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Patterns of Inheritance
Chapter 9
An Old Genetic Experiment
• Genetics is the study of
heredity/inheritance
• Dogs bred for specific traits
• Genome completed in 2003
• Wolves and domestic dogs
share a common ancestor
History of Inheritance
• Blending hypothesis
– Information (cells) from each parent produce mixed
offspring
• Tall and short adults had medium height children
– Didn’t explain disappearance/reappearance of traits
b/w generations
• Gregor Mendel 1860
– Father of genetics
– Parents pass on specific heritable factors to offspring
• ‘Genes’ don’t blend, but remain the same over generations
Useful
Genetics Terminology
Alleles
• Alternate versions of the same gene, located at the
same loci on a specific chromosome
– Dominant alleles mask others when both are present
(UPPERCASE LETTERS)
• Dominance implies it determines phenotype, not superiority or
increased prevalence
– Recessive alleles are easily masked by others (lowercase
letters)
• Recessive traits often more common
• Individuals inherit 2 alleles  1 maternal and 1 paternal
– Actual combination determines genotype
– Resulting physical expression determines phenotype
Phenotype and Genotype
• Homozygous dominant
(PP)
– 2 dominant alleles
– Express dominant trait
• Heterozygous (Pp)
– 1 dominant and 1
recessive allele
– Express dominant trait
• Homozygous recessive
(pp)
– 2 recessive alleles
– Express recessive trait
• Phenotypic vs.
genotypic ratios
Why a Pea? Characters
• Peas (Pisum sativum) have several
characters that vary among
individuals
• Have distinct traits, or variants, of
each character
• Can control types of mating/crosses
that occurred
– Self-fertilize  natural, involves 1
plant
– Cross-fertilize  artificial, involves 2+
different plants
• Can create true-breeding lines
– All individuals genetically identical
– Only contain 1 character variant
Traits
Mendel’s Initial Work
• Started with monohybrid crosses
– Differ by only 1 trait
– Can use Punnett squares to represent hypothetical crosses
• All crosses produced same results
– Crossing true-breeding tall and short (P) = only tall (F1)
– Cross any resulting tall hybrids (F1) = 3:1 ratio (type of
ratio?) of tall to short (F2)
– Short phenotype disappears but reappears in next
generation
• Held true for all 7 tested characters
Constructing Punnett Squares
• Allows determination of all possible genotypes
and phenotypes
• Remember:
– All individuals have 2 alleles for every gene
• 1 from mom and 1 from dad
– Meiosis produces haploid gametes from diploid cells
• Aa mother = A or a eggs
• Steps
– Place gametes (haploid) of one parent along top,
other along the left side
– Combine all possible female gametes with all
possible male gametes = fertilization
– Boxes with 2 alleles = possible offspring (diploids)
T
T
T
1.
2.
T
3.
4.
Practice
• Background:
– Tall (T) and short (t)
• Fill in the boxes to
show genotypes
• For each box,
identify the
phenotype
• For each punnett
square list the
genotypic and
phenotypic ratios
1._________
3._________
2 ._________
4 ._________
T
T
T
1.
2.
t
3.
4.
1._________
3._________
2 ._________
4 ._________
T
t
T
1.
2.
t
3.
4.
1._________
3._________
2 ._________
4 ._________
t
t
t
1.
2.
t
3.
4.
1._________
3._________
2 ._________
4 ._________
Adopted from:
http://www.exploringnature.org/db/detail.php?dbID=22&detID=2290
Identifying Individuals in Crosses
• P (parental) generation
• F1 (first filial) generation
• F2 (second filial) generation
• And so on …
Mendel’s Work (cont.)
• Mendel wanted to be able to
identify genotypes of all
individuals
• Designed a testcross
– Cross recessive phenotype
with a dominant phenotype
• Why is recessive phenotype
required?
– Determine genotype of a
dominant trait
– Still used in current research
Mendel’s Work (cont.)
