Ch 14: Genetics
2016
Chapter 14: Genetics
From Topic 3.1
Essential idea: Every living organism inherits a blueprint for life from its
parents.
Understandings:
• A gene is a heritable factor that consists of a length of DNA and influences a
specific characteristic.
• A gene occupies a specific position on a chromosome.
• The various specific forms of a gene are alleles.
• Alleles differ from each other by one or only a few bases.
• New alleles are formed by mutation.
Applications and skills:
• Application: The causes of sickle cell anemia, including a base substitution
mutation, a change to the base sequence of mRNA transcribed from it and a
change to the sequence of a polypeptide in hemoglobin.
From Topic 3.2
Essential idea: Chromosomes carry genes in a linear sequence that is shared
by members of a species.
Understandings:
• In a eukaryote species there are different chromosomes that carry
different genes.
• Homologous chromosomes carry the same sequence of genes but not
necessarily the same alleles of those genes.
From Topic 3.3
Applications and skills:
• Application: Description of methods used to obtain cells for karyotype
analysis e.g. chorionic villus sampling and amniocentesis and the associated
risks.
Utilization:
• An understanding of karyotypes has allowed diagnoses to be made for the
purposes of genetic counselling.
Aim 8: Pre-natal screening for chromosome abnormalities gives an indication
of the sex of the fetus and raises ethical issues over selective abortion of
female fetuses in some countries.
From Topic 3.4
Essential idea: The inheritance of genes follows patterns.
Nature of science: Making quantitative measurements with replicates
to ensure reliability. Mendel’s genetic crosses with pea plants
generated numerical data (3.2).
Understandings:
•Dominant alleles mask the effects of recessive alleles but codominant alleles have joint effects.
• Many genetic diseases in humans are due to recessive alleles of
autosomal genes, although some genetic diseases are due to
dominant or co-dominant alleles.
Applications and skills:
•Application: Inheritance of ABO blood groups.
• Application: Inheritance of cystic fibrosis and Huntington’s disease.
• Skill: Analysis of pedigree charts to deduce the pattern of inheritance
of genetic diseases.
• Skill: Construction of Punnett grids for predicting the outcomes of
monohybrid genetic crosses.
• Skill: Comparison of predicted and actual outcomes of genetic
crosses using real data.
Guidance:
• The expected notation for ABO blood group alleles is
From Topic 7.2
Nature of science:
Looking for patterns, trends and discrepancies—there is mounting
evidence that the environment can trigger heritable changes in
epigenetic factors (3.1).
Chapter 14: Genetics
From Topic 10.2
Understandings:
• Variation can be discrete or continuous.
• The phenotypes of polygenic characteristics tend to show continuous variation.
• Chi-squared tests are used to determine whether the difference between an observed and expected frequency distribution is
statistically significant.
Applications and skills:
• Application: Completion and analysis of Punnett squares for dihybrid traits.
• Application: Polygenic traits such as human height may also be influenced by environmental factors
• Skill: Calculation of the predicted genotypic and phenotypic ratio of offspring of dihybrid crosses involving unlinked autosomal genes.
Utilization:
• An understanding of inheritance allowed farmers to selectively breed their livestock for specific characteristics.
Aim 4: Use analytical skills to solve genetic crosses.
Aim 8: Ethical issues arise in the prevention of the inheritance of genetic disorders.
Genetics Before Mendel
• Blended Theory of Inheritance: genetic material contributed
by the two parents mixes, “blends” together
- Over many generations, the population should reach
uniform appearance.
- Also fails to explain why some traits reappear after
skipping a generation
• Particulate Theory of Inheritance (aka the gene idea):
parents pass heritable units (aka genes) that retain their
separate identities in offspring  this is what Mendel was
working on…
Gregor Mendel’s Bio
• In 1851-1853, he studied
at University of Vienna
• Taught by physicist
Christian Doppler (aka the
Doppler effect) and
encouraged him to use
math in science
• Also was taught by a
botanist Franz Unger, who
sparked Mendel’s interest
in variation in plants
• In 1857, Mendel bred
garden peas to study the
pattern of inheritance.
Why Garden Peas???
• Flower color
– Purple or white
• Flower position
– Axial or Terminal
• Seed color
– Yellow or Green
• Seed shape
– Wrinkled or round
• Pod Shape
– Inflated or constricted
• Pod Color
– Green or yellow
• Stem Length
– Tall or dwarf
Garden Pea Experiment
• Produced True Breeding plant varieties
• Always produced same offspring
• True Breeding parents are called P generation
• Hybrid offspring of the P generation is called the F1
• If they are fertilized and produce offspring the offspring are
called the F2
Terms to Remember
• Gene
• Allele
• Chromosomes
• Chromatin
• Loci
• Haploid/Diploid
• Characteristic/
trait
• Homozygous/
True-breeding
• Heterozygous/
hybrid
• Phenotype
• Genotype
• Test Cross
Mendel’s Laws
• Law of Segregation: Two
alleles are packed into
separate gametes.
