Principles of Inheritance 5 Flashcards
define mendelian disorder/genetic disorder
Genetic or Mendelian disorders are mainly determined by the alteration or mutation in the single gene. These mutations are transmitted to the offspring.
The pattern of
inheritance of such Mendelian disorders can be
traced in a family by the pedigree analysis.
Most
common and prevalent Mendelian disorders are
Haemophilia, Cystic fibrosis, Sickle-cell anaemia,
Colour blindness, Phenylketonuria, Thalassemia, etc.
It may be dominant or recessive. It may be sex linked or autosomal.
describe haemophilia
This sex linked recessive disease, which shows its
transmission from unaffected carrier female to some of the male progeny
has been widely studied. In this disease, a single protein that is a part
of the cascade of proteins involved in the clotting of blood is affected.
Due to this, in an affected individual a simple cut will result in non-stop
bleeding. The heterozygous female (carrier) for haemophilia may transmit
the disease to sons.
why do females rarely suffer from haemophilia
The possibility of a female becoming a haemophilic
is extremely rare because mother of such a female has to be at least
carrier and the father should be haemophilic (unviable in the later stage
of life). The family pedigree of Queen Victoria shows a number of
haemophilic descendents as she was a carrier of the disease.
why are men more affected by haemophilia than women?
for a female to be affected, she should carry this haemophilic gene on both her X chromosomes but in male only one X chromosome is present so one copy of this gene will make him haemophilic and this is the reason why many more men are affected than women
phenylketonuria
This inborn error of metabolism is also inherited as
the autosomal recessive trait. The affected individual lacks an enzyme
that converts the amino acid phenylalanine into tyrosine. As a result of
this phenylalanine is accumulated and converted into phenylpyruvic acid
and other derivatives. Accumulation of these in brain results in mental
retardation. These are also excreted through urine because of its poor
absorption by kidney.
describe sickle cell anemia
This is an autosome linked recessive trait that can
be transmitted from parents to the offspring when both the partners are
carrier for the gene (or heterozygous). The disease is controlled by a single
pair of allele, HbA
and HbS
. Out of the three possible genotypes only
homozygous individuals for HbS
(HbSHbS
) show the diseased phenotype.
Heterozygous (HbAHbS
) individuals appear apparently unaffected but they
are carrier of the disease as there is 50 per cent probability of transmission
of the mutant gene to the progeny, thus exhibiting sickle-cell trait
chromosomal disorders
The chromosomal disorders on the other hand are caused due to absence
or excess or abnormal arrangement of one or more chromosomes.
describe aneuploidy
Failure of segregation of chromatids during cell division cycle results
in the gain or loss of a chromosome(s), called aneuploidy,abnormal number of single chromosomes in a diploid set.
Failure of segregation during Anaphase I of meiosis, also called as non-disjunction
describe polyploidy
Failure of cytokinesis after telophase stage of cell division
results in an increase in a whole set of chromosomes in an organism and,this phenomenon is known as polyploidy. This
condition is often seen in plants.
what is monosomy or trisomy
The total number of chromosomes in a
normal human cell is 46 (23 pairs). Out of these
22 pairs are autosomes and one pair of
chromosomes are sex chromosome.
Sometimes, though rarely, either an additional
copy of a chromosome may be included in an
individual or an individual may lack one of any
one pair of chromosomes. These situations are
known as trisomy or monosomy of a
chromosome, respectively. Such a situation
leads to very serious consequences in the
individual.
describe down’s syndrome
The cause of this genetic
disorder is the presence of an additional copy
of the chromosome number 21 (trisomy of 21).
This disorder was first described by Langdon
Down (1866). The affected individual is short
statured with small round head, furrowed
tongue and partially open mouth (Figure 5.16).
Palm is broad with characteristic palm crease.
Physical, psychomotor and mental
development is retarded.
symptoms:
-flat back of head
-many loops on fingertips
-palm creases
-congenital heart disorder
-big wrinkled tongue
-broad flat face
describe klinefelter’s syndrome
This genetic disorder
is also caused due to the presence of an
additional copy of X-chromosome resulting into
a karyotype of 47, XXY. Such an individual has
overall masculine development , however, the
feminine development (development of breast,
i.e., Gynaecomastia) is also expressed (Figure
5.17 a). Such individuals are sterile.
describe turner’s syndorme
Such a disorder is
caused due to the absence of one of the X
chromosomes, i.e., 45 with X0, Such females
are sterile as ovaries are rudimentary besides
other features including lack of other secondary
sexual characters (Figure 5.17 b).
GENETIC DISORDERS(Mendelian) CHROMOSOMAL DISORDERS
1 SINGLE GENE involved- a single base pair MANY genes because it involves entire chromosome
2 1 character affected
3 Cause disease
4 Due to point mutation
5 Haemophilia, colourblindness, sickle cell anaemia, phenylketonuria, thalassemia, cystic fibrosis
MANY
genes because it involves entire chromosome
many
Cause syndrome
Due to aneuploidy
Down’s syndrome, Klinefelter’s syndrome, Turner’s syndrome
describe colour blindness
It is a sex-linked recessive disorder due to defect in
either red or green cone of eye resulting in failure to discriminate between
red and green colour. This defect is due to mutation in certain genes
present in the X chromosome. It occurs in about 8 per cent of males and
only about 0.4 per cent of females. This is because the genes that lead to
red-green colour blindness are on the X chromosome.
