9.3.2 Independent Assortment: An Explanation

Question 1

Independent Assortment: An Explanation

Answer 1

• Using dihybrid crosses, Mendel determined whether pairs of alleles segregate dependently or independently of each other.
• The outcome of his experiments (9:3:3:1 in the F2 generation) showed that pairs of alleles segregate independently of one
another.
• The process of meiosis shows the physical basis for independent assortment:
· Alleles (alternate forms of genes) are located on chromosomes. During meiosis, the
chromosomes segregate into gametes independently of one another.

Question 2

review

Answer 2

• In performing dihybrid crosses between plants,
  Mendel observed a 9:3:3:1 ratio in the F2 offspring. This
  outcome is expected for two heterozygous pairs of alleles that segregate independently of each other

Question 3

note

Answer 3

• The F1 offspring are heterozygous for both traits. Each has the genotype YyRr. The gametes of both parents of the F1 generation form by independent assortment. Each side of the Punnett square shows the four possible gametes that each parent can form. The 16 combinations possible in the F2 generation occur in a 9:3:3:1 phenotypic ratio.
• During meiosis, homologous chromosomes segregate into different gametes. The diagram on the left shows how recombination occurs during meiosis.
• In F1 individuals, cells contain homologous chromosomes for each of the two traits. A gamete can receive one of four different combinations of two sets of alleles, depending on how the chromosomes line up during metaphase. The four possibilities are YR, Yr, yR, and yr.

Question 4

If recombination does not occur, which one of the following F2 genotypes could not be produced from the parental cross of plants with the YYRR and yyrr genotypes?

Answer 4

• yyRR

Question 5

What is the chance of a cross YySs × YYSS producing an offspring of genotype YYSs?

Answer 5

• 1/4

Question 6

Which one of the following is true of the F2 generation produced from the parental cross of plants with the genotypes YYRR and yyrr?

Answer 6

• The phenotype ratio is 9:3:3:1

Question 7

In pea plants, seed color can be yellow (Y, dominant) or green (y, recessive). Seeds can be smooth (S, dominant) or wrinkled (s, recessive). Given the cross, YySs × Yyss, what is the probability that an offspring has green, wrinkled seeds?

Answer 7

• 2/16

Question 8

In pea plants, seed color can be yellow (Y, dominant) or green (y, recessive). Seeds can be smooth (S, dominant) or wrinkled (s, recessive). Given the cross, YySs × YYSS, what is the probability of producing an offspring with the yellow, smooth seed phenotype?

Answer 8

• 16/16

Question 9

Which one of the following is the reason that a plant with the genotype YyRr can form four different gametes?

Answer 9

• Each of the different alleles Y or y can combine with each one of the different alleles R or r.