Two-Factor Crosses Mendel also wanted to see what happens when you study the inheritance of two traits at the same time. He first crossed true-breeding plants that had smooth yellow peas (RRYY) with plants that had wrinkled green peas (rryy). All of the resulting F 1 offsrping had round yellow peas (RrYy).
Two-Factor Crosses He then allowed the F 1 plants to selfpollinate and studied the F 2 offspring Both F 1 plants were heterozygous round and yellow. Mom RrYy X Dad RrYy Possible gene combinations in the gametes of each parent: RY, Ry, ry, ry, RrYy
Results: Two-Factor Crosses Phenotype ratio 9 round yellow peas : 3 round green peas : 3 wrinkled yellow peas : 1 wrinkled green peas. Some of the resulting plants had combinations of phenotypes that were not found in either parent! The principle of independent assortment: genes for different traits can segregate independently during the formation of gametes (sperm and egg).
Summary of Mendel s Principles The inheritance of traits is determined by individual units known as genes. Genes are passed from parents to offspring. When two or more forms (alleles) of a gene exist, some forms may be dominant and others may be recessive. In sexually reproducing organisms, each adult has two copies of each gene one from each parent. These genes are segregated from each other when gametes are formed. The alleles for different genes usually segregate independently of one another.
Dihybrid Practice Problem #3 In cats, short fur (S) is dominant over long fur (s) and black fur (B) is dominant over white fur (b). A male cat with short black fur (heterozygous for both traits) breeds with a female cat with long white fur. Predict the possible genotypes and phenotypes of the offspring. Dad SsBb Mom ssbb sb SB Sb sb sb SsBb Ssbb ssbb ssbb sb SsBb Ssbb ssbb ssbb sb SsBb Ssbb ssbb ssbb sb SsBb Ssbb ssbb ssbb
Mom ssbb sb SB Sb sb sb SsBb Dad SsBb Ssbb ssbb ssbb sb SsBb Ssbb ssbb ssbb sb SsBb Ssbb ssbb ssbb sb SsBb Ssbb ssbb ssbb Genotype ratio = Phenotype ratio = 4/16 SsBb, 4/16 ssbb, 4/16 ssbb, 4/16 Ssbb 4/16 short black fur: 4/16 long white fur 4/16 long black fur 4/16 short white fur
Dihybrid Practice Problem #4 In humans, brown eyes (B) are dominant over blue eyes (b) and almond-shaped eyes (A) are dominant over round eyes (a). A man with blue, almond-shaped (heterozygous) eyes is having a baby with a woman with brown (homozygous), round eyes. What kind of eyes could their child have? Dad bbaa ba ba ba ba Mom Ba BBaa Ba Ba Ba
Dad bbaa ba ba ba ba Mom Ba BBaa Ba Ba Ba Genotype ratio = 8/16 : 8/16 Phenotype = 8/16 Brown, almond-shaped 8/16 brown, round eyes
Exceptions to Mendel s Rules Incomplete dominance The heterozygous phenotype is a mixture of both of the homozygous phenotypes. Example: Flower color in snapdragons Red flowers = RR White flowers = rr Pink flowers = Rr
Incomplete Dominance Practice Problem #5 A pink-flowered snapdragon is crossed with a white-flowered snapdragon. What are the possible genotypes and phenotypes of the offspring? Red = RR R r White = rr r Rr rr Pink = Rr r Rr rr Genotype Ratio = 2/4 Rr 2/4 rr Phenotype Ratio = 2/4 pink-flowered plants 2/4 white-flowered plants
Exceptions to Mendel s Rules Codominance both alleles are expressed at the same time in an organism. Example: Hair color in cattle Red hair = RR White hair = WW Roan (Mix of red and white hair) = RW
Codominance Practice Problem #6 In chickens, the allele for black feathers (B) is codominant with the allele for white feathers (W). Heterozygous chickens appear speckled. If two speckled chickens crossed, what type of offspring would they produce? Black = BB White = WW Speckled = BW B W B BB BW W BW WW Genotype Ratio = 1/4 BB, 2/4 BW, 1/4 WW Phenotype Ratio = 1/4 black, 2/4 speckled, 1/4 white
1. Regular Dominance, Co Dominance, or Incomplete Dominance?
2. Regular Dominance, Co Dominance, or Incomplete Dominance?
3. Regular Dominance, Co Dominance, or Incomplete Dominance?
4. Regular Dominance, Co Dominance, or Incomplete Dominance?
5. Regular Dominance, Co Dominance, or Incomplete Dominance?
6. Regular Dominance, Co Dominance, or Incomplete Dominance?
7. Regular Dominance, Co Dominance, or Incomplete Dominance?
8. Regular Dominance, Co Dominance, or Incomplete Dominance?
Exceptions to Mendel s Rules Multiple alleles genes that have more than two possible alleles. Example: Human blood type Three alleles exist: A, B and O Possible blood types: Blood Type A B AB O Genotype(s) AA or AO BB or BO AB OO
Blood type Practice Problem #7 A woman with type AB blood and a man with type O blood have a child. What are the possible blood types of the child? Blood Type A B AB Genotype(s) AA or AO BB or BO AB A B O AO BO O AO BO O OO Genotype Ratio = 2/4 AO, 2/4 BO Phenotype Ratio = 2/4 Type A, 2/4 Type B
Blood type Practice Problem #8 A woman with type A and a manwith type B blood have a child with type O blood. What are the parent s genotypes? Blood Type Genotype(s) B O A AA or AO A AB AO B AB BB or BO AB O BO OO O OO Dad s Genotype = BO Mom s genotype = AO
Exceptions to Mendel s Rules Polygenic traits traits that are controlled by two or more genes. Often show a wide variety of phenotypes. Example: Human skin color The End