Download BASIC CONCEPTS IN GENETICS

Genetic Information
• Gene – basic unit of genetic
information. Genes determine the
inherited characters.
• Genome – the collection of
genetic information.
• Chromosomes – storage units of
genes.
• DNA - is a nucleic acid that
contains the genetic instructions
specifying the biological
development of all cellular forms
of life
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Chromosome Logical Structure
• Locus – location of a gene/marker
on the chromosome.
• Allele – one variant form of a
gene/marker at a particular locus.
Locus1
Possible Alleles: A1,A2
Locus2
Possible Alleles: B1,B2,B3
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Human Genome
Most human cells
contain 46 chromosomes:
• 2 sex chromosomes (X,Y):
XY – in males.
XX – in females.
• 22 pairs of chromosomes
named autosomes.
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Genotypes
Phenotypes
• At each locus (except for sex chromosomes)
there are 2 genes. These constitute the
individual’s genotype at the locus.
• The expression of a genotype is termed a
phenotype. For example, hair color, weight,
or the presence or absence of a disease.
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Genotypes
Phenotypes (example)
genotypes
phenotypes
• Eb- dominant allele.
• Ew- recessive allele.
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Dominant vs. Recessive
• A dominant allele is
expressed even if it is
paired with a recessive
allele.
•A recessive allele is
only visible when paired
with another recessive
allele.
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One Locus Inheritance
Female
A|A
Male
1
2
A|a
a|a
3
4
A| a
5
heterozygote
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a|a
a|a
homozygote
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Mendel’s 1st Law
Two members of a gene pair segregate from each other into
the gametes, so half the gametes carry one member of the
pair and the other half carry the other member of the pair.
Y/y
y/y
all y
½ y/y
Gamete
production
½y
½ Y/y
½Y
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Gamete
production
Calculating Probabilities
• We want to predict
patterns of inheritance
of traits and diseases in
pedigrees.
• E.g., we want to know
the likelihood that a
dog chosen at random
from the study
population will have blue
eyes.
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X-linked Inheritance
Different results obtained
from reciprocal crosses
between red-eyed and
white-eyed Drosophila.
Explanation: The gene
responsible for eye-color
is X-linked. Females have
2 X-chromosomes, while
males have 1 X-chromosome
and 1 Y-chromosome.
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Mendel’s 2nd Law
• Different gene pairs assort independently
in gamete formation.
This “law” is true only in some cases.
Gene pairs on SEPARATE CHROMOSOMES
assort independently at meiosis.
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Medical Genetics
When studying rare disorders, 6 general
patterns of inheritance are observed:
•
•
•
•
•
Autosomal recessive
Autosomal dominant
X-linked recessive
X-linked dominant
Codominant
• Mitochondrial
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Medical Genetics (cont.)
Autosomal recessive
• The disease appears
in male and female
children of
unaffected parents.
• e.g., cystic fibrosis
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Medical Genetics (cont.)
Autosomal dominant
• Affected males and
females appear in each
generation of the
pedigree.
• Affected mothers and
fathers transmit the
phenotype to both sons
and daughters.
• e.g., Huntington disease.
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Medical Genetics (cont.)
X-linked recessive
• Many more males than
females show the disorder.
• All the daughters of an
affected male are
“carriers”.
• None of the sons of an
affected male show the
disorder or are carriers.
• e.g., hemophilia
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Medical Genetics (cont.)
X-linked dominant
• Affected males pass the
disorder to all daughters
but to none of their sons.
• Affected heterozygous
females married to
unaffected males pass the
condition to half their sons
and daughters
• e.g. fragile X syndrome
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Medical Genetics (cont.)
Codominant inheritance
•
Two different versions
(alleles) of a gene can be
expressed, and each
version makes a slightly
different protein
• Both alleles influence the
genetic trait or determine
the characteristics of the
genetic condition.
• E.g. ABO locus
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Medical Genetics (cont.)
Mitochondrial inheritance
• This type of inheritance
applies to genes in
mitochondrial DNA
• Mitochondrial disorders
can appear in every
generation of a family and
can affect both males and
females, but fathers do not
pass mitochondrial traits
to their children.
• E.g. Leber's hereditary
optic neuropathy (LHON)
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Notes
•
DNA - a pair of molecules joined by hydrogen bonds: it is
organized as two complementary strands, head-to-toe, with
the hydrogen bonds between them. Each strand of DNA is a
chain of chemical "building blocks", called nucleotides, of
which there are four types:adenide (abbreviated A),
cytozyne (C), guanine (G) and thymine (T).
• Mitochondria, which are structures in each cell that convert
molecules into energy, each contain a small amount of DNA.
• A chromatid forms one part of a chromosome after it has
coalesced for the process of mitosis or meiosis. During
either process, the word "chromosome" indicates a pair of
two exactly identical ("sister") chromatids joined at the
central point of each chromatid, called the centromere.
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Notes -cont
• Mitosis is the process by which a cell separates
its duplicated genome into two identical halves
• Meiosis is the process that transforms one diploid
into four haploid cells.
• Reciprocal cross a cross, with the phenotype of
each sex reversed as compared with the original
cross, to test the role of parental sex on inheritance
pattern. A pair of crosses of the type genotype
A(female) X genotype B(male) and genotype
B(female) X genotype A(male).
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Question #1
1
2
• Write the genotypes in every possible place.
• If individuals 1 and 2 marry, what is the probability that
their first child will be sick?
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Question #2
• PKU is a human hereditary disease resulting from
inability of the body to process the chemical
phenylalanine (contained in protein that we eat).
• It is caused by a recessive allele with simple
Mendelian inheritance.
• Some couple wants to have children. The man has a
sister with PKU and the woman has a brother with
PKU. There are no other known cases in their
families.
• What is the probability that their first child
will have PKU ?
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Question #2-Solution Highlights
P/p
P/p
P/p
P/p
p/p
P/-
P/-
p/p
P – the normal allele
p – the mutant allele
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Question #3
• The disease is rare.
1
2
3
4
5
6
7
8
9
10
a. What is the most likely mode of inheritance ?
b. What would be the outcomes of the cousin marriages
1 x 9, 1 x 4, 2 x 3, and 2 x 8 ?
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Question #3-Solution Highlights
a.
–
–
b.
–
–
–
–
Observations:
After the disease is introduced into the family in generation
#2, it appears in every generation  dominant!
Fathers do not transmit the phenotype to their sons 
X-linked!
The outcomes:
1 x 9: 1 must be A/a
9 must be A/Y
1 x 4: 1 must be A/a
4 must be a/Y
2 x 3: 2 must be a/Y
3 must be A/a
2 x 8: 2 must be a/Y
8 must be a/a
Same
All normal
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TANK YOU
DONE BY
DR. WISAM HABHAB
GENETICS RESIDENT