Genetics U.K. College of Nursing
Genetics U.K. College of Nursing Genes and Chromosomes Each cell contains 23 pairs of matched chromosomes for a total of 46 chromosomes per cell. One chromosome from each pair is inherited from each parent. There are 22 pairs of autosomes, which control most traits in the body, and one pair of sex chromosomes, which determine gender and other traits. Genes and Chromosomes Some genes are dominant and their characteristics are expressed even if only on one chromosome. Some genes are recessive and their characteristics will be expressed only if they are carried by both chromosomes in a pair. Group Exercise Human Variation Punnet Squares Visual representation of the principles of inheritance Dominant trait--Capital letter Recessive trait--Small letter Male vs female trait/gene Patterns of Inheritance Autosomal Dominant Autosomal Recessive Sex-Linked (or X-linked) Dominant Sex-Linked (or X-linked) Recessive Chromosomal Abnormalities Congenital Anomalies Autosomal Dominant Trait appears in every generation (does not skip) Both males and females are affected Each pregnancy of an affected person has a 50% chance of producing an affected offspring Autosomal Dominant Disorders Disorders Huntington’s Disease Retinitis Pigmentosa Polycystic Kidney Disease Achodroplasia Marfan Syndrome Dominant Disorders- Marfan Syndrome Autosomal Dominant Inheritance Autosomal Dominant Clinical Situation-One parent is unaffected One parent carries the defective gene for Marfan Syndrome Draw the Punnet Square Autosomal Dominant Punnett Square Autosomal Recessive Both parents are usually unaffected, but are carriers Trait first appears only in siblings rather than in parents Trait found equally in males and females 25% risk when both parents are carriers Increased incidence with consanguinity Autosomal Recessive Disorders Phenylketonuria Fanconi’s Anemia Tay Sachs Disease Sickle Cell Anemia Cystic Fibrosis Autosomal Recessive Inheritance Autosomal Recessive Clinical Situation Male carries the defective gene for Tay Sachs disease Female carries the defective gene for Tay Sachs disease Draw the Punnett Square Autosomal Recessive Punnett Square X-linked Inheritance Sex-Modified Traits - Dominant genes are expressed in both males & females but at differing frequencies Ex: Baldness - expressed as dominant in males, but recessive in females, never as severe in females X-linked Dominant Very rare Often lethal in males therefore few males present in the pedigree Multiple miscarriages may be present No carrier status, all individuals with the gene are affected Trait appears in every generation X-linked Dominant Female children of affected males will all be affected (100% risk); no male to male transmission. Homozygous females (both X chromosomes are affected) have a 100% chance of having an affected child of either sex. Heterozygous females (only one X affected) have a 50% of having an affected child with each pregnancy. X-linked Dominant Disorders Hypophosphatemic Rickets Fragile X Syndrome Fragile X Syndrome X-linked Dominant Clinical Situation Male is affected with hypophosphatemic rickets Female is unaffected Draw the Punnet square X-linked Dominant Punnet Square X-linked Recessive Incidence of trait much higher among males in a kinship than among females Trait cannot be transmitted from father to son An affected male will pass the carrier status to all his daughters Female carriers have a 50% risk of transmitting the gene to their offspring with each pregnancy X-linked Recessive Disorders Hemophilia A Duchenne’s Muscular Dystrophy Color-Blindness Duscenne’s Muscular Dystrophy X-Linked Recessive Inheritance X-linked Recessive Clinical Situation Male is affected with Hemophilia A Female is normal (non-carrier) Draw the Punnett Square Use X1 for chomosome with normal allele and X2 for chromosome with disease allele X-linked Recessive Punnet Square X-linked Recessive Clinical Situation Male is normal Female is a carrier of colorblindness Draw the Punnett Square X-linked Recessive Punnett Square X-linked Recessive Clinical Situation Male is affected with Duschenne Muscular Dystrophy Female is carrier of Duschenne Muscular Dystrophy Draw the Punnett Square X-linked Recessive Punnett Square Genotype - The actual gene constitution of a given person. Phenotype - The observable characteristics of a given person Traits can be environmentally modified type 