Biology chapter 17
1. Definitions:
- True-breeding: Organisms that, when self-fertilized, produce offspring identical to themselves for a specific trait.
- Self-fertilization: The process where an organism fertilizes itself, leading to offspring with the same genetic makeup.
- Cross fertilization (hybridization): The mating of two different true-breeding individuals to produce offspring with mixed traits.
- Trait: A specific characteristic or feature of an organism, such as flower color or seed shape.
- Genotype: The genetic makeup of an organism, represented by alleles (e.g., AA, Aa, aa).
- Phenotype: The observable characteristics or traits of an organism resulting from the genotype and environment.
- Heterozygous: An organism with two different alleles for a trait (e.g., Aa).
- Homozygous: An organism with two identical alleles for a trait (e.g., AA or aa).
2. Monohybrid Crosses:
- P generation: True-breeding parents (e.g., AA x aa).
- F1 generation: All offspring are heterozygous (e.g., Aa).
- F2 generation: Offspring from F1 self-fertilization (e.g., Aa x Aa) resulting in a 3:1 phenotype ratio (dominant to recessive) and a 1:2:1 genotype ratio (1 AA: 2 Aa: 1 aa).
3. Phenotype Ratios:
- For a 3:1 ratio, the fraction of dominant phenotypes is 3/4.
- Out of 200 total offspring, the number with the dominant trait would be 200 * (3/4) = 150.
4. Conclusions from Monohybrid Crosses:
- Dominant vs Recessive: Dominant traits mask the expression of recessive traits in heterozygous individuals.
- Genes and Alleles: Traits are controlled by genes, which have different forms called alleles.
- Law of Segregation: During gamete formation, the two alleles for a trait segregate from each other so that each gamete carries only one allele.
5. Meiosis and Segregation: The separation of homologous chromosomes during Anaphase I of meiosis explains segregation.
6. Definitions:
- Heterozygous: Genotype Rr.
- Homozygous: Genotype RR or rr.
- If an individual has the recessive trait, their genotype must be: rr.
- If an individual is homozygous dominant, their genotype must be: RR.
- If an individual has the dominant trait, their genotype is: RR or Rr.
7. Testcross:
- Purpose: To determine the genotype of an individual with a dominant phenotype by crossing it with a homozygous recessive individual.
- Parents: Unknown genotype (e.g., R_) and homozygous recessive (rr).
- Possible outcomes: If offspring show the recessive trait, the unknown parent is heterozygous; if all offspring show the dominant trait, the unknown parent is homozygous dominant.
8. Possible Gametes:
- AA: A
- Aa: A, a
- aa: a
- AaBB: AB, Ab, aB, ab
- AABB: AB
- AaBb: AB, Ab, aB, ab
- aabb: ab
9. Dihybrid Crosses:
- P generation: True-breeding parents (e.g., AABB x aabb).
- F1 generation: All offspring are heterozygous (e.g., AaBb).
- F2 generation: Offspring from F1 self-fertilization (e.g., AaBb x AaBb) resulting in a 9:3:3:1 phenotype ratio.
10. Conclusion from Dihybrid Crosses: The inheritance of one trait does not affect the inheritance of another trait, demonstrating the principle of independent assortment.
11. The Law of Independent Assortment states that alleles for different traits are distributed to gametes independently of one another. This principle is explained during Metaphase I of Meiosis, where homologous chromosomes line up at the metaphase plate and can assort into gametes in various combinations. Some genes do not follow this law due to linkage, where genes located close together on the same chromosome tend to be inherited together, thus violating independent assortment.
12. Pedigree Analysis helps determine if a trait is dominant or recessive by observing inheritance patterns.
- Autosomal Dominant:
- Males vs females: Affected individuals can be either male or female.
- Affected children with unaffected parents: Typically, this is not possible; affected individuals usually have at least one affected parent.
- Genotype of unaffected individuals: Typically homozygous recessive (aa).
- Genotypes of affected individuals: Can be either homozygous dominant (AA) or heterozygous (Aa).
- There are no "carriers" of Autosomal Dominant traits because one copy of the dominant allele is sufficient to express the trait.
13. Autosomal Recessive:
- Males vs females: Both males and females can be affected equally.
- Affected children with unaffected parents: This is possible if both parents are carriers.
- Genotype of affected individuals: Homozygous recessive (aa).
- Genotypes of unaffected individuals: Can be either homozygous dominant (AA) or heterozygous (Aa).
- Carriers are heterozygous (Aa) genotype.
14. X-linked Recessive:
- Males vs females: Males are more frequently affected than females.
- Affected children with unaffected parents: This is possible if the mother is a carrier.
- Genotype of unaffected males: Homozygous dominant (XY).
- Genotype of affected males: Hemizygous recessive (X^aY).
- Genotype of affected females: Homozygous recessive (X^aX^a).
- Genotypes of unaffected females: Can be either homozygous dominant (XX) or heterozygous (X^aX).
- Carriers are heterozygous (X^aX) and must be female.
15. X-linked recessive traits are inherited differently in males because males have only one X chromosome. If they inherit the recessive allele on the X chromosome, they will express the trait since there is no corresponding allele on the Y chromosome to mask it.
16. - Simple Complete Dominance: One allele completely masks the other. The heterozygote looks like the dominant phenotype.
- Incomplete Dominance: Neither allele is completely dominant, resulting in a blend of traits. The heterozygote has an intermediate phenotype.
- Codominance: Both alleles are expressed equally in the phenotype. The heterozygote shows both traits distinctly.
17. The ABO blood system demonstrates both complete dominance and codominance. The blood types and corresponding genotypes are:
- Type A: Genotype AA or AO
- Type B: Genotype BB or BO
- Type AB: Genotype AB (codominance)
- Type O: Genotype OO
Using this information, you can determine genotypes based on blood type and family history.
18. Environmental conditions can influence phenotypic expression, such as temperature affecting coat color in some animals. The term 'norm of reaction' refers to the range of phenotypes that can result from a single genotype due to environmental influences.
19. Pleiotropy is when one gene influences multiple traits. An example is Marfan syndrome, which affects connective tissue and leads to long limbs, heart issues, and eye problems.
20. Dominant traits aren’t necessarily more common in a population. Their frequency depends on factors like fitness and reproductive success. For instance, achondroplastic dwarfism is dominant but can be rare due to health complications. Lethal dominant diseases are rare because individuals with them often do not survive to reproduce, and if they do, the effects typically appear later in life.
21. Lethal recessive diseases can persist in a population because carriers (who have one copy of the allele) do not show symptoms. They can pass the allele to their offspring, allowing it to remain in the gene pool despite its lethality when homozygous.