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A condition in which an extra copy of a chromosome is present in the cell nuclei, causing developmental abnormalities.  Trisomy 13 (Patau syndrome)  trisomy 21 (Down syndrome)  trisomy 18 (Edward syndrome)

 Reductional division- the child gets half of its chromosomes from each parent  Provides genetic diversity.  Separation of chromosomes follows law of independent assortment where each gamete receives a selection of parental chromosomes. The chance of getting same chromosomes in two gametes is approximately 1 in 8 million.  Because of crossing over each chromatid receives DNA derived from both parental homologous chromosomes referred to as gene shuffling

 It is the process of nuclear division that occurs during the final stage of gamete formation.  It has two cell divisions- meiosis 1 which is reductional division and meiosis 2 which is equational division resulting in 4 daughter cells.  Prophase 1 is longer and has 5 stages-leptotene, zygotene, pachytene, diplotene and diakinesis  The chromosomal complement is reduced to haploid set of chromosomes (n-23)  Duration to complete one meiosis varies from few days to years

 A karyotype refers to a full set of chromosomes from an individual arranged according to length, position of centromere, banding pattern.  Karyotype is written as total number of chromosomes followed by sex chromosomes  Normal male karyotype: 46,XY, normal female karyotype: 46,XX  Chromosomes are classified into 7 groups and pasted accordingly- Group A- 1, 2, 3; group B- 4, 5; group C- 6-12, X; group D- 13, 14, 15; group E- 16, 17, 18; group F- 19, 20; group G- 21, 22, Y

 An autosome is a chromosome not involved in sex determination and is numbered.  Humans have 22 pairs of autosomes and one pair of sex chromosomes (the X and Y)  Autosomal dominant genes are expressed in heterozygous state.  Autosomal recessive genes are expressed in homozygous state.

 A genome is an organism's complete set of DNA, including all of its genes.  Each genome contains all of the information needed to build and maintain that organism.  In humans, a copy of the entire genome—more than 3 billion DNA base pairs—is contained in all cells that have a nucleus.  The genome includes both the genes and the non-coding sequences of the DNA/RNA  The Human Genome Project was an international research effort to determine the sequence of the human genome and identify the genes that it contains.

 A gene that is phenotypically expressed in the homozygous state but has its expression masked in the presence of a dominant gene is called recessive gene.  The person with one normal and one recessive gene will not express any abnormal features and is called as carrier.  The recessive genes are expressed in the individuals of same generation leading to horizontal transmission.  Examples- Cystic fibrosis, Alkaptonuria, Albinism

 Sex chromosomes are sex determining chromosomes in males and females. In males the sex chromosomes are XY, in females it is XX.  The X chromosome in humans is submetacentric belonging to ‘C’ group.  The X chromosome is notably larger and has a more active euchromatin region.  The Y chromosome is acrocentric and belongs to ‘G’ group.  Because only males have the Y chromosome, the genes on this chromosome tend to be involved in male sex determination and development.

 Each chromosome is made up of condensed DNA which is tightly coiled many times around proteins called histones and visible under metaphase stage.  It has a constriction point called the centromere, which divides the chromosome into two “arms” - short arm (p arm) and long arm (q arm).  Depending on the position of centromere the chromosomes are classified morphologically into metacentric, submetacentric and acrocentric chromosomes

o Ultrasonography o Amniocentesis o Chorionic villus sampling

 Prenatal diagnosis is the ability to detect abnormalities in an unborn child.  Specifically, prenatal diagnosis is helpful for: Managing the remaining weeks of the pregnancy, determining the outcome of the pregnancy, planning for possible complications with the birth process, planning for problems that may occur in the newborn infant, deciding whether to continue the pregnancy and finding conditions that may affect future pregnancies.  There are a variety of non-invasive and invasive techniques available for prenatal diagnosis. Each of them can be applied only during specific time periods during the pregnancy for greatest utility.  Some of the techniques employed for prenatal diagnosis include: o Ultrasonography o Amniocentesis o Chorionic villus sampling

 Metacentric centromere is in the center of chromosome, where length of two arms is equal. Chromosomes 1, 3, 16, 19  Submetacentric o centromere is away from the center of chromosome, where one arm is shorter than the other arm. Chromosomes 2, 4, 5, 6 to 12, 17, 18, X  Acrocentric o centromere is towards one end of the chromosome, where one of the arms is too short and the other arm is too long. Chromosomes 13, 14, 15, 20, 21, Y

 The orderly arrangement of the chromosomes according to length, position of centromere, banding pattern is called karyotyping.  A karyotype refers to a full set of chromosomes from an individual which can be compared to a normal karyotype for the species via genetic testing.  A chromosome anomaly may be detected or confirmed in this manner. Procedure:  Peripheral blood is collected in a heparinized syringe.  Blood is cultured in a suitable medium with addition of phytohaemagglutinin.  The culture vials are incubated for three days  Colchicine is added after 3days which arrests the WBC’S in metaphase.  The contents are centrifuged and cells are separated and treated with hypotonic solution.This makes the cells swell up and disperse the chromosomes. These are dropped onto a chilled slide stained with Trypsin Giemsa stain.  Metaphase spreads are analysed under microscope where each chromosome is identified for the abnormality, then photographed, cut and pasted in ...

