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Genetics
Mendelian Genetics
Blue People of Kentucky
Methemoglobinemia
Gregor Mendel
• An Austrian monk who is credited as the first person to study the
science of genetics.
• So he is referred to as “the Father of Genetics.”
• Mendel worked with garden pea plants in his experiments
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Easy to grow
Had many traits to study; height, seed shape, pod shape, flower color, etc.
Reproduced rapidly
Cross pollination
Gregor Mendel
• Mendel’s Experiment
• Crossed a pure Tall plant with a pure short plant
• This is the parent generation, P1
• All offspring were tall
• Crossed two plants from the P1 offspring
• This is the first filial or first generation of offspring, F1
• Crossed two F1 plants by self-fertilization
• This is the second filial generation of offspring after the parents, F2
• Resulted in ¾ Tall plants and ¼ short plants
Mendel’s Findings
1. Concept of Unit Characters: traits are
determined by “factors” that occur in pairs.
We now know that these unknown
“factors” are genes.
2. Dominance: One factor of a pair may cover
up or prevent the expression of a trait
Example:
Your Mom gives you the gene for having a Unibrow (recessive) and
your father gives you the gene for having two eye brows (dominant)
Dad
Mom
What will you have?
Mendel’s Findings
3. Law of Segregation: two alleles for a gene
will separate during meiosis
4. Law of Independent Assortment: alleles
separate randomly or independently of each
other. This means that seed shape does not
affect plant height.
Because of independent assortment and crossing over
during meiosis, there is an infinite number of possible
combinations of chromosomes in the gametes, this is called
genetic recombination.
Genetics
• Genetics is the study of genes or traits.
• Heredity is the transmission of traits from parent
to offspring; an organisms genetic makeup.
• Genes are sections of a chromosome that code for
a specific trait. Ex: hair color, height, etc.
• A trait is any characteristic that can be passed from
parent to offspring.
Genetics
• Alleles are the form of the gene that is passed
from parent to offspring.
• Flower color = gene;
• Purple and white = alleles of the flower color gene
• Multiple alleles are genes with more than 2
alleles
• Eye color
Important Genetics Terms
• Dominant: the “stronger” allele that can hide or mask the
recessive allele; represented by an uppercase letter
B
R
• Recessive: the “weaker” allele that is only expressed when
there is no dominant allele present; represented by a
lowercase letter
b
r
Important Genetics Terms
• Genotype: the genes on a pair of homologous
chromosomes represented by a combination of
letters. We generally use the first letter of the
dominant gene.
• HH, Hh, hh
Important Genetics Terms
• Homozygous (purebred): gene pairs that are the
same on both chromosomes
• Homozygous dominant: TT, AA, HH
• Homozygous recessive: tt, aa, hh
Important Genetics Terms
• Heterozygous (hybrid): gene pairs that are
different on both chromosomes. The
dominant gene will be expressed in most
heterozygous organisms.
• Tt, Aa, Hh
Important Genetics Terms
• Phenotype: the expression of the
gene/trait; what is actually seen or
expressed (phenotype = physical trait)
• Tall/short, green/yellow, smooth/wrinkled
Genotype & Phenotype
• Genotype of alleles:
= hair
= hairless
• All genes occur in pairs, so 2
alleles affect a characteristic
• Possible combinations are:
Genotypes:
HH
Hh
Phenotypes: HAIR
hh
HAIR hairless
17
Probability
• The traits you received from your parents were all randomly
combined due to chance.
• We use the mathematical study of probability in genetics to help
us determine the likelihood of passing on certain traits to future
offspring.
Mendelian Inheritance and Probability
• How do we know what the probability of a cross
will be?
• We use a device called a Punnett square to see
possible gene combinations.
Mendelian Inheritance and Probability
• Monohybrid Cross:
a cross between
organisms that
involves one trait.
• Dihybrid Cross:
a cross between
organisms that
involves two traits
Mendelian Inheritance and Probability
• The probability of a gene combination can be
represented by:
• A fraction: # of one kind of combination
# of all possible combinations
• A ratio: 1:4, 1:2, 3:4, 4:0, 1:2:1, etc.
• Or a percentage: 25%, 50%, ect.
Let’s practice…
Punnett squares
WHEN HEREDITY
ISN’T SIMPLE
INCOMPLETE DOMINANCE
• The phenotype of the heterozygous organism has an “in between”
phenotype.
• Example: Snapdragon flowers
• Homozygous Dominant = Red (RR)
• Homozygous Dominant = White (WW)
• Heterozygous = Pink (RW)
CODOMINANCE
• Both alleles express themselves independently in the phenotype. This means
that both alleles are expressed equally; both are dominant in a heterozygous
organism.
• Cows, horses, people (sickle cell anemia)
• Homozygous dominant = Black (BB)
• Homozygous dominant = White (WW)
• Heterozygous = Checkered (BW)
Multiple Alleles
• Traits controlled by more than two alleles.
• Blood Type: IA and IB are codominant and the
third allele, i, is recessive.
Polygenic Traits
• Traits controlled by more than one gene.
• Eye color, hair color, skin color
• 3 main eye colors: blue, green, and brown
• Many variations/combinations of these three colors
Sex-Linked Traits
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Traits carried on the sex chromosome
Sex-linked traits can only be carried on an X chromosome.
Color Blindness
Male Pattern Baldness
• Can a father pass a sex-linked trait to his son?
Autosomal Disorders
(chromosomes 1-22)
• Nondisjunction: the failure of
homologous chromosomes to
separate properly during meiosis.
• Monosomy
• Turner Syndrome
• Trisomy
• Trisomy 21 = Down’s Syndrome
Recessive Autosomal Disorders
• Cystic Fibrosis
• Sickle Cell Anemia
• Albinism
Dominant Autosomal Disorders
• Achondroplasia (dwarfism)
• Glaucoma
• Polydactyly
• Brown Teeth
Sex Chromosome Disorders
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Turner’s Syndrome - XO
Kleinfelter’s Syndrome - XXY
Superfemale – XXX
Supermale - XYY
Pedigrees
Pedigrees
• A graphic representation that shows the pattern of
inheritance within a family.
• Carrier – A person who is heterozygous for a trait. They
do not express the trait but can pass it on to future
offspring.
1. Is this a sex-linked or autosomal disorder?
2. Which individuals are carriers?
3. What is the probability that II-1 and II-2 will have any
children with the disorder?
1. Which individuals are carriers for
the trait?
2. What percentage of individual 8’s
sons will be carriers of the trait?
3. What percentage of individual 8’s
daughters will be carriers of the
trait?
4. What percentage of individual 8
and 9’s children will be affected by
the trait?
5. Is this a sex-linked trait or an
autosomal trait?
DNA Fingerprinting
Just like a fingerprint, everyone’s DNA sequences are unique.
DNA Fingerprinting Uses
• To compare DNA found at a crime
scene to possible suspects using gel
electrophoresis.
Gel Electrophoresis
• Use enzymes to cut DNA into
sections that are then separated by
size using an electric current.
DNA Fingerprinting Uses
• A process called PCR allows
forensic scientists to get enough
DNA from the smallest of samples
to identify a person.
• Personal Identification (registry)
DNA Fingerprinting Uses
• Diagnosis (and possibly cures for)
of Inherited Diseases
• Linking blood relatives
• Biological parents (paternity,
adoption)