 Chapter 5: Patterns of Inheritance
Section 5.1
Mendel’s Monohybrid Experiments
Mendel started every experiment with plants that were true breeding for a
trait but that exhibited a different form of the trait. He called this the
parental, or P generation. Offspring were called the first filial (F1)
generation. These experiments were called monohybrid crosses because
only one (mono) trait was monitored at a time. However, Mendel studied
seven different traits in his experiments.
UNIT 2 Chapter 5: Patterns of Inheritance
Section 5.1
Mendel’s Results
Example 1: P generation of male yellow-pea-producing plant
and female green-pea-producing plant
P generation cross results: All offspring (F1 generation) were
the same seed colour: yellow, i.e., one parent’s seed colour
trait seemed to disappear. This result was the same for each
of the seven traits he studied.
Continued…
UNIT 2 Chapter 5: Patterns of Inheritance
Section 5.1
Mendel’s Results
Example 2: F1 generation of yellow-pea producing plants
F1 generation cross results: In the F2 generation, some peas
were yellow and some green. Mathematically, the ratio was
3:1 yellow:green. This ratio was the same for all seven traits
that Mendel studied.
UNIT 2 Chapter 5: Patterns of Inheritance
Section 5.1
The Law of Segregation
Mendel concluded that there must be two hereditary “factors”
for each trait. Today we call those factors “alleles.” Recall
that diploid organisms have two alleles for each gene.
He also concluded that one factor/allele is always dominant,
and one is recessive. In the example, yellow colour is
dominant over green when it comes to the colour of seeds in
the pea plant.
Mendel proposed the “law of segregation” to explain this:
Traits are determined by pairs of alleles that segregate during
meiosis so that each gamete receives one allele (updated
terminology).
UNIT 2 Chapter 5: Patterns of Inheritance
Section 5.1
Genotype and Phenotype
To express alleles easily in written form, upper and lower case letters are
used. A dominant allele is represented by the first letter of the allele’s
description. The recessive allele then receives the lower case of the same
letter.
Yellow pea allele: Y
Green pea allele: y
In each plant, two alleles are present so the possible combinations are:
YY, Yy, or yy. This is the plant’s genotype.
The actual colour of the peas is the plant’s phenotype.
UNIT 2 Chapter 5: Patterns of Inheritance
Section 5.1 Review
Section 5.1
UNIT 2 Chapter 5: Patterns of Inheritance
Section 5.2
5.2 Studying Genetic Crosses
The possibility of a certain allele packaged in a gamete is ½ since there are
two alleles in a diploid cell and only one is packaged in a haploid gamete.
Thus, when determining
the possible outcomes of a
monohybrid cross, there is
½ X ½ = ¼, or a 25%
chance of each combination
of alleles in the offspring.
We use a grid called a
Punnett square to show the
law of segregation and
possible cross outcomes.
UNIT 2 Chapter 5: Patterns of Inheritance
Section 5.2
Using Punnett Squares
A Punnett square demonstrates the possible F1 outcomes
from a cross between two heterozygous parents. In this case,
the parents are heterozygous for flower colour. The
phenotype ratio is 3:1 for flower colour (purple to white).
UNIT 2 Chapter 5: Patterns of Inheritance
Section 5.2
Test Crosses
When geneticists want to know if an individual is
heterozygous or homozygous for a dominant phenotype, they
do a test cross. A test cross is a cross between an individual
of unknown genotype for a trait and an individual that is
homozygous recessive for that trait. Analyzing the phenotype
should provide insight into the unknown genotype.
In a test cross, if any
of the offspring show
the recessive
phenotype, the
unknown genotype of
the parent must be
heterozygous.
UNIT 2 Chapter 5: Patterns of Inheritance
Section 5.2
Mendel’s Dihybrid Crosses
Mendel also designed experiments to follow the inheritance pattern of
two traits to determine if the inheritance of one trait affected another.
He crossed true-breeding plants that produced yellow, round seeds
(YYRR) with true-breeding plants that produced green, wrinkled seeds
(yyrr). The peas in the F1 generation all displayed the dominant trait for
both traits (yellow and round).
What do you think the F2 generation looked like?
Explain your answer.
Continued…
UNIT 2 Chapter 5: Patterns of Inheritance
Section 5.2
Mendel’s Dihybrid Crosses
The F1 generation self-fertilized to create the F2 generation. It had a mix
of four phenotypes but came close to the ratio 9:3:3:1 (yellow, round to
yellow, wrinkled to green, round to green, wrinkled).