 Sex chromosomes are sex determining chromosomes in males and females. In males the sex chromosomes are XY, in females it is XX.  Both males and females retain one of their mother's X chromosomes, and females retain their second X chromosome from their father.  The X chromosome is notably larger and has a more active euchromatin region.  Because only males have the Y chromosome, the genes on this chromosome tend to be involved in male sex determination and development.  Genes in areas known as pseudoautosomal regions are present on both sex chromosomes. As a result, men and women each have two functional copies of these genes. Many genes in the pseudoautosomal regions are essential for normal development.

Edward syndrome (trisomy 18, 47,XX+18 or 47,XY+18): numerical chromosomal abnormality, autosomal trisomy. Features are  Infants usually die in the first few weeks after birth  Delayed developmental milestones  Mental retardation  Prominent occiput  Short palpebral fissures  Narrow palatal arch  Micrognathia  Clenched hands with over-riding of fingers.  Short sternum  Umbilical or inguinal hernia  Congenital heart defects Chromosome  Each chromosome is made up of condensed DNA which is tightly coiled many times around proteins called histones and visible under metaphase stage.  Each chromosome has a constriction point called the centromere, which divides the chromosome into two “arms.” The short arm of the chromosome is labeled the “p arm” and the long arm “q arm.”  Depending on the position of centromere the chromosomes are classified morphologically into o Metacentric- centromere is in the center of chromosome, where length of two arms is equal. C...

 Prenatal diagnosis is the ability to detect abnormalities in an unborn child.  Without knowledge gained by prenatal diagnosis, there could be an untoward outcome for the fetus or the mother or both.  Congenital anomalies account for 20 to 25% of perinatal deaths.  Specifically, prenatal diagnosis is helpful for: o Managing the remaining weeks of the pregnancy, o determining the outcome of the pregnancy, o planning for possible complications with the birth process, o planning for problems that may occur in the newborn infant, o deciding whether to continue the pregnancy o finding conditions that may affect future pregnancies.  There are a variety of non-invasive and invasive techniques available for prenatal diagnosis. Each of them can be applied only during specific time periods during the pregnancy for greatest utility. Some of the techniques employed for prenatal diagnosis include: o Ultrasonography o Amniocentesis o Chorionic villus sampling o Fetal blood cel...

 Trait is expressed even when individual has one abnormal allele (heterozygous state).  Males and females are equally affected.  If one parent is affected, each of their children has a 50% chance of being affected.  If one parent is homozygous for the dominant allele, all of the children will express the trait.  It can be traced through many generations and is called vertical transmission  All form of transmission seen- male to male, male to female, female to female, female to male  Examples: Achondroplasia, osteogenesis imperfecta, polydactyly, Marfan's syndrome, and some neuromuscular disorders.

a. X- linked dominant (X-LD) inheritance:  Uncommon, but disorders, manifest in the heterozygous female as well as in the male, with the mutant allele on his single X chromosome.  An excess of affected females may be seen in families with X-LD disorders.  A mosaic pattern demonstrated in affected heterozygous females for some X-LD disorders.  Both the daughters & the sons of an affected female with X-LD disorder, have 1 in 2 (50%) chance of being affected.  Affected male transmit the trait to the daughters but none to his sons.  Examples: Vitamin D-resistant rickets, Incontinentia pigmenti. b. X-linked recessive(X-LR) inheritance:  It usually manifests only in males.  A male with a mutant allele on his single X is called hemizygous.  A male can’t transmit X-linked trait to his son.  X-linked recessive diseases are transmitted by healthy heterozygous female carriers to male.  Trait is transmitted to male grandchild through the obligate carrier daugh...

Y CHROMOSOME  The Y chromosome is one of the two sex chromosomes in humans (the other is the X chromosome).  The sex chromosomes form one of the 23 pairs of human chromosomes in each cell.  The Y chromosome is present in males, who have one X and one Y chromosome.  Sex is determined by the SRY gene on the Y chromosome, which is responsible for the development of a fetus into a male.  Other genes on the Y chromosome are important for male fertility. Many genes are unique to the Y chromosome.  Genes present in areas known as pseudoautosomal regions take part in pairing during meiosis.

X CHROMOSOME   The X chromosome is one of the two sex-determining chromosomes and is found in both males and females.  It is submetacentric chromosome classified under ‘C’ group chromosomes  Females have two X chromosomes, whereas males have one X and one Y chromosome.  Both males and females retain one of their mother's X chromosomes, and females retain their second X chromosome from their father.  Early in embryonic development in females, one of the two X chromosomes is randomly and permanently inactivated in nearly all somatic cells. This phenomenon is called X-inactivation or Lyonization. The inactive X- chromosome is tightly coiled and hence stains dark purple and is seen as Barr body within the nucleus of a female. The X chromosome is notably larger and has a more active euchromatin region.  Numerical alteration in Sex chromosomes result in abnormalities eg Turner’s syndrome (XO), Klinefelter’s syndrome (XXY) and Triple X syndrome (XXX).  , S...