Genetics II and Genetic Disorders

Genetics II Dihybrid Crosses ____________________________________ Examples: –Pea shape and pea color –Coat length and coat color in rodents –Plant height and flower color Solve using Punnett square –Punnett square has 16 boxes

Sample Problem In guinea pigs, the allele for short hair (S) is dominant to long hair (s), and the allele for black hair (B) is dominant over the allele for brown hair (b). What is the probable offspring phenotype ratio for a cross involving two parents that are heterozygotes for both traits?

Sample Problem Short hair = dominant = SS or Ss Long Hair = recessive = ss Black coat = dominant = BB or Bb Brown coat = recessive = bb Parents are heterozygous (____________) ______ x ______ (gametes done by the FOIL method) –SB, Sb, sB, sb and SB, Sb, sB, sb

Sample Problem Cross SBSbsBsb SBSSBBSSBBSSBbSsBBSsBbSsBb SbSSBbSSBbSSbbSsBbSsbbSsbb sB sb

Sample Problem Cross SBSbsBsb SBSSBB short, black SSBb short, black SsBB __________ SsBb short, black SbSSBb short, black SSbb short, brown SsBb __________ Ssbb __________ sBSsBB short, black SsBb short, black ssBB __________ ssBb __________ sbSsBb short, black Ssbb __________ ssBb long, black ssbb long, brown

Sample Answer What is the probable offspring phenotype ratio for a cross involving two parents that are heterozygotes for both traits? –9/16 Short coats, black –3/16 ____________________ –1/16 Long coats, brown

Incomplete Dominance ______________________: one allele is not completely dominant over another allele –Heterozygous is intermediate (blending)

Codominance Codominance: both alleles of a gene contribute to the phenotype –____________________________________

_________________ Karyotype: diagram of chromosomes by size and number –Humans have 22 pairs of autosomes and 1 pair of sex chromosomes (XX or XY) Normal Human Male Karyotype

Genetic Disorders Nondisjunction: failure of chromosomes to separate during meiosis –EX: ________________________________________ Down syndrome: (trisomy 21) Genetic disorder in which there is an extra copy of the 21 st chromosome –Mild to severe physical and mental defects –___________________________________________ Turner Syndrome: (monosomy X) Genetic disorder in which females only have 1 X chromosome –reproductively sterile (can’t reproduce) –underdeveloped ovaries –Occurs in about 1 in every 2,500 females

Down Syndrome

Turner Syndrome