Chapter 3: Independent Assortment

How many chromosomes do human cells have at the end of meiosis I and meiosis II?

At the end of meiosis I and II, there are 23 chromosomes present in the human cell

How many chromatids do human cells have at the end of meiosis I and meiosis II

At the end of meiosis I, there are 92 chromatids present in the human cell

At the end of meiosis II, there are 46 chromatids present in the human cell

What are the two directions that the law of segregation can be applied to?

• Inferring genotypes from phenotypic ratios

• Predicting the phenotypic ratios from parents with known genotypes

How did Mendel establish the law of independent assortment?

He performed dihybrid crossing with pea plants containing two different traits, producing a F2 that demonstrated all 4 possible combinations of the traits that were sorted into a 9:3:3:1 phenotypic ratio

What is the law of independent assortment?

Mendels second law that states alleles for different genes can independently segregate from each other

How does the independent assortment during meiosis explain the 9:3:3:1 ratio in a dihybrid cross?

During metaphase I, when homolog chromosome pairs align randomly and separate independently, they produce four possible gamete combinations that generates the 9:3:3:1 phenotypic ratio

How can recombinants be detected in diploids?

By performing test-cross to differentiate the recombinant gametes from the parental type gametes

When test-crossing is performed on a dihybrid heterozygote with a tester containing recessive alleles…

Test cross progenies are produced consisting of 2 parental types and 2 recombinant gametes

• with the parental types resembling the original phenotype that was used to generate the dihybrid

How does meiosis provide a mechanical explanation for Mendel’s two laws?

The random alignment and independent separation of chromosomes during meiosis (especially anaphase I) explain Mendel’s law of equal segregation and independent assortment

How are pure lines of plants or animals formed?

through the performance of selfing

What is produced when two heterozygous plants from different backgrounds are crossed?

They produce a hybrid vigor, an offspring that has improved or increased function of any biological quality compared to the parents but isnt always guaranteed

What occurs when a hybrid vigor undergoes meiosis?

Gene assortment breaks up the favorable allelic combination, resulting in the next generation having fewer vigor members

What is recombination?

A process during meiosis that produces haploid cells containing new allele combinations that are different from the original parental haploid genotypes

How do you determine the # of gamete genotypes that will be produced from a hybrid?

By taking the # of gamete genotypes of each gene pair and squaring them by the # of gene pairs present

• Ex: A/a ; B/b = 2² = 4

How do you determine the # of distinct genotypes a cross can produce?

By taking the # of genotypes of each gene pair and squaring them by the # of gene pairs present

• Ex: A/a ; B/b x A/a ; B/b —> 3² = 9

How do you determine the # of distinct phenotypes a cross can produce?

By taking the # of phenotypes of each gene pair and squaring them by the # of gene pairs present

• Ex: A/a ; B/b x A/a ; B/b —→ 2² = 4

How do you predict the progeny ratio?

A punnet square is used to predict each possible allele combination and sort them into a ratio