• Continued with dihybrid crosses
– Peas differed by 2 characters
• All crosses produced same results
– Crossing true-breeding round yellow and wrinkled
green (P) = only round yellow (F1)
– Cross any resulting round yellow hybrids (F1) = 9:3:3:1
ratio (type of ratio?) of round yellow to round green
to wrinkled yellow to wrinkled green(F2)
– Wrinkled green phenotype disappears but reappears
in next generation along with 2 new phenotypes
Mendel’s Law of Segregation
• Two alleles of a trait separate
during gamete formation
• Remember
– Homologous chromosomes
each carry 1 allele
– Meiosis separates these
chromosomes  forms haploid
gametes
Mendel’s Law of Independent Assortment
• Genes located on different chromosomes are
inherited independently
• E.g. hair color doesn’t determine eye color
Autosomal Recessive Disorders
• Only affects homozygous recessive individuals
– Heterozygous is “carrier”
– Prevents complete removal of allele from a
population
• Albinism
– Lack of normal amounts of melanin (pigment) in
body
• Cystic fibrosis
– Thick mucus in lungs & digestive tract
• Cl- channel abnormality
– Most common lethal genetic disorder among
Caucasians
Autosomal Dominant Disorders
• Affects all dominant phenotypes
• Lethal types less common
• Achondroplasia
– Embryonic cartilage in skeleton doesn’t
develop properly
– “Dwarf”, average 4’ tall
• Huntington’s Disease
– Nervous system deteriorates
– Symptoms often not seen until after 30
– Die in 40s or 50s
Pedigrees: an application practice problem
Recessive trait:
attached earlobe
• Diagram family relationships and
phenotypes
• Allow human heredity to be studied
– Can’t control human mating, so look at
those naturally occurring
– Can indicate type of gene responsible
• Sex-linked or autosomal recessive/dominant
• Can deduce genotypes of most female male
affected
members from phenotypes
unaffected
– Mendelian genetics and logic
carrier
Not All Genetics Are Simple
• Mendel used characters exhibiting complete
dominance, offspring look like one of the two
parents (simple)
– Not applicable to all characters
– Genotype and phenotype relationship not so simple
• Single genes can have alleles that aren’t completely
dominant or recessive
• Characters can have 1+ genes (complex)
– Basic principles of segregation and independent
assortment still apply
Variations on Mendelian Genetics
•
•
•
•
•
Incomplete dominance
Codominance
Epistasis
Polygenic inheritance
Ignore environmental influences
– Nutrition can effect height, sun exposure can alter
skin color, exercise can change build, etc.
– Nature vs nurture still major debate
Incomplete Dominance
• Heterozygote offspring has an
intermediate of parent’s phenotype
– Doesn’t support blending
– Each genotype has own phenotype
• True breeding red and white cross
– Homozygous red and white offspring
– Heterozygous pink offspring
– 1:2:1 is and genotypic and phenotypic
ratio
Codominance
• Heterozygote offspring
expresses two alleles at the
same time
• Blood type
– 3 alleles
– 4 phenotypes
– 6 genotypes
• Universal donor?
• Universal acceptor?
Epistasis
• Gene at one locus alters the
phenotypic expression of
another gene at a second locus
• Coats of mice and Labrador
retrievers
– B (black) & b (brown)
– C (melanin) & c (no melanin)
• Possibilities
– B_C_ = black
– bbC_ = brown
– B_cc and bbcc = white or yellow
Polygenic Inheritance
• An additive effect of 2+ genes on
one phenotypic trait
• Range of small differences in a trait
• Skin color due to different amounts
and types of melanin
• Height, weight, and iris (eye) color
too
CHROMOSOMAL INHERITANCE
Human Sex Determination
• 2 sex chromosomes and 44 autosomes
• XX = female and XY = male
– Eggs = all X and sperm = X or Y
– Sperm cell determines sex
• Gene on Y chromosome responsible
for ‘maleness’
– SRY gene (TDF production) triggers testes
development
– Without, ovaries develop
• Default sex is female
Other Sex Determining Systems
• Insects have 1 sex (X) chromosome
– Females XX, males X0
• Bees and ants are haploid or diploid
– Queen decides
– Diploid females, haploid males
• Marine fish commonly change
– Social hierarchy and balance of sex’s
• Alligators and turtles rely on
incubation temperature
• Plants are complex
Sex-linked Genes
 Genes that reside on sex chromosomes, but unrelated to genetic sex
 X chromosome in humans (generally)
• Fathers pass X to all daughters, but no sons
• Mothers pass X to all offspring
– Can you justify these statements?
 X-linked disorders more common and most likely to affect males




X chromosome is larger
Male affected with 1 = hemizygous
Females affected with 2; 1 = carrier
Represented in crosses differently
 Need sex chromosome and UPPER or lower case letter to imply affected or not
(XnY = affected male, XNXn = carrier female)
• Y-linked is rare
– Used to track ancestry through male lines
Sex-linked Genes
• Genes that reside on sex chromosomes but unrelated to genetic sex
– X chromsome in humans (generally)
– Fathers pass X to all daughters, but no sons
– Mothers pass X to all offspring
• Can you justify these statements?
• X-linked disorders more common and most likely to affect males
–
–
–
–
X chromosome is larger
Males affected with 1 = hemizygous
Females affected with 2; 1 = carrier
Represented in crosses differently
• Need sex chromosome and UPPER or lower case letter to imply affected or not
(XnY = affected male, XNXn = carrier female)
• Y-linked is rare
– Used to track ancestry through male lines
Red-Green Color Blindness
F: normal; M: affected
F: carrier; M: normal
F: carrier; M: carrier
• Involves several X-linked genes
– Some heterozygous females affected (rare)
• N represents color-blind gene carried at an X chromosome
loci
• XN = trait not present, Xn = trait present
Hemophilia
• X-linked recessive
• Allele for clotting
factor VIII mutated
• Introduced into ruling
houses of Russia and
Europe