Lecture 9: Genetics
Gregor Mendel’s Discoveries
Second: The law of independent assortment
 Mendel’s experiments that followed the inheritance of flower
color or other characteristics focused on only a single
characteristic via monohybrid cross ‫التزاوج أحادي الصفة‬
 Mendel also analyzed the inheritance pattern of two traits at the
same time via a dihybrid cross ‫التزاوج ثنائي الصفة‬. For example, he
examined plants that differed in both the shape and color of
their peas.
 He crossed homozygous plants that produced yellow, round
seeds (YYRR) with plants that produced green, wrinkled seeds
(yyrr). He founds that:
 Yellow seeds (Y) was dominant to green seeds (y).
 Round seeds (R) was dominant to wrinkled seeds (r)
Dihybrid cross ‫التزاوج ثنائي الصفة‬
 It is a mating ‫ تزاوج‬between two
parent plants different in two
characteristics.
 The two pairs of alleles segregate
independently of each other.
 When the F1 plants were selffertilized, they produced an F2
generation of plants that had all four
phenotypic ratios (9:3:3:1) of the two
seed characteristics:
 9 plants with yellow, round seeds.
 3 plants with green, round seeds.
 3 plants with yellow, wrinkled
seeds.
 1 plants green, wrinkled seeds.
Genetic disorders in human
A) Recessive genetic disorders ‫االعتالالت الوراثية المتنحية‬
 Thousands of genetic disorders ‫اعتالالت وراثية‬, including disabling ‫ اإلعاقة‬or





deadly genetic diseases ‫األمراض الوراثية ال ُمميتة‬, are inherited as simple
recessive traits ‫صفات ُمتنحية‬.
Heterozygotes have a normal phenotype because one “normal” allele
produces enough of the required factors (for normal trait).
Thus, individuals who lack the disorder are either homozygous dominant
or heterozygous.
Heterozygous member may have no clear phenotypic effects, but is a
carrier who may transmit a recessive allele to their offspring.
Two carriers have a 1/4 (25%) chance of having a child with the disorder,
1/2 (50%) chance of a carrier, and 1/4 (25%) free.
Recessive genetic disorders include albinism ‫البهاق‬, cystic fibrosis ‫التليف‬
‫الكيسي‬, Tay-Sachs disease ‫ مرض تاي ساكس‬and sickle cell anemia ‫األنيمياء المجلية‬
1- Albinism ‫البهاق‬
 X-linked recessive ‫متنحي مرتبط بكروموسوم اكس‬
 Absence of melanin pigment ‫ صبغة الميالنين‬in the
hair, skin and eyes.
 Albinism results from inheritance of recessive
gene alleles.
 Albinism is caused by an alteration of the gene
that makes the melanin pigment
 Albinism is associated with vision ‫الرؤية‬
problems and skin cancer ‫سرطان الجلد‬.
2- Cystic Fibrosis ‫التليف الكيسي‬
 Autosomal recessive ‫متنحي جسدي‬, gene located in chromosome 7
 It is caused by the presence of mutations in both copies of the gene for the
protein Cystic Fibrosis Transmembrane Conductance Regulator (CFTR).
 CFTR gene controls chloride ion movement in and out of the cell.
 It affects mostly the lungs but also the pancreas, liver and kidneys.
 Without treatment, affected children die before five, but with treatment
can live between 37 and 50 years.
Cystic fibrosis is inherited in the autosomal recessive pattern
3- Tay-Sachs disease ‫مرض تاي ساكس أو البله المميت‬
 Autosomal recessive ‫متنحي جسدي‬
 Tay–Sachs results from mutations in the HEXA gene on chromosome 15
 Mutation of HEXA gene leads to accumulate lipids in the brain's nerve cells
leading to the death of the cells.
 The symptoms ‫ األعراض‬begin with seizures ‫نوبة عصبية‬, blindness ‫عمى‬, and
mental and physical disabilities ‫ إعاقة عقلية وجسدية‬six months after birth and
usually results in death by the age of four.
4- Sickle-cell anemia ‫األنيمياء المجلية‬
 Autosomal recessive ‫متنحي جسدي‬
 Sickle cell anemia results from mutations in the HBB gene ‫ هيموجلوبين بيتا‬on




chromosome 11
It is caused by the substitution ‫ استبدال‬of a single amino acid in hemoglobin.
People with sickle-cell anemia have abnormal hemoglobin in their red
blood cells.
Hemoglobin is a protein that carries oxygen throughout the body.
Doctors can use regular blood transfusions ‫ نقل الدم‬to prevent brain damage
and new drugs to prevent or treat other problems.
B) Dominant genetic disorders ‫االعتالالت الوراثية السائدة‬
1- Achondroplasia ‫القزمية‬
 Autosomal dominant ‫سائد جسدي‬
 Achondroplasia is caused by a mutation in fibroblast growth factor
receptor 3 (FGFR3) gene on chromosome 4
 People with achondroplasia have short stature ‫قصر القامة‬, with an average
adult height of 131 centimeters for males and 123 centimeters for females.
 Heterozygous individuals have the dwarf ‫قزم‬
phenotype.
 Lethal dominant alleles ‫ األليالت السائدة المميتة‬are
much less common ‫ أقل شيوعا‬than lethal
recessives, because if a lethal dominant kills
an offspring before it can mature and
reproduce, the allele will not be passed on
future generations ‫أجيال المستقبل‬.
2- Huntington’s disease ‫مرض هنتنغتون‬
 Autosomal dominant ‫سائد جسدي‬
 Huntington’s disease is caused by a mutation in HD gene.
 HD gene is located in chromosome 4
 The dominant lethal allele has no obvious phenotypic effect
until an individual is about 35 to 45 years old.
 Huntington's disease is an inherited condition that damages
nerve cells in the brain.
 Huntington's disease results in an losing of both mental and
physical control.
Summary of the human genetic disorders
 Dominant trait ‫الصفة السائدة‬: Describes a trait that is expressed when only
one copy of the allele is present in the genotype.
 Recessive trait ‫الصفة المتنحية‬: Describes a trait that is masked by the
dominant trait and is only expressed in the homozygous condition.
 Autosome ‫الكروموسوم الجسدي‬: Any chromosome that is not a sex chromosome.
 Humans have 22 pairs of autosomes and one pair of sex chromosomes (the
X and Y).
 Genetic disorders caused by genes on autosomes are called autosomal
disorders.
 Some genetic disorders are autosomal dominant ‫سائد جسدي‬
 An individual with AA has the disorder
 An individual with Aa has the disorder
 An individual with aa does not have disorder
 Other genetic disorders are autosomal recessive ‫متنحي جسدي‬
 An individual with AA does not have disorder
 An individual with Aa does not have disorder, but is a carrier ‫حامل‬
 An individual with aa has the disorder