Chapter 11
Genetics
Genetics
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All living things have a set of characteristics
inherited from its parent or parents
Genetics – the study of heredity
Trait – a specific characteristic, such as seed
color
Genes – chemical factors (DNA segments)
that control traits. Usually code for proteins
Alleles – forms of a gene, like T (tall) or t
(short)
Gregor Mendel
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Mendel was a monk in charge of the garden
monastery
◦ Mendel had stocks of plants that produced
only tall, only short, only green seeds, and only
yellow seeds
◦ He studies cross-bred plants with different
characteristics and studied the results
◦ The offspring (F1) of crosses between parents
(P) with different traits are called hybrids
Gregor Mendel
Gregor Mendel
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All of the F1 generation had characteristic of
only one parent (no blending)
◦ Each characteristic was controlled by 1 gene
with two contrasting forms, called alleles
◦ Principle of dominance states that some
alleles are dominant while others are
recessive
Gregor Mendel
Gregor Mendel
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He then crossed F1 plants to produce F2 plants
◦ Recessive traits reappeared, roughly ¼ had
the recessive traits
◦ The two alleles of the F1 plant segregate from
each other so that a gamete only has 1 copy
of the gene
◦ Each F1 plant gamete has either the dominant
or recessive (tall or short)
Gregor Mendel
Mendel
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Gregor Mendel concluded:
◦ Inheritance is controlled by factors passed
from one generation to the next.
◦ Some alleles are dominant and others are
recessive.
◦ Segregation - the alleles (separate) during
the formation of gametes.
◦ Independent assortment - genes for
different traits can segregate independently
during the formation of gametes.
Punnett Squares
Punnett Squares – tool used to predict the
probability of offspring with a certain trait or
genotype.
 Monohybrid cross – Punnett square with one
trait, 4 squares.
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Genetic Terms
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Dominant – occurs in the phenotype if one
allele is present, T
Recessive – must be homozygous to occur in
the phenotype
Homozygous – having two identical alleles for
a trait, TT or tt
Heterozygous – having two different alleles
for a trait, Tt
Phenotype – physical characteristic, tall
Genotype – genetic makeup, TT or Tt
Monohybrid Cross
Dihybrid Cross
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Since alleles can segregate independently, the
inheritance of one trait doesn’t affect the
inheritance of another (seed color and seed
shape)
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A dihybrid cross involves crossing two different
traits of the F1 generation (both parents are
heterozygous for both traits)
Dihybrid Cross
Complex Forms of Inheritance
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Some alleles are neither dominant or recessive,
and many traits are controlled by multiple
alleles or multiple genes
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Codominance – both alleles are present in
the phenotype, the black and white chicken,
blood types
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Incomplete dominance – the alleles blend to
produce the phenotype, red + white = pink
Blood Types: Codominance
Incomplete dominance
Complex Forms of Inheritance
Multiple alleles – more than 2 alleles are
possible choices, although only 2 can be used at
a time, ex. ABO blood groups, rabbit coat color
 Polygenic traits – controlled by 2 or more
genes, ex. skin color
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Sometimes expression is also affected by the
environment; height and flower color of a
sunflower are affected by genes and climate, soil,
and water availability
Multiple Alleles
Sex-linked
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Genes that are located on sex chromosomes
are said to be sex-linked genes
◦ Since males only have 1 X chromosome, all Xlinked alleles are expressed, even if recessive
◦ For females, a recessive allele must be with
another recessive to be expressed
◦ Colorblindness and hemophilia is an example
Sex-linked
Chapter 14
Heredity
Human Heredity
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A karyotype is a picture of chromosomes
arranged in their pairs (46, 23 pairs in humans)
Male
Female
Human Heredity
All egg cells carry an X chromosome, half
sperm cells carry an X, half a Y
◦ Leads to half combined becoming either XY,
or XX
 Autosomes are non-sex chromosomes, labeled
pairs 1-22 chromosomes
 Sex chromosomes are the 23rd pair of
chromosomes
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Human Heredity
To study how traits are passed from generation
to generation a pedigree chart is used
◦ Males are squares
◦ Females are circles
◦ Shaded circle/squares have the trait
◦ Horizontal lines represent marriage
◦ Vertical lines connect parents to their
offspring
 White forelock (lock of hair just above the
forehead) is dominant
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Human Heredity
Human Heredity
Many disorders are caused by an autosomal
recessive allele (disease is only apparent in the
homozygous recessive condition)
 Some disorders are expressed with only one
allele (dominant), which means the disease is
apparent in the heterozygous condition
 Some disorders are caused by a codominant
alleles
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Human Heredity
Disorder(s)
Cystic fibrosis
Huntington’s
disease
Inheritance pattern
Simple (autosomal)
recessive
Simple (autosomal)
recessive (codominant on
molecular level)
Simple (autosomal)
dominant
Colorblindness
Sex-linked recessive
Hemophilia
Sex-linked recessive
Sickle cell disease
Down syndrome
(Trisomy 21)
Symptoms
Mucus in lungs, digestive
tract, liver
Sickle shaped red blood
cells; joint pain, anemia
Mental deterioration and
uncontrollable movements;
onset -35-50
Difficulty discerning colors
Blood doesn’t clot
correctly, missing Factor
VIII
Trisomy (3 chromosomes Mild to severe mental
instead of 2)
retardation
Human Heredity
Cystic fibrosis leads to serious digestive
problems
◦ Thick, heavy mucus clogs lungs and breathing
passages
◦ Since it is recessive, must have 2 alleles to be
affected
 Sickle cell disease leads to bent/twisted shape of
RBCs (can clog in capillaries)
◦ Heterozygous individuals are generally healthy
and more resistant to malaria
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Chromosomal Disorders
Down syndrome (trisomy 21)
◦ Nondisjunction occurs when homologous
chromosomes don’t separate during meiosisleads to abnormal number of chromosomes
◦ 3 copies of chromosome 21
◦ Mild to severe mental disability, and higher
susceptibility to many diseases
 Sex chromosome nondisjunction
◦ Turner syndrome (X): sterile
◦ Klinefelter’s syndrome (XXY): generally sterile
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