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BIOLOGY 11 Mrs. Garrison MENDELIAN GENETICS GENETICS -- science of how inherited traits are passed from parent child Each chromosome contains many different genes gene - allele - Eggs and sperm carry only one set of chromosomes genotype - phenotype – dominant allele – recessive allele – First person to work out idea of dominant and recessive traits was Gregor Mendel purple flowers x white flowers F1 all purple flowers F1 x F1 1 ¾ purple flowers ¼ white flowers F2 (ratios approx.) heterozygous homozygous NON-MENDELIAN GENETICS Since Mendel’s time, we’ve found many traits which aren’t inherited in such simple fashion: polygenic inheritance - more than one gene (locus) controls the trait multiple alleles - more than 2 alleles possible in whole population incomplete dominance - heterozygote looks different than either homozygote. codominance - 2 different alleles do not show simple dominant/recessive relationship; heterozygote shows both traits 2 sex-linked traits - carried on the sex chromosomes (X, unless told diff.) X chromosome is larger than Y chromosome has many more genes (incl. all genes nec. for survival) males only get one copy of these genes Y chromosome has genes to determine “maleness” GENETICS APPLICATIONS GENETIC COUNSELING - look at family history to determine if individual has a greater than average risk of carrying a certain allele, and what the risk to their children would be. (THIS TYPE OF COUNSELING REQUIRES AN EXTENSIVE KNOWLEDGE OF GENETICS, AND SHOULD BE DONE BY EXPERTS!) amniocentesis - karyotype - TYPES OF DISORDERS AUTOSOMAL DOMINANT DISORDERS- single gene (although individual has 2 of that gene), dominant allele AUTOSOMAL RECESSIVE DISORDERS - single gene, recessive allele cystic fibrosis - 1 in 2000 births (1 in 20 adults are carriers) 3 PKU sickle cell anemia albinism Tay Sachs SEX-LINKED DISORDERS - usually on X chromosome red-green colorblindness hemophilia (2 types) Duchenne’s muscular dystrophy cleft palate night blindness severe combined immunodeficiency disease (SCID) CHROMOSOMAL ABERRATIONS - problems w/all or part of a chromsome (broken, extra, missing chromosomes); often due to non-disjunction during meiosis. Down syndrome - trisomy 21; increases w/age of mother 1 in 3000 for mothers under 30 yrs. 1 in 9 for mothers over 48 yrs. Turner’s syndrome - only one X, no second sex chromosome 1 in 2000 female births phenotypic female, sterile short stature, some mental impairment (not much) Klinefelter’s sysndrome - two or more X, one Y 1 in 1000 male births phenotypic male, sterile tall, slender, enlarged breasts, underdev. genitalia, sparse body hair Cri du chat (cat cry syndrome) - missing part of chromosome 5 deletion - occurs in 1 in 50,000 births small head, wide-spaced eyes, cry sounds like kitten 4