1. If non-disjunction occurs during meiosis, what is the possible result?
(select all correct answers)
An egg with an extra chromosome
A sperm with an extra chromosome
A sperm with a missing chromosome
An egg with a missing chromosome
2. Which of the following could be the genotype of a “true-breeding” organism?
RrXX
Dd
CcRr
CCRR
3. When Mendel crossed two dihybrid parents from the F1 generation, what famous phenotype ratio in the F2 generation did he observe?
9:3:3:1
1:2:1
0:4
3:1
4. Which of the following is true of genes and alleles?
(select all correct answers)
Pairs of alleles are found on homologous chromosomes
Alleles code for different versions of the same trait
Both alleles on a chromosome come from one parent
A pair of alleles give the genotype
1. If non-disjunction occurs during meiosis, it can result in the following:
- An egg with an extra chromosome: This happens when both homologous chromosomes fail to separate during meiosis I or sister chromatids fail to separate during meiosis II. As a result, the resulting egg will have an extra copy of that particular chromosome.
- A sperm with an extra chromosome: Similar to the case with eggs, non-disjunction can also occur with sperm, resulting in a sperm cell with an extra chromosome.
- A sperm with a missing chromosome: Non-disjunction can also lead to a missing chromosome in a sperm cell when the chromosomes fail to separate properly during meiosis.
- An egg with a missing chromosome: Again, non-disjunction during meiosis can result in an egg cell with a missing chromosome due to the failure of chromosomes to separate accurately.
2. A "true-breeding" organism refers to an organism that, when self-crossed or crossed with another true-breeding organism, consistently produces offspring with the same phenotype as the parent. True-breeding organisms are homozygous for the trait in question.
Out of the given genotypes, the only one that represents a true-breeding organism is "CCRR". In this case, both alleles for each trait (represented by "C" and "R") are the same, indicating that the organism is homozygous for each trait.
3. When Mendel crossed two dihybrid parents from the F1 generation, he observed a phenotype ratio of 9:3:3:1 in the F2 generation. This ratio is also known as the "dihybrid cross ratio" and represents the phenotypic proportions of the different combinations of traits resulting from the cross.
4. Genes are specific segments of DNA that code for particular traits, while alleles are alternative forms of a gene. Here are the correct statements regarding genes and alleles:
- Pairs of alleles are found on homologous chromosomes: Each individual has pairs of homologous chromosomes, and each homologous chromosome carries one allele of a gene. One allele comes from the individual's mother, and the other allele comes from the individual's father.
- Alleles code for different versions of the same trait: Alleles represent different versions or variations of a particular gene. For example, there can be different alleles for eye color gene, such as "brown" and "blue" alleles.
- Both alleles on a chromosome come from one parent: False statement. As mentioned earlier, the pairs of alleles on homologous chromosomes come from both parents. One allele comes from the individual's mother, and the other allele comes from the individual's father.
- A pair of alleles gives the genotype: Yes, the combination of two alleles (one from each parent) determines an individual's genotype for a particular trait.