2 diabetes PKU Traits can be medically modified Sickle cell disease (bone marrow transplant) Polycysitc kidney disease (kidney transplant) However, genotype stays the same so next generation are not saved from condition Group Exercise Punnet Squares and Patterns of Inheritance Karyotypes The arranged representation of the chromosomal make-up of a cell nucleus Chromosomal Abnormalities Abnormalities in number of chromosomes Caused by nondisjunction: failure of homologous chromosomes or sister chromatids to separate properly into different progeny cells Monosomy - condition in which one chromosome of a pair is missing from a somatic cell Monosomy X Turners Syndrome Monosomy--Turner’s Syndrome Chromosomal Abnormalities Trisomy - condition in which one chromosome in the pair is pesent in three copies in a somatic cell Down Syndrome (21), Trisomy 13 or 18 Klinefelter’s Syndrome - XXY Abnormalities of Chromosome Number Trisomy Chromosomal Structural Abnormalities Deletions - absence of normal chromosomal material; can be terminal or interstitial Duplications - presence of an extra copy of a chromosomal segment Inversions - Intrachromosomal re-arrangement such that the rearranged section is inverted Ring Chromosome - Fusion of the ends of a chromosome that forms a circle or ring Chromosomal Structural Abnormalities Translocations Interchromosomal rearrangement; can be balanced (all chromosomal material is present) or unbalanced (chromosomal material has been gained or lost); can be reciprocal or Robertsonian Structural Abnormalities Group Exercise Karyotype CD-Rom Congenital Anomalies Structural abnormalities present at birth Are usually not identified with a known genetic cause Cause may be a combination of genetic and environmental factors Children at Risk for Congenital Anomalies Positive family history of structual anomalies Child with one known structural anomaly The IUGR infant The mentally retarded child The unusual appearing child Maternal Risk Factors Diabetes Phenylketonuria (PKU) Seizure disorder Alcohol and substance abuse Recurrent pregnancy loss Teratogens Environmental substances or exposures that result in functional or structural disability. Any agent which when given to or ingested by a pregnant woman can produce a permanent morphologic or functional abnormality. Agents Which Cause Teratogenesis Drugs and Chemicals; Alcohol Infections (viruses, TORCH) Radiation exposure Fat-Soluble Vitamins Nicotine Heat Fetal Susceptibility to Teratogens Gestational age at the time of exposure Drug dosage Route of administration of agent Genetic predisposition of fetus to respond to a particular agent Gestational Susceptibility Factors Days 1-17 Little effect Days 18-60 Period of organogenesis Extreme sensitivity to major structural abnormalities Days 61-270 Considerably reduced risks Functional abnormalities can still occur Pedigrees A pictorial representation or diagram of the family history. Allows visualization of relationships of affected individuals to other family members. May indicate a pattern of inheritance Helps pinpoint persons who should be examined or tested. Pedigree Format 3 generations AT LEAST!! Note name of informant Roman numerals for generations Number individuals on pedigree across families Pedigree Pointers Seek a balance between the need for asking specific versus general questions. Ask specific questions about each individual as you construct the pedigree (birth defects, mental retardation, specific traits relevant to the diagnosis or concern) Ask general questions about the whole family or section of a family. Can you think of any family characteristics (traits) or medical problems in more than one family member? Pedigree Reminders Multiple reproductive relationships “Have you had children/pregnancies by anyone else?” “Did you have any pregnancies prior to this relationship?” Don’t forget half-sibs, abortions, miscarriages, stillbirths, previous marriages. Pedigree Pointers Indicate possible relationships “Sometimes when there is one family member with cleft lip and palate, there are others in the family with little indentations in their lower lip, heart problems at birth, or poor vision and joint pain. Can you think of anyone in your family with anything like that?” Use words the clients will understand seizures = fits = fainting spells Group Exercise Final Group Work
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