3 Mendelian Genetics Flashcards
A plant with genotype AaBb produces seeds with phenotypes in the following proportions: 60% round and yellow, 20% round and green, 10% wrinkled and yellow, and 10% wrinkled and green. What can you infer about the dominance relationships and how these traits are inherited?
The results suggest that both seed shape and color are governed by two genes, with round (R) being dominant to wrinkled (r) and yellow (Y) being dominant to green (y). The 9:3:3:1 ratio typical of a dihybrid cross between heterozygous individuals (RrYy x RrYy) supports this inference. The proportions indicate that the traits segregate independently according to Mendelian inheritance.
You have a tall plant that is phenotypically dominant (Tall) but you are unsure if it is homozygous or heterozygous for the height trait. What experimental approach could you use to determine its genotype?
You could perform a testcross by crossing the tall plant with a homozygous recessive (short) plant. If any offspring are short, the tall plant must be heterozygous (Tt). If all offspring are tall, the tall plant is likely homozygous dominant (TT).
Explain how the law of independent assortment can lead to genetic diversity in offspring and provide an example involving two traits.
The law of independent assortment states that the alleles for different traits segregate independently of each other during gamete formation. For example, if a plant with genotype AaBb (where A and a represent the gene for seed shape, and B and b represent seed color) is crossed, the alleles for seed shape (A or a) and seed color (B or b) assort independently, leading to a variety of combinations in the offspring, such as AB, Ab, aB, and ab.
During gamete formation, how does the law of segregation ensure that each gamete carries only one allele for a gene? Illustrate this with an example.
The law of segregation states that the two alleles for a gene separate during meiosis, so each gamete receives only one allele. For example, in a plant with genotype Aa, during meiosis, the alleles A and a separate, resulting in gametes that are either A or a, ensuring that each gamete has only one allele.
A researcher has a flower with a dominant purple color but is unsure if it is homozygous or heterozygous. They cross it with a homozygous recessive white flower. What results would indicate that the purple flower is heterozygous?
If the purple flower is heterozygous (Pp), the offspring would include both purple and white flowers in a 1:1 ratio. If the purple flower is homozygous dominant (PP), all offspring will be purple. Therefore, the presence of white flowers among the offspring indicates that the purple flower is heterozygous.
Using the laws of probability, calculate the expected phenotypic ratio of a monohybrid cross between two heterozygous individuals (Aa x Aa).
In a monohybrid cross (Aa x Aa), the possible genotypes of the offspring are AA, Aa, and aa. The phenotypic ratio is 3:1, where 3 parts show the dominant phenotype (AA and Aa) and 1 part shows the recessive phenotype (aa). The probability of each genotype is 1/4 AA, 1/2 Aa, and 1/4 aa.
In a flower species exhibiting incomplete dominance, red (RR) and white (WW) flowers produce pink (RW) flowers. Predict the phenotypic ratio of a cross between two pink flowers.
Crossing two pink flowers (RW x RW) produces the following phenotypic ratio: 1 red (RR): 2 pink (RW): 1 white (WW). This is because the possible genotypes are 1/4 RR, 1/2 RW, and 1/4 WW.
Explain the inheritance of the ABO blood system and why the I^A and I^B alleles are co-dominant. How does multiple allelism contribute to blood type diversity?
The ABO blood system involves three alleles: I^A, I^B, and i. The I^A and I^B alleles are co-dominant, meaning both are expressed simultaneously in individuals with genotype I^AI^B, resulting in AB blood type. The i allele is recessive and only expressed in the absence of I^A or I^B. Multiple allelism increases blood type diversity, as there are four possible blood types (A, B, AB, and O) based on combinations of these alleles.
Compare the phenotypic expression of a heterozygote in incomplete dominance versus co-dominance. Provide an example for each.
In incomplete dominance, the heterozygote exhibits a blended phenotype, such as pink flowers in a cross between red and white flowers. In co-dominance, both alleles are fully expressed simultaneously, such as the AB blood type where both A and B antigens are present on the red blood cells.
Describe how pleiotropy, epistasis, polygenic inheritance, and multifactorial inheritance differ in their effects on phenotype. Provide an example for each.
Pleiotropy: One gene influences multiple traits. Example: Marfan syndrome, where a single gene mutation affects connective tissue, leading to cardiovascular, skeletal, and ocular problems.
Epistasis: One gene affects the expression of another gene. Example: Coat color in Labrador retrievers, where the E gene affects the expression of the B gene, determining whether the coat is black, brown, or yellow.
Polygenic Inheritance: Multiple genes contribute to a single trait. Example: Human height, influenced by several genes contributing to the overall height.
Multifactorial Inheritance: Traits are influenced by both genetic and environmental factors. Example: Skin color, which is determined by multiple genes and also influenced by environmental factors like sun exposure.
