Pre Med III Genetics
Guri Tzivion, PhD
tzivion@windsor.edu
Extension 506
Winter 2016
Windsor University School of Medicine
Pre Med III Genetics
Class 2
Genes, Chromosomes and
Heredity
1. Intro to genetics: from
Mendelian to modern genetics
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Ancient Greek: genetikos, genesis, origin
The science of heredity and variation in
living organisms
Living organisms inherit traits from their
parents. Beginning from prehistoric times,
people improved crop plants and animals
through selective breeding
Modern genetics basis by Gregor Mendel
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Greek (chroma, color) and (soma, body):
chromosomes strongly stain by particular dyes
An organized structure of DNA and protein
A single piece of coiled DNA containing
many genes, regulatory elements and
other nucleotide sequences
In eukaryotes, nuclear chromosomes are packaged
by proteins into a condensed structure
called chromatin
The complex structure helps a very long DNA
molecules to fit into the cell nucleus
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The basic unit of heredity in a living organism
Contains the information to build and maintain
the cells and passes genetic traits to offspring
In general terms, a gene is a segment of nucleic
acid that, taken as a whole, specifies a trait
Genome:
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The full set of chromosomes or genes in a gamete
In diploid organisms, somatic cells contain two full
sets of genomes, while gamete cells contain a single
set
In haploid organisms, including bacteria, viruses,
and mitochondria, a cell contains only a single set of
the genome
Genomics:
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The study of the genomes of organisms
The entire DNA sequence of organisms and fine-scale
genetic mapping efforts
Research of single genes does not fall into the
definition of genomics
Human Cell
The 46 human chromosomes
What is heredity?
The passing on of characteristics from parents to
offspring.
Genetics is the branch of biology that studies
heredity
the characteristics that are inherited are traits.
Mendel was the first person to success in
predicting how traits would be transferred from
one generation to the next.
phenotype
The way an organism looks and behaves.
from Greek: phainein, meaning "to show",
and typos, meaning "type”. It is the
composite of an organism's observable
characteristics or traits
Example: what color are your eyes, or
your hair?
What you see is the phenotype.
Factors contributing to the phenotype
Genotype
Activities of
genes & gene
products
Environment
&
development
Phenotype
Genotype: collection of genes (and alleles) in an organism
Phenotype: observable properties of an organism
Is inheritance blending or particulate?
1.
In the mid 19th century, biologists believed that inheritance involved blending of
traits, meaning that traits of offspring were the average of their parents.
2.
This view was problematic since it suggested that new genetic variations would
quickly be diluted and could not be accumulated and passed to subsequent
generations as the theory of evolution predicted.
3.
The blending theory of inheritance was quickly discredited by Mendel’s
experiments, which showed that inheritance is particulate.
F1
F2
Mendelian Genetics:
Gregor Johann Mendel (1822-1884)
Augustinian monk, Czech Republic
Founding of modern genetics
Studied segregation of traits in the garden
pea (Pisum sativum) beginning in 1854
Published his theory of inheritance in 1865:
“Versuche über Pflanzen-Hybriden”
“Experiments in Plant Hybridization”
Mendel was “rediscovered” in 1902
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Gregor Mendel (1822–1884) a priest and scientist
Considered the father of genetics for his study in the 1860s
of the inheritance of specific traits in pea plants
Hypothesized that factors convey traits from parents to
offspring
Spent over 10 years on one experiment
Showed that the inheritance of certain traits follow
particular laws
Mendel's work was not recognized until the turn of the
20th century
Didn’t use the term gene, explained results in terms of
inherited characteristics
Dominant and recessive traits, the distinction between
a heterozygote and homozygote, genotype and phenotype
Mendel’s Subjects
Mendel chose pea plants, which
reproduce sexually, meaning that they
have two distinct sex cells, male and
female.
Sex cells are called gametes.
Pollination: the transfer of male pollen
grains to the pistil of a flower
Fertilization: combination of male and
female gametes
Advantages of peas
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They reproduce sexually
They have two distinct: male and
female sex cells called gametes
Their traits are easy to isolate
Mendel’s subject
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Available in many varieties, inexpensive, short
life cycle, simple procedures, easy to grow.
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The use of plants also allowed strict control over
the mating.
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He chose to study only characters that varied in
an ‘either-or’ rather than a ‘more-or-less’
manner.
Crosses
A hybrid is the offspring of parents that
have different forms of a trait, such as tall
and short.
Mendel first did monohybrid crosses,
which means he was looking at only one
trait.
homozygous vs heterozygous
An organism that is homozygous for a
trait has two alleles for the trait that are
the same.
An organism that is heterozygous for a
trait has two different alleles (dominant
and recessive).
Mendel’s Experiments:
1. Used self-fertilization in 34 different garden pea
strains (phenotypes).
2. Focused on 7 well-defined traits by crossing one
phenotypes at a time.
3. Counted offspring of each phenotype and
analyzed the results mathematically.
Some basic terminology:
Generations:
P: parental generation
F1: 1st filial generation, progeny of the P generation
F2: 2nd filial generation, progeny of the F1 generation (F3 and so on)
Crosses:
Monohybrid cross: cross of two different true-breeding strains
(homozygotes) that differ in a single trait.
