Patterns of inheritance

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Biology 1030
Patterns of inheritance
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Family resemblance - Habsburgs of Europe
Maternal homologue
Paternal homologue
Haploid sperm
Ferdinand II, Holy Roman Emperor, 1620
Maximilian I, Holy Roman Emperor, 1530
Konstanze, queen of Poland, 1604
Rudolf II, Holy Roman Emperor, 1590
Family resemblance has long been noted and
commented on
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Inheritance a historical perspective

• Until the middle ages people thought bizarre
  composite animals could result from breeding
  widely different species
• The giraffe (Giraffa camelopardalis) was once
  thought to have resulted from a camel breeding
  with a leopard
• Also, species were thought to have been
  maintained without significant change since the
  time of their creation
• Constancy of species - Variation and heredity
  occur within the boundaries of a species

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Inheritance a historical perspective

• Constancy of species is false
• 1760 - Josef Koelreuter conducted hybridization
  experiments with different strains of tobacco
  plants and obtained fertile offspring
• Well known animal hybrids include, ligers (lion x
  tiger), mule (donkey x horse)
• Hybridization may produce fertile or sterile
  offspring depending on genetic compatibility

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Inheritance a historical perspective

• Inheritance was thought to occur via pangenesis
  in which particles (pangenes) traveled from
  different parts of the body to sperm and egg
• People also believed that acquired traits or
  characteristics throughout life were inherited
• During the 19th century the idea of blending was
  popular traits from parents were thought to be
  blended together

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Blending
black horse
white horse
x
gray horse
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Inheritance a historical perspective

• Historical ideas about inheritance
• Constancy of species
• Blending
• Acquired traits
• Pangenesis
• All shown to be false during the mid 1800s
• So, how does inheritance actually work?

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How does inheritance actually work?

• Gregor Mendels experiments
• Mendel conducted the first quantitative studies
  of inheritance
• Pure breeding adult peas of different flower
  colors were crossed
• Flower color is a trait - an observable
  characteristic

Mendel was an Austrian monk
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How does inheritance actually work?

• Gregor Mendels experiments
• Crossing experiment Purple X White
  Purple
• All offspring produced had purple flowers

Why were peas used?
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Mendel used a monohybrid cross

• What is a monohybrid cross?
• Monohybrid cross - A cross between two
  individuals involving to observe inheritance of
  one trait
• Hybridization terminology
• P generation True breeding parents
• F1 generation First generation offspring
  resulting from a cross between pure breeding
  individuals (parents)
• F2 generation Second generation offspring
  resulting from a cross between F1 plants

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Results of Mendels monohybrid cross

• F1 generation peas had purple flowers
• F2 generation peas consisted of mostly purple
  flowers (3/4) and small number of white flowers
  (1/4)
• Why?

F1
F2
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So, what is the mechanism of inheritance?

• Genes code for traits, each gene has two
  different forms called alleles
• Law of segregation Alleles separate during
  meiosis sperm and egg possess one allele for
  every gene
• Genotype Combination of alleles one has

Mendels experiments showed that purple flower
color is dominant over white
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So, what is the mechanism of inheritance?

• Genotype Combination of alleles one has
• Two forms of alleles DOMINANT and recessive
  alleles
• Two alleles two possible phenotypes
• Phenotype Outward appearance expressed by a
  gene
• Dominant vs. recessive whats the difference?

Mendels experiments showed that purple flower
color is dominant over white
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Punnett square

• Letters represent alleles, typically the first
  letter of a word that defines a trait
• Capital P for purple (dominant trait) , lowercase
  p for white (recessive trait)
• Genotypes are either homozygous or heterozygous

Punnett square
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Punnett square

• Punnett square A tool developed by Reginald
  Punnett used to predict the number and variety of
  genetic combinations
• Homozygous Having the same two alleles for one
  gene (either both dominant or both recessive)
• Heterozygous Having two different alleles for
  one gene (one dominant allele, and one recessive
  allele)

Reginald Punnett
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Dihybrid crossing

• Mendel did not know if 2 traits were inherited
  together or separately
• If inherited together phenotypic ratio of the F2
  generation would be 31 (dependent assortment)
• In other words, were dominant alleles inherited
  together and were recessive alleles inherited
  together?

