Chapter 11 Mendel and Punnett Squares Key Vocabulary to review: Genotype Dominate Alleles Homozygous Phenotype Recessive Alleles Heterozygous What is Genetics? Who is Gregor Mendel? Why did he study pea plants? Generations Genotype Phenotype Punnett Square Example Parent F1 F2 1
Mendel s Conclusions: 1. Principle of Independent Assortment: 2. Principle of Dominance: 3. Principle of Segregation: Probability and Punnett Squares Probability: Punnett Squares: For each of the following traits, observe and record your phenotype and possible genotypes. Use the class copy of the phenotypes to help you understand the dominant and recessive traits. Human Phenotypes and Traits Dominant Recessive 1. Bent little Curves toward Straight (b) Finger others (B) 2. Face Shape Round (R) Square (r) 3. Mid-digit hair Hair present (H) Hair absent (h) 4. Cleft Chin Present (L) Absent (l) 5. Hair Line Widow's Peak (W) Widow's Peak Absent (w) 6. Eyebrows I Thick (T) Thin (t) 7. Eyebrows II Separate in the Center (S) Connected in the Center (s) 8. Eyelash Long (E) Short (e) Length 9. Hair type Curly (C) Straight (c) 10. Thumb Hitchhiker's (T) Straight Thumb (t) 11. Tongue dexterity Can roll (R) Unable to roll (r) Your Phenotype Your 2
Mendel s Principle of Dominance: Some alleles are dominant while others are recessive. Dominant alleles will always show if they are present. Monohybrid Cross Examples REMEMBER: Homozygous Phenotypes Heterozygous Phenotype Genotype Phenotypes Phenotypes 3
Mendel s Principle of Independent Assortment: The principle of independent assortment states that genes for different traits can segregate independently during the formation of gametes. Independent assortment helps account for the many genetic variations observed in plants, animals, and other organisms. The Punnett square of two HETEROZYGOUS plants always predicts a 9 : 3 : 3 :1 ratio in the F2 generation. 9: Dominant both traits 3: Dominant trait one, recessive trait two 3: Dominant trait two, recessive trait one 1: Recessive both traits Dihybrid Cross Example ** FOIL each parent s genotype independently sorts to set up the cross *** Key: 1. Dimples and the ability to roll your tongue are both dominant traits. Cross a man who is heterozygous for dimples and cannot roll his tongue with a woman who does not have dimples but is homozygous dominant for tongue rolling. 4
Incomplete Dominance Crosses When one allele is not completely dominant over another it is called incomplete dominance. In incomplete dominance, the heterozygous phenotype is between the two homozygous phenotypes. BLEND = RED + WHITE = Pink Ex: Snapdragon Flowers Red = White = Pink = 1. What results may be expected in a cross between a red flowered snapdragon and a white flowered snapdragon? 2. What results may be expected in a 3. Try a cross between two pink flowered snapdragons 5
Codominance Crosses In codominance, both alleles contribute to the phenotype. BOTH SHOW = BLACK + WHITE = SPECKLED For Example: In certain varieties of chicken, the allele for black feathers is codominant with the allele for white feathers. Heterozygous chickens are speckled with both black and white feathers. The black and white colors do not blend to form a new color, but appear separately. 1. A black cat crossed with an orange one results in a striped condition know as tortoiseshell. If two tortoise-shell cats should mate, what would be the genotypic and phenotypic outcome? Multiple Alleles Genes that are controlled by more than two alleles are said to have multiple alleles. An individual can t have more than two alleles. However, more than two possible alleles can exist in a population. Blood types are an example of multiple alleles 1. A man with blood type AB marries a woman with blood type O. Work out the possible blood types of their offspring. 2. Work out the possible blood types of the offspring according to a cross between a parent who is AO and BO Blood. 6
Sex Linked Crosses 1. Eye color is a sex-linked trait in fruit flies carried on the X chromosome R for dominant red and r for white. Determine the sex and eye color for a father who has white eyes and mother who is homozygous dominate for red eyes. 2. Hemophilia is a recessive sex linked disorder that is carried on the X chromosome - h for hemophilia and H for normal blood clotting. Cross a father who has hemophilia with a mother who is a carrier (heterozygous) for the disease, but does not show symptoms for hemophilia. 7