Download Chapter 15 Chromosomal Inheritance

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 – 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
• A genetic map is an
ordered list of the
locations of genes on a
chromosome.
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
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