GENETIC CROSSES. Monohybrid Crosses

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1 GENETIC CROSSES Monohybrid Crosses

2 Objectives Explain the difference between genotype and phenotype Explain the difference between homozygous and heterozygous Explain how probability is used to predict the results of genetic crosses Use a Punnett Square to predict the results of monohybrid crosses Explain the difference between codominance and incomplete dominance

3 Genotype Genotype is the genetic makeup of an organism consists of the alleles that the organism inherits from its parents Remember Capital letters are dominant traits Lower case letters are recessive traits Examples: GG Gg gg

4 Genotype An organism is homozygous when both alleles are alike Also called purebred Ex: GG, gg An organism is heterozygous when the two alleles are difference Ex: Gg

5 Phenotype Phenotype is the appearance of an organism as a result of its genotype The genotype is expressed (shown) as the phenotype Examples GG, Gg = green gg = yellow

6 Probability Probability is the likelihood that a specific event will occur Mendel s experiments showed that the F 2 generation will show the dominant trait 75% (or ¾, 3:4) of the time and the recessive trait will be shown 25% (or ¼, 1:4) of the time

7 Monohybrid Crosses A monohybrid cross is a cross between individuals that involves one pair of contrasting traits Biologists use Punnett squares to predict the probability that certain traits will be inherited by offspring

8 Monohybrid Cross Examples Homozygous X Homozygous Homozygous X Heterozygous

9 Monohybrid Cross Examples Heterozygous X Heterozygous Genotypic Ratio vs. Phenotypic Ratio The genotypic ratio is the ratio of genotypes that appear in the offspring The phenotypic ratio is the ratio of the offspring s phenotypes

10 Punnett Square Practice A black coat (B) is a dominant trait in guinea pigs A brown coat (b) is a recessive trait in guinea pigs What is the genotype of a homozygous black guinea pig? BB What is the genotype of a heterozygous black guinea pig Bb What is the genotype of a homozygous brown guinea pig? bb

11 Punnett Square Practice Predict the offspring that will result from a cross of two heterozygous black guinea pigs What are the genotypes? BB, Bb, bb What are the phenotypes? Homozygous black (25%) Heterozygous black (50%) Homozygous brown (25%)

12 Testcrosses A testcross is used to determine whether an organism is homozygous or heterozygous for a trait To do this cross, an organism of an unknown genotype is crossed with a homozygous recessive individual If the offspring all show the dominant trait, then the unknown genotype of the parent is homozygous If at least one of the offspring show the recessive trait, then the unknown genotype of the parent is heterozygous

13 Incomplete Dominance If the F1 offspring have a phenotype in between that of the parents, then the relationship is called incomplete dominance Ex: A plant with red flowers and a plant with white flowers produce pink flowers RR = red, rr = white, Rr = pink

14 Codominance Codominance occurs if both alleles are expressed in the offspring Ex: Red flowers (RR) and White flowers (WW) make Red and White flowers (RW)

15 Review Questions Explain the difference between genotype and phenotype. Genotype is the genetic makeup of an organism and consists of the alleles that the organism inherits from its parents Phenotype is the appearance of an organism as a result of its genotype Explain the difference between homozygous and heterozygous. Homozygous alleles are the same Heterozygous alleles are different

16 Review Questions What is probability? Probability is the likelihood that a specific event will occur What is the difference between incomplete dominance and codominance? Incomplete dominance offspring have a phenotype that is in between two parents Codominance both alleles are expressed in the offspring