UNIT 2 Chapter 5: Patterns of Inheritance
Mendel’s Results
A Punnett square can
show the segregation of
the gametes for two traits.
Each parent can package
the alleles in the gametes
in four different ways.
Section 5.2
UNIT 2 Chapter 5: Patterns of Inheritance
Section 5.2
The Law of Independent Assortment
Mendel found the 9:3:3:1 ratio for every dihybrid cross he
performed. This is expected only if the inheritance of one
trait has no influence on the inheritance of another trait. He
described these events in the law of independent
assortment. Using current terminology, this law states that
the alleles for one gene segregate or assort independently of
the alleles for other genes during gamete formation.
UNIT 2 Chapter 5: Patterns of Inheritance
Section 5.2
The Chromosome Theory of Inheritance
When Mendel performed his experiments and formulated his
laws of inheritance, the process of meiosis and the existence
of chromosomes had not been discovered. By the early
1900s, scientists began to see the link between both.
Continued…
UNIT 2 Chapter 5: Patterns of Inheritance
Section 5.2
The Chromosome Theory of Inheritance
In 1902, Walter Sutton showed that the behaviour of
chromosomes during meiosis was related to the behaviour of
Mendel’s factors. He realized that during gamete formation,
alleles segregate just as homologous chromosomes do, and
proposed that genes are carried on chromosomes. This
formed the basis of the chromosome theory of inheritance:
Genes are located on chromosomes, and chromosomes
provide the basis for the segregation and independent
assortment of alleles.
UNIT 2 Chapter 5: Patterns of Inheritance
Section 5.2 Review
Section 5.2
UNIT 2 Chapter 5: Patterns of Inheritance
Section 5.3
5.3 Following Patterns of Inheritance
in Humans
Geneticists who study human inheritance collect as much
information as they can and use it to create a diagram called a
pedigree. A pedigree is a type of flow chart that uses
symbols to show the inheritance patterns of traits in a family
over many generations. They help uncover the genotype of a
particular member of a family, and they can be used to
predict phenotypes and genotypes of future offspring.
How is human genetic research different from genetic
research on plants and animals?
Continued…
UNIT 2 Chapter 5: Patterns of Inheritance
Section 5.3
Following Patterns of Inheritance in Humans
UNIT 2 Chapter 5: Patterns of Inheritance
Section 5.3
Autosomal Inheritance
Autosomal inheritance refers to the inheritance of traits
whose genes are found on the autosomes (chromosomes 1 –
22). These chromosomes hold normal, functioning genes
(hair colour, freckles) as well as disorder genes (cystic
fibrosis, Huntington disease).
An autosomal dominant disorder occurs when the diseasecausing allele is dominant and an individual has one or both
copies of the allele. An autosomal recessive disorder occurs
when the disease-causing allele is recessive and an individual
has both copies of the allele.
UNIT 2 Chapter 5: Patterns of Inheritance
Section 5.3
Autosomal Inheritance
When using a pedigree to study a disorder, you can determine
if the pattern is autosomal dominant or autosomal recessive.
Huntington Disease: Autosomal Dominant
An unaffected child born of two affected
parents indicates autosomal dominant
inheritance.
This pedigree shows the inheritance pattern
for an autosomal dominant disorder. Notice
that an affected child must have at least one
affected parent to be affected.
UNIT 2 Chapter 5: Patterns of Inheritance
Section 5.3
Autosomal Inheritance
Cystic Fibrosis: Autosomal Recessive
In autosomal recessive inheritance, if
both parents are heterozygous for the
disorder, they will have an affected child.
This pedigree shows the inheritance pattern
for an autosomal recessive disorder.
Notice that the appearance of the recessive
phenotype can skip generations, and that two
unaffected parents can have an affected child.
UNIT 2 Chapter 5: Patterns of Inheritance
Tests for Genetic Diseases
Section 5.3
UNIT 2 Chapter 5: Patterns of Inheritance
Section 5.3
Genetic Counselling
A genetic counsellor has special training in human genetics
and in counselling. A family may seek a counsellor when
there is a history of a genetic disorder in the family.
Counsellors often use pedigrees to determine offspring risk.
UNIT 2 Chapter 5: Patterns of Inheritance
Section 5.3 Review
Section 5.3