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11-3 Exploring Mendelian Genetics: Independent Assortment and Dihybrid Crosses
Transcript of 11-3 Exploring Mendelian Genetics: Independent Assortment and Dihybrid Crosses
Henderson Independent Assortment and
Dihybrid Crosses Dihybrid Crosses After first looking at monohybrid crosses Mendel already showed how alleles segregate during the formation of gametes. Now he wanted to know if they segregated independently. Did the segregation of one pair of alleles affect the segregation of another pair of alleles? For example does the gene that determines if a seed is wrinkled or round affect the gene for seed color? Learning objectives: Students will understand the principle of independent assortment and be able to apply this principle to a two-factor cross (dihybrid cross) What have we learned so far about Mendel's Principles? The inheritance of biological characteristics is determined by individual units known as _______________.
Offspring _____________ them from parents. Essential Question: 1. parental generation: RRYY x rryy What is the genotype for the F1 generation? F1 generation: RrYy The F1 generation all have the same heterozygous genotype They all express the dominant phenotype: yellow and round The results of this experiment gave Mendel the hybrids he needed to cross to determine if the genes segregate independently. Do dominant genes Round (R) and Yellow (Y) segregate? Do recessive genes wrinkled (r) and green (y) segregate? How did he test if dominant and recessive genes segregated? By allowing the F1 hybrids (RrYy) to self-pollinate. Principle of If 2 or more alleles for a gene exist:
one can be dominant
one can be recessive In most sexually reproducing organisms:
each adult has 2 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 ___________________ of one another. Describe the principle of independent assortment. remember: When F1 plant flowers and produces gametes, the 2 alleles segregate from each other so that each gamete carries only 1 copy of each gene. If a seed is wrinkled, will it also always be yellow? Independent Assortment Each gamete (of our example F1 plants) produces 2 types of gametes-those with allele for short and those will allele for tall plants. CA Content Standards in 9-12 grade Science : Biology
2.g. Students know how to predict possible combinations of alleles in a zygote from the genetic makeup of the parents.
3.b. Students know the genetic basis for Mendel's Laws of Segregation and Independent Assortment. Example (p. 270 fig 11-9): He crossed peas that were homozygous dominant for round (RR) and yellow (YY) with homozygous recessive peas that were wrinkled (rr) and green (yy). F1 = all hybrids The Two-factor Cross F1: crosses involving only 1 trait Mendel wanted to study dihyrbid crosses crosses involving 2 traits Again, he began with "pure parents." This time the parents were pure for both of the traits he was studying. This means they were homozygous So one parent was YYRR and the other parent was yyrr. Results He produced plants that were all yellow with round seeds. dominant traits Mendel know that these plants were hybrids/heterozygous Genotype: RrYy Do the dominant and recessive genes segregate (always stick together)? 2 hypotheses of Independent Assortment Mendel's Observations: 556 seeds: 315 yellow and round (both dominant traits) 32 wrinkled and green (both recessive traits) BUT...another 105 had yellow and wrinkled and 105 had green and round (one dominant and one recessive trait) Mendel's Conclusions Based on his findings, Mendel concluded that genes for different traits are separated and distributed to gametes interdependently of one another. Phenotype ratios in a dihybrid cross: parents being crossed: RrYy x RrYy Summary of Mendel's Principles: The inheritance of biological characteristics is determined by individual units known as genes. Genes are passed from parents to their offspring. Summary of Mendel's Principles In cases where 2 of more forms (alleles) of the gene for a single trait exist, some forms of the gene may be dominant and others may be recessive. Independent Assortment Summary of Mendel's Principles: In most 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.