The Chromosomal Basis of Inheritance Chapter 15
Dec 26, 2015
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 – Common in Genetics Experiments
• 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
Linked Genes
• Genes on the same chromosome tend to be inherited together “linked genes”
So why do offspring look different from parents?
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
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.
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.
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.
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?
Cytogenic Map
• A cytogenic map locates genes on chromosomes with respect to visible features like stained bands.
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.
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.
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
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?
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?
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.
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
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
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
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
Other Diseases
• Cri du chat: deletion from chromosome 5, mentally disabled, die early
• Chronic myelogenous leukemia: 22 and 9 trade pieces, cancer of white blood cells results