Chromosomes and Human Genetics 10.4-10.5 12.2-12.3 Prenatal Diagnosis College Prep Biology Mr. Martino

Introduction Karyotype: photograph of special organization of metaphase chromosomes Paired with homologue Autosomes arranged from largest to smallest Sex chromos. are last pair Used to detect genetic disorders

10.4 Chromosomal Basis of Inheritance Autosomes: the chromsomes (numbered 1 – 22) that are determine all traits but sex - homologous Sex chromsomes: 23rd pair of chromos. that determine sex in humans Females: XX (homo. chromos.) Males: XY (non homo. chromos.)

10.4 Sex Determination in Humans Different species have different sex determination systems Humans have XY system – males determine sex as do spinach, grasshoppers, crickets, and roaches Females determine sex in fishes, birds, strawberries, and butterflies Most plants (monoecious) and some animals (hermaphrodites) do not have separate sexes. Ex. Corn, earthworms and snails

10.5 Linked Genes 1908 – William Bateson and Reginald Punnett discovered inheritance that contradicted Mendelian principles F2 generation should have been 9:3:3:1 – but it was 3:1 Discovered these genes are on the same chromosome Linked Genes: genes found on the same chromosomes inherited together

Early 1900’s, Thomas Hunt Morgan discovered crossing over Used Drosophila melanogaster Offspring should have been 1:1:1:1, but were not Crossing over: is the exchange of corresponding segments between two homologous chromosomes Effects several genes at once Genetic Recombination: the production of gene combos different from those carried by the original chromosomes

Pedigrees Males are squares Females are circles Pedigree: a tool used to assemble a family’s history in a family tree Males are squares Females are circles Full shade represent afflicted Partial (if present) represent carriers Marriage lines join parents on sides Sibling lines join siblings at top

10.5 Inheritance Patterns Autosomal Recessive: any recessive trait carried on a regular chromosome Ex. Cystic fibrosis, PKU, albinism Autosomal Dominant: any dominant trait carried on a regular chromosome Ex. Polydactyly, Huntington’s, achondroplasia

Sex-linked traits Sex-linked traits: are determined by genes located on the sex chromosomes Usually the X Ex. Fruit fly eye color, hemophilia, some colorblindness, red feline coat color

Hutchinson-Gilford progeria syndrome Autosomal dominant- individual only needs one dominant allele to show the trait Result of mutation Usually die in early teens Both boys pictured are not yet 10…both died while in their teens of complications with old age

Changes in Chromosome Structure Occasionally chromo. structure is altered 1. Deletion: loss of a chromosome segment 2. Duplication: gene sequences that are repeated 3. Inversion: section of DNA is reversed 4. Translocation: broken piece of chromo. attaches to nonhomo. chromo.

Do Changes in Chromosome Structure Ever Evolve? Most chromo. changes tend to be selected against Duplications seem to be the exception Duplications may provide opportunity for beneficial mutations – still have at least one good gene Several seem to be pivotal in evolution Duplications, inversions, & translocations helped Of our 23 pairs of chromos. 18 are virtually identical to chimps & gorillas – other 5 differ at inverted and translocated regions

Changes in Chromosome Number Aneuploidy: when there is an incorrect number of chromos. Major cause of reproductive failures Polyploidy: having 3 or more of each type of chromo. Possible in plants Occurs in some insects, fishes, etc. Lethal in humans – only 1% survive to birth

Nondisjunction: one or more pairs of chromosomes fail to separate during meiosis Down’s syndrome (Trisomy 21) Turner’s syndrome – females that have only 1 X Klinefelter’s Syndrome – males have 2 X’s

Prospects in Human Genetics Abortion: removal of a fetus from the uterus Genetic Screening: carriers are identified in order to learn the probability of certain disorders in offspring Genetic Counseling: diagnosis, pedigree, genetic testing for parents with potential to pass on severe genetic disorders

Ultrasound: uses sound waves to produce image of fetus Prenatal Diagnosis: there are several methods available to determine many different genetic disorders before birth Ultrasound: uses sound waves to produce image of fetus No known risks takes measurements Routine in pregnancies and may detect fetal problems Amniocentesis: withdrawal of some amniotic fluid containing fetal cells for karyotype must be 14-16 weeks Results take several weeks 1% risk of miscarriage

Chorionic Villi Sampling (CVS): a tube is inserted into cervix & a small amount of chorion is removed This is fetal tissue and DNA 8 – 10 weeks Results in hours 2% miscarriage risk Fetoscopy: a needle-thin tube is inserted into uterus for direct veiwing 10% miscarriage risk In-vitro fertilization: fertilize egg in lab and select zygotes with no disorder to implant into mother

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