Download (Microsoft PowerPoint - BehavGenTopic03BeyondMendel.ppt

Calculate the percentage of carriers for a
recessive trait, if the allele frequency of the
dominant allele is 10%.
18%
T – is the dominant allele: frequency = 0.1
t – is the recessive allele for frequency = 0.9
Carrier: Tt (including tT) frequency =
2 x 0.9 x 0.1 = 0.18
Mendel’s second law:
Independent Assortment
Different pairs of alleles are passed to
offspring independently of each other.
The result is that new combinations of
genes present in neither parent are possible.
Today, we know this is due to the fact that
the genes for independently assorted traits
are located on different chromosomes.
An exception to Mendel's 2nd law
100% linkage
But
what if genes for two traits are
located near each other on the same
chromosome?
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Meiosis
Recombination
Process of gametic cell production (gametogenesis) in which
genetic material is reduced by half (from diploid to haploid)
Genes AB and ab cross over together, but not with C & c
New variations evolve (previously not present in parental sets)
Recombination
Chromosomal mapping
Linkage analysis: technique to identify the
chromosomal location of a specific gene
e.g gene for HD:
chromosome 4
PKU locus on
chromosome 12
Another way of introducing genetic diversity
Crossovers more likely to occur between genes
that are further away
Average of 30-40 crossovers (1-2 per
chromosome) occur during a meiotic division
4 Sex-Linked Traits:
NORMAL:
A: 29, B: 45,
C: --, D: 26
Red-Green mix:
A: 70, B: --,
C: 5, D: -Red blind:
A: 70, B: --,
C: 5, D: 6
Green blind:
A: 70, B: --,
C: 5, D: 2
http://waynesword.palomar.edu/colorbl1.htm
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Color blindness
Sex-linked inheritance
Males are XY and females are XX
Two sex chromosomes are not genetically equivalent
(Y is about ¼ the size of the X)
Most common form: red-green color blindness
(more frequent 8% in males)
SKIP-A-GENERATION PHENOMENON:
If mother is colorblind, & the father is not:
all of the sons but
none of the daughters
are affected!
However, half the daughter’s
sons are likely to be affected!
Traits associated with genes
on the X chromosome
- X-linked
Traits associated with genes
on Y chromosome
- Y-linked
Consistent pattern of Heredity:
recessive allele on the X chromosome!
X-linked traits
X-linked Recessive
Males
Females
One
X chromosome
mother
Two possible genotypes
X+Y
XmY
Have trait/do not have trait
Hemizygous
Inherited from
Males
Two X chromosomes
Inherited from both parents
Three possible genotypes
X+X+
X+Xm
XmXm
Heterozygotes are carriers of
recessive traits.
Females transmit their X
randomly to either their sons
or daughters
transmit their X to
their daughters, Y to their
sons
Two copies of “deviant” gene in
females but only one copy is males is
needed to develop the disorder.
cc
Aff
mm
Females
cC
CC
Carrier Un
m+
++
Males
c
C
Affect Unaff
m
+
Males are more likely to be affected than females
regarding X-linked recessive traits
Heredity of Color blindness
Male
Draw the pedigree of a color blind
mother and an unaffected father!
Female
cc
Mating
Hemophilia – Low levels or complete absence
of a blood protein essential for clotting
C
Parents
Children
Other examples of X linked
recessive diseases
Duchenne muscular dystrophy – causing
progressive and degenerative muscle weakness.
c
cC
c
cC
Affected
Carriers
Heterozygotes for autosomal trait
X-linked carrier
Carrier of X-linked recessive trait
Lesch-Nyhan syndrome – rare disorder
caused by a deficiency of the enzyme HPRT
1/380,000 births, developmental delay in sitting,
crawling, Low IQ
Plomin, 2000 Fig 2.1 (Pg. 6)
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Meiosis
Process of gametic cell production (gametogenesis) in which
genetic material is reduced by half (from diploid to haploid)
Problem!
Things can go wrong in meiosis:
e.g. nondisjunction of chromosomes: new egg and sperm
should have only member of each chromosome pair (haploid
set). When the division does not occur properly, an egg (or
sperm) may have both members of the chromosome
resulting in trisomy!
Chromosomal abnormalities responsible for
more than half of spontaneous abortions
(miscarriages)
Some fetuses with chromosomal anomalies survive,
though with developmental abnormalities
Most common: Down’s syndrome
Trisomy & Monosomy
as results of Nondisjunction
Both parental chromosomes
(should only have one or other)
Trisomy 21 - Down’s syndrome
Two copies of one parental chromosome
(should have only one)
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Down’s syndrome
Clinical features: growth retardation, mental
retardation, distinct head and facial characteristics,
heart problems, premature aging
95% of the time, nondisjunction occurred in mother
1/1000 birth
increased rates with advanced maternal age (>35)
Nondisjunction is more likely to occur as female grows
older and activates immature eggs that have been
dormant for decades
Chromosome 21 is the smallest
• Exception to Mendel’s second law I.
Independent Assortment: when genes for two traits are
linked (located near each other on the same chromosome) and
crossover together during recombination in meiosis.
• Exception to Mendel’s laws II.
Colorblindness
• Exception to Mendel’s laws III.
chromosomal changes
(Nondisjunction in meiosis,
trisomy, monosomy, Down's syndrome)
Selected readings
Textbook:
Ch 2 page 14-19 & Ch 3 page 20-25.
The outline from this lecture presentation
will be available at the course website:
and other X-linked traits
Practice for exam:
Draw the pedigree of a color blind father
and an unaffected (non-carrier) mother!
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