• Dominance
• Recessive
• A or a
• T or t
• Law of Independent
Assortment: Each pair of
alleles segregates to the
gametes independently.
Punnett Square Review
• Monohybrid
• Dihybrid
• Phenotypic Ratios?
• Genotypic Ratios?
http://www.sumanasinc.com/webcontent/animations/content/mendel/mendel.html
Probability Rules
• Rule of Multiplication: The
probability that
independent events will
occur simultaneously is the
product of their individual
probabilities.
Question: Cross plants that are
heterozygous for flower color.
What is probability of them being
homozygous recessive?
Pp female
Formation of eggs
1/
1/
2
P
2
P P
P
1/
Formation of sperm
P
1/
2
1/
1/
4
1/
4
p
1/
P
4
2
P
P
P
P
Answer:
Probability that egg will get a p: ½
Probability that sperm will get a p: ½
Solve: ½ x ½ = ¼
Pp male
4
P
• Dihybrid answer
- Chance that egg and sperm will
pass Y and R at the same time:
– YyRr x YyRr
– Probability of YYRR • ¼
– Egg have Y and R
• ½x½=¼
– Sperm have Y and R
• ½x½=¼
x
¼
= 1/16
Probability Rules
• Rule of Addition: The probability of an event that can occur
in two or more independent ways is the sum of the
separate probabilities of the different ways.
Question: Cross two Heterozygous for flower color. What is the
probability that the offspring is heterozygous?
Answer:
Two ways to be heterozygous:
(1) Dom Egg with Rec Sperm
½x½=¼
Overall: ¼ + ¼ = 2/4 = ½
(2) Dom Sperm with Rec Egg
½x½=¼
Beyond Simple Inheritance
• Incomplete Dominance: results in “blending” when neither
allele is completely dominant over the other
Ex: Four O’clock Flowers
• Codominance: when both alleles affect the phenotype, so you
see both at the same time.
Ex: MN blood groups
Note: Dominance doesn’t have to do with which can subdue the
other allele, but rather a phenotypic expression. It’s also not a
matter of which allele is more common.
• Multiple Allele Traits: when a gene is determined by
multiple alleles (3 or more) but an individual only gets two
out of the possible alleles.
Ex: ABO Blood Group
Blood Group
Genotypes
(Phenotype)
O
ii
A
IA IA
or
IA i
B
IB IB
or
IB i
AB
IA IB
• Polygenic inheritance: an additive effect of two or more
genes on a single phenotype.
Fraction of population
– Quantitative characters: vary in the population in gradations
Ex. Skin color or height
– AABBCC: dark
– AaBbCc: intermediate
– Aabbcc: light
Skin pigmentation
Nature Vs. Nurture
• Environmental impact on
phenotype
• Altitude on blood etc…
• behavior
• Phenotype integrates the
environment and the
genotype.
Pedigree Chart Review
• Review various pedigrees to determine
- Dominant
- Recessive
• Consanguinity: relationship by descent from common
ancestor
- Autosomal
- Sex linked
- Non-nuclear inheritance: not pass from the nucleus’ DNA
Pedigree: Dominant
Pedigree: Dominant
Non-Nuclear
Pre-Natal Screening
• Fetal Testing
• Amniocentesis:
• Chorionic Villus sampling:
• Ultrasound
• Fetoscopy
• Newborn screening
• PKU: PKU is an inherited disease in which the body cannot metabolize a
http://www.doctoroz.com/videos/amniocentesis-animation
http://www.muschealth.com/video/Default.aspx?videoId=10068&cId=34&type=rel
protein called phenylalanine. It is estimated that one baby in 25,000 is
born with PKU in the US. Without treatment, PKU can cause mental
retardation. Newborn screening for PKU is required in all 50 states.
Human Genetic Disorders
Recessive Genetic Disorders
– Cystic fibrosis
• Is the most common lethal genetic disease in the United
States
• Is caused by a recessive allele carried by about one in 25
people of European ancestry
– Prolonged geographic isolation of certain
populations can lead to inbreeding, the mating of
close relatives.
• Inbreeding increases the chance of offspring that are
homozygous for a harmful recessive trait.
Dominant Genetic Disorders
– Some human genetic disorders are dominant.
• Huntington’s disease, which leads to degeneration of the
nervous system, does not begin until middle age.
• Achondroplasia is a form of dwarfism.
– The homozygous dominant genotype causes death of the embryo.
– Thus, only heterozygotes have this disorder.