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Dihybrid crossing

• Dihybrid cross Breeding individuals having for
  to observed inheritance of two different traits
• For example, seed color (yellow, green) and seed
  texture (round, wrinkled)
• Independent assortment each pair of alleles
  segregates independently of the other pairs of
  alleles during meiosis

P
F1
F2
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Both segregation and independent assortment are
explained by the distribution of chromosomes
during meiosis
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Other inheritance patterns

• Mendels experiments illustrate complete
  dominance offspring always resembled one of the
  two parents
• Dominant allele had the same phenotypic effect
  whether present in one or two copies
• Incomplete dominance - heterozygous individuals
  have an intermediate phenotype

Crossing red and white snap dragons produces pink
snap dragons
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Blending inheritance vs. Particulate inheritance
F1
F2
Offspring remain pink Return of parental
phenotypes
WRONG RIGHT
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Examples of inherited traits in humans

• Many human traits are controlled by alleles which
  seem to be inherited according to Mendellian
  inheritance laws
• Some of the most obvious traits include straight
  hairline vs. widows peak, straight thumb or
  hitchhikers thumb
• Are you dominant or recessive for such traits?

Widows peak
Hitchhikers thumb
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Some genes have more than one allele

• Blood phenotypes are controlled by a combination
  of two of three different alleles
• Three blood type alleles i, IA, IB
• The alleles for blood types A and B are codominant

Blood clumping results when certain antibodies
bind to specific antigens
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Blood groups and blood types

• Erythrocytes (red blood cell) have cell-surface
  proteins called antigens
• Blood types are based upon the types of antigens
  one has
• Defensive chemicals called antibodies are
  produced by your body to protect itself from
  foreign cells or organisms
• Antibodies circulate through the body in the
  fluidic blood plasma

Type A
Type B
Type AB
Type O
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Sex linkage

• Some traits are controlled by alleles found on
  sex chromosomes
• These traits are commonly referred to as either
  X-linked or Y-linked
• Use a punnett square to figure out genotypic and
  phenotypic proportions
• Each allele of each sex chromosome written as a
  superscript

Mother
A
A
a
a
A
A
a
Father
A
A
Punnett square for sex determination
A dominant allele a recessive allele
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Sex-linked disorders and pedigrees

• Sex-linked disorders are caused by genes located
  on sex chromosomes (usually X chromosome)
• Sex-linked disorders and other traits may be
  traced through a pedigree
• Sex-linked disorders include red-green color
  blindness, hemophilia, and Duchenne muscular
  distrophy
• Are males or females mostly affected by such
  disorders?

Pedigree shows inheritance of traits among
family members Circle female,
Square male
Pedigree for normal hearing and deafness
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Autosomal disorders

• Recessive disorders
• Cause recessive alleles
• Albinism, Cystic fibrosis, Sickle-cell anemia,
  Tay-Sachs disease
• More common than dominant disorders

• Dominant disorders
• Cause dominant alleles
• Dwarfism (Achondroplasia), Alzheimers disease,
  Huntingtons disease
• Less common than recessive disorders

Autosomal disorders are more prevalent in certain
geographic regions or cultures
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Human chromosomes

• Lets look at this again
• There are 46 chromosomes (23 homologous
  pairs) in each somatic cell
• 22 pairs of autosomes
• 1 pair of sex chromosomes
• XX Female, XY Male
• Karyotype - chromosomes are arranged according to
  shape and size

Normal human karyotype
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Nondisjunction and chromosomal disorders

• Nondisjunction failure of chromosomes to
  separate and segregate into daughter cells
• Nondisjunction may occur during meiosis 1 or
  meiosis 2
• Abnormal number of chromosomes may result (such
  as a trisomy disorder like Downs syndrome)

Nondisjunction can cause more significant
problems causing the fetus to die
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Autosomal nondisjunction-related disorders

• Down syndrome (Trisomy-21)
• Patau syndrome (Trisomy-13)
• Edward syndrome (Trisomy-18)
• Again, individuals with these disorders have an
  extra chromosome

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Nondisjunction and chromosomal disorders

• Nondisjunction may also affect sex chromosomes
• Sex chromosome disorders include Klinefelter
  syndrome (XXY, or also XXYY, XXXY) in males, and
  Turner syndrome (X) in females
• These disorders cause poor genital development,
  and various physical abnormalities such as breast
  development in men (Klinefelter syndrome), poor
  breast development in women (Turner Syndrome)
• Other abnormalities may occur which produce
  normal male or normal female offspring (XYY, or
  XXX)

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Synopsis of monohybrid crosses
C
C
c
C
c
c
C
C
C
What type of monohybrid cross is this an example
of?
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Synopsis of monohybrid crosses
A
B
B
A
A
B
What type of monohybrid cross is this an example
of?
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Synopsis of monohybrid crosses
What type of monohybrid cross is this an example
of?
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Synopsis of monohybrid crosses
What type of monohybrid cross is this an example
of?
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