the chromosomal basis of inheritance
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The Chromosomal Basis of Inheritance. Chapter 15. The importance of chromosomes. In 1902, the chromosomal theory of inheritance began to take form, stating: genes have specific locations (loci) on chromosomes, and you randomly get one chromosome from each parent. Fruit flies. - PowerPoint PPT PresentationTRANSCRIPT
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The Chromosomal Basis of Inheritance
Chapter 15
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The importance of chromosomes
• In 1902, the chromosomal theory of inheritance began to take form, stating: genes have specific locations (loci) on chromosomes, and you randomly get one chromosome from each parent.
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Fruit flies
• Fruit flies are Drosophila melanogaster• They have 4 pairs of chromosomes• They use the same XX, XY sex determination
as people• They breed quickly (every 2 weeks, hundreds
of offspring)• Wild type is the phenotype most common in
natural populations
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Linked genes
• Genes on the same chromosome tend to be inherited together “linked genes”
So why do offspring look different from parents?
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Genetic Recombination
• Genetic recombination is the production of offspring with combinations of traits different from those found in either parent.
• 1. Independent Assortment• 2. Crossing Over
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Independent Assortment
• The phenotypes of the parents are called parental types.
• The offspring, with new and different phenotypes, are called recombinant types or recombinants.
• This happens because offspring receive one chromosome from each parent, and end up looking different.
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Crossing Over
• In prophase I of meiosis, genes of homologous chromosomes switch around, called crossing over.
• This creates recombinant chromosomes, which makes offspring look different than parents.
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Linkage Mapping
• A genetic map is an ordered list of the locations of genes on a chromosome.
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Linkage Mapping
• Based on a linkage map, one can assume: the farther apart 2 genes are, the more likely a crossover will occur between them, therefore the recombination frequency is higher.
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Linkage Mapping
• A linkage map is a genetic map based on recombination frequencies.
• Units are called map units and show the distance between genes.• 1 map unit = a 1% chance of recombination.• If two genes are 50 map units apart, how likely is recombination?
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Cytogenic Map
• A cytogenic map locates genes on chromosomes with respect to visible features like stained bands.
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Sex linked traits
• Men are XY, Women are XX• Eggs only carry X, sperm carry X or Y• Significant genes are only carried on the X
chromosomes, which is larger and longer.
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Sex-linked genes
• A gene located on the X chromosome is called a sex-linked gene.
• Females ONLY express the trait if they inherit the trait on both chromosomes: XaXa
• If they are XAXA (normal) or XAXa (carrier) they do not have it
• Men ONLY need ONE copy of the recessive gene to have the trait: XaY
• If they are XAY, they don’t have it.
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Sex-linked genes
• Therefore, men are much more likely to have a sex-linked trait or disease, they only need one gene to get it, women need two!
• Color-blindness• Baldness• Muscular dystrophy• Hemophilia
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Practice: Hemophilia is sex linked.
• Cross a female carrier with a normal male.• XAXa x XAY• If they have a girl, what are the chances she is
a carrier? Normal? Has hemophilia?• If they have a boy, what are the chances he
has hemophilia? Is normal? Is a carrier?
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Practice: Hemophilia is sex linked.
• Cross a man with hemophilia with a carrier woman.
• XaY x XAXa
• If they have a girl, what are the chances she is a carrier? Normal? Has hemophilia?
• If they have a boy, what are the chances he has hemophilia? Is normal? Is a carrier?
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X inactivation in females
• Sometimes, the second X is inactivated in females, and is called a Barr body.
• She is still normal and fertile, but sometimes has patchy skin.
• This is what causes calico cats, every calico cat you see MUST be female.
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Abnormal Chromosome Number• Nondisjunction is when chromosomes do not separate
correctly during meiosis.• This causes an abnormal chromosome number, called
aneuploidy• Trisomy is when you have 3 chromosomes instead of 2 (2n
+ 1)• Monosomy is when you have 1 chromosome instead of 2
(2n – 1)• Polyploidy is having more than one complete set of
chromosomes• If any of the above organisms survive to birth, it will have
major developmental abnormalities
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Down Syndrome
• Trisomy 21 is Down Syndrome, where the child has 3 copies of chromosome 21 (instead of 2)
• Different facial features• Short stature• Mental disabilities• Prone to leukemia and Alzheimer’s• Sexually underdeveloped or sterile• Reduced Life expectancy
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Aneuploidy of Sex chromosomes
• XXY is Klinefelter Syndrome: male sex organs, small testes, sterile
• XYY: taller than average• XXX: trisomy X, healthy and normal• XO: Turner Syndrome, Monosomy X, sterile,
appear female, no secondary sex characteristics develop without hormone therapy
• All of the above have normal intelligence
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Alterations of chromosome structure
• Deletion: chromosomal fragment is deleted
• Duplication: a chromosomal fragment is doubled
• Inversion: chromosomal fragment gets reversed
• Translocation: chromosomal fragments get switched around
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Other Diseases
• Cri du chat: deletion from chromosome 5, mentally disabled, meows, die early
• Chronic myelogenous leukemia: 22 and 9 trade pieces, cancer of white blood cells results
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2 exceptions to this chapter
• 1. Genomic imprinting: a zygote expresses only one allele (either mom’s or dad’s) instead of both.
• 2. Inheritance of organelle genes: we have DNA in our mitochondria, and it comes only from mom