Ch 14: Genetics: Pile Driver of
Knowledge
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.
Chapter 14: Genetics
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 co-dominant 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:
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
• 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.
• 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
Terms to Remember
• Gene
• Allele
• Chromosomes
• Chromatin
• Loci
• Haploid/Diploid
• Character/Trait
• Homozygous/ True-breeding
• Heterozygous/ hybrid
• Phenotype
• Genotype
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
Punnett Square Review
• Monohybrid
• Dihybrid
• Phenotypic Ratios?
• Genotypic Ratios?
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?
Answer:
Probability that egg will get a p: ½ Probability that sperm will get a p: ½ Solve: ½ x ½ = ¼
Pp female Pp male
Formation of eggs Formation of sperm
•
Dihybrid answer
–
YyRr x YyRr
–
Probability of YYRR
–
Egg have Y and R
• ½ x ½ = ¼–
Sperm have Y and R
• ½ x ½ = ¼- Chance that egg and sperm will pass Y and R at the same time:
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 (2) Dom Sperm with Rec Egg
½ x ½ = ¼ ½ x ½ = ¼
Beyond Simple Inheritance
• Incomplete Dominance: results in “blending” when neither allele is completely dominant over the other
• Codominance: when both alleles affect the phenotype, so you see both at the same time.
Ex: MN blood groups
O
A
B
AB
ii
IA IA
or
IA i
IB IB
or
IB i
IA IB
• 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
• Polygenic inheritance: an additive effect of two or more genes on a single phenotype.
– Quantitative characters: vary in the population in gradations Ex. Skin color or height
– AABBCC: dark
– AaBbCc: intermediate
– Aabbcc: light
Fraction of population
Nature Vs. Nurture
• Environmental impact on phenotype
• Altitude on blood etc…
• behavior
• Phenotype integrates the environment and the
Pedigree Chart Review
• Review various pedigrees to determine
- Dominant - Recessive
• Consanguinity: relationship by descent from common
ancestor - Autosomal - Sex linked
• Fetal Testing
• Amniocentesis: http://www.doctoroz.com/videos/amniocentesis-animation
• Chorionic Villus sampling: http://www.muschealth.com/video/Default.aspx?videoId=10068&cId=34&type=rel
• Ultrasound
• Fetoscopy
• Newborn screening
• PKU: PKU is an inherited disease in which the body cannot metabolize a 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.
–
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.
–
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.
