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 CrossSB Sb sB sb
SB SSBBshort, black
SSBbshort, black
SsBB__________
SsBbshort, black
Sb SSBbshort, black
SSbbshort, brown
SsBb__________
Ssbb__________
sB SsBBshort, black
SsBbshort, black
ssBB__________
ssBb__________
sb SsBbshort, black
Ssbb__________
ssBblong, black
ssbblong, 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 ____________________– 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 21st 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
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