Mendelian Genetics and Inheritance Patterns (1C) - Bio & Biochem

1. What is the phenotypic ratio for $Aa \times Aa$ under incomplete dominance?

   $1:2:1$ (three distinct phenotypes). Incomplete dominance produces three phenotypes in the ratio due to the intermediate heterozygote differing from both homozygotes.

2. Which cross is a testcross for a dominant-phenotype individual of genotype $A\_$?

   Cross with homozygous recessive: $A\_ \times aa$. A testcross reveals unknown genotypes by crossing with homozygous recessive, exposing hidden recessive alleles in offspring.

3. What does pleiotropy mean for the effect of a single gene?

   One gene influences multiple phenotypic traits. Pleiotropy occurs when a single gene mutation affects multiple systems, as in sickle cell anemia impacting blood and organs.

4. What is the expected phenotypic ratio for a monohybrid cross $Aa \times Aa$ with complete dominance?

   $3:1$ (dominant:recessive). In complete dominance, the monohybrid cross yields three dominant phenotypes to one recessive due to heterozygote masking.

5. What is the definition of homozygous at a gene locus?

   Having two identical alleles (e.g., $AA$ or $aa$). Homozygosity occurs when both alleles at a locus are the same, resulting in consistent expression regardless of dominance.

6. What is the expected genotypic ratio for a monohybrid cross $Aa \times Aa$?

   $1:2:1$ ($AA:Aa:aa$). The genotypic ratio reflects the probabilities of homozygous dominant, heterozygous, and homozygous recessive outcomes from segregation.

7. What is incomplete dominance (as judged by heterozygote phenotype)?

   Heterozygote shows an intermediate phenotype. Incomplete dominance results from neither allele fully dominating, blending traits in heterozygotes like in snapdragon flower color.

8. What is the definition of a dominant allele in a heterozygote?

   An allele expressed in the phenotype of $Aa$. Dominant alleles override recessive ones in heterozygotes, determining the observed trait in Mendelian inheritance.

9. What does Mendel's law of segregation state about allele separation?

   Alleles separate into different gametes during meiosis. Mendel's law of segregation explains how paired alleles divide during gamete formation, ensuring each gamete receives one allele.

10. What is the definition of heterozygous at a gene locus?

    Having two different alleles (e.g., $Aa$). Heterozygosity arises from differing alleles at a locus, allowing for interactions like dominance or codominance in phenotype.

11. What inheritance pattern is suggested by a trait that appears more often in males and has no father-to-son transmission?

    X-linked recessive inheritance. X-linked recessive traits skip generations and affect males more due to their single X chromosome lacking a compensating allele.

12. What is the difference between genotype and phenotype?

    Genotype = alleles; phenotype = expressed traits. Genotype refers to the genetic makeup, while phenotype is the observable outcome influenced by both genetics and environment.

13. What is epistasis in the context of two-gene inheritance?

    An allele at one gene masks/modifies another gene's phenotype. Epistasis involves gene interactions where one gene's product affects another's expression, altering expected dihybrid ratios.

14. What is the probability that $Aa \times Aa$ produces an $aa$ offspring?

    $\frac{1}{4}$. The probability of homozygous recessive offspring is $\frac{1}{2} \times \frac{1}{2}$ from each parent's contribution of the recessive allele.

15. What is the definition of a gene in Mendelian genetics?

    A heritable DNA unit that affects a trait. Genes serve as the fundamental units of heredity, encoding information in DNA that determines specific traits passed from parents to offspring.

16. What is the definition of an allele?

    An alternative version of a gene at a locus. Alleles represent variations of a gene occupying the same chromosomal locus, leading to potential differences in trait expression.

17. What is the definition of a recessive allele in a heterozygote?

    An allele masked in $Aa$; expressed only in $aa$. Recessive alleles require homozygosity for expression, as they are suppressed by dominant alleles in heterozygotes.

18. What does variable expressivity mean for individuals with the same genotype?

    Phenotype severity varies among individuals with the genotype. Variable expressivity results from modifiers causing differing degrees of trait severity among individuals with identical genotypes.

19. What does Mendel's law of independent assortment state (with unlinked genes)?

    Alleles of different genes assort independently into gametes. Independent assortment occurs when genes on different chromosomes segregate randomly, producing varied gamete combinations.

20. What is codominance (as judged by heterozygote phenotype)?

    Heterozygote fully expresses both alleles. Codominance allows both alleles to contribute equally to the phenotype, as seen in ABO blood types with distinct expressions.

21. What is the probability that $Aa \times Aa$ produces a heterozygous $Aa$ offspring?

    $\frac{1}{2}$. Heterozygous offspring require one dominant and one recessive allele, occurring in two of four possible combinations.

22. What is the expected phenotypic ratio for a dihybrid cross $AaBb \times AaBb$ with complete dominance and unlinked genes?

    $9:3:3:1$. The dihybrid ratio results from independent assortment of two genes, combining monohybrid ratios for four phenotypic classes.

23. What offspring phenotypic ratio indicates that a dominant-phenotype parent is $Aa$ in a testcross $A\_ \times aa$?

    $1:1$ (dominant:recessive). The 1:1 ratio in a testcross confirms heterozygosity, as half the offspring inherit the recessive allele from the tested parent.

24. What does variable penetrance mean for a genotype in a population?

    Not all individuals with the genotype show the phenotype. Incomplete penetrance means environmental or genetic factors prevent phenotype manifestation in some genotype carriers.

25. What is the probability that $AaBb$ produces a gamete $ab$ assuming independent assortment?

    $\frac{1}{4}$. With independent assortment, each allele pair segregates separately, so the chance of both recessive alleles in a gamete is $\frac{1}{2} \times \frac{1}{2}$.

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