Biology 9 Lesson 5: Crossing two pairs of traits (cont’d)
1. Theoretical Summary
1.1. Mendel explains the results of the experiment
– Through the result of hybridization, we have: the rate of segregation of each pair of traits in F2 to be:
- Yellow: Green ≈ 3: 1
- Smooth: Wrinkled 3: 1
– From the experimental results on Menden, each pair of traits is determined by a genetic factor.
- Gene A is responsible for yellow seeds, gene a is for green seeds
- Gene B for smooth grain, gene b for wrinkled grain
- Purebred golden, smooth genotype: AABB
- Purebred blue, wrinkled genotype: aabb
Mendel made a diagram to explain the results of the experiment as follows:
- Genotype AABB during gametogenesis for an AB gamete, genotype aabb for an ab gamete → fertilization AaBb
- → F1 forms gametes, due to independent segregation and free combination of corresponding gene pairs to produce 4 gametes with equal proportions Ab, AB, aB, ab
- Due to the random fusion of 4 paternal and 4 maternal gametes F2 there are 16 zygotes
– Analyze the results:
The ratio of each genotype in F2:
- Yellow, smooth beads: 1AABB, 2AABb, 2AaBB, 4AaBb à 9A-B-
- Yellow seeds, wrinkled: 1Aabb, 2Aabb à 3A-bb
- Green, smooth seeds: 1aaBB, 2aaBb à 3aaB-
- Green, wrinkled seeds: 1aabb à 1aabb
+ Ratio of each phenotype in F2: 9 yellow seeds, smooth: 3 yellow seeds, wrinkled: 3 green seeds, smooth: 1 green seed, wrinkled
- Ratio Gold: Green: 3: 1; Smooth: wrinkle ratio: 3: 1
- Phenotypic ratio in F2: 9 yellow – smooth: 3 yellow – wrinkled: 3 green – smooth: 1 green- wrinkled ⇒ 9: 3: 3:1
⇒ Law of Independent Assortment: Genetic factors have segregated independently during gametogenesis.
1.2. What does the law of independent assortment mean?
– The law of independent assortment has shown one of the reasons for the appearance of extremely rich combinatorial variations in mating species. This type of variation is one of the important raw materials for breeding and evolution.
– When pairs of alleles segregate independently, sexual reproduction will produce a large amount of combinatorial variation, which explains the diversity of organisms.
* Combination Variation: A new phenotype appears in offspring due to the recombination of alleles from both parents. Combinational variation depends on the number of gene combinations (gametophyte combinations) in the hybrid, the larger the number of gamete combinations, the higher the combinatorial variation.
The number of gamete combinations = number of male gametes x number of female gametes in that cross
– If it is known that genes for certain traits segregate independently, it is possible to predict the outcome of phenotypic segregation in the next generation. Therefore, through crossbreeding, humans can recombine genes, creating new varieties with high yield and good quality.
1.3. Contents of the law of independent assortment
– When crossing parents that differ in two pairs of purebred traits that are genetically contrasting independently for F2 the ratio of each phenotype is equal to the product of the proportions of its constituent traits.
1.4. True test condition
– Crossbred parents must be purebred for the trait to be monitored.
– A gene determines a trait, the dominant gene must be completely dominant.
– The number of individuals in hybrid generations must be large enough for accurate statistics.
– Each pair of genes for a trait is located on different pairs of chromosomes.
– Same chromosome segregation during gamete formation and random fusion of gametes during fertilization.
– Gametes and zygotes have the same vigor, expression of the trait must be complete.
2. Illustrated exercise
2.1. Type 1: Knowledge related to the law of independent assortment
Question 1: How did Mendel explain the results of his experiments?
– Mendel explained his experimental results by the independent assortment and free combination of pairs of genes that regulate those pairs of traits during gametogenesis and fertilization.
Verse 2: According to Mendel, given n pairs of heterozygous genes that segregate independently, what is the number of gametes?
A. The number of gametes is 2n.
B. The number of gametes is 3n.
C. The number of gametes is 4n.
D. The number of gametes is 5n.
According to Mendel, for n pairs of heterozygous genes that segregate independently, the number of gametes is 2 .n.
2.2. Form 2: Applying the law of independent assortment to solve some genetic problems
Question 1: In chickens, the gene for leg hair is dominant over the gene for no leg hair; Light blue feathers are an incomplete dominant trait between black (dominant) and white fur. For two purebred chicken breeds with white feathers and no leg feathers, black feathers can mate with each other Ffirst.
1. F chickenfirst continue to interbreed, the phenotypic ratio in F2 how?
2. F chickenfirst mated with chickens without leg feathers, white feathers. Determine the result of the hybridization. Show that the genes for these traits are located on autosomes and segregate independently.
1. F chickenfirst continue to interbreed, the phenotypic ratio in F2:
– Because the hairy trait is completely dominant to the hairless trait => the ratio of F2 is 3:1
– Because green fur trait is intermediate between black and white => ratio in F2: 1:2:1
=> ratio in F2 of the hybrid is: (3:1) x (1:2:1) = 3:6:3:1:2:1
→ The phenotypic ratio is: 6 with leg hair, light green hair: 3 with leg hair, black hair: 1 with leg hair, white hair: 2 without leg hair, light green hair: 1 without leg hair, Black feathers: 1 hairless legs, white fur.
