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

SB Sb sB sb

SB SSBB SSBb SsBB SsBb

Sb SSBb SSbb SsBb Ssbb

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

Down Syndrome

Turner Syndrome