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Part 6: The Chromosomal Basis of InheritanceChapter 15
Date: 12/1/16
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AP Bio Vocabulary of the Day(Stems, Prefixes, and Suffixes…Oh my!)
echin- spiny, prickly
Example: echinoderm (a phylum of sea creatures many of whom have spiny skin)
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Fabulous Fact
The brain of an adult human weighs around 3 pounds (1.5 kg). Although it makes up just 2% of the body's weight, it uses around 20% of its energy.
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What you must know:How the chromosome theory of inheritance connects the physical movement of chromosomes in meiosis to Mendel’s laws of inheritance.
The unique pattern of inheritance in sex-linked genes.
How alteration of chromosome number or structurally altered chromosomes (deletions, duplications, etc.) can cause genetic disorders.
How genetic imprinting and inheritance of mitochondrial DNA are exceptions to standard Mendelian inheritance.
AP Biology Standards addressed: Big Idea 3
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How are chromosomes involved in inheritance???(Hint: think about Mendel’s laws)
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Chromosome theory of inheritance:
Genes have specific locations (loci) on chromosomes
Chromosomes segregate and assort independently
Chromosomes tagged to reveal a specific gene (yellow).
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Thomas Hunt Morgan
Drosophila melanogaster – fruit flyFast breeding, only 4 prs. Chromosome, with 1 pair of sex chromosomes (XX/XY)
Sex-linked gene: located on X or Y chromosome
Red-eyes = wild-type; white-eyes = mutantSpecific gene carried on specific chromosome – white eye gene carried on X chromosome
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Sex determination varies between animals
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Sex-linked genes
•Sex-linked gene on X or Y
•Females (XX), male (XY)– Eggs = X, sperm = X or Y
•Fathers pass X-linked genes to daughters, but not sons
•Males express recessive trait on the single X (hemizygous)
•Females can be affected or carrier
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Transmission of sex-linked recessive traits
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Sex-linked disordersColorblindness
Duchenne muscular dystrophy
Hemophilia
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Types of Chromosomal InheritanceV. X-inactivation in Females
In females one X in a cell is deactivated and condenses into a Barr body
Process occurs randomly in each embryonic cell
This results in a mosaic of two types of cells in the body
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X-InactivationBarr body = inactive X chromosome; regulate gene
dosage in females during embryonic development
• Cats: allele for fur
color is on X
• Only female cats can
be tortoiseshell or
calico.
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.. in human females affected by anhidrotic ectodermal dysplasia in which a mutant gene on one X chromosome results in patches of skin with no sweat glands.
X Chromosome Inactivation in Humans
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Types of Chromosomal Inheritance
I. Linked Genes
Genes located near each other on the same chromosome are inherited together and considered “linked”- the frequency of being inherited together is greater than 50%
If genes are far enough apart on the same chromosome the probability of crossing over is high and they behave as unlinked genes
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Human developmentY chromosome required for development of testes
Embryo gonads indifferent at 2 months
SRY gene: sex-determining region of Y
Codes for protein that regulates other genes
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Genetic Recombination: production of offspring with new combo of genes from parents
If offspring look like parents parental types
If different from parents recombinants
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• If results do not follow Mendel’s Law of Independent
Assortment, then the genes are probably linked
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Linked genes: located on same chromosome and tend to be inherited together during cell division
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Crossing over: explains why some linked genes get separated during meiosis
the further apart 2 genes on same chromosome, the higher the probability of crossing over and the higher the recombination frequency
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Types of Chromosomal Inheritance
II. Chromosomal Disorders
1. Abnormal Chromosome Number
Results from nondisjunction in meiosis
At fertilization leads to aneuploidy (too many or too few chromosomes)
Examples: Down Syndrome, Kleinfelter Syndrome, Turner Syndrome
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Karyotyping can detect nondisjunctions.
Down Syndrome = Trisomy 21
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Nondisjunction: chromosomes fail to separate properly in Meiosis I or Meiosis II
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Klinefelter Syndrome: 47XYY, 47XXY
Nondisjunction
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Nondisjunction
Turner Syndrome = 45XO
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Types of Chromosomal Inheritance
II. Chromosomal Disorders (cont.)
2. Alterations to Chromosome Structure
Result from chromosome breakage
Deletion- chromosome fragment lost
Insertion- chromosome fragment attached
Inversion- order reversed
Translocation- chromosome segment moved to a nonhomologous chromosome
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Chromosomal Mutations
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Chromosomal Mutations
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Types of Chromosomal InheritanceIII. Genomic Imprinting
During gamete formation, the allele from one parent is silenced
The result is expression of only the other parent’s allele
Leads to variation in phenotype
Unique to mammals and flowering plants
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Genomic Imprinting in Humans
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The Curious Case of the Calico and Tortoiseshell…
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Calico or Tortoiseshell??? Dense or Dilute???
Dilute Dense
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Genetics in the Real World: The Trouble Twins
Ms. Van’s Twins!