2. The result of the hybrid
– Gene convention:
- A: hairy
- A: hairless
- B: black feathers
- B: white feathers
=> Ffirst have genotype: AaBb
– Ffirst hybrid with hairless and white => AaBb X aabb
Ffirst: AaBb x aabb
G: Ab, aB, AB, ab ab
Genotype: 1 AaBb, 1 Aabb, 1aaBb, 1aabb
Phenotypic: 1 with leg hair, light blue hair: 1 with leg hair, white hair: 1 without leg hair, light blue hair: 1 without leg hair, white hair.
Verse 2: In humans, gene A is for curly hair, gene a for straight hair, gene B for black eyes, and gene b for blue eyes. Please choose the mother with the appropriate genotype in the following cases to give birth to a child with curly hair and black eyes?
P: Mom with curly hair, black eyes (AABB) x Dad with straight hair, blue eyes (aabb)
G: AB ab
KH: 100% curly hair, black eyes
⇒ Choose the answer EASY
3.1. Essay exercises
Question 1: What is the content of the law of independent assortment?
Verse 2: In tomato, gene L is for red stem, 1 for green stem (green), N for split leaf, n for whole leaf (called potato leaf).
Here are the results of the 5 crosses:
1. Red stem, split leaves x Green stem, split leaves
319 red stems, split leaves: 103 red stems, whole leaves: 315 green stems, split leaves: 105 green stems, whole leaves
2. Red stem, split leaves x Red stem, whole leaves
216 red stems, split leaves: 209 red stems, whole leaves: 68 green stems, split leaves: 72 green stems, whole leaves
3. Red stem, split leaves x Green stem, split leaves
720 red stems, split leaves: 238 red stems, whole leaves: 0 green stems, split leaves: 0 green stems, whole leaves
4. Red stem, split leaves x Green stem, whole leaves
301 red stem, split leaf: 0 red stem, whole leaf: 305 green stem, split leaf: 0 green stem, whole leaf
5. Red stem, whole leaves x Green stem, split leaves
78 red stems, split leaves: 82 red stems, whole leaves: 79 green stems, split leaves: 86 green stems, whole leaves
Determine the genotype of P in the above 5 crosses.
Question 3: When crossing two purebred varieties of a plant species Ffirst. For Ffirst continue to pollinate each other, at F2 obtained 3202 trees, including 1801 tall trees with red fruit. Knowing that the corresponding traits are short plant, yellow fruit; inherited by the rule of complete dominance and no gene permutations occur.
1. Determine the genotype and phenotype of P, write a hybrid diagram from P to F2.
2. Determine the number of individuals (mean) of each possible phenotype in F2.
3.2. Multiple choice exercises
Question 1: In peas, the gene for smooth seeds is dominant, wrinkled seeds are recessive; Yellow seeds are dominant, green seeds are recessive. Two pairs of genes are located on two different pairs of chromosomes. Pairs of peas heterozygous for seed shape genes and seed color genes crossed with wrinkled seed plants and heterozygous for seed color; The phenotypic segregation of the hybrid seeds will follow which of the following ratios?
A. 3:1 B. 3:3:1:1
C. 9:3:3:1 D. 1:1:1:1
Verse 2: What is the practical significance of the law of segregation?
A. Shows the segregation of traits in the progeny.
B. Determine the mode of inheritance of the trait.
C. Identify dominant and recessive traits for application in breeding.
D. Identify pure lines.
Question 3: According to Mendel’s experiment, when purebred peas were crossed with smooth yellow seeds and wrinkled green seeds, Ffirst smooth, golden grain. When giving Ffirst pollination then F2 has a phenotypic ratio of
A. 9 yellow, smooth: 3 yellow, wrinkled: 3 green, wrinkled: 1 green, smooth.
B. 9 yellow, smooth: 3 green, smooth: 3 green, wrinkled: 1 yellow, wrinkled.
C. 9 yellow, smooth: 3 yellow, wrinkled: 3 green, smooth: 1 green, wrinkled.
D. 9 yellow, smooth: 3 yellow, wrinkled: 3 green, wrinkled: 1 yellow, smooth.
Question 4: In Mendel’s experiment on crossing two pairs of traits, when Ffirst What will be the results obtained in terms of phenotype?
A. 1 yellow, smooth: 1 green, wrinkled.
B. 3 yellow, smooth: 1 green, wrinkled.
C. 1 yellow, smooth: 1 yellow, wrinkled: 1 green, smooth: 1 green, wrinkled
D. 4 yellow, smooth: 4 green, wrinkled: 1 yellow, wrinkled: 1 green, smooth.
Question 5: According to Mendel, the essence of the law of independent assortment is
A. pairs of separately inherited traits.
B. traits of different types combine to form combinatorial variation.
C. pairs of independently inherited traits.
D. pairs of genetic factors segregate independently during meiosis.
After studying the lesson Hybrid two pairs of traits (period 2), you need to:
– Explain the results of the experiment of two pairs of traits according to Mendel’s point of view
– State the law of independent assortment
– Analyze the meaning of the law of independent assortment for selection and evolution