Chapter 2 Boy or Girl Females XX Males XY ovum 22 of the 23 chromosomes are matching pairs called autosomes Number them from longest 1 to shortest 22 23rd pair consists of sex chromosomes The X is relatively large and the Y is short and carries little genetic material Determine sex by whether an X bearing or a Y bearing sperm fertilizes the Y chromosome is crucial for male sexual development One that switches on the production of male hormones Two involved in the formation of male sex organs Multiple Births Fraternal dizygotic twins the most common type of multiple birth resulting from the release and fertilization of two ova Genetically no more alike than ordinary siblings account for 1 in every 62 births in U S and 1 in every 80 births in Canada Older maternal age fertility drugs and in vitro fertilization are major causes of the dramatic rise in fraternal twinning and other multiple births Table 2 2 page 55 a Ethnicity occurs more commonly in blacks b Family history occurs more often among women whose mothers and sisters gave birth to fraternal twins c Nutrition occurs less often with poor diets occurs more often in women who are tall and overweight d Number of births more likely with each additional birth e Season and geographic region increases with exposure to sunlight and during summer months Identical monozygotic twins zygote that has started to duplicate separates into two clusters of cells that develop into two individuals Same genetic makeup Same around the world about 1 in every 330 births Animal research has discovered environmental influence that induces this type temperature changes variation in oxygen levels and late fertilization Children of single births often are healthier and develop more rapidly than Unclear in humans twins Twins are mostly born early Genetic Inheritance The way genes from each parent interact One chromosome inherited from the mother and one from the father Each form of gene allele If allele s from both parents are alike homozygous If allele s are different the child is heterozygous Dominant Recessive Inheritance Occurs in many heterozygous pairings Only one allele affects the child s characteristics dominant Second allele which has no effect is recessive Table 2 2 page 57 Example dark hair is dominant D and blond hair is recessive b A child who inherits a homozygous pair of dominant alleles DD and a child who inherits a heterozygous Db will both be dark haired Db can still pass that trait so it is called carriers of the trait Blond hair results from a pair of recessive alleles bb Recessive disorders PKU affects the way the body breaks down proteins Born with two recessive alleles lack an enzyme that damages the CNS By year one they are permanently retarded In U S and Canada require that each newborn tested for PKU Children normally have light hair and blue eyes Modifier genes enhance or dilute the effects of other genes Incomplete Dominance A pattern of inheritance in which both alleles are expressed in the phenotype resulting in a combined trait or one that is intermediate between the two Sickle cell trait sickle cell anemia a heterozygous condition present in many black Africans causes red blood cells to become crescent moon shaped especially under low oxygen conditions Heterozygous individuals are protected from the disease under most circumstances Common in Blacks because carriers of it are more resistant to malaria Polygenic Inheritance Many influences and interactions among genes A pattern of inheritance in which many genes affect the characteristic in question height weight intelligence and personality Complex and much is still unknown Down Syndrome Most common chromosomal disorder occurring 1 out of every 1 000 births 95 of cases are from failure of the 21st pair of chromosomes to separate during meiosis so the baby inherits 3 of these chromosomes rather than 2 Sometimes called trisomy 21 Mosaic pattern causing some but not all body cells to have the defective chromosomal makeup less genetic material less extreme Consequences a Mental retardation memory and speech problems limited vocabulary and slow motor development b Physical features short stocky build flattened face protruding tongue almond shaped eyes and unusual crease running across the palm of hand Infants born cataracts hearing loss heart and intestinal defects c Fewer individuals die early because of medical advances More than half of affected individuals who live past 40 show symptoms of Alzheimer s disease most common form of dementia Caring for a baby poses extra challenges for parents a Facial deformities often lead to breathing and feeding difficulties b Rarely smile show poor eye contact and explore objects less persistently c When parents encourage engagement they develop favorably in emotional social and motor skills clearly environmental factors affect how well children with Down syndrome fare Down syndrome rises dramatically with maternal age because geneticists believe that the ova present in a woman s body since her own prenatal period weakens over time As a result chromosomes do not separate properly 5 to 10 of cases the extra genetic material originates from the father Prenatal Diagnosis and Fetal Medicine Prenatal diagnostic methods medical procedures that permit detection of development problems before birth Doctors can insert a needle into the uterus and administer drugs to the fetus Surgery has been performed to repair hearts lungs and diaphragm malformations Fetuses with blood disorder have been given blood transfusions These techniques result in complications premature labor and miscarriages Least aggressive method Ultrasound high frequency sound waves are beamed at the uterus Maternal Blood Analysis second month of pregnancy developing organism s cells enter the maternal bloodstream Isolated cells can be examined for genetic defects Prenatal tests 1 Ultrasound high frequency sound waves are directed at the woman s abdomen noninvasive gives information about structural abnormalities of the fetus 2 Fetal MRI uses a magnet and radio images to generate pictures of the organs 3 CVS 9 12 weeks a small sample of the placenta is removed more invasive small risk of limb deformity Can detect genetic and chromosomal abnormalities 4 Amniocentesis 14 20 weeks a sample of amniotic fluid is withdrawn and analyzed Small risk of miscarriage can detect genetic and chromosomal abnormalities 5 Maternal blood screening 16 18th week identifies elevated risk of birth
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