Genetics

CHAPTER 2


Genetics





INTRODUCTION




The National Human Genome Project



Definition: Genetics is a medical science concerned with the transmission of characteristics from parent to child (Nussbaum, McInnes, & Willard, 2007)



Foundation of inheritance



1. Cell division: all beings begin life as a single cell (zygote). The single cell continues to reproduce itself by the process of either mitosis or meiosis.



a. Mitosis is the process of cell division in which new cells are made. The new cells have the same number and pattern of chromosomes as the parent cell (46 chromosomes comprising 44 autosomes and 2 sex chromosomes); mitosis occurs in five stages (Figure 2-1).




b. Meiosis is a process of cell division that occurs involving the sperm and ova and is known as gametogenesis; this process decreases the number of chromosomes by 50% (from 46 to 23 per cell) and occurs in two successive cell divisions (Figure 2-2)




(1) The first division consists of four phases.



(2) Second division



(a) The chromatids of each chromosome separate and move to the opposite poles of each of the daughter cells.


(b) This is followed by each of the cells dividing into two cells, which results in four cells (spermatogenesis and oogenesis).



(c) During the meiotic division, two of the chromatids might not move apart when the cell divides; this lack of separation is called autosomal nondisjunction; this is also the stage at which breakage can occur, resulting in abnormalities of chromosomal structure such as that producing cri du chat syndrome and Down syndrome


(d) Thus meiosis mixes up the chromosomes and crossing over mixes up the genes within a chromosome.


2. Genetic information is present on the chromosomes (Jarvi & Chitzyat, 2008; Lashley, 2007; Nussbaum et al., 2007).



a. Chromosomes are composed of DNA, a complex protein that carries the genetic information.


b. DNA occurs as a double-stranded helix found in the cell nucleus.



c. Genes are the smallest known unit of heredity.



3. Chromosomes form a genetic blueprint that is composed of tightly coiled structures of DNA.



a. Chromosomes are threadlike structures within the nucleus of the cell that carry the genes.


b. Humans have 46 chromosomes in each body cell (22 pairs of autosomes and 1 pair of sex chromosomes [diploid]).


c. Chromosomes have a primary central constriction called the centromere (Figure 2-3).




d. The sex cells contain 23 chromosomes (haploid).


e. Abnormalities of chromosome number are as follows (Lashley, 2005, 2007; Nussbaum et al., 2007):



(1) Paired chromosomes fail to separate during cell division (nondisjunction).


(2) If nondisjunction occurs during meiosis (before fertilization), the fetus usually will have abnormal numbers of chromosomes in every cell (trisomy or monosomy).



(3) If nondisjunction occurs after fertilization, the fetus might have two or more chromosomes that evolve into more than one cell line (mosaicism), each with a different number of chromosomes (Lashley, 2007).



f. Abnormalities in chromosome structure are as follows:



(1) Abnormalities of the chromosomes involving only a part of the chromosome


(2) Abnormalities that can occur by translocation, by deletions, or by additions (Lashley, 2007)



(a) Translocation occurs when the individual has 45 chromosomes, with one of the chromosomes fused to another chromosome (usually number 21 fused to number 14) (see Figure 2-3).



(b) Additions and/or deletions: a portion of a chromosome can be added or lost, which will result in adverse effects on the infant



Modes of inheritance (mendelian)



1. Many diseases are caused by an abnormality of a single gene or a pair of genes.


2. Autosomal dominant inheritance: autosomal dominant disorders occur when an individual has a gene that produces an effect whenever it is present (homozygous or heterozygous); this gene overshadows the other gene of the pair



a. Mode of transmission (Figure 2-4)



b. Characteristics



(1) Affected individuals generally have an affected parent; the family tree (genogram) might show several generations of individuals with the condition


(2) The affected individual has a one in two (50%) chance with each pregnancy of passing the abnormal gene on to his or her child.


(3) Males and females are equally affected.


(4) An unaffected individual cannot transmit the disorder to his or her children.


(5) A mutation (a gene that has been spontaneously altered) can result in a new case.


(6) Autosomal dominant disorders vary greatly in the degree of characteristics that are seen within a family (e.g., in Marfan syndrome, the parent might have only elongated extremities but the child might have a more involved condition, including dislocation of the lens of the eye and severe cardiovascular abnormalities).


3. Autosomal recessive inheritance: an individual has an autosomal recessive disorder if he or she has a gene that produces its effects only when there are two genes on the same chromosome pair (homozygous trait).



a. A carrier state can occur (heterozygote).



b. Mode of transmission (Figure 2-5)



c. Characteristics



(1) An affected individual has clinically normal parents, but the parents are both carriers for the abnormal gene.


(2) The carrier parents have a one in four (25%) chance with each pregnancy of passing the abnormal gene on to their offspring; in this case, when a recessive gene is received from each parent, the child will have the disorder.


(3) If the offspring of two carrier parents is clinically normal, there is a one in two (50%) chance that he or she will be a carrier, like the parents.


(4) Males and females are affected equally.


(5) The family genogram usually shows siblings affected in a horizontal pattern.


(6) There is an increased risk if intermarriage occurs (consanguineous matings); individuals who are closely related are more likely to have the same genes in common.


(7) Recessive disorders tend to be more severe in their clinical manifestations.


(8) The presence of certain autosomal recessive genes can be detected in the normal carrier parent; examples of diseases for which carrier screening is available are sickle cell anemia, Tay-Sachs disease, and cystic fibrosis.

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Oct 29, 2016 | Posted by in NURSING | Comments Off on Genetics

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