Chromosomes and Human Genetics: A Comprehensive Overview

1. Chromosomes and Human Genetics10.4-10.512.2-12.3Prenatal Diagnosis College Prep Biology Mr. Martino

2. 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

3. 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.)

4. 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

5. 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

6. 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

7. Pedigrees • 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

8. 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

9. 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

12. 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

13. 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.

15. 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

16. 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

17. 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

18. 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

19. 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

20. 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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