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Chapter 12
Inheritance Patterns
and Human Genetics
How is your sex determined?
Thomas Hunt Morgan (1866-1945)
– Won the Nobel Prize in 1933 for
work with fruit flies Drosophila &
their white-eyed mutation.
– Found that Drosophila had 4 pairs
of chromosomes with 1 mismatched
pair
• Males XY
• Females XX.
– True for all mammals and most insects
• Always a 50:50 chance of getting
a male or female offspring
Why work with fruit flies?
1. *.5mm long
2. *Fast reproductive cycle
Embryo develops 1 day after fertilization
to become a larva
Pupa 2 days later
4 more days to emerge as an adult
3. Fertile with 8 - 12 hours after hatching
4. *Need only a small area to culture
many
5. *Reproduce many offspring
Since Y chromosome is so small, could genes
be located on the X chromosome that are not
on the Y chromosomes?
Sex – Linkage
– traits that are determined by whether you
are male or female
Normal fruit flies (wild type) are Red eyed
Morgan crossed a white eyed male and a
red eyed female fruit fly.
•
All F1 were wild type fruit flies
•
Did an F1 cross.
– ½ the males were white eyed but no females had
the white eyes
P: XRXR x Xr Y
Xrr
Y
Y
XR
XRXr
XRY
XRXR
XRXr
XRY
Phenotype ratio: 100% wild type
F1 cross: XRXr x XRY
XR
Y
XR
XRXR
XRY
Xr
XRXr
XrY
Phenotype ratio: 75% wild type 25% white eyed
100% of females will be wild type
50% of males could be white eyed
What cross would yield a white eyed female?
XRXr x XrY
XR
Xr
Xr
XRXr
Y
XRY
XrXr XrY
Phenotype:
50:50 for males
and females for white
or wild type eyes
<>
Human Sex
Chromosomes
• The X chromosome is much larger than
the Y chromosome.
• Genes that are located on the top of the
X chromosome are not on the Y
chromosome
• Males either have the trait or are normal
• Females can be carriers for the traits.
Linked Genes
• Genetic linkage is the tendency of alleles found
on a chromosome to be inherited together
during meiosis
• Genes nearer to each other are less likely to be
separated onto different chromatids during
crossover, and are genetically linked
• The nearer two genes are on a chromosome,
the lower is the chance of a swap occurring
between them, and the more likely they are to be
inherited together.
Chromosome Mapping
• A genetic map is a map based on the
frequencies of recombination between markers
during crossover
• The greater the frequency of recombination
between two genetic markers, the further apart
they are assumed to be
• Conversely, the lower the frequency of
recombination between the markers, the smaller
the physical distance between them.
Sex-linked traits in humans
1. Red-Green color blindness –
recessive. Can’t distinquish between
colors
2. Muscular dystrophy- recessive –
weakens then destroys muscle tissue
3. Hemophilia – recessive – lacks ability
to produce clotting factor – bleeders
– Ran rampant through the royal families
of Europe – “Royal Hemophilia”
Queen Victoria
Czar Nicholas II
Detecting Genetic Diseases
• Amniocentesis
• The Pedigree
How could you trace a trait
throughout a family?
• Pedigree
– shows how a trait is inherited over several
generations
= Female
= Female with the trait
= Male
= Male with the trait
If half colored in, they are a carrier of the trait
Sample Pedigrees
Human Genetic Disorders
• Color blindness
• Hemophilia
• Cystic fibrosis
• Huntingdon’s disease
• Tay Sachs
• Sickle cell anemia
Here is a test for you. Look at the figures below and
write down what you see. Do not talk or make any
comments4. during this test!
If you could not see the 29, 45,
56, 6 or 8, you are color blind!!
12 – 20% of the population has
this trait.
If normal color vision, you will see a 5. If
colorblind, you see a 2
Youtube test
Sex influenced trait
Males and females express trait differently
with the same phenotype
Male Pattern Baldness – recessive –
speaks for itself!
On autosomes, not sex linked
• XNXN XNYN - Normal hair
• XNXn XNYn - Normal female, Bald male
• XnXn XnYn - Bald
How does the inheritance of traits located on the same
chromosome affect how they are inherited?
Linkage groups
• If genes are located on the same chromosome,
they tend to be inherited together
• Morgan - genes for wing length and body color
were on the same chrom (linked)
• Crossing over during Prophase I of Meiosis
unlinks linked genes
• The closer two genes are on a chromosome,
the less likely they are to cross over. >
probability of crossing over the further away
they are on a chrom
Traits controlled by 3 or more alleles
for a given trait
– Blood type: IA IB i (A, B, O)
Polygenic inheritance
2 or more genes determine a single
trait. Ends up blending these genes
– Height
– Skin pigmentation
– Eye color
– Hair color
What can go wrong? AKA Mutations
1. Germ cell mutations
– affect gametes
– may be passed on to offspring
2. Somatic mutations
– affect body cells
– not inheritable
–Some cancers & leukemia
3. Chromosomal mutations
– changes in structure of a chromosome or
addition or loss of an entire chromosome
–
–
–
Deletion/addition – portion of chrom is missing
or added
Inversions – pieces of chroms flip flop
Translocations – pieces of nonhomologous
chroms are exchanged
A lovely British animation
Nondisjunction
– type of Chromosomal mutation
– entire chromosomes are missing
or extra chromosomes
- Chromosomes fail to separate
during meiosis
- Results in gametes with extra
chromosomes or missing
chromosomes
Animation
Trisomy 21
– Down syndrome
– Extra #21
Chromosomes
– If Sex chromosomes fail to disjoin:
• Turner’s syndrome – X0 female
• Klinefelter’s syndrome – XXY male
46XX
Descriptions
45X0
47XXY
46XY
Disorder
Symptom
Pattern of
Inheritance
Frequency at birth
Huntington's
disease
Deterioration of brain
tissue - middle age
Autosomal
dominant
1/10,000
Cystic fibrosis
Mucus clogs lungs,
liver & pancreas doesn't make it to
adulthood
autosomal
recessive
1/2,080 (whites)
Sickle cell
anemia
Impaired blood
circulation - organ
damage
autosomal
recessive?
1/500 (African
decent)
Tay-Sachs
disease
Deterioration of
central nervous
system - childhood
death
autosomal
recessive
1/1,600
(European
Jewish decent)
Phenylketonuria
Brain failure - doesn't
have enzyme for
phenylalanine
digestion
autosomal
recessive
1/18,000
Hemophilia
Failure to clot blood
X-linked
recessive
1/7,000
Muscular
dystrophy
wasting away of
muscles
X-linked
recessive
1/10,000
What to do to help?
GENE THERAPY
Other neat stuff…
X chromosome inactivation and
Barr bodies