Reciprocal cross: sexes for the two strains are reversed (and if the
results are the same, trait is not sex-linked).
Dihybrid cross: cross of two different true-breeding strains
(homozygotes) that differ in two traits.
*Genetics etiquette - female conventionally is written first
Results of Mendel’s
monohybrid parental cross:
“Mendel’s Principle of
Uniformity in F1”
F1 offspring of a monohybrid
cross of true-breeding strains
resemble
only one of the parents.
Why?
Smooth seeds (allele S) are
completely dominant to
wrinkled seeds (allele s).
Smooth and wrinkled
parental seed strains
crossed.
Punnett square
F1 genotypes
4/4 Ss
F1 phenotypes
4/4 smooth
F1 x F1 crosses:
Mendel also discovered
that traits that disappear
in the F1 generation
reappear in the F2
generation in a 1:3 ratio.
“Mendel’s Principle of
Segregation”
F1 x F1 Punnett square:
F2 genotypes
1/4 SS
1/2 Ss
1/4 ss
F2 phenotypes
3/4 smooth
1/4 wrinkled
Crosses also can be represented with branching diagrams
What about the six other phenotypic traits?
1. Results of reciprocal crosses always were the same.
2. F1 progeny always resembled the parental strain.
3. In the F2 progeny, parental strains lost in the F1 generation
always reappeared at a ratio of 1:3.
“Mendel’s Principle of Segregation”:
Each hereditary characteristic is controlled by two factors (alleles)
that separate and pass into reproductive cells (gametes).
Recessive characters masked in the F1 progeny of two true-breeding
strains, reappear in a specific proportion of the F2 progeny.
Modern formulation of Mendel’s Principle of Segregation:
Two members of a gene pair segregate (separate) from each other
during the formation of gametes.
Confirming the Principle of Segregation with test-crosses:
 SS x SS  true breeding (100% homozygous
dominant)
 ss x ss  true breeding (100% homozygous
recessive)
How do you determine whether an individual with the
dominant phenotype is homozygous or heterozygous?
Cross it with homozygous recessive:
SS x ss
 4/4 dominant trait
Ss x ss
1/2 dominant trait + 1/2 recessive trait
Test Crosses
Mendel’s dihybrid crosses:
1. Mendel also performed crosses involving two pairs of traits,
e.g., seed shape (smooth vs. wrinkled) and color (yellow vs.
green).
2. If alleles sort independently, four possible phenotypes (2n)
appear in the F2 generation in a 9:3:3:1 ratio.
“Mendel’s Principle of Independent Assortment”:
Pairs of inheritance factors (alleles) segregate independently of each
other when sex cells (gametes) are formed.
Modern formulation of independent assortment:
Genes on different chromosomes behave independently in gamete
production.
Dihybrid cross:
F1 generation
Dihybrid cross:
F2 generation
Ratio:
9:3:3:1
Trihybrid crosses:
1. Involve three independently assorting character pairs.
2. Results:
1. 64 combinations of 8 different gametes (4n)
2. 27 different genotypes (3n)
3. 8 different phenotypes (2n)
4. Predicted ratio of phenotypes = 27:9:9:9:3:3:3:1
Summary of Mendel’s 3 Principles:
 Mendel’s Principle of Uniformity in F1:
F1 offspring of a monohybrid cross of true-breeding strains
resemble only one of the parents.
Why? Smooth seeds (allele S) are completely dominant to
wrinkled seeds (allele s).
 Mendel’s Principle of Segregation:
Recessive characters masked in the F1 progeny of two true-breeding
strains, reappear in a specific proportion of the F2 progeny.
Two members of a gene pair segregate (separate) from each other
during the formation of gametes. Inheritance is particulate, not
blending as previously believed.
 Mendel’s Principle of Independent Assortment:
Alleles for different traits assort independently of one another.
Genes on different chromosomes behave independently in gamete
production.
Mendel’s laws explain inheritance in terms of discrete
factors (genes) which are passed from generation to
generation according to simple rules of chance.
• These principles apply to all sexually reproducing
organisms for simple patterns of inheritance.
• Later experiments using various organisms indicated that
more complicated patterns of inheritance exist.
• These patterns of inheritance include situations where one
allele is not completely dominant over another allele, where
there are more than two alleles for a trait, or where the
genotype does not always dictate the phenotype in a rigid
manner
Rediscovery of Mendel’s Principles:
 William Bateson’s (1902) experiments with fowls demonstrated
that Mendel’s principles applied also to animals.
 Bateson argued that mutation (not selection) was the most
important force shaping variation in plants and animals.
 William Bateson also coined the terms: Genetics, Zygote, F1, F2,
Allelemorph ( allele).
1907 - Reginald Punnett and William Bateson
Exceptions To Mendel’s Original Principles
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Incomplete dominance
Co-dominance
Multiple alleles
Polygenic traits
Epistasis
• Pleiotrophy
• Environmental effects on
gene expression
• Linkage
• Sex linkage