Basic Principles of Inheritance

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Basic Principles of Inheritance
Principle of Segregation
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Inheritance The first hunch

• Why do children look like their parents?
• The male contributes the most to offspring
  features homunculus inside the sperm
• Qualities of the parents blended to form the
  qualities of the child

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Gregor Mendel (1822-1884)
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3 basic questions

• What is inherited?
• How is it inherited?
• What is the role of chance in heredity?

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Mendels observations

• Variation, expressed in alternative forms of a
  trait, is widespread in nature.
• ? basis for evolution
• Observable variation is essential for following
  the distribution of traits in the offspring

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Mendels observations
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Mendels observations

• Traits dissapeared in one generation and
  reapeared in another generation of inbred plants.
• Variation seems not to be distributed by chance

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Hypothesis

• Observable traits are determined by independent
  units of inheritance
• A particle with no physical feature whose
  function is to control a visible trait
• Today GENE
• Region of DNA that encodes a specific proteinor
  a particular type of RNA

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The Mendelian Experiments

• Isolation of pure- breeding lines
• Generation of hybrids
• Control over fertilization allowing reciprocal
  crosses

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Mendels PeasThese are the seven
characteristics, each having two contrasting
traits, that Mendel identified.
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Mendels Experiments

• He experimentally crosses different strains of
  pure plants (parental generation) to develop
  hybrids (F1 generation).
• He then crossed these hybrids (F1 gen.) and
  analyzed the results (F2 gen.)

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Mendels Results

• The hybrids (F1 gen.) only displayed one of the
  parental traits. (tall)
• When crossing two hybrids, some of the resulting
  offspring (F2 gen.) displayed one of the parental
  traits and some displayed the other. (some tall
  some short)
• These traits in the F2 generation consistently
  occurred in a 3 to 1 ratio. (3 tall 1short)

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Mendelian Laws

• Principle of Segregation
• Mendel's Principle of Independent Assortment

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Principle of Segregation

• Monohybrid crossing
• Evidence that blending did not occur
• Yellow-pea hybrids should have the information to
  produce green peas

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Principle of Segregation
P
Grows into plant
Gamete formation
P
PP
p
Grows into plant
Gamete formation
p
pp
p
Seed formation
FERTILIZATION
Pp
P
Pp
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Principle of Segregation

• The factors of inheritance (genes) come in
  alternative forms (alleles)
• The 2 alleles for each trait are separated or
  segregated in the formation of gametes (eggs and
  sperm), then unite at radom one from each parent
  during fertilization.

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Definitions

• DNA Deoxyribonucleic acid the molecule in which
  genetic information is encoded.
• Allele One alternative of a pair or group of
  genes that could occupy a specific position on a
  chromosome.
• Chromosome A linear strand of DNA harboring many
  genes.
• Gene A unit of genetic information that occupies
  a specific position on a chromosome and comes in
  multiple versions called alleles.
• Genotype The genetic constitution of an
  organism.
• Phenotype The physical or observable
  characteristics of an organism.

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Definitions

• Heterozygous Having a genotype with two
  different and distinct alleles for the same
  trait.
• Homozygous Having a genotype with two of the
  same alleles for a trait.
• Dominant An allele producing the same phenotypic
  effect whether inherited heterozygously or
  homozygously an allele that "masks" a recessive
  allele.
• Recessive An allele producing no phenotypic
  effect when inherited heterozygously and only
  affecting the phenotype when inherited
  homozygously an allele "masked" by a dominant
  allele.

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Mendelian Laws

• Principle of Segregation
• Mendel's Principle of Independent Assortment

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Principle of segregation

• For any particular trait, the pair of alleles of
  each parent separate and only one allele passes
  from each parent on to an offspring.  Which
  allele in a parent's pair of alleles is inherited
  is a matter of chance.  We now know that this
  segregation of alleles occurs during the process
  of meiosis

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Principle of Independent Assortment

• Different pairs of alleles are passed to
  offspring independently of each other.  The
  result is that new combinations of genes present
  in neither parent are possible

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• Inheritance of Bood Types
• 1 gene
• 3 alleles (A,B,i)
• Possible Genotypes

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Remember

• GENE
• Nucleic acid (DNA) segments that determine a
  specific characteristic
• LOCUS
• Localization of a gene in a chromosome
• Singular locus/ PluralLoci
• ALLELE
• Diferent forms of a gene
• MUTATION
• Change in the DNA sequence
• KARYOTYPE
• the chromosomal constitution of the cell nucleus
  by extension, the microphotograph of chromosomes
  arranged.

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Gene, locus y allele

• Genes
• A,B,C
• Alleles
• Gene A A1, A2
• Gene B B1
• Gene C C3, C1
• Locus
• Gene A
• Gene B
• Gene C

A
A1 B1 C3
A2 B1 C1
B
C
CROMOSOME 5
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DEFINITIONS
LOCUS Physical site occupied by a gene. EXAMPLE
Angiotensin 1q42 43
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Definitions
1

• Autosomic Trait
• Inherited characteristic whose allele is is
  present in the chromosomes different from X and Y
  (autosomic)
• In humans pair 1 to 22

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22
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• Trait linked to sex
• Inherited characteristic whose allele is is
  present in the X or Y chromosomes (pair 23 in
  humans XX o XY)

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Paterns of inheritance

• Autosomic dominant
• Phenotype is determined by dominant allele and
  present in all sexes
• Autosomic Recessive
• Phenotype is determined by recesive allele and
  present only in homozygous state
• Linked to sex
• Only present in one sex

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Generation of variability

• During Metaphase I crossing-over results in a
  physical exchange of equivalent segments of
  chromosomes
• If there are allelic differences between these
  segments, then the products of crossing-over are
  genetic recombinants for those alleles

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